Effect of PEEP on the relation of PLdrive to Pdrive

1

Effect of PEEP on the relation of P

Ldrive

to P

drive

The airway driving pressure (Pdrive) is distributed over the lung (PLdrive) and the chest wall (PCWdrive) as follows: Pdrive = PLdrive + PCWdrive. In order to protect the lung, PLdrive

should be low. Clinically, this is approximated using Pdrive, as in most settings PCWdrive

is low. PCWdrive increases, however, with rising abdominal pressures, as during

laparoscopy.¹ In this setting, therefore, Pdrive may be a less reliable estimate of PLdrive. During laparoscopy with head-down tilt, we observed an increase in PLdrive as PEEP increased. Furthermore, there was a variable change in effect size, even when PEEP was increased by a constant value of 5 cmH2O (Figure 3). We, therefore,

hypothesized that PEEP influences the relation between Pdrive and PLdrive.

The effect of 3 levels of PEEP (15, 10 and 5 cmH2O) on the relation between the airway driving pressure (Pdrive) and the transpulmonary driving pressure (PLdrive) was analyzed post-hoc as an exploratory endpoint. Raw ventilation data was summarized per breath and stored in a separate database. Transpulmonary pressure outliers, as identified using the boxplot rule, were assumed to be due to surgical artifacts and removed for each PEEP level. All data recorded before the start of each

measurement period, or containing clinically implausible values were discarded.

Implausible values were defined as PEEP > PIP, PLee > PLpeak, Pdrive > PLdrive, PLee >

PEEP or PLpeak > PIP. All breaths with a measured PEEP outside the range of set PEEP ± 1cmH2O were also removed. The effect of PEEP level on the relation between PLdrive and Pdrive was studied in a linear mixed effects model. A clinically important effect of PEEP was assumed if there was a significant interaction between PEEP level and Pdrive (p<0.05).

2 The analysis included 6440 breaths from 19 patients at 3 levels of PEEP. The

relationship between Pdrive and PLdrive at each PEEP level is illustrated in Figure 5.

A mixed effects model predicting PLdrive by Pdrive and PEEP level with a random slope per PEEP level and a random intercept per patient had a conditional R² of 64.8% and a marginal R² of 25.5%. The interaction between Pdrive and PEEP level was

significant (p<0.001).

This result suggests that in the setting of increased intra-abdominal pressure and head-down tilt, PEEP attenuates the distribution of Pdrive in the lung.

Reference

1. Brandão JC, Lessa MA, Motta-Ribeiro G, et al. Global and Regional Respiratory

Mechanics During Robotic-Assisted Laparoscopic Surgery: A Randomized Study [published correction appears in Anesth Analg. 2020 Apr;130(4):e118]. Anesth Analg. 2019;129(6):1564-1573. doi:10.1213/ANE.0000000000004289