- The upper airway normally warms, moistens and filters inspired gas. When these functions are impaired by disease, or when the nasopharynx is bypassed by endotracheal intubation, or when using high flow gases in non-invasive settings, artificial humidification of inspired gases must be provided.
- Absolute humidity - the mass of water vapour per unit volume of a gas.
- Humidity at saturation - the maximum mass of water which can be present in a gas per unit volume at a specified temperature
- Relative humidity - the ratio of absolute to saturation humidity at a specified temperature expressed as a percentage.
- Latent heat of vapourisation - the energy absorbed per gram in the phase change from liquid to gas
- Air at 37 ̊ with a 100% relative humidity contains 44gm-3 of water (SVP=47 mmHg)
- Inspired gas is normally humidified in the nose and mouth before entering the lower respiratory tract.
- As the upper airway is bypassed in intubated patients, RH of inspired gas falls below 50% with adverse effects, including:
- Increased mucus viscosity
- Depressed ciliary function
- Cytological damage to the tracheo-bronchial epithelium, including mucosal ulceration, tracheal inflammation and necrotising tracheobronchitis
- Microatelectasis from obstruction of small airways, and reduced surfactant leading to reduced lung compliance
- Airway obstruction due to tenacious or inspissated sputum with increased airway resistance