Inhalation anaesthesia according to the B


	San Sebastian 2004 Session 3-2
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Inhalation anaesthesia according to the B.E.T. scheme: A simple method for quantitativeanaesthesia / Anestesia inhalatoria según el esquema B.E.T.: Un método sencillo para anestesia cuantitativa.

Prof. Jan Baum, Damme, Germany.

The B.E.T.-Scheme in Inhalational Anaesthesia

One of the main advantages of Total Intravenous Anaesthesia and, especially, of Target Controlled Anaesthesia is the economic and efficient use of anaesthetics. In the very beginning of anaesthesia a bolus is applied establishing the desired blood concentration. In the following only that amount of drug is delivered to the patient replacing just that amount transferred into the tissues together with that amount eliminated by metabolism or excretion. This technique is called the drug application according to a B.E.T.-scheme (Bolus, Elimination, Transfer).

The application of volatile anaesthetics and anaesthetic gases in general follows quite different rules: The desired gas and anaesthetic concentration has to be established not only in the lung and tissues of the patient, but also in all gas containing parts of the anaesthetic machine. The most easy way to reach this aim in an appropriate short time is initially to ventilate the patient with gas of the desired composition, either by using non-rebreathing systems or by using rebreathing systems with a fresh gas flow as high as to prevent from any significant rebreathing. Thus, initially the fresh gas is delivered in significant excess. Even if the volatile anaesthetics could be delivered indipendently from the fresh gas flow, the use of high flow would be the only wash-in technique when intending to use precisely composed gas mixtures consisting either of nitrogen or of nitrous oxide with oxygen. The use of low flows via rebreathing systems from the very beginning would significantly prolong the wash-in phase. Even in the most sophisticated anaesthetic machines, delivering the different gas components independently by electronic control, satisfying rapid wash-in only can be realized by increasing the flow and, thus, working with significant excess.

A quite different situation results if pure oxygen is used as fresh gas and high inspiratory oxygen concentrations are accepted. The only use of oxygen significantly facilitates the approach towards a B.E.T.-scheme in inhalational anaesthesia. As soon as the whole anaesthetic system and the ventilator´s bellows is filled sufficiently with gas the flow is reduced to just that oxygen volume taken up by the patient. The individual oxygen uptake can roughly be estimated by applying Brody´s formula. Then, but never before flow reduction to this basal rate, an amount of fluid inhalation anaesthetic is injected into the breathing system establishing the desired anaesthetic´s concentration within the whole gas containing space comprising the breathing system, the hosing, the ventilator and the functional residual capacity. When using the newer volatile anaesthetics metabolism and, thus, elimination becomes negligible. It is only that amount of anaesthetic being transferred into the tissues which has to be replaced continuously with the aid of a motor-driven syringe.

To give an idea how to simply realize B.E.T.-scheme in clinical practice following method will be demonstrated by examples: A normal body weight adult patient is to be anaesthetized. Induction is accomplished in the routine way intravenously. After intubation or placement of a laryngeal mask airway the patient is connected to a breathing system. The volume of the whole gas containing system is estimated to be about 7.5 litre. This volume is sufficiently filled with gas by increasing the oxygen flow to 4.0 L/min. A sufficient filling is reached if the ventilator is readily filled with gas at the end of the expiratory phase. 1.0 to 1.5 mL fluid isoflurane or sevoflurane are injected into the expiratory limb of the breathing system as the initial bolus. This, generally, will lead to an expiratory concentration equalling the MAC concentration of the respective volatile anaesthetic. In the followig period fluid anaesthetic is injected continuously into the expiratory limb of the breathing system with a motor-syringe. The delivery rate is initially set to 12 mL/hour and then adapted to the measured concentration. Due to increasing saturation within the body´s tissues the amount of volatile being transferred declines continuously. Thus, the delivery rate of the motor-syrynge has to be reduced from time to time. This is accomplished by stepwisely reducing the delivery rate by 2 mL/hour. Rapid increase of the anaesthetic concentration can be realized by injecting a small additive bolus of 0.5 mL into the system to rapidly enhance transfer, rapid decrease of the anaesthetic, however, only can be gained by switching to high fresh gas flow, between 2.0 to 4.0 L/min to enhance elimination.

The disadvantage of this scheme is the lack of precise control of the gas composition. As the flow has to be reduced to the oxygen uptake as soon as possible after sufficiently filling the system with gas, the anaesthetist cannot influence the gas composition circulating within the breathing system. The oxygen concentration, finally establishing within the sytem, is influenced by several factors: The gas composition contained in the anaesthetic machine at the very moment of connecting the patient to the breathing sytem, the gas composition contained in the lung of the patient after induction, and finally, the composititon of the gas used to fill up the breathing system and the ventilator at the very beginning. The resulting gas composition, circulating within the sytem after flow reduction, hardly can be influenced rapidly without increasing the flow for wash-in of the desired gas composition, i.e. without leaving the B.E.T.-scheme.

On the other hand, the anaesthetist will be surprised by the fact how easily the B.E.T.-scheme can be applied even in inhalation anaesthesia, realizing the most economic and efficient way to use anaesthetics.