99mTc Hynic-rh-Annexin V scintigraphy for in vivo imaging of apoptosis in patients with head and neck cancer treated with chemoradiotherapy

Purpose The purpose of this study was to determine the value of 99mTc Hynic-rh-Annexin-V-Scintigraphy (TAVS), a non-invasive in vivo technique to demonstrate apoptosis in patients with head and neck squamous cell carcinoma.
Introduction
Apoptosis is an important mechanism of cell death in response to treatment with radiation and many chemotherapeutic agents [1]. To what extent apoptosis contributes to the overall cytotoxic effect of an anti-cancer treatment modality has been the topic of intense research during the last decade [2–4]. The relative contribution of apoptosis to the occurrence of cell death varies greatly both between different tumour types and normal tissues [5].
Recently, in vivo imaging of apoptosis has proven to be feasible by using radiolabelled Annexin V [6–10]. This endogenous human protein has a high affinity for membrane-bound lipid phosphatidylserine (PS), which becomes exposed at the outer leaflet of the cell membrane bilayer at an early stage of the apoptotic process [11, 12]. PS then serves as a recognition site for macrophages that digest and remove apoptotic cells. The reproducibility of the 99mTc-Hynic-Annexin-V scintigraphy has been demonstrated in head and neck squamous cell carcinoma (HNSCC) patients by serial imaging in untreated patients [13] in which the mean difference in uptake was 6%. We have recently demonstrated that 99mTc-Hynic-rh-Annexin V scintigraphy (TAVS) correlates with radiation-induced cytologically confirmed apoptosis in non-Hodgkin lymphoma [14] and can be used to identify patients that have a favourable prognosis [15].
The current standard of practice for patients with advanced stage HNSCC is treatment with concurrent cisplatin-based chemoradiation [16, 17]. Although this is an effective treatment, it also is accompanied by more severe toxicity than radiation alone. We reasoned that TAVS early during treatment might be used to monitor apoptosis induction in tumour and normal tissue and to give an indication of the radiosensitivity of these structures. In future, this would also offer the possibility to adapt the treatment strategy at an early stage on a patient-by-patient basis.
In the present study, we therefore aimed at assessing the feasibility of TAVS as a non-invasive technique to demonstrate treatment-induced apoptosis in vivo, in patients with HNSCC, early during treatment with concurrent chemoradiation. The purpose was to determine the degree of uptake on TAVS in normal tissue, primary tumour and lymph node metastases and to evaluate the treatment-induced Annexin uptake in relation to radiation dose. Furthermore, we questioned whether the differences in uptake would correlate with treatment response.
Materials and methods
Patients included in this study were recruited between January 2004 and March 2005 from a randomised phase III trial in advanced stage HNSCC, investigating the optimal route of cisplatin delivery during cisplatin-based chemoradiation (RADPLAT). Randomisation was between two treatment arms: Arm 1, standard intravenous (IV) administration of cisplatin 100 mg/m2 (1 h before radiotherapy at days 1, 22 and 43) or arm 2, high-dose selective intra-arterial (IA) delivery of cisplatin at a dose of 150 mg/m2 (on days 2, 9, 16 and 23, within 1 h after radiotherapy delivery). Main eligibility criteria included: inoperable squamous cell carcinoma of the oral cavity, oropharynx or hypopharynx, TNM stage T3-4 status of primary oral cavity or oropharyngeal tumours and T2-3-4 for hypopharyngeal tumours, with any N-status, (functionally) inoperable disease, no distant metastases, age at least 18 years, ability to give informed consent and no prior cerebro-vascular accident. Both the randomised trial and the TAVS study were approved by the medical ethics committee of the hospital. Patients were informed about the nature of the study protocol and signed informed consent before enrolment separately for both the randomised trial and the TAVS protocol.
Radiotherapy was given with 4–6 MV photon linear accelerators. Target volume included the primary tumour and the bilateral neck for a dose of 46 Gy in 23 fractions. A boost was given to the known macroscopic tumour extensions at the primary tumour site and lymph node metastases to a dose of 24 Gy in 12 fractions. The total dose delivered was 70 Gy in 35 fractions, five fractions per week, with an overall treatment time of 7 weeks. The radiation technique was either a conventional three-field beam setup (using conventional simulation or virtual simulation with CT scan) or an intensity-modulated radiotherapy (IMRT) plan depending on resources and tumour extent. The IA cisplatin delivery was accomplished by a selective catheterisation procedure using the femoral artery according to the earlier described RADPLAT protocol [18, 19]. Concurrently, with IA cisplatin sodium-thiosulphate was administered to neutralise systemic cisplatin. In both arms, prehydration and posthydration were given.
99mTc-Hynic-rh-Annexin V scintigraphy, diagnostic imaging, radiation treatment planning and image fusion
Baseline TAVS was performed within 2 weeks before the start of concurrent chemoradiation (average interval 5 days, range 1–8). Post-treatment TAVS was done within 48 h after the start of cisplatin chemotherapy. At the time of post-treatment TAVS, patients had received 6 Gy of radiation in case of IV cisplatin, 8 Gy in case of IA cisplatin. Each patient received an average of 847 MBq (range 714–1032 MBq) of 99mTc-Hynic-rh-Annexin V (Theseus Imaging Corporation, Boston, USA) by slow intravenous injection 4 h before the planar imaging and single-photon emission tomography (SPECT) imaging. Planar images were used to assess the biodistribution. SPECT of the head and neck region was acquired by the step-and-shoot mode, one step per 3°, 30 s per frame, matrix size 128 × 128, using a dual-head gamma camera (Genesis, Philips, Best, The Netherlands) equipped with low-energy, high-resolution collimators. For SPECT reconstructions, an iterative algorithm was used and the images were postfiltered using a Butterworth filter (cutoff frequency 0.35, order 5). The use of absolute quantitative analysis of SPECT data was validated by comparison of iterative and FBP reconstruction methods. Transaxial, coronal and sagittal slices were visually examined to evaluate tracer uptake at the tumour sites and in normal tissues. The intensity of the obtained images was corrected by normalisation for the injected radioactive dose and body weight.
Baseline diagnostic imaging with MRI (1.5-T system; Somatom; Siemens Medical Systems, Erlangen, Germany) or spiral CT (Tomoscan AVE1, Philips, Best, The Netherlands or HiSpeed CT, GE Medical Systems, USA) was performed within 3 weeks before the start of treatment and repeated 6–8 weeks after the end of the treatment for the evaluation of treatment response.
Radiation treatment planning was done with our clinical treatment planning system (TPS; U-MPlan, University of Michigan, Ann Arbor, USA). Six patients were treated by IMRT, and the treatment plans were recalculated for the dose at the time of first post-treatment TAVS. The regions of interest (ROI) were delineated manually for each patient on the CT scan. These included the gross tumor volume (GTV) of the primary tumour and/or lymph nodes, the parotid glands and the submandibular glands. In the other seven patients, a standard three-field technique was used by virtual simulation with a CT scan. In these latter patients, the CT scan was imported into the TPS in which the delineations were done. The treatment fields were reconstructed, and the clinically applied dose distribution at the time of TAVS was recalculated in the TPS. The primary tumour and lymph node volumes were calculated by 3-D reconstruction of the delineated GTV.
SPECT, MRI and CT were performed separately. For the SPECT scan, we obtained reproducibility of the positioning of the head and neck as during RT by the use of immobilisation mask fixed to the table by adhesive tape. All images and the radiation dose data were transferred for image fusion to our in-house developed workstation for co-registration (Worldmatch Workstation) in DICOM format [20]. Keeping the limitations of the mask fixation in mind, it was decided that for more accuracy, matching was performed on different anatomical regions for different ROIs, e.g. for the analysis of parotid glands or oropharyngeal tumours, the co-registration was done in such a way that it ensured adequate matching for that area only. For co-registration of ROIs in the neck (e.g. lymph nodes or hypopharyngeal tumours), only the neck and vertebral column was used in the matching procedure. Accurate matching of body contours and bone structures was visually verified. SPECT, composite SPECT/SPECT, CT and SPECT/CT images, obtained using a colourwash technique, were simultaneously examined using linked cursor to evaluate Annexin V uptake in tumour and normal tissues.
The tumour and normal tissue uptake at baseline and post-treatment were calculated as follows. ROIs were delineated in the planning CT. Each ROI was projected using the registration transformation onto the baseline and post-treatment SPECT scans. The area was next sampled by 10.000 random points, and for each point, the Annexin uptake was determined in the SPECT scans by trilinear interpolation. Of these 10.000 samples, the maximum and mean values were computed. In this way, the uptake in the ROI was accurately sampled, although the pixel size in the SPECT scans is relatively large (about 5 mm). For the quantification of Annexin uptake, no attenuation correction was performed because the images were obtained on a conventional gamma camera without a hybrid system. Subsequently, the difference (ΔU) between the post-treatment and baseline uptake was determined by subtraction of the baseline scan from the post-treatment scan. The subtraction was performed on a point by point basis for all 10.000 points. Then, the mean or maximum value of the difference of both scans (subtraction scan) was computed and expressed as ΔU. No correction for background activity was made, as this was automatically eliminated by subtraction of the baseline activity from the post-treatment activity, assuming that the background activity is equal in the baseline and post-treatment scan. Radiation dose parameters (mean and maximum dose in cGy) within each ROI were calculated and correlated with corresponding ΔU parameters. It was observed that the maximum ΔU and the mean ΔU were closely related: a linear relationship between both two parameters was found for each ROI (tumour: r = 0.93, p < 0.0001; parotid gland: r = 0.94, p < 0.0001; submandibular gland: r = 0.90, p < 0.0001; lymph node r = 0.94, p < 0.0001), indicating that both values would have been representative for the whole ROI.
