Instrumental dead space in ventilator management

Michela Botta and colleagues described ventilator management in a large cohort of patients with COVID-19 on mechanical ventilation.1 Their study provides important data on ventilator settings in this population and shows that high tidal volumes on the first day of ventilation are associated with a higher risk of 28-day mortality. These data reinforce the notion that low tidal volumes should be used in these patients, especially when low lung compliance is measured.2 The minute ventilation used in mechanically ventilated critically ill patients is frequently around 150 mL/min per kg predicted bodyweight or above, in contrast to surgical patients undergoing mechanical ventilation in the operating room (in whom minute ventilation is around 100 mL/min per kg predicted bodyweight).3 When a tidal volume of 6 mL/kg predicted bodyweight is set in patients with COVID-19-associated acute respiratory distress syndrome (ARDS), a respiratory rate of around 25 breaths per min or above should be used to reach a minute ventilation of 150 mL/min per kg predicted bodyweight, to allow adequate CO2 removal.3 Consequently, we were surprised that the low minute ventilation delivered, 126 mL/min per kg predicted bodyweight, was associated with relatively normal partial pressure of CO2 (PaCO2) in Botta and colleagues' study.1 In several studies describing the respiratory mechanics of mechanically ventilated patients with COVID-19, PaCO2 values that were similar to or higher than the PaCO2 values in Botta and colleagues' study were obtained despite much higher minute ventilations, frequently above 150 and up to 198 mL/min per kg predicted bodyweight (appendix). This discrepancy might be explained either by underestimation of the set respiratory rate relative to the patients' respiratory rates, or by the systematic reduction of the instrumental dead space described in Botta and colleagues' study. The minute ventilation required for CO2 removal depends on CO2 production, which is linked to metabolism and is particularly influenced by the patient's core body temperature; it also depends on CO2 elimination, which is related to alveolar ventilation (minute ventilation minus dead space ventilation). The volume of dead space includes the instrumental dead space, the anatomical dead space, and the alveolar dead space.3 It would be interesting to know more about the type of circuits used in the patients in this study, and the instrumental dead space after the Y-piece in these patients.