CARDIAC OUTPUT AND VENOUS ADMIXTURE DURING INTERMITTENT POSITVE PRESSURE BREATHING: Influence of Respiratory Frequency and Arterial Carbon Dioxide Tension

Abstract

Cardiac output, venous admixture and pulmonary gas exchange were measured in ten human subjects. Five were healthy and were studied under routine light anaesthesia, five were in respiratory or cardiac failure and undergoing prolonged respirator treatment. The subjects were investigated during four different conditions of artificial ventilation (accelerating gas flow pattern), the minute volume being kept constant in all phases of the study: (1) 12 b.p.m.; (2) 24 b.p.m.; (3) 12 b.p.m. with CO₂ added to the inspired gas mixture to elevate Pa_co2 to that measured in phase 2; (4) 12 b.p.m. (control). It was found that a small rise in Pa_co2 (5 mm Hg) was followed by a small but significant rise in cardiac output which is attributed to an increase in catecholamine release. Venous admixture was unchanged, except for an increase in one patient with a large pre-existing shunt. The VD/VT ratio increased markedly with an increase in frequency, but was not influenced by the addition of CO₂ to the inspired gas mixture.