Distribution of ventilation and oxygenation in surgical obese patients ventilated with high versus low positive end-expiratory pressure: A substudy of a randomised controlled trial

BACKGROUNDIntra-operative ventilation using low/physiological tidal volume and positive end-expiratory pressure (PEEP) with periodic alveolar recruitment manoeuvres (ARMs) is recommended in obese surgery patients.OBJECTIVESTo investigate the effects of PEEP levels and ARMs on ventilation distribution, oxygenation, haemodynamic parameters and cerebral oximetry.DESIGNA substudy of a randomised controlled trial.SETTINGTertiary medical centre in Geneva, Switzerland, between 2015 and 2018.PATIENTSOne hundred and sixty-Two patients with a BMI at least 35kg per square metre undergoing elective open or laparoscopic surgery lasting at least 120min.INTERVENTIONPatients were randomised to PEEP of 4cmH²O (n=79) or PEEP of 12cmH²O with hourly ARMs (n=83).MAIN OUTCOME MEASURESThe primary endpoint was the fraction of ventilation in the dependent lung as measured by electrical impedance tomography. Secondary endpoints were the oxygen saturation index (SaO²/FIO²ratio), respiratory and haemodynamic parameters, and cerebral tissue oximetry.RESULTSCompared with low PEEP, high PEEP was associated with smaller intra-operative decreases in dependent lung ventilation [-11.2%; 95% confidence interval (CI)-8.7 to-13.7 vs.-13.9%; 95% CI-11.7 to-16.5; P=0.029], oxygen saturation index (-49.6%; 95% CI-48.0 to-51.3 vs.-51.3%; 95% CI-49.6 to-53.1; P<0.001) and a lower driving pressure (-6.3cmH²O; 95% CI-5.7 to-7.0). Haemodynamic parameters did not differ between the groups, except at the end of ARMs when arterial pressure and cardiac index decreased on average by-13.7mmHg (95% CI-12.5 to-14.9) and by-0.54lmin^-1m^-2(95% CI-0.49 to-0.59) along with increased cerebral tissue oximetry (3.0 and 3.2% on left and right front brain, respectively).CONCLUSIONIn obese patients undergoing abdominal surgery, intra-operative PEEP of 12cmH²O with periodic ARMs, compared with intra-operative PEEP of 4cmH²O without ARMs, slightly redistributed ventilation to dependent lung zones with minor improvements in peripheral and cerebral oxygenation.TRIAL REGISTRATIONNCT02148692, https://clinicaltrials.gov/ct2.