The carbon dioxide rate of rise in awake apneic humans

Currently available estimates of the PaCO₂ rate of rise in resting humans with resting lung volume were gathered during general anesthesia. The PaCO₂ rate of rise during apnea in awake subjects was determined to acquire a value that may be more applicable to awake, ventilator-dependent, critically ill patients. Clinically, apnea occurs at functional residual capacity. With F_iO₂ = 1.0, 20 volunteers held their breaths at functional residual capacity for 0, 10, and 20 seconds, and then for as long as possible. They exhaled through an infrared CO₂ analyzer after each interval to determine end-tidal pCO₂. An estimate of the logarithmic PaCO₂ rise during breath holding at functional residual capacity was 7 mmHg during the first 10 seconds (43 mmHg/minute), 2 mmHg during the next 10 seconds (13 mmHg/minute), and 6 mmHg/minute thereafter. In conclusion, PaCO₂ increases more rapidly in awake apneic humans than earlier thought. The values reported herein probably are better for estimating duration of apnea in conscious, critically ill patients than are values obtained during general anesthesia.