Carbon dioxide elimination from each lung during endobronchial anaesthesia. Effects of posture and pulmonary arterial pressure.

The ventilation and carbon dioxide elimination of each lung, and pulmonary arterial pressure, were studied in 17 patients during the early phases of anaesthesia for pulmonary surgery. The patients were ventilated mechanically to moderate hypocapnia. Expired tidal volume and carbon dioxide elimination rate of the lung to be operated on, and of the other lung, were similar in the supine position. There was a significant (P less than 0.01) increase in ventilation and a decrease in end-tidal PCO2 of the upper lung after turning the patient on to the side. Simultaneously, the physiological deadspace fraction of tidal volume (VD/VT) increased from 42 to 45% (P less than 0.05). Mean pulmonary arterial pressure (MPAP) increased slightly as surgery on the chest wall commenced. A concomitant increase of carbon dioxide elimination from the upper lung occurred also, although the distribution of ventilation, between the lungs, was unchanged in comparison with the conditions during undisturbed anaesthesia. Individual changes in MPAP (delta MPAP) and corresponding changes in VD/VT (delta (VD/VT)) were negatively correlated (r = -0.68, P less than 0.01). The regression equation was delta (VD/VT) (%) = 0.7 - 0.83 X delta MPAP (mmHg). It was concluded that variations in pulmonary arterial pressure during surgical stimulation may significantly affect the pattern of carbon dioxide elimination in the lungs. However, there was no evidence that these effects were important clinically.