Second Gas Effect and Diffusion Hypoxia

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Chapter: Essential pharmacology : General Anaesthetics

In the initial part of induction, diffusion gradient from alveoli to blood is high and larger quantity of anaesthetic is entering blood. If the inhaled concentration of anaesthetic is high, substantial loss of alveolar gas volume will occur and the gas mixture will be sucked in, independent of ventilatory exchange—gas flow will be higher than tidal volume.


SECOND GAS EFFECT AND DIFFUSION HYPOXIA 

 

In the initial part of induction, diffusion gradient from alveoli to blood is high and larger quantity of anaesthetic is entering blood. If the inhaled concentration of anaesthetic is high, substantial loss of alveolar gas volume will occur and the gas mixture will be sucked in, independent of ventilatory exchange—gas flow will be higher than tidal volume. This is significant only with N2O, since it is given at 70–80% concentration; though it has low solubility in blood, about 1 litre/min of N2O enters blood in the first few minutes—gas flow is 1 litre/min higher than minute volume. If another potent anaesthetic, e.g. halothane (1–2%) is being given at the same time, it also will be delivered to blood at a rate 1 litre/min higher than minute volume and induction will be faster—second gas effect.

 

The reverse occurs when N2O is discontinued after prolonged anaesthesia—N2O having low blood solubility rapidly diffuses into alveoli and dilutes the alveolar air—PP of oxygen in alveoli is reduced. The resulting hypoxia, called diffusion hypoxia, is not of much consequence if cardiopulmonary reserve is normal, but may be dangerous if it is low. This can be prevented by continuing 100% O 2 inhalation for a few minutes after discontinuing N2O, instead of straight away switching over to air. Diffusion hypoxia is not significant with other anaesthetics because they are administered at low concentrations (0.2–4%) and cannot dilute alveolar air by more than 1–2%.

 

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