Evaluation of treatment
Six to eight weeks after the end of treatment, the results of therapy were evaluated by means of radiological investigations (by MRI or CT scan and/or ultrasound) and examination under general anaesthesia, with biopsies taken in case of suspicious findings. For residual disease in the neck at the time of evaluation, salvage neck dissection was performed if the patient was judged operable. Follow-up visits were planned every 3 months in the first year after therapy, every 4 months in the second year and less frequent thereafter. A follow-up chest X-ray was performed annually.
Statistical analysis
For quantitative comparison of continuous data Student’s t test was applied. Chi-squared and Fisher’s exact test were used for analysis of categorical data. The Pearson and Spearman rank correlation coefficient r were used to calculate correlations between Annexin uptake and treatment parameters. Locoregional control and survival data were calculated from the start of treatment using the Kaplan–Meier method and log rank testing. Two-sided P values of <0.05 were considered statistically significant.
Results
Sixteen patients gave their consent for participating in the TAVS study. However, one patient refused post-treatment TAVS scans; in one patient, no CT scan in radiation treatment position was available, and in one patient, co-registration with SPECT was unsuccessful due to misalignment because no immobilisation mask and no tongue depressor could be used during SPECT due to pain (Fig. 1). The number of patients included in this analysis on 99mTc-Hynic-rh-Annexin V scintigraphy in advanced stage HNSCC is therefore 13. The mean age of patients was 53 years (range 26–66 years). See Table 1 for the patient, tumour and treatment characteristics.
Fig. 1Sagittal projection of TAVS imaging co-registered with planning CT scan. The solid line represents the contour of the patient during RT-planning CT scan, including tongue depressor. The projected image is the TAVS. a Baseline TAVS; b post-treatment TAVS. Note the mismatch of contours of the head and neck mostly due to different position of the chin in b. The misalignment of the lower neck in a is due to the flexion of the neck. For the analysis, the matching procedure was focused on the mandibular region. This patient was excluded from analysis because of misalignment in bTable 1Patient, tumour and treatment characteristicsPatient numberGenderAge (years)Tumor siteTNM stageTumor volume (cm3)Max ΔU in primary tumourInterval cisplatin infusion and Annexin scan (h)RT techniqueMode of cisplatin administration1M64Oral cavityIII216252.2Conventional 3 fieldIntra-arterially2M58OropharynxIII4720350.3IMRTIntra-arterially3F65OropharynxIV2511253.1IMRTIntra-arterially4M55OropharynxIII263653.5IMRTIntra-arterially5M49HypopharynxII134353.3IMRTIntra-arterially6F66Oral cavityIV3213553.6Conventional 3 fieldIntra-arterially7M60OropharynxIV14719154.3Conventional 3 fieldIntra-arterially8M46OropharynxIV2817052.7Conventional 3 fieldIntra-arterially9M55OropharynxIV414249.9IMRTIntravenously10M57OropharynxIV255151.7IMRTIntravenously11M48OropharynxIV9111751.5Conventional 3 fieldIntravenously12M50HypopharynxIV10013250.3Conventional 3 fieldIntravenously13M26OropharynxIV1055250.1Conventional 3 fieldIntravenouslyM Male, F female, IMRT intensity modulated radiotherapy
Annexin uptake in normal tissue and tumour
At baseline scintigraphy, reproducible physiologic Annexin uptake was detected on the planar images in liver, kidneys, spleen, gall bladder, bone marrow, colon and urinary bladder, as described earlier [7, 14, 21].
In the parotid glands, weak baseline uptake was present in all cases. The post-treatment TAVS showed moderate to strong increase in uptake in 24 of 26 parotid glands (Fig. 2a). The average of the mean number of counts increased from 55 to 88 (p < 0.001). Visual analysis of the increase in uptake showed that these changes were related to the radiation portals and the dose given to the parotid glands (Fig. 3). The difference in Annexin uptake between the post-treatment and baseline TAVS (ΔU) in each parotid gland was correlated with the radiation dose at the time of post-treatment scintigraphy (n = 13 patients, n = 26 parotids). The mean ΔU showed a positive correlation with the mean radiation dose (Pearson coefficient r = 0.59, p = 0.002): parotid glands that received a higher dose of radiation showed a higher Annexin uptake (Fig. 4a). The increase in Annexin uptake in parotid glands that were treated with parotid-sparing IMRT was less than in parotid glands that were treated with a conventional three-field technique (IMRT 29 counts, three-field technique 59 counts, p = 0.02). The given radiation dose at the time of post-treatment TAVS (6 or 8 Gy) and the mode of cisplatin administration (IA or IV) did not affect the uptake in parotid glands. As xerostomia scoring was not documented sufficiently detailed in the files, we interviewed patients alive at last follow-up for xerostomia grading for the purpose of this study. In nine patients alive at last follow-up, the treatment-induced Annexin uptake in parotid glands was related to the subjective xerostomia rating using the EORTC QLQ HN-35 questionnaire [22]. No relation could be established in this small set of patients (data not shown).
Fig. 2Baseline (diamonds) and post-treatment (squares) Annexin activity in parotid glands (a), submandibular glands (b) and tumour (c) for all 13 patients. The data for 26 parotid and submandibular glands represent the right and left gland within each patientFig. 3TAVS imaging co-registered with planning CT scan in frontal plane and axial plane, a and c at baseline, b and d after treatment, from patient number 2. The Annexin uptake is represented by colourwash. In b and d, the isodose lines show the dose distribution in relation to the parotid gland and primary tumour (isodose lines shown, 40, 60, 80 and 95% from outer to inner side of patient). Note the increased treatment-induced Annexin uptake in the right parotid gland, in correspondence to the higher radiation dose distribution, when compared to the left parotid gland. Also note the weak increase in primary tumour Annexin uptake in the right oropharynx after treatment and in the anterior floor of mouth (d)Fig. 4Correlation plot of mean radiation dose (cGy) and the post-treatment increase in Annexin uptake, mean ΔU, in parotid glands (a), normalised for tracer dose and body weight. Circles represent parotid glands treated with IMRT, triangles parotids treated with conventional technique. In b, the correlation between mean radiation dose (cGy) and the post-treatment increase in Annexin uptake, mean ΔU, in submandibular glands is given
For the submandibular glands, a similar pattern as with the parotids was observed: absent/weak baseline uptake and moderate to strong increase after the start of chemoradiation (Fig. 2b). The average of the mean number of counts increased from 85 to 132 (p < 0.001). No correlation between the ΔU and radiation dose was noted (Fig. 4b), probably as all submandibular glands were located within the high-dose region.
The baseline TAVS showed moderate to strong Annexin uptake in the primary tumour in all patients, indicative of spontaneous apoptosis or necrosis. On the post-treatment TAVS, the uptake in tumour clearly increased in nine cases; in the other four, little or no changes occurred (Fig. 2c). The average of the maximum number of counts increased from 127 to 163 (p = 0.007). The difference between post-treatment and baseline TAVS (ΔU) in primary tumour was not correlated with primary tumour volume (Pearson coefficient r = 0.39, p = 0.21) nor with the mode of chemotherapy administration (p = 0.21). In nine patients, 17 lymph node metastases were present in the neck (mean lymph node volume 5 cm3, range 1–19). The treatment-induced Annexin uptake (maximum ΔU) in primary tumour was positively correlated with the uptake in corresponding lymph node metastases (r = 0.73, p = 0.004) as shown in Fig. 5. To verify that the observed effects of Annexin uptake in different structures could not be attributed solely to the administered radioactive dose per patient but was tissue-specific, we calculated the correlation between the ΔU in tumour and parotid glands. No evidence of such an effect was found (r = 0.28, p = 0.18).
Fig. 5Correlation plot of the treatment-induced increase in Annexin uptake, maximum ΔU, in primary tumour and lymph node metastases. Each colour represents an individual patient. Some patients had multiple lymph node metastases originating from the same primary tumour. In these cases, multiple values of treatment-induced Annexin uptake in lymph nodes (on the y-axis) correspond to a single value of the primary tumor (on the x-axis)
Response to treatment, locoregional control and survival
The median follow-up of all patients alive at last follow-up was 30 months (range 24–38). The complete response (CR) rate at the primary tumour site was 85%. CR rate in the neck in the case of nodal metastases was 100%. The overall CR rate was 85%. The estimated locoregional (LR) tumour control rates were 68% at 2 and 3 years. The rates for disease-free survival (DFS) were 54% at 2 and 3 years. Overall survival (OS) rates at 2 and 3 years were 62 and 40%, respectively. In this small subset of 13 patients from a larger randomised phase III trial, there were no differences in response rates, LR control, DFS or OS between the two treatment arms (IV vs IA). No correlation could be established between baseline or treatment-induced Annexin uptake (ΔU) and any outcome parameter (response rate, recurrences and/or survival).
Discussion
This study represents our first clinical experience on in vivo imaging of apoptosis with TAVS in HNSCC patients in which we applied co-registration of multiple imaging modalities. By this, we were able to analyse treatment-induced changes in normal and tumour tissue, early during therapy, after one cycle of cisplatin chemotherapy and 6–8 Gy of radiotherapy. Treatment-induced Annexin uptake in the parotid glands could be visualised, indicative of early treatment related apoptosis at mean radiation doses as low as 3–8 Gy (see Fig. 4a) and one course of cisplatin. The Annexin uptake in the parotids showed a radiation dose-response relationship: Glands that had received higher doses of radiation demonstrated increased Annexin uptake.
