The Influence of Prolonged Inspiratory Time on Respiratory Mechanics and Oxygenation in Obese Patients Undergoing Spine Surgery in the Prone Position
Phase: NA
Conditions: Hypoxia
Sponsor: Gachon University Gil Medical Center

 
 
 
 
 
Title : The Effect of Prolonged Inspiratory Time on Pulmonary Mechanics in Obese 
Patients 
 
 
 
 
NCT number :  NCT02961920 
 
Document date : March 6, 2018 
 
 
 
 

Methods 
Subjects 
This study was approved by the institutional Ethics Committee (Gachon University 
Gil Medical Center Institutional Review Board 2016-211, Incheon, Korea) and registered at the 
US Clinical Trials Registry (NCT02961920). After obtaining written informed consent, we 
assessed 54 patients scheduled for lumbar spine surgery in our institution between November 
2016 and October 2018. Fifty adult patients, ASA physical status 1 or 2 with a body mass 
index (BMI) of ≥ 25 kg/m2, were randomly allocated to one of two groups: CRV (n=25) or 
ERV (n=25). BMI was not used as the basis for the ASA classification. Patients who had 
severe pulmonary disease (history of chronic obstructive pulmonary disease, asthma, 
bronchopleural fistula, or pneumothorax) or haemodynamic instability and/or hypovoloemia 
were excluded. The enrolled patients were randomly allocated according to a predetermined 
allocation sequence to receive an I:E ratio of either 1:2 (CRV) or 1:1 (ERV). A researcher in 
our institution who was not involved this study generated the allocation sequence without 
blocking using a randomisation plan generator (http://www.randomisation.com). This study 
was performed in accordance with relevant guidelines and regulations including CONSORT 
guidelines (Fig. 1). 
Anaesthesia 
For anaesthesia induction, intravenous propofol 1.5 mg/kg and rocuronium 0.8 mg/kg were 
administered. For mechanical ventilation, volume-controlled ventilation was applied with a 
tidal volume of 10 mL per ideal body weight (kg), an I:E ratio of 1:2 or 1:1 according to the 
allocated group, and no external PEEP. For maintaining an end-tidal carbon dioxide tension 
(EtCO2) between 33 and 36 mmHg during surgery, the respiratory rate was adjusted. For 

anaesthesia maintenance, sevoflurane of an end-tidal concentration 2-2.5 vol% was used and 
adjusted to maintain the bispectral index score between 40 and 60. Inspired oxygen fraction 
(FiO2) was 0.4 using oxygen and air. For repeated blood sampling and monitoring continuous 
blood pressure, radial artery catheterization was conducted. 
Study design 
Standard monitoring during anaesthesia included continuous electrocardiograph, systemic 
arterial pressure, heart rate, EtCO2, and peripheral oxygen saturation. We measured 
intraoperative respiratory mechanics including peak airway pressure (Ppeak), mean airway 
pressure (Pmean), plateau airway pressure, positive end-expiratory pressure (PEEP), and 
arterial blood gas at three time-points: ten minutes after anaesthesia induction in the supine 
position (Tsupine), 30 min after in the prone position (T30), and 90 min after in the prone position 
(T90). We calculated driving airway pressure (Pdriving), lung compliance (Cdyn), 
and static lung compliance (Cstatic) from respiratory data, and alveolar to arterial oxygen 
gradient (D(A-a)O2) from arterial gas analysis data. Pdriving was defined as the difference 
between plateau airway pressure and PEEP. Cdyn and Cstatic were calculated using the 
following formula: tidal volume/ (Ppeak-PEEP) and tidal volume/ Pdriving, respectively. 
Alveolar oxygen tension was calculated using the following formula: FiO2 (PB – 47 mmHg) – 
(PaCO2/ 0.8), where PB is the barometric pressure (760 mmHg). 
Statistical analysis 
A previous study reported that during CRV with an inspired oxygen fraction of 0.4, the mean 
value (SD) of partial arterial oxygen pressure (PaO2) was 181 (28) mmHg in obese patients 
after prone positioning20. We assumed a mean difference of PaO2 between CRV and ERV of 
15%. With an α-value of 0.05 and a statistical power of 90%, a sample size of 23 patients was 

required for each group. Anticipating a 10% drop out rate, we recruited 50 patients. 
Continuous variables are expressed as mean ± standard deviation and categorical variables as 
number of patients (%). Continuous variables were analysed using the Student t-test or Mann- 
Whitney U test. Categorical variables were analysed using the Fisher’s exact test. Serial 
changes in respiratory variables and haemodynamic changes were analysed using repeated 
measures ANOVA followed by Bonferroni correction25. After analysing group x time 
interaction, we reanalysed serial changes in respiratory variables in each group using repeated 
measures ANOVA followed by Bonferroni correction. SPSS 21.0 (SPSS Inc., Chicago, IL) 
was used for statistical analyses.