Respiratory volume monitoring in an obese surgical population and the prediction of postoperative respiratory depression by the STOP-bang OSA risk score

Study Objective To evaluate use of a respiratory volume monitor (RVM; ExSpiron, Respiratory Motion, Inc., Waltham, MA, USA) that provides minute ventilation (MV), tidal volume (TV) and respiratory rate (RR) measurements in obese surgical patients, hitherto undescribed. Design Prospective, IRB-approved observational study of RVM parameter accuracy in obese surgical patients, designed to test the ability of the RVM to detect predefined postoperative respiratory depression (PORD) and apneic events (POA) and to correlate STOP-Bang scores with PORD and POA. Setting Pre-, intra-, and post-op patient-care areas, including the post-anesthesia care unit (PACU) in 2 academic centers with bariatric populations. Patients 80 patients (47 ± 12 years), BMI of 43 ± 7 kg/m² undergoing elective surgery were enrolled. Interventions Data collected included patient characteristics, STOP-Bang scores and RVM data from immediately preoperatively through PACU completion without effecting standard clinical care. Measurements Low minute ventilation (LMV) was defined as 40% of predicted MV, and PORD was defined as sustained LMV for 5 minutes. Appropriate parametric and non-parametric statistical analyses were performed, P <.05 considered significant. Main Results In 56 patients with complete intraoperative ventilator data, correlation between RVM and ventilator MV measurements was r = 0.89 (measurement bias 1.5%, accuracy 11%). Measurement error was 0.13 L/min (95% confidence interval - 0.93 L/min - 1.20 L/min). In PACU, 16.3% and 31% of patients had PORD and POA respectively. There were no significant differences in the incidence of PORD and POA in 3 STOP-Bang risk categories (P >.2). Conclusions There was excellent correlation and accuracy between the RVM and ventilator volumes in obese surgical patients. A considerable number of patients exhibited PORD and POA in the PACU. The STOP-Bang risk scores correlated poorly with PORD and POA which suggests that obese surgical patients remain at risk for early post-operative respiratory events irrespective of the STOP-Bang score.