Carbon dioxide dynamics during apneic oxygenation: The effects of preceding hypocapnia

Study Objective: To compare the rate of increase of arterial carbon dioxide tension (PaCO₂) during apneic oxygenation preceded by acute respiratory hypocapnia with that during apnea preceded by respiratory eucapnia. Design: Randomized, prospective, single crossover study. Setting: Operating room at a teaching hospital. Patients: 19 ASA physical status I, II, and III patients requiring general endotracheal anesthesia and invasive monitoring for elective surgery. Interventions: Two ventilatory states preceding apneic oxygenation were studied in each patient. The first respiratory state was established using controlled mechanical ventilation in the stable, anesthetized patient, followed by a 5-minute period of apneic oxygenation. Arterial and mixed central venous blood gas samples were obtained simultaneously prior to and at 1-minute intervals during apnea. Ventilatory parameters were then changed to establish the second respiratory state. During the subsequent period of apnea, the study was repeated as above. Measurements and Main Results: Arterial and mixed central venous partial pressure of carbon dioxide (pCO₂) levels were measured. A greater increase in PaCO₂ was found during the first minute of apneic oxygenation in the hypocapneic group compared with the eucapneic group. The venoarterial gradient of pCO₂ was also greater in the hypocapneic group prior to apnea. Conclusions: Acute hypocapnia compared with eucapnia prior to apneic oxygenation is associated with a greater rise in PaCO₂ in the anesthetized patient. This finding may be due in part to widening of the venoarterial gradient of pCO₂.