STUDIES OF ANAESTHESIA IN RELATION TO HYPERTENSION IH: PULMONARY GAS EXCHANGE DURING SPONTANEOUS VENTILATION

Ventilation, pulmonary gas exchanges and oxygen transport werestudied in a group of treated and untreated hypertensive elderlypatients, before, during and after nitrous oxide-halothane anaesthesiawith spontaneous ventilation. During anaesthesia minute andalveolar ventilation were depressed (— 30 per cent) outof proportion to the decrease in oxygen uptake (-18 per cent)and carbon dioxide production (— 19 per cent), and moderatehypercapnia ensued (mean Paoo3 50.3 mm Hg). All these variablesreturned to the pre-anaesthetic levels within 1 hour of terminatinganaesthesia. VD/VT was increased following induction of anaesthesiaas a result of decreased tidal volumes (— 47 per cent),but did not change progressively during the course of anaesthesia.Total deadspace (VD) was reduced by an average of 44 ml as aresult of intubation. Based on measurements of arterial Poaand the alveolar-arterial Poj difference, mean pulmonary venousadmixture was 10.4 per cent before anaesthesia, 10.9 per centduring anaesthesia but before surgery, 13.1 per cent after surgery,and 15.6 per cent 1 hour after the termination of anaesthesia.Although the average postoperative arterial Poa was slightlylower ( - 7.4 rnm Hg) than before anaesthesia, the differencewas due to many factors, and no evidence of a deteriorationin the over-all pulmonary gas exchange could be found. The conceptof "airway closure" in the supine elderly subject as a causefor the increased pulmonary venous admixture at rest is discussed.Impairment of pulmonary function during anaesthesia in hypertensiveelderly patients causes less concern than the severe changesin cardiovascular function ^*Present address: Hospital Notre Dame, Montreal133, P.Q., Canada