“Intraoperative PEEP for Obese Patients: A step forward, a long road still ahead”

The ideal Positive End-Expiratory Pressure (PEEP) management during surgery is still elusive. In the current issue of Anesthesiology, Simon et al.¹ and the PROVEnet investigators provide another step forward on the long road of searching for a physiology-supported PEEP strategy. Such PEEP strategy aims to optimize alveolar patency, minimize lung strain, and improve gas exchange, but the ideal long-term goal of any intraoperative management would be to reduce postoperative pulmonary complications (e.g., atelectasis, pneumonia) and improve overall clinical outcomes. Pursuing the appropriate PEEP strategy is even more imperative in obese patients because of their known reduced expiratory reserve volume and functional residual capacity, increased risk of atelectasis, and hypoxemia. For an in-depth review of the consequences of obesity on the respiratory system, we suggest the reader to excellent available reviews on this topic.^2,3

Clinical investigations focused on intraoperative PEEP management have traditionally compared two fixed levels of PEEP, typically 0-5 cmH₂O versus a higher PEEP level, variable depending on the surgical population and procedure studied. The PROBESE study,⁴ the largest multicenter trial focused on obese patients during abdominal surgery, randomized them to receive either a fixed 4 cmH₂O PEEP without recruitment maneuvers, or intermittent recruitment maneuvers with a constant PEEP 12 cmH₂O. The 12 cmH₂O PEEP ventilatory strategy was not associated with a significant reduction of postoperative pulmonary complications within 5 days after surgery, compared to 4 cmH₂O PEEP.⁴ An increasingly popular concept for protective mechanical ventilation is that of ‘best PEEP’, ‘optimal PEEP’ or ‘individualized PEEP’, as opposed to pre-selected PEEP levels. An individualized PEEP protocol refers to selecting a PEEP level that optimizes a specific respiratory parameter for individual surgical patients at a particular time point. Respiratory parameters that have been used to identify individualized PEEP levels in surgical patients are related to respiratory system mechanics (e.g., respiratory system compliance, driving pressure, transpulmonary pressure), lung aeration (assessed with lung ultrasound) or changes in lung volume calculated from ventilation-induced fluctuations in impedance (assessed with electrical impedance tomography) (Figure 1).

Figure 1: Techniques used for intraoperative PEEP optimization.

(PEEP=Positive End-Expiratory Pressure; Plateau P_aw=Plateau Airway Pressure; V_T=Tidal Volume)

Simon et al.¹ present a secondary analysis of two previously published prospective studies focused on PEEP management in obese patients undergoing laparoscopic abdominal surgery. They compared parameters of gas exchange, respiratory system mechanics, and electrical impedance tomography dorsal-ventral ventilation distribution from three groups of patients receiving care at the same institution: (1) those receiving intermittent recruitment maneuvers with constant intraoperative PEEP 12 cmH₂O participating in the PROBESE study (n=21), (2) those from the PROBESE study receiving a constant PEEP 4 cmH₂O combined with patients from patients receiving a fixed PEEP 5 cmH₂O without recruitment maneuvers from a comparable single-center study by Nestler et al.⁵ (n=44), and (3) those patients from the Nestler et al.⁵ study receiving a recruitment maneuver, performed before pneumoperitoneum insufflation, followed by an electrical impedance tomography-guided PEEP level that was then maintained throughout surgery (individualized PEEP) (n=25). The authors found that electrical impedance tomography-guided PEEP resulted in greater oxygenation (Pa_O2/F_IO2) before extubation, better intraoperative respiratory system mechanics (lower median driving pressure, greater respiratory system dynamic compliance), and greater tidal volume redistribution to the dependent areas of the lung, compared to both PEEP 4-5 cmH₂O and PEEP 12 cmH₂O patients.

