PEEP Titration Is Markedly Affected by Trunk Inclination in Mechanically Ventilated Patients with COVID-19 ARDS: A Physiologic, Cross-Over Study

Abstract

Background: Changing trunk inclination affects lung function in patients with ARDS. However, its impacts on PEEP titration remain unknown. The primary aim of this study was to assess, in mechanically ventilated patients with COVID-19 ARDS, the effects of trunk inclination on PEEP titration. The secondary aim was to compare respiratory mechanics and gas exchange in the semi-recumbent (40° head-of-the-bed) and supine-flat (0°) positions following PEEP titration.

Methods: Twelve patients were positioned both at 40° and 0° trunk inclination (randomized order). The PEEP associated with the best compromise between overdistension and collapse guided by Electrical Impedance Tomography (PEEP_EIT) was set. After 30 min of controlled mechanical ventilation, data regarding respiratory mechanics, gas exchange, and EIT parameters were collected. The same procedure was repeated for the other trunk inclination.

Results: PEEP_EIT was lower in the semi-recumbent than in the supine-flat position (8 ± 2 vs. 13 ± 2 cmH₂O, p < 0.001). A semi-recumbent position with optimized PEEP resulted in higher PaO₂:FiO₂ (141 ± 46 vs. 196 ± 99, p = 0.02) and a lower global inhomogeneity index (46 ± 10 vs. 53 ± 11, p = 0.008). After 30 min of observation, a loss of aeration (measured by EIT) was observed only in the supine-flat position (-153 ± 162 vs. 27 ± 203 mL, p = 0.007).

Conclusions: A semi-recumbent position is associated with lower PEEP_EIT and results in better oxygenation, less derecruitment, and more homogenous ventilation compared to the supine-flat position.

Keywords: COVID-19; acute respiratory distress syndrome; mechanical ventilation; trunk inclination; ventilator-induced lung injury.

Figure 1

Study protocol.

Figure 2

Overdistension, collapse and driving pressure during PEEP titration performed in supine-flat and semi-recumbent positions. The results of collapse and overdistension during PEEP titration performed in supine-flat position and semi-recumbent position are presented in Panel (A) and Panel (B), respectively. Red dots represent the percentage of collapse, while black dots represent the percentage of overdistension. A two-way ANOVA for repeated measures was performed to evaluate the effect of trunk inclination and PEEP both on collapse and overdistension (p < 0.001 for both). * = p < 0.05 compared to the percentage of overdistension in supine-flat position for the same PEEP value; † = p < 0.05 compared to the percentage of collapse in supine-flat position for the same PEEP value. The values of driving pressure recorded during PEEP titration performed in supine-flat position and semi-recumbent position are presented in Panel (C) and Panel (D), respectively. A two-way ANOVA for repeated measures was performed to evaluate the effect of trunk inclination and PEEP on driving pressure (p < 0.001). ‡ = p < 0.05 compared to the best PEEP in supine-flat position (13 cmH₂O). § = p < 0.05 compared to the best PEEP in semi-recumbent position (8 cmH₂O).

Figure 3

“Best” PEEP based on EIT in supine-flat and semi-recumbent position. Individual pairs of “best” PEEP identified through EIT in the two positions are reported. The p-value refers to the paired t-test. Data of 12 patients are reported; however, 2 pairs of patients have the same pair of “best” PEEP values in supine-flat and in semi-recumbent position. As a consequence, data of only 10 patients are visible.

Figure 4

Pressure-Impedance variation curves in supine-flat and semi-recumbent position. Panel (A) Pressure-impedance variation curves recorded during the decremental PEEP trial in supine-flat position. Panel (B) Pressure-impedance variation curves recorded during the decremental PEEP trial in semi-recumbent position. In both panels, impedance represents changes in impedance compared to the impedance value measured at PEEP of 6 cmH₂O. Data are presented as mean ± standard deviation. Notably, for data points referring to inspiratory pressure, the standard deviation is reported for both the change in lung impedance and plateau pressure. A two-way ANOVA for repeated measures was performed to evaluate the effect of trunk inclination (factor 1) and PEEP (factor 2) on end-inspiratory impedance variation (red dots, vertical standard deviation bars) and end-expiratory impedance variation (black dots) and plateau pressure (red dots, horizontal standard deviation bars) (p < 0.001 for all). * = p < 0.05 compared to end-expiratory impedance variation in supine-flat position for the same PEEP value; † = p < 0.05 compared to plateau pressure in supine-flat position for the same PEEP value; ‡ = p < 0.05 compared to end-inspiratory impedance variation in supine-flat position for the same PEEP value.

Figure 5

Representation of the implications of a position-related shift of the pressure–volume curve on respiratory mechanics. The theoretical effect on the respiratory system’s pressure–volume curve of the change in trunk inclination in patients on volume-controlled ventilation is represented. On the Y-Axis tidal volume is represented as the distance between two consecutive horizontal dashed lines, which represent end-expiratory (red) and end-inspiratory lung volume (black). Please note that the distance between two consecutive horizontal dashed lines is constant along the Y-axis, since tidal volume is constant in volume-controlled ventilation. Changing patients’ position from supine-flat (continuous line) to semi-recumbent (short-dashed line), will shift the pressure–volume curve leftwards (and vice versa). If PEEP titration is performed in supine-flat position, a specific “best” PEEP value is identified (in the example 13 cmH₂O, Point A). If the patient’s trunk inclination is now changed to semi-recumbent, the pressure–volume curve is shifted to the left and, for the same pressure, a higher end-expiratory lung volume (EELV) will be achieved (from A to C). Overdistension might occur, leading to a reduced respiratory system compliance (lower slope of the red-marked part of the pressure-volume curve). On the contrary, if PEEP titration is performed in semi-recumbent position, the identified “best” PEEP will have a lower value (in the example 8 cmH₂O, Point B). If the patient’s trunk inclination is now changed to supine-flat (without changing PEEP), the pressure-volume curve is shifted to the right and the patient will have a lower EELV for the same pressure (from B to D). Alveolar collapse might occur, potentially leading to a reduced respiratory system compliance. Finally, if trunk inclination is changed from supine-flat to semi-recumbent position and PEEP is optimized (from A to B), a similar EELV will be maintained, resulting in similar compliances of the respiratory system.