Triggering for Submaximal Exercise Level in Gastric Exercise Tonometry: Serial Lactate, Heart Rate, or Respiratory Quotient?

Gastric exercise tonometry is a functional diagnostic test in chronic gastrointestinal ischemia. As maximal exercise can cause false-positive tests, exercise buildup should be controlled to remain submaximal. We evaluated three parameters for monitoring and adjusting exercise levels (heart rate [HR], respiratory quotient [RQ], and serial lactate measurements) in 178 tests in both healthy volunteers and patients suspected of gastrointestinal ischemia. Exercise levels above submaximal occurred in 20% of HR-, 2% of RQ-, and 5% of lactate-monitored tests (P<0.05 for HR vs. RQ and lactate). Low levels were seen in 5% of HR-, 10% of RQ-, and 41% of lactate-monitored tests (P<0.01 for lactate vs. HR and RQ). High levels resulted in 43% false-positive tonometry results compared to 19% of all tests (P<0.001); low levels did not result in more false negatives (5% vs. 6%). Although RQ monitoring yielded the greatest proportion of optimal exercise tests, serial lactate monitoring is our method of choice, combining optimal diagnostic accuracy, low cost, and simplicity.
Introduction
For diagnosis in patients with suspected chronic gastrointestinal ischemia, we have previously shown that gastric exercise tonometry can be used as a functional test providing information about the adequacy of the gastrointestinal mucosal perfusion [1, 2]. In these studies it was demonstrated that during 10 min of submaximal exercise, gastric ischemia occurred only in patients with splanchnic artery stenosis. An extreme exercise level may cause false-positive tests, as shown in a study in healthy volunteers, where lactate levels exceeding 8 mM resulted in gastric ischemia in 50% [3]. Furthermore, it may be conceivable that exercise of very low intensity can lead to false-negative results in exercise tonometry used for diagnosing chronic gastrointestinal ischemia. In order to prevent false-positive and false-negative tonometry tests, the exercise intensity should be monitored throughout the test, and if necessary, adjustments of the workload should be made in order to obtain an optimal exercise test.
Exercise intensity can be monitored by various parameters including arterial plasma lactate concentration, decrease in serum arterial base excess (BE) or bicarbonate concentration (which are both directly related to lactate level), heart rate (HR), and respiratory parameters [respiratory gas exchange ratio (RQ) = carbon dioxide output (VCO2)/oxygen uptake (VO2)] [4].
In this study we evaluated and compared three consecutive time periods in each of which a different parameter was used for monitoring the exercise intensity and adjusting the workload in order to obtain a submaximal exercise test. Initially HR monitoring was used; in the second period exercise intensity monitoring and adjustment were guided by monitoring of RQ. In the final period exercise intensity was monitored by serial rapid lactate measurements.
Subjects and methods
In 10 volunteers (5 females, 5 males; mean age, 25 years; range, 23–28 years) and 157 patients (59 males, 98 females; median age, 55 years; range, 13–82 years), 178 tonometry exercise tests were performed. The volunteers were tested as part of a more extensive study investigating the effect of two different exercise levels on gastric tonometry [3]. All patients were suspected of having symptomatic chronic gastrointestinal ischemia. Their clinical presentation was that of unexplained abdominal pain, weight loss, diarrhea, or gastric ulcers. Gastric tonometry exercise testing was performed as a diagnostic function test in addition to duplex sonography and selective angiography of the splanchnic vessels. Some of the patients in this study have been previously described in a publication by our group investigating the diagnostic potential of gastric exercise tonometry [1].
The procedure of gastric tonometry exercise testing was described in detail in a previous study [3]. In short, a standard nasogastric tonometry catheter was inserted and connected to an automated air tonometry device that was set up to measure intragastric partial carbon dioxide pressure (PCO2) every 10 min. All subjects were studied after a fasting episode of 4 hr. Ranitidine, 100 mg, was given intravenously 90 min prior to exercise testing. A radial artery catheter was introduced in the nondominant arm to allow sequential arterial blood sampling.
Exercise was performed on an electromagnetically braked bicycle ergometer (Lode, Groningen, Netherlands).
Monitoring exercise intensity and adjusting the workload
The maximal workload (Wmax) was estimated using standard criteria, i.e., age, sex, and weight [5]. The exercise episode was started at 10% of Wmax, and in the first 4–6 min the workload was increased every minute by 10% of Wmax. The workload was intended to remain constant thereafter at a submaximal exercise level. Three different approaches for monitoring the exercise intensity and—if necessary—guiding additional adjustments of the workload were evaluated in consecutive periods.
First period: HR-triggered tests
Exercise intensity was monitored by measuring HR, aiming for a HR in the last 4–5 min of 80% of maximal predicted HR (HRmaxpred). HRmaxpred was calculated by the following formula: HRmaxpred = 210 − 0.65, age [6].
Second period: Respiratory gas exchange ratio (RQ)-triggered tests
The workload adjustments were guided by measurement of the RQ, aiming for an RQ of 1.0 in the last 4–5 min of the exercise episode. This RQ was chosen as it indicates the anaerobic threshold
Third period: Lactate-triggered tests
In 40 of the RQ-triggered tests serial rapid lactate measurements were done during exercise at t = 0, 4, 6, 8, and 10 min. These measurements were not used for guiding the exercise intensity, but to develop a lactate-based exercise triggering protocol. From the results of rapid serial lactate measurements in the RQ-triggered period, an algorithm was developed for triggering the exercise intensity (see Table 1). We aimed for a lactate concentration of 4 mM in the last 4 min of the test. In parallel with RQ triggering at an RQ value of 1.0, this lactate level resembles the anaerobic threshold, at which redistribution of blood flow patterns is initiated.Table 1Algorithm for lactate-guided exercise intensity adjustmentsStart at 10% of predicted WmaxIncrease workload by 10% of Wmax every minute (at t = 1, 2, 3, 4, 5, and 6 min)Lactate measurements at t = 4, 6, 8, and 10 minIf lactate >3 mM, decrease workload by 10% of WmaxIf lactate not >1 mM, increase 2 × 10% of Wmax
In the first two periods (HR and RQ triggering) the tests were performed at the pulmonary function department of our hospital. During these exercise tests a 12-lead electrocardiogram was recorded (Case 12; Marquette Electronics Inc., Milwaukee, WI, USA). HR was recorded every minute. Breath-by-breath oxygen uptake (VO2) and RQ were measured by a respiratory gas analyzer system (Oxycon-α; Jaeger, Bunnik, the Netherlands) and recorded every 30 sec.
In the third period (rapid serial lactate triggering) the test was performed at the gastroenterology function department; HR was monitored, but respiratory parameters were not monitored.
Arterial blood samples for determination of base excess (BE), bicarbonate (blood gas analyzer; Radiometer ABL520, Copenhagen, Denmark), and lactate (enzymatic assay; Cobas Fara; Roche Diagnostics, Branchburg, NJ, USA) were drawn, in parallel with the 10-min tonometry interval, before and immediately at the end of the 10-min exercise episode. Rapid serial lactate measurements were performed using a small portable lactate analyzer (Accutrend; Roche Diagnostics, Almere, the Netherlands), specifically developed for use during exercise testing. Using this device, measurement results were available within 60 sec after blood sampling.
The tests evaluated in the present study were all aimed at a submaximal level: lactate level (or BE decrease) at or just above 4 mM (the anaerobic threshold) and not exceeding 8 mM. After comparison with arterial lactate measurements (see Results), BE decrease during exercise was used as the parameter assessing the exercise intensity (Fig. 1). The optimal exercise level was a BE decrease of between 3 and 7 mM. Less than 3 mM and more than 7 mM were regarded as low and high exercise levels, respectively.Fig. 1Relationship between arterial lactate concentration after exercise and BE decrease at the end of the exercise period
For comparing the results of HR and RQ measurements with BE decrease as the indicator of the exercise intensity reached, the following parameters were used: maximum RQ reached (RQmax), total time RQ>1 (RQt>1), maximum heart rate reached (HRmax) as a percentage of HRmaxpred, and total time of HR >80% of HRmaxpred (HRt>80).
