Carboxyhaemoglobin and closed-circuit anaesthesia

Abstract

With a change in concept of the uptake of inhalation anaesthetics based on common physiological principles [1], we are able to provide simple, easily applicable closed-circuit anaesthesia with an existing anaesthesia machine. Following intubation, the anaesthesia circuit and FRC were primed with the desired O2 and anaesthetic concentration with high flow anaesthetic/O2/air mixture for several minutes. Following this, only the supply of calculated anaesthetic vapour, dependent on the desired inspired anaesthetic concentration, and 200-300 mL min−1 of O2 are required for maintenance of anaesthesia. Recent pubications [2,3] indicate the possible production of carbon monoxide (CO) during closed-circuit anaesthesia with inhalational anaesthetics. We have examined the possibility of CO production in the circuit by determining blood carboxyhaemoglobin levels during closed-circuit anaesthesia. With IRB approval, we administered general anaesthesia with inhalation anaesthetics using a closed circuit. After several minutes of high flow gas mixture to prime the anaesthesia circuit and FRC, we closed the circuit with a minimum total gas flow of less than 500 mL min−1. We obtained arterial blood samples for measurement of carboxyhaemoglobin levels during the initial stages of anaesthesia and at the end of surgery. We recorded the length of closed-circuit, fresh gas flow rate and changes in the soda lime. We studied 73 patients. During the mean duration of 3.74 ± 0.92 h of closed-circuit anaesthesia, very little change occurred in carboxyhaemoglobin level (from 0.88 + 0.57 to 0.90 + 0.59). Average fresh gas flow during closed-circuit anaesthesia was 322 + 99 mL min−1. No correlation was found between the changes of soda lime canister and the level of carboxyhaemoglobin. During 73 consecutive cases with closed-circuit anaesthesia, we failed to demonstrate an increase in blood carboxyhaemoglobin level. Possible explanations are: (1) the limited use of inhalation anaesthetic agents despite maintaining similar adequate inspired anaesthetic concentration; and (2) humidity (water content) of soda lime is much higher than in the conventional high fresh gas flow anaesthesia. We consider that closed-circuit anaesthesia is a simple, safe anaesthetic technique and able to provide a considerable reduction in environmental pollution as well as anaesthetic consumption. Closed-circuit anaesthesia is also able to maintain airway humidity and body temperature with good haemodynamic stability.