The PaCO₂ rate of rise in anesthetized patients with airway obstruction

Apneic, anesthetized patients frequently develop airway obstruction or may be disconnected from ventilatory support. The rate of PaCO₂ rise is usually assumed to be equal to that of anesthetized humans who are receiving apneic oxygenation. Apneic oxygenation may eliminate CO, because it requires a continuous O₂ flow. The CO₂ rate of rise in anesthetized humans with airway obstruction was measured. Fourteen consenting healthy adults were monitored continuously with pulse oximetry and EKG. Enflurane-O₂ anesthesia was established for at least 10 minutes with normal PaCO₂ without neuromuscular blockade so that anesthesia was deep enough to prevent spontaneous ventilation. Then, patients' tracheal tubes were clamped. Arterial blood samples were obtained before and after 0, 20, 40, 60, 120, 180, 240, and 300 seconds after clamping, provided that oxyhemoglobin saturation exceeded 0.92. The equation that best described the PaC0₂ rise was a logarithmic function. Piecewise linear approximation yielded a PaC0₂ increase of 12 mmHg during the first minute of apnea, and 3.4 mmHg/minute thereafter. These values should be employed when estimating the duration of apnea from PaC0₂ change for anesthetized patients who lack ventilatory support. In addition, it appears that the flows of O₂ that most earlier investigators used when delivering apneic oxygenation probably did not eliminate significant C0₂ quantities.