The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [367]
Humidification
Definition
Humidity is the amount of water vapour present in a gas. The terms used to define humidity are absolute humidity, maximum capacity and relative humidity. Absolute humidity is the mass of water vapour that a given volume of gas can carry at a set temperature. When a gas is at its maximum capacity, it is said to be fully saturated. Relative humidity is the ratio of the absolute humidity to the maximum capacity. The warmer the gas, the more vapour it can hold but if the temperature of the gas falls, water held as vapour will condense out of the gas into the surrounding atmosphere.
Anatomy and physiology
The respiratory tract is lined with ciliated epithelial cells that secrete mucus. Each cell has about 200 hair-like structures known as cilia, whose role is to remove unwanted mucus and secretions. Less than optimal humidification results in the slowing of the mucociliary transport system and pooling of mucus in the lower airways. Pooling of mucus may obstruct a small airway, restricting gas exchange. Pools of mucus also provide an ideal site for bacterial colonization. If less than optimal humidification continues, cell damage occurs and gas conditioning moves deeper into the lung (Estes and Meduri 1995).
Related theory
Normal room air has an approximate temperature of 22°C with a relative humidity of 50% and a water content of 10 mgH2O. For effective gas exchange to occur in the lungs, the air would need to be at a temperature of 37°C with 100% humidity and a water content of 44 mgH2O per litre by the time it reaches the bifurcation in the trachea, which is referred to as the isothermic point. When the temperature falls below 37°C and humidity falls below 100%, several changes take place in the airways. With a drop in temperature and humidity, the mucus that collects in the airways thickens and movement of the cilia is reduced. If there is no improvement the mucus will become thicker and immobile; the cilia will also lose their mobility so clearance of all secretions will stop and infection can set in (Marieb et al. 2008).
If there is a continuing lack of humidity further damage occurs. The cilia can break off, causing damage to the mucosal lining of the respiratory tract. The isothermic point of saturation moves from the bifurcation of the trachea to a lower point in the lungs, resulting in further damage which can lead to collapse of the alveoli, decrease in lung function and hypoxaemia (Carroll 1997, Fell and Boehm 1998, Ward and Park 2000).
Evidence-based approaches
Many devices can be used to supply humidification; the best of these will fulfil the following requirements.
The inspired gas must be delivered to the trachea at a room temperature of 32–36°C with 100% humidity and should have a water content of 33–43 g/m3 (Bersten et al. 2009).
The set temperature should remain constant; humidification and temperature should not be affected by large ranges of flow.
The device should have a safety and alarm system to guard against overheating, overhydration and electric shocks.
It is important that the appliance should not increase resistance or affect the compliance of respiration.
It is essential that whichever device is selected, wide-bore tubing (elephant tubing) is used to allow efficient formation of water vapour (Figure 10.7C).
Rationale
Inhalation of oxygen used during respiratory therapy, which is a dry gas, can cause evaporation of water from the respiratory tract and lead to the damage of mucosal lining if humidification is not provided. In patients who are intubated or have a tracheostomy, the natural pathway of humidification is bypassed (Wilson 2006).
Indications
Used for mechanical ventilation and non-invasive ventilation such as continuous positive airway pressure (CPAP)/non-invasive positive end expiration (NIPEE).