Studies of anaesthesia in relation to hypertension. IV. The effects of artificial ventilation on the circulation and pulmonary gas exchanges.

The cardiovascular responses, ventilation and pulmonary gas exchanges of 8 treated and 6 untreated hypertensive patients were studied during artificial ventilation under nitrous oxide/oxygen/relaxant anaesthesia, with and without halothane (1%). Mean arterial pressure (m.a.p.) fell to 69% and 62% of the awake values in the treated and untreated patients respectively during hypocapnia (mean PaCO2 23 mm Hg) induced by IPPV under nitrous oxide anaesthesia. When halothane (1%) was added under the same conditions of IPPV, m.a.p. fell to 60% and 53% of the awake values respectively, as a result of reduced cardiac output (50% and 53% respectively of awake values), whereas systemic vascular resistance was raised above the awake values in all patients. Electrocardiographic evidence of myocardial ischaemia was observed during these periods of arterial hypotension in 50% of the treated patients and in all the untreated patients. Increased alveolar-arterial Po, differences during IPPV were due to desaturation of mixed venous blood associated with raised arteriovenous oxygen content differences. Pulmonary venous admixture did not change significantly during the course of anaesthesia with IPPV, and recovery. VD/VT increased during spontaneous ventilation following induction of anaesthesia, but decreased significantly below awake values during IPPV, returning to control values in the postoperative period.