Statistics
All values are given as mean ± SD unless otherwise stated. P values <0.05 were considered statistically significant. Correlations between lactate and BE and lactate and bicarbonate decrease, and between both HR and RQ parameters and exercise intensity as measured by BE decrease, were calculated using Spearman’s rank test. For comparison of HR and RQ parameters with exercise level (as divided into low, optimal, or high), one-way ANOVA with Bonferroni post hoc analysis was used. Group comparisons of the tonometry results of high and low exercise levels versus all tests and the diagnostic accuracy of tonometry in the different monitoring episodes were performed using chi-square testing.
Results
HR triggering
In 39 tests (10 in healthy volunteers and 29 in patients) HR triggering was used. In 22 (56%) of these HR-triggered tests the mean HR in the last 4 min equaled the target HR of 80% ± 5% of HRmaxpred. In nine tests (23%) the target HR was not reached; in five of these the subjects were using β-blocking medication. In eight tests (20%) the HR was >85% of HRmaxpred.
The resulting exercise levels as measured by BE decrease are listed in Table 2. In three tests the exercise intensity was too low. In two the target HR was not reached; one of the subjects used β-blockers. Two tonometry results were true negative; the other was false positive.Table 2Resulting exercise intensities for the three triggering regimesExercise level(BE decrease, <3 mM)(BE decrease, 3–7 mM)(BE decrease, >7 mM)Triggering methodLowTargetHighHR (39 tests)3 (8%)27 (69%)9 (23%)*RQ (84 tests)8 (10%)74 (88%)2 (2%)Lactate (55 tests)23 (41%)**29 (53%)*3 (5%)Note. Values are given as numbers (percentage of total for triggering method). *P < 0.05 and **P < 0.001 comparing the three triggering regimes.
Of the eight tests with an erroneously high exercise level, in only one test was the HR too high. The other six tests in which the HR was too high resulted in the submaximal exercise levels aimed for. When analyzing the 110 tests in which both HR and RQ were recorded, only a weak correlation was observed both between HRmax and exercise intensity and between HRt>80 and exercise intensity as measured by BE decrease (r=0.23 and r=0.20, respectively; P<0.05). There was no significant difference for the HR-derived parameters comparing low, target, and high exercise intensity. Figure 2 displays the comparison of HRmax for the three exercise intensity groups.Fig. 2HRmax (95% confidence interval) at low, target, and high exercise levels HRmax is expressed as percentage HRmaxpred
RQ triggering
In 84 tests RQ triggering was used. In 67 (80%) of these tests the target RQ of 1.0 was reached while the mean RQ in the last 4 min was 1.0 ± 5%. In 11 tests (13%) the target RQ was not reached and in 6 tests (7%) the mean RQ in the last 4 min exceeded the target RQ by more than 5%.
Of the eight tests with a low exercise level, the target RQ was not reached in four. In both tests with a too high exercise intensity, the mean RQ in the last 4 min was >1.05.
When analyzing all 110 tests in which RQ was monitored, positive correlations were found for comparing RQmax and RQt>1 with BE decrease (r = 0.34 and r = 0.38, respectively; P < 0.0005). When divided into low, target, and high exercise intensity groups, both RQmax and RQt>1 showed significant differences among the three groups (P < 0.0005; one-way ANOVA with Bonferroni post hoc analysis). Figure 3 displays the comparison of RQmax for the three exercise intensity groups.
Lactate triggering
In 55 tests lactate triggering was used. In 26 of these tests the target lactate level of 4.0 mM in the last 2–4 min was not reached. This resulted in a BE decrease below 3 mM in 19 tests. In two tests the rapid lactate measurements exceeded 8 mM at t = 6 or 8 min. In one, the resulting exercise level was too high (BE decrease>7 mM). Of the 27 tests in which the target lactate levels were reached (at least 4 mM at t = 6 or 8 min, and remaining below 8 mM), the resulting exercise levels were too low in 7 and too high in 2.
Arterial lactate vs BE decrease and bicarbonate decrease
In 78 tests both arterial lactate levels and blood gas parameters were obtained. A high degree of correlation of both BE decrease and bicarbonate decrease with lactate level after exercise was found: r = 0.93 and r = 0.89, respectively (P < 0.0005). Figure 1 displays the relation between arterial lactate and BE decrease after exercise. Mean paired difference between lactate and BE decrease was 0.7 (SD, 0.7) mM, and that between lactate and bicarbonate decrease was 1.4 (SD, 0.9) mM.
Comparison accuracy of exercise tonometry at low, target, and high exercise levels
As shown in Table 1, lactate triggering resulted in significantly more low exercise levels compared to the other two regimes (P < 0.001).Fig. 3RQmax (95% confidence interval) at low, target, and high exercise levels
False-negative tonometry results were found in 10 of all 178 tests (6%). Low exercise levels did not result in an increase in false-negative tonometry tests: in only 2 of 10 (5%) false-negative tests was the exercise level too low; 1 test was RQ-triggered, the other lactate-triggered.
Tonometry results were false positive in 34 of all 178 tests (19%). Of the 14 tests with a too high exercise level, in 6 (43%) the tonometry tests were false positive (P < 0.001 comparing high exercise level vs. all tests). Three of these tests were HR-triggered (but HR did not exceed the target HR), two RQ-triggered, and one lactate-triggered.
The sensitivity and specificity of tonometry exercise testing for gastrointestinal ischemia did not differ among the three groups (82% and 73%, respectively).
Discussion
For optimal diagnostic accuracy, the exercise level in gastric exercise tonometry can be monitored and adjusted by RQ measuring or, alternatively, by serial arterial lactate measurement. Although the latter resulted in more tests with lower than desired exercise levels, this had no influence on diagnostic accuracy.
The greatest problem relating to exercise level in gastric exercise tonometry is the above-submaximal, or maximal, test, as this is associated with 43% false-positive tests in this study. In maximal exercise, splanchnic blood flow is severely reduced and can lead to reduction below the level required to maintain aerobic metabolism. Although data on the splanchnic vascular responses to maximal exercise have not been established, ischemia has been established frequently in these circumstances [3, 7]. In a study on splanchnic blood flow in trained volunteers, mean superior mesenteric artery blood flow fell by 38–49% after a 10-min exercise test at 70% of maximal workload [8]. It has been shown that the flow reduction in the celiac artery exceeds that of the superior mesenteric artery but still does not exceed about 50% [9]. Therefore the normal splanchnic flow after submaximal exercise remains well above the normal lower level of normal splanchnic blood flow, which is estimated as approximately 30% of the normal baseline value [10]. This explains why submaximal exercise does not result in ischemia in subjects with normal splanchnic vessels. Gastric ischemia during submaximal exercise can be seen in patients with splanchnic stenoses or in patients with reduced cardiac output [1, 11]. Thus, for practical clinical purposes 10 min of submaximal exercise seems adequate to distinguish between normal and pathological gastrointestinal vascular responses.
Some exercise should be performed to allow for blood flow redistribution and reduction of the splanchnic blood flow and, thus, provocation of ischemia. However, the minimally required level is currently unknown. Even with very low-intensity exercise in healthy humans a significant shift in blood flow from the abdominal viscera to the exercising muscles was observed [12]. Thus submaximal exercise as currently advocated might not be necessary, and lower lactate levels may suffice for diagnostic testing.
Triggering on RQ measurements resulted in the highest proportion of tests within the target range but has disadvantages. It requires a pulmonary function laboratory with specific devices for measurement of exhaled carbon dioxide and inhaled oxygen, making the test expensive and more complicated. The advantage of RQ monitoring in minimizing the proportion of below-target exercise level did not result in a lower number of false-negative tests in this study.
The major advantages of the serial lactate measurement are its simplicity and low cost. This rapid test can be performed by paramedic personnel and enables determination of lactate within 60 sec. This enables adequate monitoring and triggering of exercise levels. A gastric tonometry exercise test, a routine procedure at our hospital in patients suspected of chronic gastrointestinal ischemia, takes 15 min of a doctor’s time and 2 hr for a registered nurse. Moreover, the currently available air tonometry device allows for accurate and reproducible measurements without the hassle and problems associated with classically used saline tonometry [13].
