Oxygen saturation instability in suspected covid-19 patients; contrasting effects of reduced V_A/Q and shunt

Patients in the UK at risk of Covid-19 pneumonia, but not needing immediate hospital attention, are to be given pulse oximeters to identify a fall in oxygen saturation (SaO2 or SpO2) at home. A recent finding in Covid-19 pneumonia is a dominant reduction in ventilation to perfused alveoli (V_A/Q). A mathematical model of gas exchange was used to predict the effect of shunt or reduced V_A/Q on SaO2 stability inferred from the slope of the PIO2 vs SaO2 curve as it intersected the line representing ambient PIO2. A ±1 kPa variation in PIO2 predicted a 1.5% and 8% change in SpO2 with 15% shunt and 0.4 V_A/Q respectively. As a consistency check, two patients with pre-existing lung disease and 12 hour continuous SpO2 monitoring breathing air had gas exchange impairment analysed in terms of shunt and reduced V_A/Q. The patient with 16% shunt and normal V_A/Q had a stable but reduced SpO2 (circa 93±1%) throughout the 12 hr period. The patient with a V_A/Q reduced to 0.48 had SpO2 ranging from 75-95% during the same period. SpO2 monitoring in suspected covid-19 patients should focus on SpO2 varying >5% in 30 minutes. Such instability in at risk patients is not diagnostic of Covid-19 pneumonia but this may be suspected from a dominant reduction in V_A/Q if episodic hypoxaemia has progressed from a stable SpO2.