Second Gas Effect and Diffusion Hypoxia

 

Second Gas Effect

For anaesthesia using inhalational agents, it is a common practice to use an anaesthetic inhalation mixture of 70% N2O + 25-30% O2 + 0.2- 2% other anaesthetic

Oxygen is used for maintaining blood oxygen levels while nitrous oxide (the first gas) and other anaesthetic gas (the second gas) are the anaesthetic agents

The lipid solubility of nitrous oxide is low and therefore, high concentration of nitrous oxide (70%) are required in the alveoli to achieve anaesthesia.

At such high concentration, a large amount of nitrous oxide is uptaken into the alveoli capillary at the rate of 1 litre/min

This creates a negative suction in the alveoli and the second gas is sucked into the alveoli. 

This speeds up the induction of anaesthesia because higher amount of the second gas is able to reach the brain. 

This is known as second gas effect

Diffusion Hypoxia

 Reverse of second gas effect occurs when N2O is discontinued after prolonged anaesthesia.

Large amount of nitrous oxide diffuses across alveolo-capillary wall into the alveoli.

Partial pressure of oxygen in alveoli falls. There is risk of hypoxia in the patient. This is known as Diffusion Hypoxia.

Diffusion Hypoxia is easily avoided by 100% oxygen inhalation in the first few minutes after discontinuing nitrous oxide.

Diffusion hypoxia is not significant with inhalation anaesthetics other than nitrous oxide because only nitrous oxide is required to be given at high concentration (70%), while other  inhalation anaesthtics are used at concentrations of 0.2 -2%