In conclusion, by using RQ and serial lactate measurements, adequate exercise levels for gastric exercise tonometry can be achieved. Rapid lactate measurements and the presented algorithm for gastric exercise tonometry are feasible for daily clinical practice.

exercise level
gastric exercise tonometry
tonometry
chronic gastrointestinal ischemia
exercise test
diagnosis


Melanoma of the Anus Disguised as Hemorrhoids: Surgical Management Illustrated by a Case Report

Introduction
Anal mucosal melanoma is a rare tumor that constitutes only 0.4–1.6% of all melanoma manifestations [1]. Prognosis is very poor, with a median survival of less than 2 years, despite curative surgery [2, 3]. Females are more likely to be affected than males and most patients present in the sixth or seventh decade of their lives [1]. Diagnosis is not always straightforward, and is often accidental after surgical treatment for presumed benign disease such as hemorrhoidectomy or lateral internal sphincterotomy, provided that tissue is submitted to the department of pathology [4]. Unfortunately, the tumor is often widely metastasized at the time of initial diagnosis [5, 6]. Here we present a patient who was referred to the surgeon for treatment of hemorrhoids but was diagnosed with anal melanoma.
Case report
A 62-year-old black man with a history of prostatism was referred to our hospital by his general practitioner with the diagnosis of bleeding hemorrhoids. For a 10-week period he noticed a swelling protruding from the anus with daily bleeding from the anus. Defecation was problematical because of pain, but the stool had a normal aspect. Inspection of the anus showed a painful swelling with a diameter of 1.5 cm with a dark necrotic aspect surrounding the anus below the dentate line. Rectal palpation was impossible because of pain. At the outpatient department the swelling was excised under local anesthesia of the entire anus (anal block). Because of the atypical aspect of the swelling, the material was submitted for pathologic examination, which revealed a melanoma of the anus. Further staging of the tumor was not possible based on this material because of fragmentation of the tissue.
A sigmoidoscopy was performed but showed no further lesions. In addition, an abdominal and thoracic computed tomographic (CT) scan with contrast showed no signs of tumor around the anorectal region and no signs of intraabdominal, lymphatic or thoracic metastases. After the patient was diagnosed with having melanoma of the anus, examination under general anesthesia was performed. Inspection of the anal mucosa showed two additional lesions at 2 and 3 o’clock with the sacrum at 6 o’clock (Fig. 1). A wide local excision (WLE) including these lesions was performed with a margin of 2 cm, en bloc with 25% of the circumference of the rectal wall (Fig. 1). The total excised tissue was 6×7 cm. Pathologic examination revealed two small foci of the melanoma with a depth of maximal 3.5 mm. All margins were free of tumor. The patient recovered well from the operation. After 1-year follow-up by means of outpatient department visits and proctoscopy every 3 months, and thoracic and abdominal CT-scan twice a year, there were no signs of recurrent tumor.Fig. 1Melanoma of the anus: wide local excision. A=melanoma
Discussion
Patients with anorectal melanoma usually present with symptoms of rectal bleeding and an anorectal mass and are often misdiagnosed as having hemorrhoids, as was illustrated by the present case report [4]. Therefore, on any atypical anorectal lesion, biopsy should be performed in order to prevent delayed diagnosis.
For the operative management of anorectal melanoma, two options are available: a wide local excision (WLE) or a more extensive abdominal perineal resection (APR). The choice between these surgical procedures is controversial. The main arguments in favor of APR are its ability to control lymphatic spread and to create bigger excision margins, resulting in an assumed lower local recurrence rate. But, in contrast to WLE, APR is associated with mortality, considerable postoperative morbidity (4% hemorrhage, 11–16% wound infection and 14–24% wound dehiscence) and the need for a permanent colostomy [7–11].
Despite attempted curative surgery, the median survival for anorectal melanoma is only 20 months and most patients die within 5 years regardless of the type of intervention used [12]. Therefore, quality-of-life issues must be given consideration when making treatment decisions.
Droesch et al performed a systematic review of the literature, including 14 studies with a total of 301 patients [12]. Wide local excision was performed in 129 patients, and 172 patients underwent APR. Local recurrence developed in 47% of patients after WLE and in 23% of patients after APR. Although these data suggest a trend towards better locoregional control after APR, the difference was not statistically significant. In addition, there was no difference in overall survival between WLE and APR (median survival 21 months for WLE and 17 months for APR).
Bullard et al found an opposite trend with a rate of 50% of local recurrence after APR, compared to 18% after WLE in 15 patients [5]. Postoperative radiotherapy may improve locoregional control after wide local excision. In the retrospective study of Ballo et al, actuarial 5-year local control was 74% and nodal control was 87% in 23 patients who received postoperative radiotherapy after wide local excision [13]. Definitive assessment of the efficacy of adjuvant radiation therapy requires further prospective studies.
In general, it is assumed that the stage of the disease, especially the tumor thickness, is the main prognostic factor [13–15]. Therefore, Weylandt et al suggested that the decision between WLE and APR should be governed by the tumor thickness [16]. In patients with a tumor thickness below 1 mm, a local sphincter-saving excision with 1 cm safety margin would be appropriate; and in cases of a tumor between 1 and 4 mm, a wide local excision with a margin of 2 cm seems to be adequate. Patients with a tumor thickness above 4 mm or invasion of the internal sphincter muscle should be treated with APR. In the case report mentioned in this article, the tumor depth was 3.5 mm and excision margins were free of tumor, thus no additional APR was performed.
In order to evaluate the deepness of the lesion preoperatively, endorectal ultrasonography can be performed [6]. Accuracy in evaluating tumor depth of rectal cancer ranges from 81 to 94%, and accuracy in detecting lymph node metastasis ranges from 58 to 80% [17]. However, its accuracy in evaluation of anorectal melanoma remains unproved and is still evaluated [5].
At the time of diagnosis, up to one-third (16–33%) of the patients have disseminated disease [3]. For these patients, palliative treatment with chemotherapy might be a treatment option. Kim et al reported a series of 18 patients with metastatic anorectal melanoma treated with cisplatin, vinblastine, dadabazine, interferon alpha-2b and interleukin-2 [18]. Major response was seen in 44% of the patients, and complete response occurred in 11%, with a median overall survival of 12.9 months. Yeh et al used a different treatment regimen of temozolomide, cisplatin and liposomal doxorubicin in a patient with stage IV anal mucosal melanoma [19]. This patient showed a remarkable response to chemotherapy, with minimal residual disease and excellent quality of life at 12 months after the start of treatment. However, this therapy still has to be further evaluated and currently no standard systemic therapeutic regimen exists for metastatic anorectal melanoma.
In conclusion, anorectal melanoma represents both a diagnostic and therapeutic challenge to physicians given its non-specific presentation and rarity. It is associated with poor prognosis, regardless of the type of intervention used. Therefore, the overall treatment goal should be to optimize the quality of life. Since wide local excision is a more limited intervention associated with at least comparable survival compared to APR and no need for permanent colostomy, wide local excision is recommended as primary therapy if negative surgical margins can be achieved. APR should be reserved for patients in whom the tumor is thicker than 4 mm and/or involves the anal sphincter.

diagnosis
treatment
wide local excision
melanoma anus
abdominoperineal resection


Exploring Barriers to ‘Respondent Driven Sampling’ in Sex Worker and Drug-Injecting Sex Worker Populations in Eastern Europe

Respondent driven sampling (RDS) has been used in several counties to sample injecting drug users, sex workers (SWs) and men who have sex with men and as a means of collecting behavioural and biological health data. We report on the use of RDS in three separate studies conducted among SWs between 2004 and 2005 in the Russian Federation, Serbia, and Montenegro. Findings suggest that there are limitations associated with the use of RDS in SW populations in these regions. Findings highlight three main factors that merit further investigation as a means of assessing the feasibility and appropriateness of RDS in this high risk population: the network characteristics of SWs; the appropriate level of participant incentives; and lack of service contact. The highly controlled and hidden nature of SW organizations and weak SW social networks in the region can combine to undermine assumptions underpinning the feasibility of RDS approaches and potentially severely limit recruitment. We discuss the implications of these findings for recruitment and the use of monetary and non-monetary incentives in future RDS studies of SW populations in Eastern Europe.
