Introduction: Since the outbreak of SARS-Cov2 in December 2019, the number of patients affected by ARDS has risen to unprecedented proportions. Yet little is known thus far concerning the physiological effects of the prone position on these patients despite experts having recommended this treatment for all intubated ARDS patients with a PaO2/FIO2 ratio < 150 mmHg. The objective of this study was to analyse the physiological effects of prone positioning on intubated patients affected by SARS-Cov2 by analysing a set of relevant physiological parameters, in particular PaO2/FIO2, alveolo-arterial gradient, and blood gases.
Methodology: Retrospective analysis. Eighty COVID-19 patients hospitalized for ARDS in the Adult Intensive Care Unit (ICU) of Lausanne University and intubated. Fifty-two were proned during intubation without extra-corporeal membrane oxygenation (ECMO) and 44 were included in the analysis (2 were excluded because they were proned under ECMO only and 6 refused reutilization of their data). Demographic and general patient data, invasive mechanical ventilation data as well as arterial blood gas analysis data were collected. Descriptive statistics were performed in the global study population and in the different ARDS severity groups.
Results: 72.7% (n=32) patients were males and 27.3% (n=12) were females. The median age was 61.5 y.o. [57.0-72], their median admission weight was 85kg [71.3-104]. Hypoxia severity distribution at ICU admission was: PaO2/FiO2 >300 (n=1), PaO2/FiO2 200 ≤300 (n=1), PaO2/FiO2 100 ≤200 (n=20), PaO2/FiO2 ≤100 (n=22). Patients stayed in the ICU for a median of 18.0 [12.1-27.5] days and ICU mortality for the 44 included patients was 25% (n=11). The median number of prone sessions per patient was 3 [2, 6] - min 1, max 14 - and their median duration was 1037 minutes [973, 1120] for a total of 196 prone sessions. The difference in physiological data between pre- and post- proning over all sessions were: an increase in PaO2/FiO2 of 63 [34, 92] (p value = <0.0001, n=195) and a slight increase in compliance 1.7 [-1.8, 5.5] mL/cmH2O (p value = 0.0022, n=160), a decrease in alveolo-arterial gradient by 127 [-196, -71] mmHg (p value = <0.0001, n=193), in PaCO2-EtCO2 by -1.6 [-4.7, 2.9] mmHg (P value = 0.0132, n=118) and in alveolar dead space fraction by – 2.1 [-6.3 – 1.89]% (P value < 0.0001, n=128). Ventilatory ratio 0.05 [- 0.22, 0.30] (p value = 0.1044, n=193) and PaCO2 -0.1 [-6.7, 6.0] (p value = 0.8143, n=195) did not change.
Conclusion: PaO2/FiO2 ratio increased by prone positioning as observed in most studies in non-SARS-Cov2 related ARDS. Alveolo-arterial gradient markedly decreased with proning and compliance slightly increased. PaCO2 and the ventilatory ratio were not significantly altered, whereas alveolar dead space fraction was slightly but significantly decreased. PaCO2-EtCO2 also decreased. The significant decrease in PaCO2-EtCO2 and increase in compliance in the presence of a marginal decrease in alveolar dead space fraction supports the theory that a decrease in V/Q mismatch related to proning is the main factor contributing to oxygenation improvement.