Simultaneous variations of PaCO2 and PACO2 in assisted ventilation.

Thirty-two patients receiving artificial ventilation of the lungs were studied to determine if variations in PACO2 could be reflected in variation in PaCO2. Eleven patients had chronic obstructive lung disease, eight had suffered acute respiratory failure, and 13 had neurological disturbances but normal lungs. PaCO2 and PACO2 were measured concurrently as ventilatory patterns, haemodynamic state or inspired gas concentrations changed. Neither change in inspired oxygen concentration nor change in haemodynamic state had much effect PaCO2 or PACO2. Change in PaCO2 induced by ventilatory change and by change in inspired carbon dioxide concentration were well described by linear regression of PaCO2 on PaCO2. In patients with chronic lung disease, (PaCO2-PACO2) was the same at all values of PACO2 whereas, in the other patients the ratio PaCO2/PACO2 did not change.