Introduction
In many countries in Eastern Europe large scale social and economic transition over the past 15 years has been associated with widespread poverty and increasing levels of unemployment and population migration, particularly among women.1,2 Such transition has also been associated with shifts in patterns of sexual behaviour and sexual relationships, including increased commercial sex work, the growth of informal sex work economies and associated human trafficking.3 Additionally, a number of countries in the former Soviet Union have experienced a rapid or explosive spread of HIV associated with injecting drug use.4–7 These epidemics are potentially exacerbated by close links between sex work and injecting drug use.2,8–11 This has caused concerns that high levels of sexual mixing between injecting drug users (IDUs) and their sexual partners in settings of high HIV and sexual transmitted infection (STI) prevalence may transform HIV infection from concentrated epidemics among risk populations to generalised epidemics.9,10 It is therefore essential to assess the diffusion of HIV and related risk behaviour in hidden vulnerable populations, such as IDUs and sex workers (SWs), as accurately as possible.
There has been increased and recent interest in an approach to sampling hidden populations known as respondent driven sampling (RDS).12,13 One of the claimed advantages of RDS over other methods of sampling hidden populations, such as time–location sampling, is that it requires little in-depth formative research among study populations.13,14 RDS begins with a set number of non-randomly selected seeds (members of the target population). Seeds recruit their peers (other target population members who make up their social network) who in turn recruit their peers into the study. This occurs through successive waves of recruits which, it is argued, become increasingly more representative of the underlying population as the recruitment progresses. Since RDS claims to generate a representative sample from non-randomly selected seeds, many researchers have assumed that sampling can proceed without detailed prior knowledge about the local target population.13,14
RDS has proved feasible and successful in recruiting hidden populations of IDUs in a variety of settings, resulting in the rapid acquisition of long and varied recruitment chains.11,14–19 RDS has also been used in several countries to collect both behavioural and biological data from SWs (e.g., Vietnam, India) and men who have sex with men (e.g., Bangladesh, Cambodia, Uganda, United States).20–24 However, little is known about the feasibility of RDS as a method of recruiting hidden populations of SWs in settings where the organisation of sex work and access to SWs is highly controlled and where there is little or no contact between the target population and local services. Drawing upon three case studies in Eastern Europe targeting SWs and IDUs involved in SW (IDU/SWs), we explore the feasibility of RDS, identifying some barriers to successful recruitment. These findings have implications for the direction of future research exploring the feasibility of RDS methods.
Methods
Using RDS, data on HIV prevalence and associated risk factors were gathered from SWs in Serbia and Montenegro and from IDUs involved in sex work in Russia (see Table 1). SWs in Serbia and Montenegro were given three uniquely numbered coupons with which to recruit other SWs. In Russia, although no RDS study directly targeting SWs was carried out, RDS was used to sample SWs/IDUs in an RDS study among IDUs. Incentives were provided to participants completing the survey (primary incentive) and for recruiting other eligible participants (secondary incentive). Participants in Belgrade were also offered a rapid HIV test and pre- and post-test voluntary and confidential counselling (VCT). Those with preliminary-positive tests for HIV were referred to a local clinic for confirmatory testing. The studies were undertaken by local fieldwork teams coordinated as part of larger internationally funded research initiatives.
Table 1.Three RDS studies among sex workers and injection drug using sex workersCountryCity  (population size)Formative researchRecruitment criteria  for SWsNumber of  seedsSample reached  (target)Year, study  durationSerbiaBelgrade  (about 2 million)Literature search;  questions on network  characteristics and RDS  study feasibility posed  during individual  interviews with SWs in  2005 (n=10)Exchanged sex for  drugs, money or  other goods in  last 30 days  (irrespective of gender  or sexual orientation)Six initially,  six added later209 (400)2005,  2 monthsMontenegroPodgorica  (about 300,000)Literature search;  questions on network  characteristics and RDS  study feasibility posed  during in-depth interviews  with SWs in 2005 (n=4)Exchanged sex for drugs,  money or other goods  in last 30 days  (irrespective of gender or  sexual orientation)One to start with,  one added  later2 (150)2005,  2 monthsRussian  FederationTogliatti  (740,636)Literature search [relied on  previous qualitative study  of IDUs conducted in 2001  (n=57) and epidemiological  survey of IDUs including SWs  (n=426)]Exchanged sex for drugs,  money or other goods  and injected drugs in  last 4 weeksOne, 35 SWs recruited  by a female non-SW  IDU seed36 (not set)2004,  1.5 months
All studies were approved by appropriate local ethical committees and international human subject review boards. Participants were required to agree to an informed consent before participating. Participants were not linked to the study with any personal identifiers, and all participant information was kept confidential and anonymous.
Prior to data collection, formative research, comprising a combination of key informant interviews and a literature review, was undertaken to assess the local feasibility of the RDS study identifying the optimal study set up (Table 1, column 3). The formative research elicited data on size estimates of the sex work population; description of how sex work is organised (sectors of sex work, freedom of movement, average earnings and prices charged for services in each sector); and the legal situation surrounding sex work. In Serbia and Montenegro, researchers also asked SWs about the size of their social networks in order to fine-tune questions on which RDS analysis would be based. A social network was defined as the number of SWs, 18 years and older, each SW knows and has seen in the past week, month, three months, six months, or year.
Results
As shown in Table 1, a total of 209 SWs were recruited in Belgrade (Serbia), two in Podgorica (Montenegro) and 36 in Togliatti (Russian Federation).
Case 1: Serbia
RDS was conducted in the autumn of 2005 in Belgrade, Serbia’s capital, by a local fieldwork team with a limited access to SW networks. Estimates of the size of the SW population in the municipality, based on police records and anecdotal estimates, vary widely from 900 to 3,000 SWs.2,24,25
Sex work is illegal. Findings from in-depth interviews of SWs suggest that there are approximately ten areas located near the main transit routes in and out of Belgrade where street SWs operate in small groups. Qualitative data suggests that the majority of SWs operate independently, advertising their services in newspapers or through referrals via existing client networks, potentially making them harder to reach through RDS.
Formative interviews also found that there was little mixing of SWs across different sex work sectors (street, agency, and SWs working independently), among street SWs themselves (transvestite, Roma and Serbian/Montenegrin heterosexual SWs) and among SWs working independently. However, in addition to a lack of strong connection among SWs and their independent working situations, the study team undertaking formative qualitative research associated SWs’ reluctance to participate in the study to perceptions of inadequate incentive (10 euros for 1 hour of their time) and a general mistrust associated with coming into contact with ‘official’ agencies. However, the same interviews suggested that in principle SWs would be interested in participating in a survey involving the collection of both behavioural and biological data through HIV testing. Interviewees indicated that they knew approximately 20 other SWs, which again suggested that RDS would be feasible.
The RDS study had a target of 400 SWs, both male and female. The sample needed to be large enough for a statistically reliable analysis of risk factors and HIV prevalence, should RDS analysis show that the sample is reasonably representative. The interview site was on the premises of a non-governmental organisation (NGO) located in central Belgrade easily accessible by public transport. It was a part of a large apartment block with a separate entrance. The site was open everyday from 3 to 9 p.m. and could accommodate three to four participants an hour because of the computer administered questionnaire. After two months, the study ceased, having reached only 209 SWs. In total, 12 seeds were used during the study. Of the initial six seeds only three produced any subsequent waves. The rate of recruiting increased noticeably after an RDS study of IDUs being implemented at the same site had ceased. Due to the difficulties experienced by the research team in locating new seeds through other means, four participants with dense social networks were drawn from the seventh wave of two of the most productive recruitment chains and used as new seeds. This resulted in two more productive chains that ultimately reached seven waves.
The average SW network size reported during the study was 12. In response to the question, “where do you most often solicit your clients?,” the majority of SWs replied that they solicited on the street (43%), followed by referral from an agency, pimps, friends or other SWs (28.9%) and by telephone (25.2%). Importantly, the take-up of VCT provided as part of the RDS study was high (71.8%), and SWs indicated that they participated because of the HIV testing, rather than the monetary incentive.