Loss of parotid gland function, leading to xerostomia, is an important long-term side effect of radiotherapy, affecting quality of life of patients. It has well been shown by scintigraphy studies [23] and by salivary flow rate studies [24] that reduction of radiation dose to the parotid glands can maintain parotid function, decrease xerostomia and improve quality of life. Most studies on salivary gland function are performed after a full course of RT (typically 60–70 Gy in 6–7 weeks) [23–26]. Our study design with early in vivo imaging indicated that already after low dose of RT (6–8 Gy), parotid glands may be affected and that the physiologic process leading to loss of parotid gland function already starts early. This is in agreement with observations in experimental rodent studies in which apoptosis was induced early, after doses of up to 5 Gy [27]. Similar results were obtained in a monkey model, with early radiation induced apoptosis of serous acinar cells of salivary glands [28]. It was suggested that these early effects are mainly responsible for the acute sialoadenitis, which, at higher RT doses, might progress in chronic xerostomia by damage to and depletion of ductal stem cells within the gland. In the nine patients assessed for xerostomia, no relation could be established with subjective salivary gland function after radiotherapy and parotid gland Annexin uptake. However, the significance of this analysis is limited, as we assessed both parotid glands separately by TAVS, and xerostomia questionnaires give an overall impression of function. We did not include follow-up salivary flow studies. In short, our results indicate indirect evidence of early radiation-induced salivary gland damage and, in future studies, this needs to be correlated with functional outcome parameters.
Due to the large inter-patient variability in the baseline TAVS (Fig. 2c), the subtraction method for assessing treatment-induced changes was chosen over a relative method (e.g. percentage change). In this figure, it is important to notice that the maximum uptake value of the baseline and post-treatment scan might be located in different points within the tumour ROI. Therefore, these data cannot be simply subtracted, but for the calculation of the maximum ΔU in primary tumour and lymph nodes, the baseline scan was first subtracted from the post-treatment scan, and, subsequently, these parameters were calculated from the subtraction scan. The treatment-induced Annexin uptake in primary tumour and pathological lymph nodes showed a positive correlation: Patients with primary tumours with a high Annexin uptake also appeared to have lymph node metastases with high Annexin uptakes. The ΔU in primary tumour and lymph node metastases showed large inter-patient differences (Fig. 5). This variation could not be attributed to differences in the route of cisplatin administration (intravenous or intra-arterial chemoradiation), nor to the radiation doses given (6 or 8 Gy), nor to the primary tumour volume. This positive correlation for ΔU in primary tumour and lymph nodes as well as the large inter-patient differences might represent a tumour-specific apoptotic response.
In our series, repositioning of the patient during SPECT scan in RT position was standard procedure, but due to the limitations in fixation, not optimal. The repositioning facilitated the co-registration of SPECT and CT scan. In one patient, misalignment occurred because the positioning was not accurate due to the fact that the RT position could not be reproduced (Fig. 1). So, for co-registration of (functional) imaging to RT-planning CT scan data in HNSCC, we recommend improvements from our protocol to obtain a reproducible positioning, like the use of an immobilisation mask fixed to the SPECT table, the use of the same mattress under the patient and a laser alignment system. In ongoing studies, we have now incorporated these. In the current series, no attenuation correction was applied because the images were obtained on a conventional gamma camera without a hybrid system. For the purpose of this study, the quantification of Annexin uptake remains valid, as we used the relative increase from baseline to post-treatment uptake, and the degree of attenuation will be equal on both scans.
In this small series of HNSCC, the treatment-induced Annexin uptake (ΔU) in primary tumour did not predict outcome: No correlation between Annexin uptake and early response was observed probably partly due to the high initial response rates (85%). In addition, TAVS also did not predict the locoregional control rates within the first 2 years of follow-up. In our previous experience on TAVS in other tumour sites (mainly lymphoma), a strong correlation has been found between early response (within 3 months) and Annexin uptake [15]. One explanation for the lack of correlation might be that in the current series, these advanced stages HNSCC with large tumours harbour more necrosis, which is also detected by TAVS because of accessibility of PS at the necrotic cell membrane [29]. Van de Wiele et al. [8] showed that in HNSCC, Annexin scintigraphy correlated with histopathological apoptosis (using TUNEL assay) only in the absence of necrosis. Indeed, in this series, baseline Annexin uptake in the HNSCC patients was clearly demonstrated, whereas it was not present in our previous study in lymphoma patients [15]. This may suggest the presence of necrosis in large tumours from solid origin. Other factors that may affect Annexin uptake are the intra- and peri-tumoural lymphocyte infiltration [30]. For the future, we plan to increase the number of HNSCC patients to undergo the TAVS imaging to prove whether or not it can be used for outcome prediction as it was shown in patients with malignant lymphoma and non-small cell lung cancer [14, 15].
In conclusion, co-registration of Annexin V scintigraphy with radiotherapy-planning CT scan showed a radiation-dose-dependent uptake in parotid glands, indicative of early apoptosis during treatment. The inter-individual spread in Annexin uptake in primary tumours could not be related to differences in treatment schedule or tumour volume, but the Annexin uptake in tumour and lymph nodes were closely correlated. This effect might represent a tumour-specific apoptotic response.

annexin
scintigraphy
apoptosis
head and neck cancer
parotid gland


A newly developed tool for intra-tracheal temperature and humidity assessment in laryngectomized individuals: the Airway Climate Explorer (ACE)

The aim of this study is to develop a postlaryngectomy airway climate explorer (ACE) for assessment of intratracheal temperature and humidity and of influence of heat and moisture exchangers (HMEs). Engineering goals were within-device condensation prevention and fast response time characteristics. The ACE consists of a small diameter, heated air-sampling catheter connected to a heated sensor house, containing a humidity sensor. Air is sucked through the catheter by a controlled-flow pump. Validation was performed in a climate chamber using a calibrated reference sensor and in a two-flow system. Additionally, the analyser was tested in vivo. Over the clinically relevant range of humidity values (5–42 mg H2O/l air) the sensor output highly correlates with the reference sensor readings (R2 > 0.99). The 1–1/e response times are all <0.5 s. A first in vivo pilot measurement was successful. The newly developed, verified, fast-responding ACE is suitable for postlaryngectomy airway climate assessment.
Introduction
Total laryngectomy results in a permanent disconnection of the upper and lower airways, and leads to an increase in chronic pulmonary complaints like frequent involuntary coughing, sputum production, and repeated daily forced expectoration in order to clear the airways [5]. Heat and moisture exchangers (HMEs) were developed in an attempt to compensate for the lost functions of the upper airway and have been found to reduce these symptoms and improve the quality of life [1, 4, 7].
In vitro, the moisture exchanging capacity of the HME can be specified with the ISO 9360 standard. However, this standard does not incorporate a standardized method for determining the heat exchange of the HME, and it is not possible to translate the results obtained with the lung model of the ISO 9360 standard to the true in vivo situation. Therefore several questions remain unanswered. Fundamentally: what is the in vivo heat and moisture exchanging capacity of the HME? And clinically, how does the in vivo heat and moisture exchanging capacity relate to that of the upper airway, i.e. where does the HME stand in the range from open stoma to nose breathing? Is a further improvement of the HMEs heat and moisture exchanging capacity possible and desirable, and does this result in a further decrease of the pulmonary complaints due to the use of this rehabilitation device?
Measuring intra airway temperature and humidity is a technically complex issue [15]. Assessing the temperature and humidity variations during normal breathing requires fast sensor response characteristics. No appropriate commercial systems for this purpose are available, and only a few self-constructed validated systems have been published [6, 8, 10, 11, 13, 14]. Two groups evaluated the influence of an HME on intra airway temperature and humidity in laryngectomees. In one study [10] the sensor response time was up to 9 s and in the other study [8, 9] a response time of 2 s is reported which has been shown to be sufficiently fast for normal breathing volunteers, but which might be insufficient for patients.
Our goal was to develop a verified intra-tracheal airway climate analyser with response characteristics fast enough for assessment of end tidal intra-airway temperature and humidity variations during normal breathing in laryngectomized individuals, and for evaluation of the influence of HMEs on these parameters.
Design considerations
Placement of a humidity sensor in the airway itself was rejected. Humidity sensors must be kept clean, and a construction to protect a sensor would be so large that the aerodynamics within the airway would be disturbed. A high airflow around the sensor is required to minimize the response time of the sensor. A constant airflow is necessary for a reliable interpretation of the time dependent signal, and in addition facilitates the use of deconvolution to optimize system response characteristics. Therefore, a sensor environment had to be created in which airflow remains constant and high during the whole breathing cycle, i.e. in a “chamber”. In order to disturb airway aerodynamics as little as possible, a small diameter sample catheter for transportation of breathing air outside the airway to a humidity sensor is required. We decided that the lumen surface of the sample catheter should not exceed 10% of the lumen surface of the trachea. A decrease in lumen area of maximal 10% will result, by the law of mass conservation, in an increase in local flow velocity of 10%. This on its turn will lead to a transbronchial pressure drop of maximal 20% which is supposed to be small enough to avoid substantial deformation of the trachea wall. This means that based on a diameter of 2 cm, which is representative for the trachea of an adult person, the maximal diameter of the sample catheter should not be more than 6 mm.
In order to prevent condensation of water, the temperature of the measurement system must not be below the temperature of the breathing air. In addition, high humidity levels should be avoided to prevent hysteresis of the humidity sensor. Based on the maximal, i.e. end expiratory, temperature of about 36°C, with a relative humidity (RH) of about 97% [16], the breathing air must be heated to at least 38°C in order not to exceed 90% RH (http://www.thunderscientific.com/web_humicalc/index.php). On the other hand, a heated sample catheter should be thermally isolated in order to prevent patient discomfort and artificial heating of inspired air.
