Inspiratory Airways Resistance in Respiratory Failure Due to COVID-19

Abstract

Rationale: Acute respiratory failure (ARF) in COVID-19 (C19) is associated with high morbidity and mortality. To date, physiologic descriptions have largely focused on gas exchange and respiratory system compliance, however our anecdotal observations suggested that increased airways resistance also commonly contributed to ARF in C19. Normal inspiratory airways resistance (iRaw) has been reported to be 1 cm H2O/L/sec, while unselected patients with ARDS have been reported to have values closer to 5 cm H2O/L/sec (https://doi.org/10.1164/ajrccm/139.5.1169). Methods: We measured iRaw in a prospective convenience sample of 55 mechanically ventilated patients with C19 in 3 adult ICUs between March and September 2020. There were no exclusion criteria. We collected baseline demographics, date of first positive C19 test, days from intubation until the first captured waveform (see below), gas exchange parameters, inflammatory biomarkers, and 90-day mortality. All C19 medical care including sedation and neuromuscular blockade was determined by the primary team. During measurements of ventilatory mechanics, patients were placed on volume control ventilation 6 ml/kg/PBW, square flow at 60 L/min, and an end-inspiratory hold time of 0.3 seconds. Screen shots of flow-time and pressure-time scalars were captured for later review. iRaw was calculated as peak airway pressure-plateau airway pressure/flow. Results: Patient characteristics are shown in the table. The median iRaw was 12 cm H2O/L/sec (IQR 10-16). iRaw was not significantly different among patients with asthma or COPD compared to those without a history of obstructive airways disease (median 12.5 vs 12 cm H2O/L/sec, respectively, p=0.66). Survival to 90 days among patients with iRaw above 12 cm H2O/L/sec was 68% compared to 60% for patients below 12 cm H2O/L/sec (p=0.58). iRaw did not correlate with CRP, ferritin, PaO2/FiO2 ratio, or static compliance (Cstat). Conclusion: Waveform analysis using a convenience sample of mechanically ventilated patients with ARF due to C19 showed a significant increase in iRaw compared to prior studies done on unselected ARDS patients without C19. Increased iRaw was independent of a history of obstructive airways disease and did not correlate with biomarkers of disease severity nor did it predict mortality. Additional studies will be needed to determine if increased iRaw prolongs the duration of mechanical ventilation in C19.