Case 2: Montenegro
There are no estimates of the number of SWs in Podgorica, the Republic’s capital. Sex work is ostensibly illegal. Through formative rapid assessment no street-based SWs were identified and the majority of SWs were found to work independently from their homes. Key informant interviews (IDUs, ex-IDUs, taxi drivers, public health officials, NGO representatives and journalists) identified the only semi-public sex work sites as being approximately six illegal brothels, registered as lap dancing venues, in which most of the SWs also lived. At least one such brothel was found not to allow SWs to leave the premises. Indications were that local police also ‘moonlighted’ as security guards in these brothels. Other links between brothels and public services included taxi drivers, beauty salons and hotels. The intersection of the police and sex work networks, the small size of the city and the high number of implicated actors suggested that SWs could be easily monitored and controlled.
Previous attempts by the local NGOs to conduct research among SWs in Podgorica had failed. Key informants, interviewed during formative rapid assessment, reported knowing at least one SW or their pimps but in almost all cases introductions were denied or never materialized. Those conducting the formative rapid assessment received strong advice from local experts and key informants—including NGOs, police, and taxi drivers—against approaching brothel owners because of the highly controlled nature of SW venues in the city. After two weeks of trying to make a contact, four SWs were accessed via their drug user networks, the only entry points into SW networks available to researchers. These SWs were interviewed at their place of work (brothel and pimp’s flat). Each of the four interviewed SWs independently estimated that there were approximately 200 SWs in the city, many of them also drug injectors, most of whom worked independently, advertising their services via newspapers and television and via client recommendation but who were not open about their SW status. All four SWs interviewed indicated that they would participate in a RDS study and would invite others to do so, and each estimated they had personal regular contact with between five and ten SWs.
The four SWs participating in the formative rapid assessment were invited to participate in the RDS study as seeds. Only one of the four participated but failed to recruit anyone else. She reported having had contact with ten SWs in 4 weeks, but no additional seeds were secured. Despite recruiting an additional SW (who required complete anonymity fearing being identified as a SW by others to his family and who had not seen other SWs in the past month), the RDS study failed to identify a sufficient pool of seeds willing to participate.
Case 3: Russia
The RDS study of IDUs and IDU/SWs was conducted in May 2004 in Togliatti City. Togliatti City is the second city of Samara Region situated approximately 1,000 km south of Moscow. Togliatti was selected as a study site because evidence suggests a recent increase in the number of IDUs and SWs and an explosive spread of HIV associated with injecting drug use.6,11
Approximately 2,000 SWs are estimated to operate in Togliatti, half of whom work from the street.6 Despite ambiguous Russian legislation towards sex work, it is not tolerated by the wider community or the police. The city has been noted for its intense street policing of drug users and SWs, often involving fines levied under administrative codes, such as possession of drugs, causing a public nuisance, or lack of official residency permit, rather than criminal codes.26,27 Street-based SWs in Togliatti operate openly along the main roads and highways. Two popular locations are Moskovsky Prospect where approximately 70 women are working at any one time, and Pobeda Street where about 100 women work.
In a study conducted in Togliatti City in 2001, 50% of female IDUs (n=155) reported ever exchanging sex for goods or money, of whom 86% (n=66) were currently SWs.6 Although no stand-alone RDS studies of SWs have been conducted in Russia, attempts have been made to boost participation of IDU/SWs in RDS studies of IDUs motivated by the evidence of high proportion of SWs who also inject drugs in other cities.28–30
A cross-sectional survey of IDUs and female IDU/SWs was undertaken in Togliatti in 2004 as a follow up to the 2001 baseline survey. A total of 476 IDUs were sampled during a 5-week period in May 2004 using RDS. Among these, 55 (11%) reported ever exchanging sex for money, drugs or goods, and of these, 36 in the past month, which was lower than expected. At the start of the field work, seven seeds were selected, of which one seed was female who had exchanged sex in the past month. SWs were explicitly asked to recruit other SWs, and non-SWs were encouraged to recruit either SWs or non-SWs (Table 1, column 4). This first SW seed produced no recruits and was lost to follow up. Another female non-SW started a chain that recruited a total of 22 female IDUs who had ever exchanged sex, of whom 13 had exchanged sex in the past month. This female non-SW seed produced a total of 135 recruits by the end of the study equivalent to 28% of the total sample, of which 15% (20) were SWs and who reported working on the street. Among the other seeds, only a handful of current SWs were recruited (n=4). Towards the end of the project more coupons were distributed to female SWs in an attempt to increase the SW sample (n=5), none of which produced any recruits. In order to increase the sample of SWs in the survey, 21 SWs were subsequently identified and interviewed without coupons.
Data on the number of female SWs within an individual’s IDU network were unfortunately not provided. The mean IDU network size among SWs was 19.5 (SD 14.2) and among non-SWs 27.1 (SD 33.0). Only one SW recruited through RDS reported working from an apartment; all others were street-based SWs. We expected that RDS would enable us to recruit more hidden populations of SWs such as those working from apartments or hotels. The fact that SW seeds failed to produce any waves of SWs or were lost to follow up, indicates that SW networks may not be as well connected.
Field notes show that initial attempts to access street SWs at Moskovsky Prospect and Pobeda Street had failed primary because of intensified police action in the area. A local officer of the Federal Security Service believed he had been recently infected with HIV by a SW from the area, and, in revenge, he and his colleagues enforced a repressive policy of fines and arrests among SWs. Recruiting SWs towards the end of the survey was more successful when policing practices returned to normal. Meanwhile, the sample target was reached for non-SW IDUs.
Discussion
Despite undertaking formative research in the planning of these RDS studies, we found that RDS methods were unable to attain sufficient samples of SWs in Serbia, Montenegro, and Russia. A number of inter-related factors emerged as important to the success of RDS: low motivation of SWs to participate due to their perception of inadequate incentive; low-density SW networks combined with high level of independence in the organisation of sex work; a tightly controlled small number of sex work venues; lack of contact between SWs and local services in the context of high levels of mistrust of ‘official’ agencies; and difficulties emerging in the organisation of the RDS studies of SWs at the same time as a parallel RDS study of IDUs.
Whereas in other settings (e.g., Vietnam, north-eastern India, Brazil) SWs have reacted favourably to nominal monetary incentives in RDS studies, including incentives which fall below the rate charged for sex,1617 it is possible that the incentives offered were perceived as being too low. In order to earn primary and secondary incentives, which together in all three studies amounted to about a half-an-hour SW’s rate, they had to leave their work and travel to and from the fixed interview site at least twice plus spend at least an hour involved in the study. It is possible that SWs in Eastern Europe may be less interested in the incentives offered since they have comparatively higher earnings.
Indications are that free HIV/STI health assessments and referral for treatment should be explored in addition to the incentive. The high acceptance of VCT in Belgrade, for example, is a good example of how provision of services can be more effective than monetary incentives in recruiting survey participants. If incentives are purely monetary, they should be adequate to entice SWs to participate; yet this is often not feasible given the risk of high incentives also encouraging the attempted participation of non-SWs.
Unlike other RDS studies conducted in Vietnam,16 SWs in Belgrade felt uncomfortable being interviewed at a location where a concurrent study of IDUs was being conducted. Based on anecdotal information and the fact that SWs in Belgrade started participating in the study once the IDU study had finished, we believe that SWs were dissuaded from going to a location that was used by IDUs. This problem can be assessed during formative research using focus groups with SWs to assess whether they would feel comfortable sitting in a waiting room with a group of male and female IDUs.
Perhaps the main reason for the poor success of RDS recruitment among SWs in these three settings is the characteristics of SW networks. RDS methodology requires that the target population form sufficiently large and dense networks. However, the information on the network sizes of a few SWs gathered through our formative research are not easy to interpret since there is no known cut-off point for determining an appropriate network size or density for RDS. Based on the results of our studies, SWs in our three study areas often work independently, thus forming smaller, more isolated, networks. Those working on the street and in brothels are also under tight control by managers, pimps and police thus restricting their movement and weakening network density. On the other hand, prevalent societal norms—a form of social control—prevent SWs who work independently from revealing their SW status and participating. The low recruitment rate encountered in our studies might be overcome by allowing studies to run for a much longer time—perhaps as long as six months or a year—allowing SWs to gain trust in the investigators.