We decided to limit the sample rate to less than 10% of the respiratory minute volume. Based on a breathing frequency of at least 12 cycles/minute and a tidal volume of 0.5 l for an adult human at rest, the sample rate should be 0.6 l/min at most.
Finally the catheter should be easy to manipulate and it should be possible to insert it into the trachea even if an HME filter is in situ. The primary measuring site for the sensor should be about 1 cm behind the HME, but preferentially, measurements deeper in the trachea should be possible as well.
Components
Airway Climate Explorer
Breathing air is sampled through a 0.8 mm diameter central canal of a multilumen tube (see Fig. 1a, b.). Four of the six surrounding canals are threaded with two resistance wire loops in order to facilitate catheter heating. Resistance wire was chosen to minimize current load in case of unexpected patient contact. Two surrounding canals were threaded with a copper wire loop in order to measure temperature for the system heating control unit. A thermocouple (MLT1402 T-type Ultra Fast Thermocouple Probe (IT-23), ADInstruments Ltd, Oxfordshire, UK) was placed just inside the tip of the central canal, in order to establish a constant airflow around this sensor. The thermal response of the thermocouple is specified as 5 ms, accuracy ±0.1°C. The distal 2 cm of the sample catheter is not actively heated in order to prevent retrograde heating of the thermocouple. The multilumen tube is surrounded by a silicone isolation tube with an outer diameter of 5 mm. In between both tubes, a small layer of air increases thermal isolation. The length of the sample catheter is 30 cm.
Fig. 1Drawings of the sample catheter; a 3D overview with enlargement of the tip. b cross section (total diameter 5 mm)
The proximal end of the catheter is connected to an aluminium sensor house, incorporating the humidity sensor (see Fig. 2). The humidity sensor most suitable for our purposes appeared to be the capacitive humidity sensor of a radiosonde (RS92K, Vaisala Oyj, Helsinki, Finland). The technical specifications report a response time of <0.5 s at 20°C and <20 s at −40°C, both at an airflow of 6 m/s. The humidity sensor is placed in an aluminium sensor house that is wrapped with copper wire for heating. The humidity sensor is mounted on a sensor beam and consists of a small chip with the sensor polymer and an aluminium wire that in the original radiosonde served as a heating wire, but which we use to measure the temperature of the humidity sensor in situ (see Fig. 3). Apart from the humidity sensor chip, the sensor beam also contains a temperature sensor, but this was not used, as it is located relatively far from the humidity sensor chip.
Fig. 2Cross section of the sensor house; I connection to catheter, II humidity sensor, III copper wire for heating of the sensor house, IV droplet interceptor with electrode for fluid detection, V valve for disconnection of pressure gradient in case of fluid detection in the droplet interceptor, VI thermocouple wireFig. 3Details of the humidity sensor (dimensions 4 × 1.5 mm2)
The sensor house is heated to avoid prolonged saturation of the sensor [12, 15]. The temperature is kept stable within 0.2°C. Therefore the output of the capacitive sensor which is sensitive for the relative humidity can be directly converted to absolute humidity. Because a radiosonde is not designed for long-term use, the sensor house was constructed such that it could be easily opened to replace the sensor beam. The diameter of the air canal that leads to the humidity sensor is 1.4 mm. The air inflow passes through a droplet interceptor in the sensor house in order to prevent sucked up sputum from being projected onto the sensor chip. In the bottom of the droplet interceptor an electrode is placed in order to detect fluid accumulation, in which case a mechanic valve instantly disconnects the pressure gradient through catheter and sensor house, which is established by a vacuum pump (DIVAC 0.6, Leybold Vacuum BV, Utrecht, the Netherlands). The flow rate is controlled and kept constant at 0.6 l/min, by a mass flow meter/controller (Smart Mass Flow Meter, type 5850S and Smart Control V1.4 Software, Brooks Instruments, Veenendaal, the Netherlands), which is placed between the sensor house and the pump.
Figure 4 summarizes the airway climate explorer in a block diagram. In this figure also the custom-made electronic module is shown. The main functions of the module are: processing of the humidity sensor signal and its associated temperature signal, i.e. heating of both the sample catheter via the resistance wire, and the sensor house via the copper wire, each to their own adjustable set points. The electronic principles of the design of the module are described in the appendix.
Fig. 4Block diagram of the airway climate explorer
For patient safety, the total assembly is connected to a safety-isolating transformer.
Data acquisition and signal processing
All signals of the total assembly, i.e. voltage output of the sensor polymer and the aluminium wire on the humidity sensor of the ACE, temperature signal of the thermocouple, sample rate of the mass flow controller, and all ancillary equipment (see below), are read out simultaneously via a multichannel data acquisition system with additional software (Powerlab and Chart 5.4.1 software, ADInstruments Ltd, Oxfordshire, UK) on a PC. For deconvolution of the raw humidity output of the ACE, the software application LabVIEW (National Instruments Netherlands BV, Woerden, The Netherlands) is used.
Cleaning and disinfection protocol
Prior to every in vivo measurement, the sample catheter is cleaned and disinfected with the following procedure. First the catheter is rinsed with tap water. Subsequently the catheter is cleaned with disinfection hand soap (Baktolin Basic, Bode Chemie, Hamburg, Germany) and rinsed again with tap water. Then the catheter is placed in a solution of Biotex (Sara Lee, Utrecht, The Netherlands) and tap water for 10 min, after which it is placed in a 70% alcohol–water solution for 5 min. During the disinfection procedure, chemicals are prevented from entering the central canal of the sample catheter by establishing retrograde airflow, in order to protect the humidity sensor polymer. The central canal is not sterilized because during use there is continuous suction of air towards the vacuum pump. Finally the catheter is dried in room air.
Ancillary equipment for frequent in hospital calibration
For frequent, in-house, temperature calibrations a can of water is used in which the thermocouple is placed, simultaneously with a calibrated thermometer (Thermalite, Electronic Temperature Instruments Ltd, Worthing, UK), accuracy: ±0.4°C, acting as a secondary reference. By using water with melting ice or warm tap water, the full temperature range can be quickly calibrated.
For frequent humidity calibrations, a small climate room was constructed. This climate room consists of a plexiglas box of 26 × 42 × 16 cm3, in which moisturized air can be brought via the distal end of the tube of a neonatal respiratory humidifier (MR730 Respiratory humidifier, Fisher & Paykel Healthcare, Papendrecht, the Netherlands). Air is mixed via an electromotor-driven propeller (25 cm diameter). The box has entries for the tip of the sample catheter and for a calibrated heated humidity and temperature sensor accuracy ±0.6°C and ±2.5% relative humidity (RH), (Testo B.V., Almere, the Netherlands), which is used as a secondary humidity reference. The system can be operated in ambient air (30–60% RH), can be connected to the hospital oxygen supply, and can be set to deliver dry or 100% humidified gas at room temperature.
Assessment of step-response characteristics was performed in a two-stream system, which consisted of two tubes that were mounted side by side, connected to—in absence of compressed room air facilities—the hospital’s oxygen supply. Through one tube, dry oxygen flowed, and through the other the oxygen was heated and 100% humidified with the respiratory humidifier. Subsequently, the sample catheter tip was manually switched between both tube lumina.
Leakage testing is performed by checking the time required to completely empty a flexible sack that is filled with a well-defined volume of air (1 l). This volume is pushed into the sack with a calibration syringe (Jaeger GmbH/VIASYS Healthcare GmbH, Hoechberg, Germany).
Environmental temperature and humidity are monitored with the heated humidity and the temperature sensor.
Calibration and verification
Full calibration of the system was performed with one single sensor beam. However, due to the disposable design of a radiosonde and the possibility of damaging the sensor during the measurements, most likely several sensor beams will be used over time. Therefore, response time behaviour was checked also for a second sensor beam, and the calibration procedure was designed such that these characteristics can be easily determined for a consecutive beam.
Operating temperature
Raising the temperatures of the system, and therefore of the humidity sensor, above room temperature, we observed an initial decrease in the response time, but above 40°C the response time increased. Therefore 40°C was chosen as the operating temperature. At this temperature, the sampled air in the catheter reaches 40°C well before arrival in the sensor house, and therefore the influence of breathing temperature variations on sensor temperature is negligible. For measurements at 1 cm deep in the trachea the heated part of the tube does not enter the trachea so that artificial heating of the inspired air will not occur.
Static calibration
The temperature of the sensor is kept stable within 0.2°C. Therefore the output of the capacitive sensor, which is sensitive for the relative humidity, can be directly converted to absolute humidity. In a climate cabinet (Heraeus Votsch HC7020), the humidity output of the ACE was plotted against a clinically relevant range of absolute humidity values, as measured with the secondary reference sensor, see Fig. 5. Since this relation appeared to be linear, a 2-point calibration is considered appropriate for further frequent humidity calibrations in the hospital. Given the accuracy of the reference sensor and the somewhat limited range between the calibration points to avoid excessive condensation or temperature disequilibrium in the calibration device, the inaccuracy of the system for measuring absolute humidity values after the 2-point calibration is less than 5%.
Fig. 5Output of the humidity sensor plotted against the secondary reference
The thermocouple measuring the temperature at the tip of the sample catheter was 2-point calibrated against the values of the reference thermometer in ice water and water at body temperature. This procedure can be performed in the hospital, prior to every in vivo measurement. Given the accuracy of the reference thermometer, the accuracy of the system for measuring temperature values after a 2-point calibration is 0.3°C.
Assessing the role of condensation in the measuring system
In order to assess the necessity of heating the system in order to prevent condense formation within the device, step response measurements were performed with an unheated system and subsequently with the system heated to 40°C. With an unheated system, a delay of up to several seconds was observed in the humidity trace relative to the thermocouple trace after a downward step. The longer the sensor is kept in a high humidity environment, this delay increases; and it is indicative of the formation and subsequent evaporation of the condensate within the system. Even when simulating clinically relevant breathing frequencies, a noticeable delay of 0.15 s was observed.
