Techniques of Inhalation of Anaesthetics

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Chapter: Essential pharmacology : General Anaesthetics

Different techniques are used according to facility available, agent used, condition of the patient, type and duration of operation.


TECHNIQUES OF INHALATION OF ANAESTHETICS

 

Different techniques are used according to facility available, agent used, condition of the patient, type and duration of operation.

 

1. Open Drop Method

 

Liquid anaesthetic is poured over a mask with gause and its vapour is inhaled with air. A lot of anaesthetic vapour escapes in the surroundings and the concentration of anaesthetic breathed by the patient cannot be determined. It is wasteful—can be used only for cheap anaesthetics. Some rebreathing does occur in this method. However, it is simple and requires no special apparatus. Ether is the only agent used by this method, especially in children.

 

2. Through Anaesthetic Machines

 

Use is made of gas cylinders, specialized graduated vaporisers, flow meters, unidirectional valves, corrugated rubber tubing and reservoir bag.

 

The gases are delivered to the patient through a tightly fitting face mask or endotracheal tube. Administration of the anaesthetic can be more precisely controlled and in many situations its concentration determined. Respiration can be controlled and assisted by the anaesthetist.

 

a) Open System 


The exhaled gases are allowed to escape through a valve and fresh anaesthetic mixture is drawn in each time. No rebreathing is allowed—flow rates are high—more drug is consumed. However, inhaled O2 and anaesthetic concentration can be accurately delivered.

 

b) Closed System 


The patient rebreaths the exhaled gas mixture after it has circulated through sodalime which absorbs CO2. Only as much O2 and anaesthetic as have been taken up by the patient are added to the circuit. The flow rates are low; especially useful for expensive and explosive agents (little anaesthetic escapes in the surrounding air) e.g. halothane, enflurane, isoflurane. However, control of inhaled anaesthetic concentration is difficult.

 

c) Semiclosed System 


Partial rebreathing is allowed through a partially closed valve. Conditions are intermediate with moderate flow rates.

 

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