In the studies described above, interview sites were rented and staff employed for a fixed amount of time. A preferable setting is one with on-going services for SWs that can be used as an interview site for a long period of time allowing recruits to come at a slow rate. However, time was unlikely a major factor contributing to difficulties in RDS recruitment in the cities where our studies were conducted.
The dearth of well-established projects targeting SWs in Eastern Europe means that greater effort should be invested prior to RDS studies to identify a greater number of seeds and engender SWs’ trust. In some cases existence of a well-established service with established contacts with local SW communities might be a decisive factor. For example, this was the case in a RDS study among SWs in Tbilisy, Georgia (January 2006, written communication, Johnston LG). In addition to promoting trust, such service organisations can provide additional networking opportunities for loosely connected SWs.
Conclusion
Given the cost of conducting second-generation sero-surveillance among highly vulnerable populations, formative research is important for establishing the appropriate type of sampling and reward system. We found the making available free and confidential HIV testing services was an effective incentive, and perhaps more so than monetary incentives alone. Although formative research would require some additional costs, it can increase efficiency in a more extensive and successful study.
During formative research, we suggest that policy makers, service providers and target populations be consulted to determine the visibility of sex work, as low visibility can be associated with tight control of the sex work industry and weak networking among SWs, indicating difficulties using RDS. If researchers find it difficult to recruit SWs for formative study, the RDS study is likely to fail. Depending on the timescale, funding and interest among existing service providers, additional services may be provided well ahead of a RDS study in order to access SW respondents.
Further investigation is needed into the ethnography of SW networks in this region, especially on network composition and service access. In the case of IDUs who exchange sex for money, goods, or drugs, it is important to understand their primary network linkage—injecting drug use or sex work—and how they represent their “other identity” within each network. Incentives need to be better identified in advance of RDS studies, specifically as they relate to level of income, opportunity costs, and trust in the investigators and seeds. Without this cultural and legal sensitivity, surveillance for HIV among this vulnerable population will not provide adequate information on epidemic spread or program effectiveness.

respondent driven sampling
sex workers
russia
serbia
montenegro
hidden populations


Association between long travel and venous thromboembolic disease: a systematic review and meta-analysis of case-control studies

The term “economy-class syndrome” defines an infrequent episode of venous thromboembolism (VTED) related to a long travel, namely by plane. However, this relation has not clearly been demonstrated by investigators. We carried out a systematic review and a meta-analysis of cases-control studies that had studied this topic. We realised a systematic review of the literature and selected all the case-control studies published. Two authors carried out a methodological evaluation according to the Scottish Intercollegiate Guidelines Network items (concordance was analysed by weighted kappa index), and a systematic analysis of the potential biases of each study was assessed. We carried out the meta-analysis with the data extracted from the studies. We recovered eight cases-control studies. The relation between the antecedent of a long travel and subsequent VTED varied from OR = 1.1 to OR = 4.0 and was found to be significant in four studies. The studies were highly heterogeneous in methodology and so the results obtained about the relation between the long travel and the VTED and the score at SIGN50. Two meta-analysis were carried out: only with travels by plane in which the relation was not significant (OR = 1.21; CI 95%, 0.95–1.55) and with all types of transport, with a slightly significant relation (OR = 1.46; CI95%, 1.24–1.72). We may deduce from this systematic review that there does exist a weak association between episodes of VTED and a long travel, but not by plane specifically. The heterogeneity and the methodological quality of the studies published preclude of more robust conclusions.
Introduction
Venous thromboembolic disease (VTED) has been associated with many risk factors, one of which is the antecedent of a long travel, most often by plane. This association was previously suggested by Homans [1] and was believed so self-evident that it gave rise to the term “economy-class syndrome” [2]. Subsequently, the description of clinical cases related to other means of transport led to the coining of another more appropriate generic term, “traveller’s thrombosis” [3].
The scenario for this syndrome has varied from the alarm caused by the apparently high frequency of fatal pulmonary embolism (PE) occurring after a prolonged flight, as reported in the media, to the scepticism of a weak, although likely association, reported in various scientific studies. The published reports have ranged from descriptions of individual cases to cohort studies and even clinical trials. However, the low incidence of episodes of VTED after a long travel makes the design of a case-control study particularly useful for evaluating the suggested association [4].
Our objective was to carry out a systematic review and meta-analysis of case-control studies to analyse the association between long travels and the development of VTED.
Materials and methods
We carried out a bibliographic search using a combination of keywords and MeSH headings in the Medline, Embase and Cochrane Library bibliographic databases. We selected only case-control studies, in any language, with no limit on the date of publication, and performed a cross search of the references cited in these studies. The corresponding author named in each study was requested to inform us of any relevant data that had not been described in the original manuscript.
Two authors (JTS and AJP) carried out a descriptive analysis of the studies identified in the search and evaluated their quality according to the checklist proposed by the Scottish Intercollegiate Guidelines Network [5]. Concordance in the evaluation of the items on this list was analysed using the respective weighted kappa indexes.
We also analysed the limitations and potential biases that might reduce the validity of the studies examined. The sources of bias analysed were grouped into three categories: exposure misclassification (recall bias), selection bias (including detection bias, effect misclassification, survival bias, self-selection bias and Berkson’s bias) and confounding factors. Memory bias was defined as the fact that when subjects know they have suffered VTED, it makes them more likely to remember prior exposure to the antecedent of a long travel. Effect misclassification was minimised by considering only those studies in which VTED was diagnosed by objective complementary examination. Detection bias was suspected when the odds ratio (OR) in the days immediately after the travel was less than that corresponding to subsequent days. We only selected data published for VTED incidence at 30 days after the travel because was the time lapse most frequently analysed in the studies. Survival bias was considered to be that occurring when the study included prevalent cases, which could give rise to confounding of the variables related to the origin and those concerning the prognosis of the disease. Self-selection bias was considered to be present if the controls participated on their own initiative in the study and were not selected in a consecutive or random way; thus, they might be related to the results being sought. Berkson’s bias would be derived from the use of hospital patients as controls.
The confounding factors were judged to be influential if no account were taken of the possibility that the exposure factor and the disease might be related through a third variable which was related to each of the other two.
From the results included in the published studies and the data supplied by the different authors, we were able to perform a meta-analysis of the studies that had estimated the risk ratio of deep-vein thrombosis (DVT) and of DVT and/or PE either after a travel (by any means of transport) or only after a travel by plane. The OR was used as a measure of association, with a confidence interval of 95%. The hypothesis of homogeneity was evaluated using X [2] and Der Simonian–Laird tests, and fixed and random effects methods were used, respectively, for the cases of homogeneity and heterogeneity between the studies. We assessed possible publication bias using Begg and Egger tests and carried out a sensitivity analysis by evaluating the influence of the absence of each individual study on the global measure of association of all the other studies.
The computer softwares used for this study were SPSS 11.5 (SPSS, Chicago, IL, USA) and EpiDat 3.1 [6].
Results
Nine case-control studies were recovered; of these, we analysed the data from seven [7–14], as the other one [15] corresponded to a preliminary analysis for one of the seven. One of these [7] was made up of three sub-studies, but for reasons of consistency, we only included the data for two of these in the meta-analysis, as in the third one, the authors assessed patients outpatients and inpatients with PE. Table 1 shows the main characteristics of all these studies. The relation between the antecedent of a long travel and subsequent VTED varied from OR = 1.1 to OR = 4.0 and was found to be significant in four studies [8, 9, 11, 14]. In them, the OR ranged from 2.1 (1.1–4.0) to 4.0 (2.0–7.9).