When measuring high humidity at operating temperature for a long time, after the downward step, a very fast response with a delay of less than 0.05 s was observed. This delay is caused by the transport of sampled air from the tip of the catheter towards the humidity sensor and is not relevant for the measurement of end tidal values. In the downward trace this fast response was followed by a shoulder at about 50% of the step, of which the length again increased with the duration at high humidity. This shoulder probably represents the evaporation of the condensate in the unheated tip of the sensor.
During in vivo measurements, however, the system will never be exposed to high humidities for an extended period of time. Therefore we simulated a range of breathing frequencies with the two-stream system resulting in a series of block functions of approximately 9, 18, and 35 cpm (cycles per minute). At 9 cpm a short shoulder is still visible, but the condensate evaporates sufficiently fast that the “end-inspiratory” value is reached. At 18 and 35 cpm no shoulder was observed.
Raw response time characteristics
Using the two-stream system the step response of the ACE was determined. Both for the temperature and the humidity signal we determined the 1–1/e response time. For the downward humidity step we could not use a plain step function because condensation in the tip would occur. Therefore we used a block function with a frequency of 18 cpm (see Fig. 6a). At this frequency the response time is still relatively short compared with the period of the block function and easy to determine from the humidity trace.
Fig. 6Response to a block function; solid black line raw humidity trace, solid grey line deconvolved humidity trace, dashed line thermocouple trace. Vertical axis: °C/mg H2O/l. Horizontal axis: seconds. a Response to a block function of approximately 18 cycles per minute. b Overview of the response to a block function with varying frequency (9 to 35 cpm)
For the thermocouple, the response to increasing and decreasing temperature steps was <0.2 s. For the first humidity sensor, the response to increasing humidity and decreasing humidity was <0.5 and <0.8 s, respectively, at a frequency of 18 cpm. We verified that the response characteristics at other frequencies were comparable. Both rise and fall times of the second humidity sensor were approximately 60% shorter.
The observed raw response times are slightly longer than we had expected based on the specifications of the Vaisala Humidity Sensor, probably because the airflow over the sensor is slower than specified. Although every attempt was made to minimize the volume of the sensor chamber we estimate that the airspeed over the sensor is between 2 and 3 m/s instead of 6 m/s.
Deconvolution of the humidity signal
Since the raw step response times of tested humidity sensors were considered too slow for patients with high breathing frequencies, it was decided to further process the raw humidity signal. Depending on the tested sensor, the raw step response can be closely approximated by a single or a double exponential function. Therefore it is possible to reconstruct the original signal by deconvolution. Although we observed a different response time for increasing and decreasing humidity steps, we nevertheless chose a single function because of its better numerical stability:
For the removal of excessive noise a low pass filter of <300 cpm was applied. Because the raw response characteristics varied in between the humidity sensors and over time, τ was adjusted for each individual measurement. The principle of adjustment of these parameters was to acquire a maximal value for τ, such that no overshoot of the deconvolved humidity trace was visible. We used the thermocouple signal as reference for the true step function. For the first sensor, the deconvolved signal was stable and optimal for increasing humidity with τ = 0.4 s. For the second humidity sensor a τ of 0.1 s appeared to be appropriate. We verified that the raw step response characteristics were reproducible, and that the deconvolution results were not influenced by the step size.
After deconvolution of the humidity signal, the response characteristics improved, see Fig. 6a. The 1–1/e step response time to a block function for the first humidity sensor was <0.2 s for increasing humidity and <0.4 s for decreasing humidity at a frequency of 18 cpm. For the second sensor the deconvolved step response characteristics were similar after deconvolution.
Figure 6b shows the raw and deconvolved humidity response to a block function over a range of breathing frequencies. It is shown that the frequency dependence of the peak-to-peak values is reduced after deconvolution. At higher frequencies, we nevertheless see that the deconvolved signal increasingly deviates from the true peak-to-peak values. This deviation is more marked in the lower peak, due to the relatively long humidity fall time of the raw trace, compared to the rise time. The magnitude of the deviation in the lower peak ranges from about 1.5 mg H2O/l at a breathing frequency of 9 cpm to 5 mg H2O/l at 35 cpm. Since the step response characteristics of the deconvolved signal were comparable for both tested sensors, the deviations are of comparable magnitude too.
At 35 cpm even the amplitude of the fast thermocouple signal does not reach the true peak-to-peak values. The deviation is less than 0.5°C so that we do not consider a correction necessary for the thermocouple signal.
The in vivo measurement
The total assembly and the influence of a heat and moisture exchanger were tested in one laryngectomized volunteer. The tip of the sample catheter was pushed 1 cm through a small hole that was punched in an HME plaster (Fig. 7). The diameter of the puncture was slightly smaller than the diameter of the sample catheter in order to avoid air leakage. Figure 8 shows a segment of the raw and deconvolved humidity trace and the thermocouple signal. In this particular volunteer τ required adjustment to 0.35 in order to eliminate overshoot. During breathing in rest, intra airway temperature and humidity ranged from 28/34°C and 15/37 mg H2O/l (typical end inspiratory/end expiratory values), respectively. During the total measurement session, breathing frequency ranged from 12 to 44 cpm (mean 17.8 cpm, SD 4.4 cpm). In addition, inspiration time may be relatively short compared to expiration time. See the beginning of the trace in Fig. 8.
Fig. 7Sample catheter in situ, pushed through a punched hole in the HME plaster, without HME filter in situFig. 8In vivo humidity trace before and after deconvolution; black trace raw humidity signal, grey trace deconvolved humidity signal. Vertical axis: mg H2O/l. Horizontal axis: seconds
Discussion
Our results show that we have succeeded in developing an intra-tracheal airway climate analyser with response characteristics fast enough for assessment of end tidal intra airway temperature and humidity variations during normal breathing in laryngectomized individuals. In the literature, only a few systems have been described that simultaneously measure temperature and humidity in the human airway [6, 8, 10, 11, 13, 14].
Ingelstedt [6] reported on intra airway climate in healthy individuals using a well- validated system based on psychrometry, with signal stability 0.3–0.4 s after sudden exposure to increased humidity. However, this method had a few disadvantages. The wet thermo element remained sufficiently moistened for only 5–8 respiratory cycles, after which it had to be retracted in order to be watered by a moistened wick again. Furthermore, even the slightest contact with tracheal secretion increased the thermal inertia of the psychrometer, after which it has to be cleaned in a 4-h lasting procedure.
Although contact of the humidity sensor of the ACE by tracheal secretion is less likely, the central canal of the sample catheter is susceptible to obstruction by tracheal secretion. When this would occur, the sample catheter can be disconnected from the sensor house and subsequently the obstructive plug can be removed by establishing reversed airflow. The electrodes in the bottom of the droplet interceptor appeared to work effectively. If the humidity sensor would be damaged, it can easily be replaced. Due to the “in hospital” calibration procedures, the measurements can be continued within a relatively short time span.
The system of Primiano et al. [13] consisted of a sample catheter that was connected to a mass spectrometer. In order to prevent condensation, the tip of their sample catheter was constricted, establishing a decrease in air pressure. Extensive validation of the total assembly revealed impressive 10–90% response time characteristics of less that 0.25 s for humidity readings and less than 0.12 s for temperature readings, respectively. Since mass flow meters are expensive and require solid maintenance, the use of such a device was not considered feasible for our project. Although their measurement system has been used for assessment of anaesthesiology heat and moisture exchanger effectiveness, we are not aware that this system has been used for airway climate assessment in laryngectomees.
McRae et al. [10, 11] and Keck et al. [8, 9] have assessed tracheal climate and the influence of HMEs in laryngectomees. The set up of Keck et al. consisted of a sample catheter, connected to a sensor house that incorporated a capacitive humidity sensor.
As described in [9] the sensor house was heated with a heat mat to 34–35°C, which is slightly lower than the maximum end expiratory temperature observed (35.2°C) so that condensation still might be a possibility. However, the sample catheter was not heated and the verification of the total assembly has not been included in the report. In the present device, evidence of condensation—probably precipitated in the unheated distal 2 cm of the catheter tip (which is in vivo somewhat heated by the body) is observed when offering block functions at a frequency of 9 cpm. Despite this, at 9 cpm the end values were not affected. It is therefore unlikely that also during in vivo measurements condensation will affect the amplitude of the peak-to-peak values. Moreover, the temperature and humidity steps will occur more gradually in the in vivo situation, theoretically decreasing the amount of condense formation in the sensor tip, compared to the offered block function.
In the studies by McRae et al. [10, 11], the response time of the humidity sensor used in the setup of McRae was up to 9 s. The frequency of human rest breathing is far too high compared to these response characteristics to obtain representative values for intra-tracheal humidity.
However, as the two-stream block function experiment has shown, also in the present measuring device, the amplitude of the deconvolved humidity trace depends on the breathing frequency. The clinical implication of this finding is that, at higher breathing frequencies, the true peak-to-peak values are slightly, but progressively underestimated by the ACE, although probably less than in vitro due to more gradual humidity and temperature variation in the latter situation. The variation in end-inspiratory humidity values and temperature signals, as indicated by the deconvolved humidity signal and the thermocouple in the in-vivo experiment may therefore be partially due to a frequency dependency and not due to a real variation in humidity and temperature values.
