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 patientsduring the carry phases of anaesthesia for pulmonary surgery.The patients were ventilated mechanically to moderate hypocapnia.Expired tidal volume and carbon dioxide elimination rate ofthe lung to be operated on, and of the other lung, were similarin the supine position. There was a significant (P<0.01)increase in ventilation and a decrease in end-tidal PCO₂ ofthe 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<0.05). Mean pulmonary arterialpressure (MPAP) increased slightly as surgery on the chest wallcommenced. A concomitant increase of carbon dioxide eliminationfrom the upper lung occurred also, although the distributionof ventilation, between the lungs, was unchanged in comparisonwith the conditions during undisturbed anaesthesia. Individualchanges in MPAP (MPAP) and corresponding changes in VD/VT ((VD/VT))were negatively correlated (r=–0.68, P<0.01). The regressionequation was (VD/VT) (%) = 0.7–0.83 x MPAP (mm Hg). Itwas concluded that variations in pulmonary arterial pressureduring surgical stimulation may significantly affect the patternof carbon dioxide elimination in the lungs. However, there wasno evidence that these effects were important clinically