Table 1Principal characteristics of the case-control studies evaluating the association between a long travel and venous thromboembolic diseaseStudy, yearFerrari, 1999Samama, 2000Dimberg, 2001Arya, 2002Hosoi, 2002Ten Wolde, 2003Martinelli, 2003Cannegieter, 2006Period of study1992–19951990–19911995–19982000–20012000–20011997–20001999–20011999–2000Origin of casesPatients admitted with VTEDDVT, symptomatic outpatientsEmployees of the World Bank with confirmed DVTDVT, symptomatic outpatientsDVT, symptomatic outpatientsDVT outpatients; PE outpatients and hospital admissionsPatients with VTED during previous 12 months and examined for possible thrombophiliaVTED, outpatients and admissionsOrigin of controlsPatients admitted in Cardiology Department, age-matchedViral syndrome, matched by age and genderEmployees of the World Bank without DVT, matched by month and year of diagnosisHospital attention for compatible symptoms; DVT excludedHospital attention for compatible symptoms; DVT excludedHospital attention for compatible symptoms; DVT or PE excludedFriends or partners, volunteers, of hospital patientsPartners of casesNo. of travel/cases (%)39/160 (24.4%)62/494 (12.6%)3/17 (17.6%)20/185 (10.8%)15/101 (14.9%)8/130 (6.2%)31/210 (14.8%)233/1906 (12.2%)No. of travel/controls (%)12/160 (7.5%)31/494 (6.3%)163/489 (33.3%)31/383 (8.1%)13/106 (12.6%)38/959 (4.1%)16/210 (7.6%)182/1906 (9.5%)OR (95% CI)4.0 (2.0–7.9)2.1 (1.4–3.4)0.4 (0.1–1.5)1.4 (0.8–2.5)1.25 (0.56–2.7)1.6 (0.7–3.5)2.1 (1.1–4.0)2.1 (1.5–3.0)Type of transportVariousVariousOnly planeVariousVariousVariousOnly planeVariousDuration of travelAt least 4 h (not stratified)Not stated (“long travel”)Not statedStratified into 3 and 8 hours>3 hoursStratified from 3–5h to >16 hStratified into 8 hAt least 4 h, stratified into 4 hLapse between travel and VTED (weeks)43442448Principal limitationsBerkson’s bias, Controls age–matchedNot specific to assess risk of travel, Confounding, Controls matched by age and genderOnly international travels were evaluated (not duration of travel)Selection of controlsConfounding, Not differentiated by duration of travelBerkson’s biasSurvival bias, Matched by age, gender and academic level, Self–selection bias, Recall biasMemory bias, NO objective clinical information about controls
With respect to means of transport, two studies [9, 12] only assessed travel by plane, while two [7] MEGA examined separately each means of transport. Another study [13] distinguished travel by plane from all other means of transport, while another two [10, 11] considered various means of transport jointly, but did not distinguish those corresponding to cases from controls, and in one study [8], any means of transport was eligible for consideration without further specification.
The duration of the travel was addressed in different ways: two studies [8, 12] made no reference to this question; two studies [11, 15] only considered travels with a duration exceeding 4 h, and another, those exceeding 3 h [13]. The other three studies [7, 9, 10] considered different durations. The lapse of time between the travel and the diagnosis of VTED was 2 weeks in one study [13] and 4 weeks for the others, except that of Samama et al. [8] which did not specify the period exactly and MEGA study [14] that included 8 weeks before. In the study by Dimberg et al. [12], two periods were analysed (15 and 30 days), but in our own analysis, only the data corresponding to 30 days were taken into consideration, this period being equivalent to the 4 weeks used in the other studies.
Four studies [7, 9, 11, 14] analysed cases of both DVT and of PE, while the others only considered patients with clinical presentation of VTED in the form of DVT. The selection of cases was generally that of patients with symptoms of DVT and/or PE. One study [12] used a register of days off work, kept by medical insurance companies for the staff of the company in which the study was carried out. The rate of prior history of a long travel, among the cases, ranged from 6.2% [7] to 24.4% [11].
Diverse methods were used for selection of controls. Three studies [7, 10, 13], selected outpatients who, after attending hospital with symptoms compatible with DVT and/or PE, were found not to have these diseases; in another study [11], the controls were hospital patients, matched by age, who had been admitted for other reasons during the same period of time as the cases. Two studies chose outpatient controls, matched by age and sex with the cases, one selecting patients with symptoms of respiratory viriasis [8] and the other using relatives and friends of patients who had suffered VTED [9]. Another study [14] chose the partners of the cases as controls. Finally, Dimberg et al. [12] chose ten control cases, matched by month and year of diagnosis of DVT, from among the staff of the same company that employed the cases. The study by Ferrari et al. [11] even used different exclusion criteria for cases and for controls such that anticoagulant treatment was an exclusion criterion for the former but not for the latter. The rate of prior occurrence of a long travel, among the controls, ranged from 4.1% [7] to 33.3% [12].
With respect to the possible biases present in each of the studies, we highlight the following:
Recall or classification bias: the study by Martinelli et al. [9] could suffer from bias in this sense, as one of the exclusion criteria was the absence of a prior occurrence of VTED during the previous 24 months. In general, this bias is consubstantial with the studies of cases and controls such that the cases tend to relate the disease with the antecedent of interest.Detection bias: only one study [12] analysed two periods of time in which DVT had occurred after a long travel, namely at 15 and 30 days. The OR for the former period was greater than that for the latter, and thus, this form of bias was not present here.Misclassification bias: in all the studies, only cases of VTED that had been objectively diagnosed and for which the appropriate complementary examinations were taken into consideration, and thus, their correct classification was ensured. In three [7, 10, 13], the controls were derived from persons who, having sought medical treatment for symptoms compatible with DVT and/or PE, were found not to suffer either of these after the same objective examination used to confirm the cases. In others, the controls presented no symptoms of DVT and/or PE and so were also assumed to be true controls.Survival bias: this bias type could have occurred in the study by Martinelli et al. [9], as the cases chosen were those patients who had suffered an episode of venous thromboembolism during the previous 2 years and for whom a study of thrombophilia was performed; thus, there was, in fact, a selection of all the potential incidental cases of DVT during this period.Self-selection bias: in the study by Martinelli et al. [9], the controls were recruited from the relatives and friends of the cases, a circumstance that could have negatively influenced the relation being studied, as the sample was obtained from a population that was socially related to the cases, and thus, exposure factors could have been shared. In MEGA study [14], the controls were partners of the cases, and the evaluated relation might have been diminished.Berkson’s bias: this bias might have been present in the study by Ferrari et al. [11], as the controls chosen were patients admitted to hospital with chest pain, which could have meant they had a lower probability of prior travel; this pathology was, therefore, a potential limiting factor. Other studies [7, 10, 13] could also have been affected by this form of bias in that the controls they chose were patients who had attended hospital with symptoms compatible with those of DVT and/or PE; objective examination subsequently determined that neither pathology was present. The study by ten Wolde et al. [7] included cases of VTED present in patients who had been admitted to hospital for other reasons, which reduced the number of days before the episode of VTED in which the patient could have travelled.
The methodological evaluation of the studies is shown in Table 2. Overall, the evaluation was appropriate in five studies [7, 10, 12–14], while results were less satisfactory in three [8, 9, 11] mainly because of the inadequate choice of controls and because the study design did not enable the authors to achieve a good estimate of the risk being studied. There was a high degree of agreement between the reviewers concerning the quality criteria of the cases and controls (weighted kappa index = 0.8).