The observed PEEP levels that produced the most homogeneous ventilation (lowest regional ventilatory delay index by electrical impedance tomography) are remarkable: the median PEEP was 18 cmH₂O [interquartile range 16-22 cmH₂O] with a range from 10 to 26 cmH₂O. These values are quite similar to the optimal electrical impedance tomography-guided PEEP (mean±SD) of 15±1 cmH₂O (range 13-17 cmH₂O) found by Erlandsson et al.⁶ during laparoscopic gastric bypass. In contrast to obese patients,^1,6 optimal respiratory system mechanics were achieved with lower PEEP (median[interquartile range] 10[8-15] cmH₂O, range 2-20 cmH₂O) in non-obese patients undergoing abdominal surgery.⁷ Of note, optimal PEEP observed by Simon et al.¹ in obese patients was higher than in the PROBESE PEEP 12 cmH₂O group.⁴ It remains unknown if the lack of improved clinical outcomes in the PROBESE PEEP 12 cmH₂O group could be related to the possibility that PEEP 12 cmH₂O was inadequate to achieve an ‘optimal PEEP’ in some patients. Furthermore, implementing certain high PEEP values as reported in the study by Simon et al.¹ may be faced with technical difficulties, as widely used anesthesia machines (e.g., Draeger Apollo) cannot achieve PEEP levels beyond 20 cmH₂O.

So, should we attempt using PEEP levels greater than 10-12 cmH₂O for obese surgical patients? It is not yet clear. Findings by Simon et al.¹ have several limitations that currently prevent introduction of this approach to clinical practice. First, while electrical impedance tomography data confirmed greater ventilation of dorsal lung regions and this can explain higher oxygenation, it is not a clinical tool routinely available at the bedside. Second, it only explored ventilation distribution at the third/fourth intercostal space level, which may not be representative of the global lung ventilation distribution or even the lung bases that are more at risk of atelectasis. Third, electrical impedance tomography-guided individualized PEEP was associated with greater oxygenation, suggesting a better ventilation-perfusion matching, compared to fixed PEEP, but multiple studies have shown that this is a temporary physiologic outcome that disappears after tracheal extubation. To some extent, a focus on intraoperative oxygenation is obsolete and no longer sufficient to be promoted as an outcome, by itself, in any ventilatory approach. Instead, we must confirm that individualized PEEP improves relevant clinical outcomes beyond purely physiologic parameters before its routine use can be recommended. A higher PEEP may optimize respiratory system mechanics, but it may simultaneously pose a cardiocirculatory challenge. In this and other studies, patients receiving higher PEEP may be more likely to have bradycardic or hypotensive episodes or require higher fluid infusion rates or vasopressor doses.^1,4 Excessive PEEP may also induce inhomogeneous alveolar overdistention, which could lead to increased alveolar capillary permeability and lung edema.⁸ The use of very high PEEP, and its interplay with the tidal volume, may overcome the physiologic limits of lung expansion, lose the intended ‘lung protection’ effect and, instead, become harmful.⁹ Therefore, short-term benefits (e.g., improved oxygenation) related to management with individualized PEEP may be offset by adverse postoperative outcomes.

The study presented by Simon et al.¹ takes us a step forward on the long road of understanding PEEP selection to improve intraoperative respiratory mechanics and oxygenation in obese patients. Unfortunately, the key question remains unanswered: how to select the physiologically-sound PEEP as part of protective ventilatory strategy that improves postoperative pulmonary outcomes. This ventilation should be achieved without negatively affecting intraoperative hemodynamics, and with resources widely available to anesthesia providers in the operating room. Future clinical trials on obese patients will be required to address the complex interaction between optimal intraoperative PEEP and reduction of postoperative pulmonary complications. For now, Simon et al.¹ have shown that individualized electrical impedance tomography-guided high levels of PEEP were associated with better intraoperative respiratory mechanics and oxygenation in the obese patients undergoing laparoscopic abdominal surgery included in their study. We believe that very high PEEP levels, as used for some patients in the Simon et al.¹ study, should not be routinely implemented in clinical practice before we obtain more definitive evidence that this ventilation approach is associated with improved pulmonary outcomes.