In our study, we found that both the PEEP level required for preventing alveolar collapse (“Open Lung PEEP”) as well as the PEEP level required for equally balancing overdistension and collapse (“Best Compromise PEEP”) was significantly higher in Trendelenburg position with capnoperitoneum than in supine position. Both Open Lung PEEP and Best Compromise PEEP were positively correlated with BMI in supine position but not in Trendelenburg position with capnoperitoneum.
Currently, there is no international consensus on best PEEP level for prevention of regional hypoventilation and PPC. Frequently, a conservative PEEP level of less than 5 cmH2O is being used for ventilation both in open abdominal and in laparoscopic surgery [16]. Our findings imply that to minimize alveolar collapse and overdistension during MV under general anesthesia, higher PEEP values in the range of 16 to 20 cmH2O can be considered in Trendelenburg position with capnoperitoneum, without additional risk for hemodynamic compromise or lung overdistension. BMI could be used as additional criterium for choosing individualised PEEP value in supine position without capnoperitoneum using the regression equations presented in Fig. 4. For Best Compromise PEEP, the regression equation can be simplified with reasonable accuracy as follows: Best Compromise PEEP = BMI / 3, yielding an equation that is easily applicable at the bedside.
Our seemingly contradictory finding of a positive correlation between the required PEEP value and BMI in the supine position but not in the Trendelenburg position with capnoperitoneum requires cautious interpretation. The mechanism by which a higher BMI leads to higher PEEP requirements is most likely closely related to intra-abdominal pressure, which is positively correlated to BMI during general anesthesia [17]. In our study, abdominal pressure during capnoperitoneum was closely monitored and kept at a steady level of 10–12 mmHg throughout the course of surgery whenever possible. This may have at least partially offset differences in intra-abdominal pressure due to BMI. On the other hand, we cannot entirely exclude the possibility that in Trendelenburg position with capnoperitoneum, some patients would have required even higher PEEP levels than the ones investigated in our study (20 – 4 cmH2O). Due to the nature of the EIT analysis performed for the present study, “Best Compromise PEEP” and “Open Lung PEEP” in these patients would still be identified within the range of 20 – 4 cmH2O, precluding any possible differentiation in the higher PEEP range.
Our suggestion of using higher PEEP values for MV in Trendelenburg position is in line with the findings of another study on EIT-guided PEEP ventilation conducted by Pereira and colleagues [15]. They describe PEEP values of 13.5 ± 1.6 cmH2O for counteraction of atelectasis formation in Trendelenburg position, although the PEEP trial was conducted before establishment of capnoperitoneum. This may explain the higher PEEP values for prevention of alveolar collapse and equally balancing overdistension and collapse reported in our study. Nevertheless, Pereira and colleagues were able to demonstrate the positive effect of EIT guided ventilation using postoperative CT scans showing less atelectasis formation compared to a control group ventilated with a fixed PEEP of 4 cmH2O.
Investigating patients undergoing robot-assisted laparoscopic prostatectomy with capnoperitoneum in Trendelenburg position, Shono and colleagues reported that a higher PEEP level of 15 cmH2O had a positive effect on regional ventilation distribution as compared to ventilation with PEEP of 5 cmH2O [18]. However, despite more homogeneous intraoperative ventilation no relevant improvement of postoperative lung function could be observed while comparing two patient groups ventilated with PEEP of 15 and 5 cmH2O.
Another study on optimal PEEP level in Trendelenburg position with capnoperitoneum during laparoscopic surgery used ventilation parameters such as dynamic compliance, dead space to tidal volume ratio and intrapulmonary shunt ratio for evaluation of lung-protective ventilation and suggested ventilation with a moderate PEEP of 8 cmH2O for improvement of pulmonary ventilation [9]. In this study, a lower PEEP level of 0 or 4 cmH2O was argued to be not effective to compensate the effects of capnoperitoneum and Trendelenburg position, while a higher PEEP of 12 cmH2O was associated with increased intrapulmonary shunt and hemodynamic depression. In contrast, we observed preserved oxygenation and hemodynamic stability with the application of EIT-guided PEEP in Trendelenburg position with capnoperitoneum. All in all, high PEEP values during ventilation with Trendelenburg position with capnoperitoneum were well tolerated in our study.