Quality control
The first sensor functioned correctly for 1 year and the second for half a year. Both sensors had to be replaced because of accidental damage during cleaning. The two humidity sensors appeared to have different response characteristics. Sensor age, variation in the production process of the humidity sensor etc. may result in variation between the characteristics of different humidity sensors, but also between the characteristics of one sensor measured over time. Linearity between reference humidity values and sensor output in all consecutive sensors is felt to be a prerequisite for serial production of radiosondes. The small climate room and the 2-point calibration is therefore considered appropriate for daily quality control. A potential source of error is leakage of air, causing dilution of sampled tracheal air with room air. Therefore the simple leakage test should be part of the regular quality control.
Although both tested humidity sensors revealed the different step response times of the raw humidity signal, the peak-to-peak amplitude of deconvolved signal was comparable between both sensors. The two-stream step response set up facilitates regular control of the sensor response characteristics and, if necessary, adjustment of the deconvolution function can be easily performed.
Limitations and further improvements
During measurements deeper in the trachea (e.g. 4 cm), contact of the catheter tip with the airway wall may lead to irresistible cough. Preferably the distal end of the sample catheter should be better pliable in the shape of the trachea, in order to facilitate better control of its tip and reducing airway wall contact. Also visual control of the catheter tip may contribute to prevention of wall contact.
The response of the sensor when decreasing the humidity is slower than the response to increasing humidity. However, the same simple deconvolution function is used for increasing and decreasing humidity. A more advanced signal processing might further improve response characteristics and decrease the remaining frequency dependence of the peak tot peak amplitude. In particular for the end-inspiratory values this is important because of the relatively short time span of the inspiration time compared to the expiration time, combined with the relatively slow response characteristics to decreased humidity compared to increased humidity.
In conclusion, the Airway Climate Explorer is an easy to use, relatively inexpensive tool for intra airway temperature and humidity measurements. When looking at the end-inspiratory and end-expiratory humidity values as measured by our system, the accuracy is adequate for assessment of intratracheal climate.

temperature
humidity
airway climate
heat and moisture exchanger
gas sampling probe


Health-related physical fitness of adolescents and young adults with myelomeningocele

To assess components of health-related physical fitness in adolescents and young adults with myelomeningocele (MMC), and to study relations between aerobic capacity and other health-related physical fitness components. This cross-sectional study included 50 adolescents and young adults with MMC, aged 16–30 years (25 males). Aerobic capacity was quantified by measuring peak oxygen uptake (peakVO2) during a maximal exercise test on a cycle or arm ergometer depending on the main mode of ambulation. Muscle strength of upper and lower extremity muscles was assessed using a hand-held dynamometer. Regarding flexibility, we assessed mobility of hip, knee and ankle joints. Body composition was assessed by measuring thickness of four skin-folds. Relations were studied using linear regression analyses. Average peakVO2 was 1.48 ± 0.52 l/min, 61% of the participants had subnormal muscle strength, 61% had mobility restrictions in at least one joint and average sum of four skin-folds was 74.8 ± 38.8 mm. PeakVO2 was significantly related to gender, ambulatory status and muscle strength, explaining 55% of its variance. Adolescents and young adults with MMC have poor health-related physical fitness. Gender and ambulatory status are important determinants of peakVO2. In addition, we found a small, but significant relationship between peakVO2 and muscle strength.
Introduction
During the last decades, life expectancy of persons with myelomeningocele (MMC) has increased and many will nowadays survive into adulthood (Bowman et al. 2001). As a consequence, lifestyle-related diseases, such as cardiovascular disease and diabetes mellitus, will be of increasing concern in this patient group. Therefore, more attention towards a healthy lifestyle is warranted. Similar to the general population, persons with MMC develop their own lifestyle during the transition from adolescence to adulthood. At this age, special attention should be paid to optimize the lifestyle in order to improve health throughout life.
Physical fitness is recognized as an important component of health (Lamb et al. 1988; Twisk et al. 2002) and it may be important for the performance of functional activities and quality of life (Noreau and Shephard 1995; Stewart et al. 1994). Low physical fitness may result in high physical strain during the performance of activities (Bruinings et al. 2007). As a consequence, activity levels may decrease due to fatigue and discomfort, exacerbating low physical fitness. Caspersen and co-workers defined several health-related components of physical fitness, i.e. aerobic capacity, muscle strength and endurance, flexibility and body composition (Caspersen et al. 1985).
Only a few studies are available on health-related physical fitness in persons with MMC. In a previous study in adolescents and young adults with MMC, we found that average peak oxygen uptake (peakVO2) was 42% lower than normative values of healthy peers, with lower values in non-ambulatory than in ambulatory persons (Buffart et al. in press). Several other studies have also reported low aerobic capacity (Agre et al. 1987; Sherman et al. 1997; van den Berg-Emons et al. 2003). Furthermore, children with MMC, and particularly non-ambulatory children, were found to have reduced strength of lower extremity muscles (Agre et al. 1987; McDonald et al. 1991; Schoenmakers et al. 2004). Hip and knee contractures have been reported in respectively 18% and 36% of adolescents and young adults with MMC and hydrocephalus (Verhoef et al. 2004). Finally, high levels of body fat have been found in persons with MMC (Bandini et al. 1991; Mita et al. 1993; Shepherd et al. 1991); previously we reported that 35% of adolescents and young adults with MMC were obese (Buffart et al. in press). Because these previous studies lack simultaneous assessment of several health-related physical fitness components, the relation between aerobic capacity and other components remains unclear. Insight in these inter-relations may show for example, whether focusing on peripheral factors such as muscle strength would have additional value for improving aerobic capacity.
Due to the scarcity of studies in persons with MMC, the first aim of this study was to describe health-related physical fitness i.e. aerobic capacity, muscle strength, joint flexibility and body composition in a relatively large group of adolescents and young adults with MMC, allowing descriptions of subgroups regarding gender and ambulatory status. Secondly, we wanted to study the relation between aerobic capacity and other components of health-related physical fitness, controlled for relevant personal and disease-related characteristics. Studies in persons with other chronic conditions such as spinal cord injury have shown relations between aerobic capacity and muscle strength (Haisma et al. 2006; Janssen et al. 1993; Zoeller et al. 2005). Therefore, also in persons with MMC, we expected aerobic capacity to be related to muscle strength.
Method
Participants
Recruitment
Adolescents and young adults with MMC, aged between 16 and 30 years, were recruited from the university hospitals in Rotterdam, Leiden, Utrecht and Amsterdam and all rehabilitation centers in the Southwest of the Netherlands. Exclusion criteria were complete dependence on an electric wheelchair, presence of disorders other than MMC that affect daily physical activity (e.g. rheumatoid arthritis), and presence of disorders that contra-indicate a maximal exercise test (e.g. exercise-induced ischemia or arrhythmias, uncontrolled hypertension and exercise limitation due to chronic obstructive pulmonary disease). We invited 171 persons of whom 50 participated in the study (29%). Main reasons for non-participation were no interest, lack of time or duration of the measurements. No differences were found between participants and non-participants regarding age, gender, level of lesion and presence of hydrocephalus, as measured with an independent t-test or Chi square test (Buffart et al. in press). All participants and parents of adolescents aged less than 18 years gave written informed consent before participating in the study. The Medical Ethics Committee of the Erasmus MC Rotterdam and of all participating institutes approved the study.
Characteristics
In total, 25 males and 25 females (mean age 21.2 ± 4.5 years) participated in the study. Table 1 presents personal and disease-related characteristics of participants. Neurological level of lesion and the presence of hydrocephalus were obtained from the medical records. Five categories of neurological level were distinguished: thoracic, thoracolumbar, lumbar, lumbosacral and sacral. We considered hydrocephalus to be present when a shunt was placed. Ambulatory status was determined according to the classification of Hoffer and co-workers (Hoffer et al. 1973): (1) community ambulator, walking indoors and outdoors, (2) household ambulator, walking only indoors, and (3) non(functional) ambulator. Non-functional ambulators walk only during therapy sessions and non-ambulators are completely wheelchair dependent. Since main mode of ambulation in daily life is similar between non-functional ambulators and non-ambulators, we combined these two groups. Educational level was categorized as low (pre-vocational practical education or lower level), medium (pre-vocational theoretical education and secondary education) or high (higher education and university) (Donkervoort et al. 2007).
Table 1Personal and disease-related characteristics of participants (n = 50) Gender (% male)50Age (mean ± SD in years)21.2 ± 4.5Height (mean ± SD in m)1.57 ± 0.12Body mass (mean ± SD in kg)67.6 ± 15.6Level of lesion (%) Thoracic2 Thoracolumbar14 Lumbar28 Lumbosacral42 Sacral14 Hydrocephalus (%)82Ambulatory status (%)a Community ambulator30 Household ambulator14 Non (functional) ambulator56Educational level (%) Low37 Medium39 High24aNo gender differences as tested with a Chi square test
Aerobic capacity
Aerobic capacity was measured in a progressive maximal exercise test, based on the McMaster All-Out Progressive Continuous Cycling and Arm test (Bar-Or 1983), on an electronically braked arm or cycle ergometer (Jaeger ER800SH and ER800 respectively; Jaeger Toennies, Breda, The Netherlands). Studying patients with cerebral palsy who were partly wheelchair-dependent, Bhambani and co-workers concluded that maximal exercise testing during the main mode of ambulation elicits the highest oxygen uptake (Bhambhani et al. 1992). Therefore, depending on their main mode of ambulation, participants performed an arm crank test (n = 33) while sitting in their own immobilized wheelchair with cranks at shoulder height, or a leg cycle exercise test (n = 17). The test was preceded by a 3-minute warm-up (5 W for arm ergometry and 20 W for cycle ergometry), followed by a resting period of 5 min. During the test, resistance was increased every 2 min with a variable load, ensuring that total individual exercise duration ranged from 8 to 12 min. The pedal/crank rate was 60 rpm and strong verbal encouragement was given throughout the test. The test was terminated when the subject stopped due to exhaustion. Gas exchange was determined continuously using a breath-by-breath portable measurement system (K4b2, COSMED, Rome, Italy). Calibration was performed before each test with reference gases. Heart rate was measured continuously using a heart rate (HR) monitor which was attached to the system, and participants were fitted with a transmitter belt around the chest (Polar Electro, Finland). Aerobic capacity was defined as the mean oxygen uptake during the last 30 s of exercise (peakVO2, in l/min). In addition, for those measured during cycle ergometry, values of aerobic capacity were expressed as percentage of reference values of Dutch able-bodied sedentary males and females of similar ages, as estimated from a submaximal exercise test on a cycle ergometer using the nomogram of Åstrand (Vos 2001). Peak work load (peakW) was defined as the highest work load maintained for at least 1 min.