Table 2Methodological evaluation of the studies included in the systematic review in accordance with SIGN 50 criteriaCharacteristicFerrari 1999Samama 2000Dimberg 2001Arya 2002Hosoi 2002Ten Wolde 2003Martinelli 2003Cannegieter 2006Internal validityClear, appropriate questionsGAGGGGAGSelection of subjectsCases and controls from comparable populationsAAA-GA-GGAPGIdentical exclusion criteria for cases and controlsP-AAAGGGAGParticipation rate by cases and controls95-NS80NSNS74-79NS91-NS83-77Comparison between participants and non-participantsNSNSNSNSNSNSNSNSCases are defined and clearly differentiated from controlsA-GGPAGG-APG-AIt is clearly stated that the controls are non-casesAGPPG-AG-APPEvaluationKnowledge of exposure did not influence designation of casesAAAPG-APAAThe exposure is measured in a standard, valid wayAPA-PG-AG-AG-APAConfoundingIdentification of main confounding factorsP-APAP-AAA-PP-AAStatistical analysisIdentification of confidence intervalsYesYesYesYesYesYesYesYesOverall assessmentControl of bias and confounding factors+/++++/+++/++/++++++++/+++Confidence that the overall effect is due to the exposure being investigated++++++++++/++++++/+++The results are applicable to the target group of patients being studied+++++/++++++++/+++++++++++A single evaluation is shown when the two reviewers agree; otherwise, both evaluations are given.G Good, A adequate, P poor, NS not stated
Results of the meta-analysis The studies were varied as regards the episode of VTED included (DVT and/or PE), the controls chosen, the means of transport evaluated and the estimated duration of the travels. These variations influenced the performance of the meta-analysis. Thus, we decided to carry out two meta-analyses, on the basis of the similarity between study designs, i.e., on the one hand, studies of patients with DVT (with or without PE), considering any type of transport, and on the other hand, those studies examining only cases in which the subjects travelled by plane (Table 3).
Table 3Results obtained in the studies, by type of transport All types of transportOnly planeOther typesCasesControlsCasesControlsCasesControlsDVTSamama62/494 (12.6)31/494 (6.3)NSNSNSNSHosoi15/101 (14.9)13/106 (12.6)9/101 (8.9)12/106 (11.3)6/101 (5.9)1/106 (0.9)Aryaa20/185 (10.8)31/383 (8.1)16/185 (8.6)29/383 (7.6)4/185 (2.2)2/383 (0.5)Ten Woldeb8/130 (6.2)38/959 (4.1)NSNSNSNSDimbergcNANA17/30 (56.7)489/891 (54.9)NANADVT and/or PEFerrari39/160 (24.4)12/160 (7.5)NSNSNSNSMartinelliNANA31/210 (14.8)16/210 (7.6)NANACannegieterd233/1906 (12.2)182/1906 (9.5)86/1906 (4.5)72/1906 (3.8)147/1906 (7.7)110/1906 (5.8)NA Not applicable, NS not statedResults are expressed as number of patients travelling / total (%).aArya: Travels of more than 3 hbTen Wolde. Restricted to patients capable of travellingcDimberg: More specific case definition of the two analyseddCannegieter: Venous thromboembolism episodes within the 8 weeks after travel
Meta-analysis of studies including patients with DVT or DVT+PE, all types of transport Six studies [7, 8, 10, 11, 13, 14] were examined. The Der Simonian–Laird test of heterogeneity gave a result of p = 0.011, with an estimated 75% total variance due to variance between the studies. The global estimator with a model of random effects produced an OR of 1.46 (95%CI, 1.24–1.72). No publication bias was recorded (the Begg and Egger tests results were not significant). The sensitivity analysis revealed that no study influenced significantly on the final OR. Figure 1a shows the forest plot graph corresponding to this meta-analysis.
Fig. 1Forest plot for studies that evaluated the antecedent of any type of transport and only travels by plane. a Travels by any type of transport. b Travels by plane
Meta-analysis of studies including patients with DVT or DVT+PE, only travels by plane This meta-analysis included five studies [9, 10, 12–14]. The results of the Der Simonian–Laird heterogeneity test were not significant, which suggests there was little heterogeneity between the variances of the studies. The OR estimator produced a non-statistically significant value of 1.21 (95%CI, 0.95–1.55). Again, no publication bias was detected, and the sensitivity analysis showed that none study was influential on the OR. Figure 1b shows the forest plot graph corresponding to this meta-analysis.
Discussion
The results from our analysis reveal a slight association between long travels and the development of DVT or PE, although this association disappears when only travels by plane are considered. The published case-control studies were characterised by considerable heterogeneity, especially in the selection of controls, and there were some errors in the study design that could have led to bias in estimating the effect of the association being studied. A meta-analysis of case-controls studies has recently been published. In that study, we did not find any relationship between a long-distance travel and venous thromboembolic disease when all the means of transportation were valued or only by plane, but not all published studies were included [14].
The pathogenic basis of the “economy class syndrome” lies in a series of predisposing environmental factors within aircraft (low humidity, relative hypoxia and low barometric pressure) that would facilitate the relative dehydration of passengers; these factors would be accompanied by mechanical ones, such as immobility, the narrowness of the seats and the prolonged maintenance of a seated position, with flexion and compression of the poplitea region [15–17]. The description of episodes of DVT related to other means of transport produces a loss of plausibility of the environmental factors associated solely with travels by plane and puts the prime focus on purely mechanical factors and on the characteristics of the patients themselves. Moreover, a recently published study [18] found no variation at all between different coagulation factors after exposure to the environmental conditions resembling in a prolonged travel by plane. Nevertheless, in one recent systematic review, Philbrick et al. [19] emphasised the presence of prothrombotic risk factors and a flight duration longer than 6 h as predisposing to suffer a VTE episode.
The patient-related variables refer to a greater frequency of VTED risk factors. Thus, various studies have reported that a high percentage of the patients involved were obese, higher or smaller, aged more than 50 years, had antecedents of VTED, were taking oral contraceptives or presented a state of hypercoagulability or thrombophilia [2, 11, 15, 18, 19]. This result implies that there is a relation between transport-related factors and those concerning to the patients themselves, and that this is relevant to the development of an episode of VTED.
Studies that have sought to analyse this association have obtained divergent results mainly because of the different study designs; this would explain the high variability in the OR obtained, which ranged from 1.1 to 4.0. Moreover, these studies were heterogeneous in various methodological respects, and this could have influenced the final results; some [8, 11] matched the cases and controls by sex and age, which limits the influence of these variables in determining the association under study; in the study by Martinelli et al. [9], the cases were a selection of all the possible incident cases within a given period; in another [11], the controls selected were not derived from the same population as that of the cases; the means of transport and the duration of the travel, as well as the type of VTED episode evaluated were different in all the studies; and, in general, no multivariate or stratified analysis was carried out to attribute the increased risk of developing an episode of VTED to the long travel.
An interesting aspect from the methodological point of view is the possible selection bias that might have entered these studies: those patients (the cases) who travelled presented fewer VTED risk factors than did the controls, i.e. they were basically more healthy. Not only could this have led to the relation between the development of an episode of VTED and the antecedent of having made a long travel being underestimated, but it might even appear that the prior travel had a protective effect, as was the case in the study by Dimberg et al. [12]. One way of minimising this type of bias could have been to include only those patients who would physically have been capable of making a long travel; this was only actually done in the case of the study by ten Wolde et al. [7]. Another possibility could be to adjust for this variable (which might be considered that of comorbidity) by means of multivariate analysis.
Given the methodological heterogeneity of the published studies of cases and controls, it was not possible to carry out a detailed meta-analysis of different aspects of the relation under study, and so its scope was limited to those studies that assessed the association between DVT or between DVT and PE and prolonged travels by plane or by any other form of transport. In any case, the association that was found in the latter case was only weak, and there was a large degree of variation in the methodological quality applied in the various studies.
A noteworthy question is the antecedent of a prolonged travel made by the controls, with variations in all the studies of 6.3–8.1%, except in the case of the study by Dimberg et al. [12]; in the latter case, the scenario chosen for analysis led to the choice of a population in which a high proportion (33.3%) made international travels, an aspect that could have weakened any potential association. Similarly, the rate of incidence of a lengthy travel among the cases ranged from 6.7 to 14.8%, although in two studies, this figure was surprisingly high, 17.6% in that by Dimberg et al. [12] and 24.4% in the study by Ferrari et al. [11].
The methodological quality of the studies examined in this systematic review means that we must be cautious concerning the results reported. Although there does seem to be a likely relation between a long travel and the development of an episode of VTED, such an association must be of such a magnitude that a small bias or modification to the study could increase or decrease the strength of the association recorded.
In conclusion, we may deduce from this systematic review that there does exist a real, but weak, association between episodes of VTED and the antecedent of a lengthy travel, and this relation with the travels by plane is only nearly significant. The heterogeneity and the methodological quality of the studies published on the question limit the robustness of the conclusions obtained.

long travel
venous thromboembolism
economy-class syndrome