Karsten and colleagues suggested ventilation with PEEP of 10 cmH2O for prevention of regional hypoventilation in patients undergoing laparascopic surgery in supine position while monitoring regional ventilation with EIT [19]. These results corresponded to our Open Lung and Best Compromise PEEP values established in supine position without capnoperitoneum.
Erlandsson and co-workers suggested ventilation with PEEP 15 ± 1 cmH2O during laparoscopic surgery in supine position for morbidly obese patients with BMI 49 ± 8 kg/m2 according to monitoring of regional ventilation with EIT, which was performed before the beginning of surgery and induction of capnoperitoneum [20]. These values are higher than the ones observed in our study during ventilation in supine position without capnoperitoneum. This may be due to the correlation between BMI and PEEP values needed to prevent alveolar collapse in supine position observed in our study. Nestler and colleagues suggested even higher PEEP levels for prevention of atelectasis formation based on intraoperative EIT measurements in obese patients undergoing laparascopic surgery in supine position [21].
Some intra-individual variation of Open Lung and Best Compromise PEEP was observed in supine as well as in Trendelenburg position. This suggests that intraoperative monitoring of regional ventilation with EIT on an individual basis could contribute to improvement of lung-protective ventilation strategies. This may not be cost-effective for patients with general low risk of PPC and operation in supine position, but might be of benefit in high risk patients and body positions with increased intra-abdominal and transpulmonary pressure.
Strengths
Our study has some strengths. To our knowledge, it is the first study that assessed Open Lung PEEP and Best Compromise PEEP with EIT both after induction of general anesthesia and in Trendelenburg position with capnoperitoneum. Previous studies performing PEEP titration with EIT did so after induction of general anesthesia but not after establishment of capnoperitoneum [15, 21, 22]. This limits the generalizability of their findings to the markedly different respiratory mechanics in patients with capnoperitoneum. The method for identifying Open Lung PEEP and Best Compromise PEEP used in our study detects both overdistension and alveolar collapse, while some previously used EIT methods are measures for alveolar collapse but are relatively insensitive to overdistension [21, 22]. With this approach, we were able to show that the PEEP levels required to prevent alveolar collapse and to provide equal balance between collapse and overdistension are positively correlated to BMI in the supine position but not in the Trendelenburg position with capnoperitoneum. The latter is an intriguing new finding that merits further investigation. We derived regression equations for calculation of Open Lung PEEP and Best Compromise PEEP according to BMI in the supine position, enabling clinicians that have no EIT available to benefit from our results.
Limitations
Our study has several limitations. As this study was conducted in the department of gynecology and obstetrics, only younger female patients were included, limiting the generalizability of our findings. Our findings should be examined in a mixed gender cohort, to eliminate possibility of gender specific findings. Moreover, as patients with BMI > 35 kg/m2 were excluded from the present study, our results cannot be extrapolated to morbidly obese patients.
In general, the perioperative risk of pulmonary complications is low in a relatively young and healthy patient population as studied in our manuscript. Therefore, it is unclear whether individualized adjustment of PEEP will have any clinically relevant effects on patient-centered outcomes in this patient population.
The effects of Trendelenburg position and capnoperitoneum were not investigated separately. The sample size was relatively small and was further reduced by a drop-out rate of 10%. However, given the relatively homogeneous patient population, we were still able to obtain robust values for median and interquartile ranges of Open Lung and Best Compromise PEEP. Post hoc power calculations using the actually achieved effect sizes revealed a statistical power of > 0.95 for detecting a difference in PEEP level between supine position and Trendelenburg position.
Our study was neither designed nor powered to investigate the influence of EIT-guided PEEP postoperative pulmonary complications. Nevertheless, none of the patients investigated in this study had additional oxygen requirements or needed invasive or non-invasive mechanical ventilation after the end of surgery.
A randomized study to assess the effect of EIT-guided PEEP settings during Trendelenburg position with capnoperitoneum on the occurrence of PPCs would be desirable to confirm our results and further specify the optimal PEEP level for prevention of PPCs after mechanical ventilation during general anesthesia.