Fig. 4
Brain and ventilation curves and metabolic hyperbola in a healthy subject (A) and when this individual suffers from pneumonia due to COVID-19 (B). A Health. Notice that brain and ventilation curves are similar (black lines) and thus the RCO/min corresponds to actual PaCO2 and ventilation, set by the intersection point (black circle) between ventilation curve and metabolic hyperbola (blue line). B This human develops severe pneumonia due to COVID-19, causing increased V’CO2 and VD/VT which move the metabolic hyperbola upward. The concomitant hypoxemia and metabolic acidosis shift the brain curve to the left and increases its slope (red line). Due to increased respiratory system elastance, a given RCO/min results in a lower ventilation and thus, the slope of the ventilation curve (dashed black line) is shifted downward. A dissociation between the ventilation curve and brain curve occurs. The desired PaCO2 is 25 mmHg (point 1) and at this level of PaCO2 RCO/min corresponds to 16.6 l/min. The actual PaCO2 is 30 mmHg (point 2) and ventilation 13.8 l/min. PaCO2 of 30 mmHg represents hypercapnia for respiratory centers which increase their activity along the brain curve. Respiratory activity stabilizes to a level corresponding to 36.6 l/min (point 3). Unmet ventilatory demands are 22.8 l/min. RCO/min: respiratory centers output per minute