Nebulizers produce a fine mist at room temperature. The humidification of inspired gases from these droplets depends on the RH. When the RH is above 80 per cent, the evaporation rate from the droplets decreases. Moreover, droplet diameter is of crucial importance. Droplets of diameter less than 1 pm can reach the alveoli. Only droplets with a diameter of 3 to 5 pm participate efficiently in the humidification of inspired gases.

There are three types of nebulizer:

1. the centrifuge generator;

2. the ultrasonic nebulizer (Fig. 5) in which a piezoelectric crystal vibrates at very high frequencies in order to produce a mist (droplet size, 2.8-4 pm);


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Fig. 5 Ultrasonic nebulizer. (Reproduced with permission from Ihomachotefal (1995))

3. nebulizers in which gas is sprayed directly towards the end of the capillary tube and nebulizes the water aspired by the Bernoulli effect (droplet size, 0.5-3 pm). Closed circuits

The conditions for humidifying and heating gases are comparable in a heated humidifier and a closed circuit with a fresh gas inflow of 0.5 l/min but adequate gas conditioning is achieved much later. The use of a higher fresh gas inflow reduces the level of humidification.

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