Comparison of low and high positive end-expiratory pressure during low tidal volume ventilation

Abstract

Editor, We read with interest the article ‘Comparison of low and high positive end-expiratory pressure during low tidal volume ventilation in robotic gynaecological surgical patients using electrical impedance tomography’ by Chun et al.1 The finding that a low positive end-expiratory pressure (PEEP) level may be sufficient in the selected study group is not surprising considering that the study population was quite young and without overweight people (mean BMI 22.4 and 23.4 kg m−2 in the two groups, respectively). Thus, the study population likely consisted of individuals with high lung elasticity and little tendency for airway closure. There was no difference between the two study groups in arterial blood gases or the widely recognised oxygenation index PaO2/FiO2. However, the authors report a significant difference between groups in the obscure oxygenation index called ‘oxygenation factor’. This factor is derived from calculating PaO2/(FiO2 x MPAW), where MPAW is the mean airway pressure, and has been found to be inversely correlated to the degree of intrapulmonary shunting.2 As the authors found a significant higher oxygenation factor in the low PEEP group compared with the high PEEP group, they suggest that a ‘PEEP of 4 cmH2O might be better for ventilation strategy in consideration of intrapulmonary shunt’.1 Several objections may be raised in regard to this proposition, apart from the fact that the oxygenation factor appears to be a post hoc analysis. First, the oxygenation factor was validated treating postoperative patients with PEEP following cardiac surgery.2 In contrast, Chun et al.1 prophylactically and randomly assigned healthy patients with no preceding impact on lung physiology to two different PEEP levels. Second, we find it very unlikely that the intrapulmonary shunt would be less with PEEP of 4 cmH2O than with PEEP 8 cmH2O.3 How could that possibly be physiologically explained when the intrapulmonary shunt primarily correlates with the amount of atelectatic lung tissue? In our opinion, the effect on the oxygenation factor is simply a mathematical consequence of the different levels of PEEP. We welcome the study by Chen et al.,1 as it adds further knowledge regarding intra-operative ventilation of the healthy and normal-weight patient. However, the discussion regarding optimal PEEP during general anaesthesia is already complex and confusing. We therefore believe it is important to be careful when discussing the physiological mechanisms and possible further implications from this study, not to add further confusion. Acknowledgements relating to this article Assistance with the letter: none. Financial support and sponsorship: none. Conflicts of interest: none.