A comparison of eleven general anaesthetics administered with 7.5 per cent carbon dioxide during spontaneous breathing

Abstract

EVIDENCE has accumulated to show that hypercapnia has serious adverse effects both in the conscious state and during anaesthesia. 1,z,8 Nevertheless, a mild degree of hypercapnia (Pco2 ~ 65) has been found useful during certain surgical procedures, particularly in patients with extensive arteriosclerosis, 4 during deliberate induced deep hypothermia 5 and during anaesthesia for carotid endarterectomy, 6 because it may prevent tissue and cerebral hypoxia, 7,sIn our previous reports of anaesthesia with added carbon dioxide, no appreciable alterations in cardiovascular dynamics and metabolic functions were observed when pulmonary ventilation was controlled. 9,t~ Since it was found that enflurane and isoflurane are potent respiratory depressants at a surgical depth of anaesthesia, this study was devised to determine the physiological and metabolic effects of these two anaesthetics when carbon dioxide was added, and spontaneous breathing was permitted. The responses with nine other general anaesthetics were also studied for comparison. MATERIALS AND METHODSSerial crossover tests were done on 18 large, trained, healthy, male, mongrel dogs (19 to 31 kg) at two-week intervals, employing the eleven anaesthetic agents shown in Table I. Each agent was used at least six times and each animal was anaesthetized with four different agents. A high protein diet was provided for one month prior to testing and during the intervals between tests.In each of 72 experiments, an unpremedicated dog, fasted overnight, was weighed first and then an 18-gauge plastic cannula was inserted in a fore-leg vein and 20 mg/kg thiopentone (2 per cent) was administered slowly. The dog was laid on its side. The cannula was attached to a regulated infusion of 0.9 per cent saline. A cuffed tracheal tube (40 French) was inserted without using a muscle relaxant and attached to a non-rebreathing type anaesthetic circuit delivering a gas mixture containing 50 per cent N20, 7.5 per cent CO2 and 42.5 per cent 02. The animal was allowed to breathe spontaneously and a respirometer was used to measure minute ventilation and respiratory rate. A femoral artery was eannulated percutaneously and the cannula was attached to a Statham strain gauge and Grass Polygraph through a 3-way stopcock to allow for blood sampling. ECG (lead II) was recorded on the polygraph. A rectal telethermometer probe (Yellow Springs Instrument Co., Inc. ) and a urinary catheter attached to a calibrated gravity trap were also inserted. Following these preparations, venous and arterial blood