In addition, the ventilatory anaerobic threshold (VAT) was estimated by the ventilatory equivalent method, when the ventilatory equivalent for O2 (Ve/VO2) and the end-tidal O2 partial pressure (petO2) increased while ventilatory equivalent for CO2 (Ve/VCO2) and end-tidal CO2 partial pressure (petCO2) remained stable (Reinhard et al. 1979; Wassermann et al. 1999). VAT was also expressed relative to the measured peakVO2 (VAT%). HR and respiratory exchange ratio (RER) were used as objective criteria for maximal exercise. Subjective strain was measured at the end of the final stage using the modified Borg scale of rating perceived exertion (RPE), which is a vertical scale labelled from 0 (no effort at all) to 10 (maximal effort) (Borg 1982; Mahler et al. 1987).
Muscle strength
We measured strength of two large muscle groups of the lower and upper extremity with a hand-held dynamometer (MicroFet, Hoggan Health Industries) using the “break” testing method. We measured strength of hip flexors and knee extensors in persons whose main mode of ambulation was walking. In persons whose main mode of ambulation was wheelchair-driving, we measured strength of shoulder abductors and elbow extensors. The positions and the performance of the measurements were according to van der Ploeg and co-workers (van der Ploeg et al. 1991). The applicator of the dynamometer was held against the distal part of the limb segment, and participants were asked to build up their force to a maximum against it. The examiner applied sufficient resistance just to overcome the force exerted by the participant, and the applicator was then immediately moved away from the limb segment and the measured force was recorded. Each trial lasted approximately 4 s, and three repetitions were performed with 1 min rest in between. We used the average value of three repetitions of the dominant side for further analyses because we found no differences between the dominant and non-dominant side (tested with a paired samples t-test). To assess whether muscle strength was subnormal and to be able to compare strength of upper and lower extremity muscles, values were normalized to Z-scores using reference values of healthy males and females. For hip flexors, shoulder abductors and elbow extensors we used reference values of Phillips and co-workers for males and females aged 20–29 years (Phillips et al. 2000) and for knee extensors, we used reference values of Bohannon and co-workers (Bohannon 1997). In case the examiner could not resist the muscle strength, Z-score was set at 2. We used the lowest Z-score of the upper or lower extremity as indicator of muscle strength, and we considered muscle weakness to be present when Z-score ≤−2.
Flexibility
As indicator of flexibility of lower extremity we assessed passive mobility of hip and knee joint while participants were lying supine and of the ankle joint while they were sitting. We considered mobility to be restricted when extension of hip and knee joint and ankle dorsal flexion did not reach neutral position. Since no differences were found in mobility restrictions between dominant and non-dominant side (tested with the Wilcoxon signed rank test) we used the results of the dominant side to calculate a sum score of joint mobility ranging from 0 (no mobility restrictions in any joint) to 3 (all three joints have mobility restrictions).
Body composition
Height was measured with a flexible tape while lying on a bed. In case of joint contractures, measurements were performed from joint to joint. Body mass of ambulatory persons was obtained while standing on a Seca scale and of non-ambulatory persons while sitting on an electronic scale (Cormier, France). Thickness of four skin-folds (biceps, triceps, subscapular, and suprailiac) was measured twice on the right side of the body with a Harpenden caliper (Burgess Hill, UK) and mean values were used for further analyses. In addition, sum of four skin-folds were expressed as percentage of reference values of Dutch sedentary males and females of similar ages (Vos 2001).
Statistical analysis
Results of health-related fitness components are presented as mean ± standard deviation (SD) for the total group and for subgroups regarding gender and ambulatory status.
Simple linear regression analyses were used to study relations between personal and disease-related characteristics and peakVO2 (in l/min), in order to detect relevant determinants to include in the multiple regression models. Multiple linear regression analyses for peakVO2 were carried out in two steps. First, we built a model including relevant personal and disease-related characteristics. In the second step, we studied whether other health-related physical fitness components, i.e. muscle strength (Z-score), sum of four skin-folds (mm), and joint mobility (number of restricted joint), were significantly related to peakVO2, controlling for the personal and disease-related characteristics from step 1. We presented the standardized regression coefficients (β) and explained variance (R2) of the linear regression models. Statistical analyses were performed using SPSS 12.0 for Windows. P ≤ 0.05 were considered significant.
In the analyses, we pooled the data of peakVO2 measured during arm and cycle ergometry, and corrected for differences in exercise mode. Due to high collinearity between type of ergometer and ambulatory status (correlation coefficient (r) = 0.92, P < 0.001), we adjusted the analyses for ambulatory status as proxy for type of ergometer, because clinically, ambulatory status would be more meaningful.
Furthermore, due to overlap between lesion level and ambulatory status (r = 0.58; P < 80.001), we chose to only correct for ambulatory status in the multiple regression analyses. Ambulatory status was determined during the study, whereas level of lesion was obtained from medical records. The national study on adolescents with spina bifida in the Netherlands (ASPINE) reported that the level of lesion as mentioned in medical records may be unreliable because lesions are determined at different ages, sometimes using the motor level and sometimes using the sensory level, and often lacking descriptions of methods (Verhoef et al. 2004).
Results
Descriptive results of aerobic capacity, muscle strength, joint mobility and body composition for the total group and for subgroups regarding gender and ambulatory status are presented in Table 2. Average peakVO2 was 1.48 ± 0.52 l/min. For persons measured during cycle ergometry (n = 17), peakVO2 corresponded to 67 ± 15% of reference values. Sixty-one percent of the participants had subnormal muscle strength as indicated by Z-scores and 61% had mobility restrictions in one or more joints. Average sum of four skin-folds was 74.8 ± 38.8 mm, corresponding to 159 ± 77% of normative values (Table 2). According to objective and subjective criteria most participants reached their peak exercise performance. Average peakHR was 174 ± 19 beats per minute, which was 90.4 ± 9.6% of the age predicted maximum (220—age for cycle ergometry; 210—age for arm ergometry) and average peakRER was 1.17 ± 0.22. Average RPE was 6.2 ± 2.2, indicating that participants experienced the exercise as heavy to very heavy.
Table 2Descriptive results of health-related physical fitness components for the total group and for subgroups regarding gender and ambulatory statusAll (n = 50) GenderAmbulatory statusMale (n = 25) Female (n = 25) Community (n = 15) Household (n = 7) Non(functional) (n = 28) Aerobic capacity (mean ± SD) PeakVO2 (l/min) 1.48 ± 0.521.78 ± 0.511.18 ± 0.301.85 ± 0.571.44 ± 0.451.29 ± 0.40% of reference valuesa67 ± 1571 ± 1361 ± 1868 ± 1654 ± 2–PeakVO2 (ml/kg min) 22.6 ± 8.228.1 ±  7.017.0 ± 4.729.0 ± 7.722.3 ± 6.619.2 ± 6.8Peak oxygen pulse (ml/bpm)8.7 ± 3.010.1 ±  2.87.3 ± 2.410.7 ± 2.87.8 ± 2.47.8 ± 2.8PeakRER1.17 ± 0.221.17 ± 2.281.18 ± 0.201.15 ± 0.221.27 ± 0.161.16 ± 0.24PeakW (W)91 ± 42113 ± 4369 ± 28123 ± 4297 ± 3573 ± 34PeakHR (bpm)174 ± 19179 ± 16169 ± 20173 ± 21183 ± 14172 ± 18PeakHR % of predicted maximum90 ± 1092 ± 889 ± 1087 ± 1095 ± 891 ± 10VAT (l/min)1.20 ± 0.431.39 ± 0.441.01 ± 0.321.55 ± 0.451.07 ± 0.291.05 ± 0.34VAT%82 ± 1580 ± 1486 ± 1684 ± 1077 ± 2283 ± 16Muscle strength (mean ± SD) Z-score −2.1 ± 1.8−2.3 ± 2.1−1.9 ± 1.5−2.7 ± 2.2−2.0 ± 1.6−1.8 ± 1.7Weak strength: Z-score ≤ −2 (%)615864795754Joint mobility (median [range]) Number of restricted joints 1 [0–3]1 [0–3] 1 [0–3] 0 [0–2] 1 [0–2] 1.5 [0–3] Impaired mobility in any joint (%)615467295782Body composition (mean ± SD)Sum of four skin-folds (mm) 74.8 ± 38.851.2 ± 24.6100.4 ± 35.159.1 ± 29.266.5 ± 34.786.0 ± 42.0% of reference values159 ± 77146 ± 79173 ± 73121 ± 52160 ± 101181 ± 75aOnly for those measured on the cycle ergometer, n = 17 (ten males, seven females)
Several personal and disease-related characteristics were related to peakVO2 (Table 3). PeakVO2 was higher in males than in females (β = −0.61; P < 0.001), higher in community ambulatory persons than in household and non-ambulatory persons (β = −0.48; P < 0.001) and higher in persons with a lower level of lesion (β = −0.43; P = 0.002). Age, presence of hydrocephalus and educational level were not related to peakVO2.
