Abstract
BackgroundWe hypothesized that as CARDS may present different pathophysiological features than classic ARDS, the application of high levels of end-expiratory pressure is questionable. Our first aim was to investigate the effects of 5–15 cmH2O of PEEP on partitioned respiratory mechanics, gas exchange and dead space; secondly, we investigated whether respiratory system compliance and severity of hypoxemia could affect the response to PEEP on partitioned respiratory mechanics, gas exchange and dead space, dividing the population according to the median value of respiratory system compliance and oxygenation. Thirdly, we explored the effects of an additional PEEP selected according to the Empirical PEEP-FiO2 table of the EPVent-2 study on partitioned respiratory mechanics and gas exchange in a subgroup of patients.MethodsSixty-one paralyzed mechanically ventilated patients with a confirmed diagnosis of SARS-CoV-2 were enrolled (age 60 [54–67] years, PaO2/FiO2 113 [79–158] mmHg and PEEP 10 [10–10] cmH2O). Keeping constant tidal volume, respiratory rate and oxygen fraction, two PEEP levels (5 and 15 cmH2O) were selected. In a subgroup of patients an additional PEEP level was applied according to an Empirical PEEP-FiO2 table (empirical PEEP). At each PEEP level gas exchange, partitioned lung mechanics and hemodynamic were collected.ResultsAt 15 cmH2O of PEEP the lung elastance, lung stress and mechanical power were higher compared to 5 cmH2O. The PaO2/FiO2, arterial carbon dioxide and ventilatory ratio increased at 15 cmH2O of PEEP. The arterial–venous oxygen difference and central venous saturation were higher at 15 cmH2O of PEEP. Both the mechanics and gas exchange variables significantly increased although with high heterogeneity. By increasing the PEEP from 5 to 15 cmH2O, the changes in partitioned respiratory mechanics and mechanical power were not related to hypoxemia or respiratory compliance. The empirical PEEP was 18 ± 1 cmH2O. The empirical PEEP significantly increased the PaO2/FiO2 but also driving pressure, lung elastance, lung stress and mechanical power compared to 15 cmH2O of PEEP.ConclusionsIn COVID-19 ARDS during the early phase the effects of raising PEEP are highly variable and cannot easily be predicted by respiratory system characteristics, because of the heterogeneity of the disease.
Highlights
We hypothesized that as COVID-19 ARDS patients (CARDS) may present different pathophysiological features than classic Acute respiratory distress syndrome (ARDS), the application of high levels of end-expiratory pressure is questionable
Our first aim was to investigate the effects of 5–15 cmH2O of Positive end-expiratory pressure (PEEP) on partitioned respiratory mechanics, gas exchange and dead space; secondly, we investigated whether respiratory system compliance and severity of hypoxemia could affect the response to PEEP on partitioned respiratory mechanics, gas exchange and dead space, dividing the population according to the median value of respiratory system compliance and oxygenation
The main findings of this study evaluating different PEEP levels, with constant tidal volume, were: (1) 15 c mH2O of PEEP significantly increased the ventilatory ratio, lung elastance and mechanical power with heterogeneous responses, (2) the arterial oxygenation increased by increasing the PEEP, (3) the compliance of the respiratory system and the level of hypoxemia at baseline did not affect the PEEP response and (4) the PEEP suggested by the PEEP/Fraction of inspired oxygen (FiO2) table was higher than 15 c mH2O
Summary
We hypothesized that as CARDS may present different pathophysiological features than classic ARDS, the application of high levels of end-expiratory pressure is questionable. Based on the available body of evidence in order to limit the ventilator induced lung injury (VILI), not-related-to-COVID-19-ARDS patients are currently managed by applying low tidal volume without overcoming an inspiratory plateau pressure of 30 cmH2O and moderate–high PEEP levels in the moderate–severe forms [3,4,5]. It has been shown that among ARDS patients similar PEEP levels should not be used, but an individualization is required because the response to PEEP differs according to the respiratory mechanics, hemodynamic, lung recruitability and shunt. CARDS patients may show a discrepancy between a relatively high respiratory system compliance, higher amount of lung gas volume and severity of hypoxemia [11,12,13]. If the vascular derangement is the major mechanism related to hypoxemia, the use of moderate–high PEEP levels is questionable
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