Driving pressure during proportional assist ventilation: an observational study

Abstract

Background: During passive mechanical ventilation, the driving pressure of the respiratory system is an important mediator of ventilator-induced lung injury. Monitoring of driving pressure during assisted ventilation, similar to controlled ventilation, could be a tool to identify patients at risk of ventilator-induced lung injury. The aim of this study was to describe driving pressure over time and to identify whether and when high driving pressure occurs in critically ill patients during assisted ventilation.

Methods: Sixty-two patients fulfilling criteria for assisted ventilation were prospectively studied. Patients were included when the treating physician selected proportional assist ventilation (PAV+), a mode that estimates respiratory system compliance. In these patients, continuous recordings of all ventilator parameters were obtained for up to 72 h. Driving pressure was calculated as tidal volume-to-respiratory system compliance ratio. The distribution of driving pressure and tidal volume values over time was examined, and periods of sustained high driving pressure (≥ 15 cmH₂O) and of stable compliance were identified and analyzed.

Results: The analysis included 3200 h of ventilation, consisting of 8.8 million samples. For most (95%) of the time, driving pressure was < 15 cmH₂O and tidal volume < 11 mL/kg (of ideal body weight). In most patients, high driving pressure was observed for short periods of time (median 2.5 min). Prolonged periods of high driving pressure were observed in five patients (8%). During the 661 periods of stable compliance, high driving pressure combined with a tidal volume ≥ 8 mL/kg was observed only in 11 cases (1.6%) pertaining to four patients. High driving pressure occurred almost exclusively when respiratory system compliance was low, and compliance above 30 mL/cmH₂O excluded the presence of high driving pressure with 90% sensitivity and specificity.

Conclusions: In critically ill patients fulfilling criteria for assisted ventilation, and ventilated in PAV+ mode, sustained high driving pressure occurred in a small, yet not negligible number of patients. The presence of sustained high driving pressure was not associated with high tidal volume, but occurred almost exclusively when compliance was below 30 mL/cmH₂O.

Keywords: Compliance; Monitoring; Protective ventilation; Tidal volume.

Fig. 1

Upper panel: Identification of periods of sustained high ΔP: representative plot from one patient, generated via an R-Shiny application built for this purpose, showing the driving pressure (ΔP) time-series signal, raw data in green, and smoothen signal in blue (marked with thick blue arrow). The thick horizontal gray arrow indicates the period of sustained high ΔP ≥ 15 cmH₂O. In x-axis is time in hours from initiation of recording. Lower panel: Identification of periods of stable compliance: representative plot of one patient showing compliance over time (x-axis showing time in seconds from initiation of recording). In the first step of this analysis, the slope change points (gray arrows) are identified, in the second step, the slope of each segment is calculated (segments numbered here from 1 to 15), and in the third step, periods with slope between − 0.001 and 0.001 are selected as periods of stable compliance (in this case periods 6 and 9 shown with blue arrows)

Fig. 2

Time as  % of the total analyzed period, with ΔP and V_T values within the range of each cmH₂O or mL/kg of ideal body weight, from less than 5 to more than 15, expressed as median and interquartile range of all patients’ values

Fig. 3

Driving pressure (ΔP, in cmH₂O) vs. tidal volume (V_T, in mL/kg ideal body weight) during all periods of stable compliance (661 periods from 60 patients), colored according to the range of respiratory system compliance (Crs, mL/cmH₂O). Dotted vertical and horizontal lines indicate thresholds of ΔP ≥ 15 cmH₂O and V_T ≥ 8 mL/kg, respectively, and solid black lines indicate the correlation lines for each range of compliance. In the upper right quarter (values ΔP ≥ 15 cmH₂O and V_T ≥ 8 mL/kg), there are 11 points (10 points with compliance 21–30 mL/cmH₂O and one point with compliance of 31 mL/cmH₂O), pertaining to four patients

Fig. 4

ROC curve for the absence of ΔP ≥ 15 cmH₂O based on compliance, for all periods of stable compliance (661 periods from 60 patients). Arrows indicate the coordinates for the specific values of compliance (AUC = 0.97)