Increased Ventilatory Ratio, Hypercapnia Common in ARDS Associated With COVID-19

Patients with acute respiratory distress syndrome (ARDS) associated with coronavirus disease 2019 (COVID-19) frequently have increased ventilatory ratio (VR) and hypercapnia while using low tidal volume ventilation, according to a case series from an intensive care unit (ICU) in China. Findings from this series were published in the American Journal of Respiratory and Critical Care Medicine.

A total of 8 consecutive patients (mean age, 63.2±11.0 years) who tested positive for severe acute respiratory syndrome coronavirus 2 were included in the series. All patients had been previously intubated at another institution prior to transfer to the ICU. Additionally, patients included in the series had diagnosed ARDS, based on the Berlin definition. The first 4 consecutive patients received a ventilation strategy that used low tidal volume of 6.0 mL/kg predicted body weight (PBW). Ultimately, these patients developed hypercapnia.

Researchers measured the VR to examine the first 4 consecutive patients’ moderately reduced respiratory system compliance. In patients with ARDS, VR is a bedside index that measures impaired ventilation efficiency and correlates with physiologic dead space fraction. The patients had a mean VR value of 2.1±0.3, indicating high dead space (VD) to tidal volume (VT) ratio.

Following application of an increased VT to the first 4 patients, the partial pressure of carbon dioxide (PaCO₂) significantly decreased compared to VT 7.0 mL/kg PBW (57.7±5.2 vs 44.1±3.6 mm Hg, respectively; P =.003). Compared to VT 7.0 mL/kg PBW, the VR in these first 4 patients significantly decreased (intermediate vs low VT: 1.7±0.2 vs 2.1±0.3 mm Hg, respectively; P =.018), whereas the partial pressure of oxygen/fraction of inspired oxygen (PaO₂/FiO₂) improved (intermediate vs low VT: 241±38 mm Hg vs 207±61 mm Hg, respectively; P =.402).

Based on their findings, the researchers suggested that low tidal volume may not be the best approach for all patients with ARDS associated with COVID-19, particularly in patients “with a less severe decrease in respiratory system compliance and inadequacy of ventilation.” 

“Clinicians must have a high index of suspicion for increased pulmonary dead space when COVID-19-related ARDS patients present with hypercapnia,” they concluded.

Reference

Liu Xi, Liu Xu, Xu Y, et al. Ventilatory ratio in hypercapnic mechanically ventilated patients with COVID-19 associated ARDS [published online March 23, 2020]. Am J Respir Crit Care Med. doi:10.1164/rccm.202002-0373LE