Table 3Regression models for aerobic capacityIndependent variablesAerobic capacity (peakVO2, in l/min)β P-valueR2Simple regression analysis Personal characteristics Gender−0.61<0.0010.36 Age−0.07 0.65– Lesion level−0.430.0020.17 Hydrocephalus−0.18 0.22– Ambulatory status−0.48<0.0010.22 Educational level0.130.39–Multiple regression analysis Step 1a  Personal characteristics0.50  Gender−0.55<0.001  Ambulatory status−0.40<0.001 Step 2b  Inter-relations  Muscle strength (Z-score)0.220.040.55  Body composition (sum of four skin-folds in mm)0.250.080.53  Joint mobility (number of restricted joints)0.000.98–Significant betas are presented in boldβ standardized regression coefficient, R2 explained variance, Gender male (0), female (1), Lesion level sacral (1), lumbosacral (2), lumbar (3), thoracolumbar (4), thoracic (5), Hydrocephalus no (0), yes (1), Ambulatory status community ambulator (1), household ambulator (2), non(functional) ambulator (3), Educational level: low (0), medium (1), high (2)aDue to large overlap between ambulatory status and lesion level, we only included ambulatory statusbRelations between aerobic capacity and other fitness components controlled for gender and ambulatory status
Fifty percent of the variance in peakVO2 was explained by gender and ambulatory status (Table 3). In addition, when controlling for both variables, we found that participants with higher muscle strength had higher values of peakVO2 (β = 0.22; P = 0.04) explaining an additional 5% of the variance in peakVO2. Furthermore, we found that participants with higher sum of four skin-folds tended to have higher values of peakVO2 (β = 0.25; P = 0.08).
Discussion
Components of health-related physical fitness
In the present study, several health-related components of physical fitness were studied simultaneously in a relatively large group of adolescents and young adults with MMC. In general, most participants had poor health-related physical fitness.
Compared to the general population and compared to other patient groups, persons with MMC had low aerobic capacity. Regarding community ambulatory persons with MMC, values of peakVO2 were 32% lower than the reference values for able-bodied people and 23% lower than the average peakVO2 of males with spastic diplegia measured during cycle ergometry (Lundberg 1978). However, peakVO2 is influenced by the amount of active muscle mass (Davies and Sargeant 1974,1975; Lewis et al. 1983), which may possibly be reduced due to paresis of lower extremity muscles. PeakVO2 in non-ambulatory persons with MMC was lower than in ambulatory persons which may be caused by the lower amount of active muscle mass during arm ergometry compared to cycling. In able-bodied people, arm exercise induces a peakVO2 ranging from 53 to 73% of that achieved with lower extremity exercise (Bar-Or and Zwiren 1975). If that ratio is also applicable to non-ambulatory persons who may be accustomed to using their arm and shoulder muscles, adapted values of average peakVO2 would range between 1.76 and 2.43 l/min, which is of comparable range to the peakVO2 in ambulatory persons with MMC. Furthermore, compared to males with spinal cord injuries with lesions below T10 measured during arm ergometry (Janssen et al. 2002), the peakVO2 in non(functional)-ambulatory males with MMC was 22% lower. We therefore assumed that the poor aerobic capacity we found in persons with MMC may be influenced by reduced active muscle mass, but also deconditioning is likely to be present. This is supported by previous studies showing that adolescents and young adults with MMC were inactive, and that inactivity was associated with lower aerobic capacity (van den Berg-Emons et al. 2001), particularly in non-ambulatory persons with MMC (Buffart et al. in press).
VAT is another indicator of aerobic capacity, which was strongly correlated to peakVO2 in the present study sample (r = 0.84; P < 0.001). In the general population, VAT roughly corresponds to 50–60% of VO2max during leg exercise (Davis et al. 1997), and values of 40–60% have been found in the able-bodied population during arm exercise (Flandrois et al. 1986; Lin et al. 1993; Schneider et al. 1999). Paradoxically, we found VAT to be at 82% of peakVO2 suggesting high aerobic capacity. Comparably, Coutts and McKenzie found that persons with tetraplegia had lower VAT than persons with paraplegia caused by smaller muscle mass, but they had higher VAT%, and it was suggested that values of peakVO2 were low in relation to VAT (Coutts and McKenzie 1995).
Fifty-four percent of the non-ambulators and 79% of the community ambulators had subnormal strength in at least one of the major muscle groups. The relatively better performance of non-ambulators compared to ambulators in this respect might be explained by their relatively higher muscle strength in the upper extremities due to the habituation of using their arms for wheelchair propulsion. Furthermore, in contrast to lower extremity muscles, the neurological level of lesion will not disturb innervations of upper extremity muscles, which suggests that the weak upper extremity strength we found in non-ambulators is due to atrophy of muscles as a consequence of disuse. Weak strength of lower extremity muscles in ambulatory persons with MMC corresponds to previous findings in children with MMC (Agre et al. 1987; McDonald et al. 1991; Schoenmakers et al. 2004) and might be related to disturbed innervations as well as to disuse of muscles.
Results of the present study support the findings of Verhoef and co-workers that a large proportion of persons with MMC have reduced joint mobility (Verhoef et al. 2004). Similar to the results of Agre and co-workers restrictions in joint mobility in the lower extremities were mainly present in non-ambulatory persons with MMC (Agre et al. 1987).
We found high sum of four skin-folds, particularly in females and non-ambulatory, which is in agreement with high levels of body fat reported in previous studies (Bandini et al. 1991; Mita et al. 1993; Shepherd et al. 1991).
Relations between aerobic capacity and other components of health-related physical fitness
The results of the regression analyses indicated muscle strength was associated to peakVO2 when controlling for gender and ambulatory status. This finding is in accordance with literature on persons with spinal cord injury (Haisma et al. 2006; Janssen et al. 1993; Zoeller et al. 2005). In contrast to gender and ambulatory status, muscle strength is modifiable. Strength training may increase muscle mass and thus metabolizing mass contributing to higher peakVO2 (Janssen et al. 1993). In persons with spinal cord injury, strength training resulted in increased peakVO2 (Cooney and Walker 1986). Because causality cannot be established with the cross-sectional design of the present study, future studies should confirm whether strength training results in increased aerobic capacity in persons with MMC. However, considering the small, but significant, contribution of muscle strength to the explained variance, we assume this specific effect to be small. In this respect, it seems that mainly aerobic training is needed in order to improve aerobic capacity; however, including strength training may have additional value.
Body composition tended to be positively related to aerobic capacity, indicating that persons with more body fat had higher values of peakVO2. This relation may be caused by the greater fat-free mass of overweight persons concomitant with a greater body size (Unnithan et al. 2006).
Joint mobility was not related to aerobic capacity. However, in non-ambulators we might have underestimated the relation because we did not measure mobility of upper extremity joints. Nevertheless, good flexibility of lower extremity is suggested to be important to prevent problems later in life, such as problems with personal hygiene and transfer capabilities (Agre et al. 1987).
Limitations of the study
The methodology of the study had some limitations. First, because it is suggested that the primary mode of ambulation elicits the highest values of peakVO2 (Bhambhani et al. 1992), we used different exercise modes to assess aerobic capacity for ambulatory and non-ambulatory persons. However, due to large differences in active muscle mass, arm and leg exercise have different physiological responses. Therefore, we corrected for ambulatory status as proxy measure for type of ergometer when analysing relations between aerobic capacity and the other health-related fitness components. Furthermore, because the main mode of ambulation of ambulatory persons is walking rather than cycling, we may have underestimated peakVO2 in some ambulatory persons with MMC who rarely cycle. However, in clinical practice, the cycle ergometer seems to be more practical because physically disabled people may experience severe balance problems on the treadmill, and on the cycle it is possible to strap the feet to the pedals (Lundberg 1978). Peripheral local fatigue may have caused exercise cessation before reaching maximum oxygen uptake, however, based on the objective (peakHR and peakRER) and subjective criteria of maximal exercise (RPE), it may be concluded that values of peakVO2 are reasonable.
Furthermore, muscle strength was measured using hand-held dynamometry. This method is known to be cheap, quickly applicable (van der Ploeg et al. 1984) and reliable (Bohannon 1997). However, for an average examiner, values above 250 N are considered too high to apply sufficient resistance (van der Ploeg et al. 1991), which may lead to less accurate results in strong muscle groups. Using absolute values of strength measured with an isokinetic device, instead of expressing muscle strength as Z-score, might provide more detailed insight into the relation between strength peakVO2. However, the current measurement method and use of Z-scores are considered adequate to determine weakness of major muscle groups.
Finally, the response rate was low, which hampers generalization of results. However, personal and disease-related characteristics did not differ between participants and non-participants. Moreover, the prevalence of middle-level (lumbosacral) and high-level (lumbar or thoracolumbar) lesions of the present study sample was comparable to the persons who participated in the national ASPINE study (Verhoef et al. 2004). Despite, a selection bias may have occurred since it could be that the more active and more fit people had higher interest in participating than the less active and less fit ones, which may have led to an overestimation of health-related physical fitness components.
In conclusion, the results of the present study show that both ambulatory and non-ambulatory adolescents and young adults with MMC have poor health-related physical fitness. A large part of the variance in aerobic capacity is explained by gender and ambulatory status. Results showed a small but significant relationship between peakVO2 and muscle strength, suggesting that adding strength training to aerobic training may have additional value in increasing peakVO2.

aerobic capacity
muscle strength
body fat
joint mobility
spina bifida