Clinical Performance of Closed System Anaesthesia
with Conventional Anaesthetic MachinesJan A. Baum
Closed system anaesthesia is the most efficient method to apply inhalation anaesthetics. It is only realized if, at any moment during the course of anaesthesia, the fresh gas volume equals that amount of gas taken up by the patient. The total gas uptake, on its part, is the sum of the gas volumes of each single gaseous component of the anaesthetic gas taken up by the patient. It becomes the more difficult for the anaesthetist to estimate the total gas uptake of the individual patient the more complex is the composition of the anaesthetic gas. If, for instance, the carrier gas consists of a mixture of nitrous oxide and oxygen, the nitrous oxide uptake follows a power function whereas the oxygen uptake remains nearly constant during the course of anaesthesia. Thus, to exactly follow the total gas uptake would require continuous adaptation of the fresh gas volume by frequent alterations at the gas flow controls. This is impossible when conventional anaesthetic machines are used, but requires technically sophisticated devices electronically controlling fresh gas supply by closed loop feedback. Thus, the main obstacle for any realisation of closed system anaesthesia with conventional anaesthetic machines is the power function characteristic of the uptake of any of the anaesthetic gas components, as it holds, for instance, for nitrous oxide.
Consistent omission of the use of nitrous oxide as a carrier gas component results in a total gas uptake being mainly determined only by the oxygen consumption of the patient, which remains nearly constant during the course of anaesthesia. This holds likewise for pure oxygen or a carrier gas mixture consisting of air and oxygen, as, due to its extremely low solubility in blood and tissues, nitrogen uptake itself is negligible. Compared with the oxygen consumption, the small amount of anaesthetic vapour, taken up by the patient, quantitatively also does not play a significant part in total uptake. In clinical practice the fresh gas flow can be reduced to just that amount of oxygen being taken up by the patient. It can be calculated by applying Brody´s formula and is about 250 mL/min in an average body weight adult patient. An initial high flow phase, however, has to preced flow reduction to establish a sufficient concentration of the volatile anaesthetic. This is indispensable as the delivery of volatile anaesthetics in conventional anaesthetic machines still is linked to the fresh gas flow. After ten minutes, however, generally a sufficient concentration of the volatile anaesthetic is already established. During that time the volatile´s uptake has become as low that even with fresh gas flows as low as 0.25 mL/min - or even lower - a sufficient amount of anaesthetic vapour can be delivered into the system to maintain the aspired concentration within the circuit.
Due to the omission of nitrous oxide, the routine supplemental injection of 0.1-0.2 mg fentanyl or 0.5-1.0 mg alfentanil during induction is recommended to replace the missing analgesic effect. The missing hypnotic effect can be replaced by increasing the anaesthetic´s expiratory concentration by only 0.2-0.25 times the MAC of the chosen anaesthetic. According to clinical experience 1.0-1.2 vol% isoflurane, 2.0-2.2 vol% sevoflurane, and 4.0-5.0 vol% desflurane (expiratory concentrations), in general, will guarantee sufficient anaesthetic depth. Thus, applying following scheme enables the anaesthetist to perform closed system anaesthesia with conventional anaesthetic machines in routine clinical practice. Initial high-flow phase: 3.0 L/min air and 1.0 L/min oxygen (alternatively 4.0 L/min oxygen), vaporizer setting for isoflurane 2.5 vol%, sevoflurane 3.5 vol%, and desflurane 6.0 vol%. After ten minutes fresh gas flow reduction to 0.25 L/min oxygen, vaporizer setting for isoflurane 5.0 vol%, sevoflurane 8.0 vol%, and desflurane 10.0 vol%. Sevoflurane and desflurane are especially suitable to be applied with closed system anaesthesia. When using isoflurane, however, at such low flows hardly a concentration of 1.2 vol% can be maintained, but also the concentration of only 0.9-0.8 vol% guarantees sufficient anaesthetic depth in most of all surgical cases. The use of conventional anaesthetic machines featuring fresh gas flow compensation of the ventilator and a gas reservoir facilitates the performance of closed system anaesthesia.
The key to realize closed system anaesthesia with conventional anaesthetic machines in clinical practice is the use of simply composed carrier gases.