To the Editor:
We read with great interest the recent article by Simon et al.1 In this study, the authors have shown that individualized positive end-expiratory pressure (PEEP) exerts lower driving pressure.1 This in turn proved the redistribution of ventilation toward dependent lung areas, as measured by electrical impedance tomography. These sound results imply great notions regarding intraoperative respiratory management. However, we highlight four concerns regarding the methodology used.
First, the study combined data from a multicenter2 and a single-center trial. This was likely to cause selection bias. The inclusion periods were separated at 4-yr intervals. The authors divided the combined cohort into three treatment groups: individualized PEEP, fixed low PEEP, and fixed PEEP of 12 cm H2O. The differences in the patient characteristics were unclear. The Assess Respiratory Risk in Surgical Patients in Catalonia (ARISCAT) score greater than 44 was noted in one patient (4%) in the individual PEEP group, which is less in comparison with the other two groups. We would like to know whether preoperative pulmonary function (forced expiratory volume in 1 s/forced vital capacity), oxygenation, and partial pressure of carbon dioxide differed among the groups. We question this because capnoperitoneum time and duration of an operation are basic information for considering postoperative pulmonary complications. To us, it seems that these might have influenced the results.
Second, the results clearly demonstrated that the individualized PEEP group needed larger amounts of fluid infusion and doses of vasoactive medication than the other two groups. There was no doubt as to whether these discrepancies were related to pulmonary management strategy. We question how intraoperative infusion management strategies differed between the period of the single-center study (2012 to 2013) and the multicenter study (2016 to 2018). Additionally, preoperative oral intake or dose of hypertensive drugs may have differed in the 4-yr interval.
Third, the complications related to individualized PEEP cannot be studied in totality. Hemodynamic depression attributable to excessive PEEP may be a risk factor for patients with cardiovascular diseases. In this study, the transpulmonary pressure was not measured, which could have been used as an alternative parameter for lung injury. It is known that intracranial pressure or perfusion in the brain is largely influenced by PEEP.3
Finally, the definition of postoperative pulmonary complications described by the authors was not relevant to the process of early recovery after surgery. The postoperative complications earlier included acute respiratory distress syndrome, bronchospasm, new pulmonary infiltrates, and so on2 . In our opinion, setting a clinically relevant outcome could be as simple as the need for oxygen therapy, including a low-flow nasal cannula. This approach would resemble ventilator-associated event surveillance for intubated mechanically ventilated patients and in turn support studies for ventilator-associated pneumonia.4 We wish to know how the length of oxygen therapy differed among the groups after surgery. Additionally, we would like to have information on new relevant criteria that matches the early recovery after surgery concept.
We thank Editage (http://www.editage.com) for English-language editing.
The authors declare no competing interests.