Carbon dioxide balance during thoracic surgery

¹ Sections of Anesthesiology and Physiology, Mayo Clinic and Mayo Foundation, Rochester, Minnesota

Measurements were made of O₂ uptake, CO₂ output and CO₂ tension of arterial and mixed venous blood and expired alveolar gas in 28 anesthetized and paralyzed dogs ventilated with air at a constant rate. Retention of CO₂ was not observed in the lateral decubitus position with either closed or open thorax. Induction of open pneumothorax resulted in an unexplained metabolic alteration characterized by an increase in O₂ uptake and CO₂ elimination and a decrease in plasma buffer base. Small amounts of CO₂ were eliminated from exposed pleura. The left pulmonary artery was occluded (after left thoracotomy) and 1 hour later the left bronchus also was occluded—or the order was reversed. CO₂ retention of major degree was observed after initial occlusion of the artery and retention of minor degree after occlusion of the bronchus. During the 2nd hour (after additional occlusion) the previously retained CO₂ was eliminated. The results suggest that the ventilatory requirements are not influenced specifically by effects of lateral position, open pneumothorax, or small right-to-left shunts, but rather are determined to a large degree by the metabolic production of CO₂ and the degree to which ventilation is distributed to perfused alveoli.