Gas is fully saturated at body temperature by the time it reaches the isothermic boundary
Expansion
Inspired air is warmed to 37 degrees Celsius and fully saturated, containing 44 mg of water per litre at a vapour pressure of 6.3 kPa (47 mmHg).
The isothermic saturation boundary is the point at which this is achieved, normally just below the carina. Above it, the airway loses heat and water on inspiration and recovers some on expiration; below it, conditions are constant.
The nose does most of the work, providing about 75 per cent of heating and humidification with its large surface area and turbinate turbulence.
Bypassing the upper airway with an endotracheal tube or tracheostomy moves the boundary distally, so dry cold gas reaches further into the lung. Consequences:
- Inspissated secretions and mucus plugging
- Ciliary damage and impaired clearance
- Atelectasis and increased infection risk
- Heat loss, significant in neonates and long procedures
Artificial humidification uses a heat and moisture exchanger (a passive condenser filter) for short-term use, or an active heated humidifier for prolonged ventilation.