Respiratory gas humidification is a method of artificially conditioning respiratory gas for the patient during therapy, and involves humidification, warming, and occasionally filtration of the gas being delivered. If these three measures are not performed to compensate for the natural conditioning of air by the respiratory system, lung infections and lung tissue damage may occur. This is particularly problematic in high gas-flow therapies such as echanical ventilation in patient populations with highly sensitive respiratory tracts (i.e.

asthma Asthma is a common long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wh ...

tics), or among those requiring ventilation for longer periods of time. The two methods currently available for this purpose are active or passive respiratory gas humidification.

Active respiratory gas humidifiers

An active respiratory gas humidifier ensures that patients on mechanical ventilation are supplied with optimally conditioned respiratory gas. In active humidifying processes, moisture and heat is input to respiratory gas by an electrically powered humidifier. Performance data and safety-related requirements for active respiratory gas humidifiers are specified by the standard ISO 8185. According to that standard, the minimum water content of inspired respiratory gas is ca. 33 mg/dm³ and the maximum respiratory gas temperature is ca. 42 °C. The aggregation of water in the gas produced by an active respiratory gas humidifier may be a suspension, or

aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...

, which is produced by a

nebulizer In medicine, a nebulizer (American English) or nebuliser (English language, English) is a drug delivery device used to administer medication in the form of a mist inhaled into the lungs. Nebulizers are commonly used for the treatment of asthma, c ...

; or particulate water, output from an

evaporator An evaporator is a type of heat exchanger device that facilitates evaporation by utilizing conductive and convective heat transfer, which provides the necessary thermal energy for phase transition from liquid to vapour. Within evaporators, a ci ...

or bubble humidifier.

Nebulizers

Nebulizers generate

s consisting of droplets of various sizes that are admixed to the inspired respiratory gas. Types of nebulizers currently on the market include #Small volume nebulizers, which are used to administer medications such as salbuterol or albuterol. #Large volume nebulizers, which are similar to bubble humidifiers except for the addition of an air entrainment port, and #Ultrasonic nebulizers, which may carry a risk of overwatering the patient. The high density mist produced by nebulizers is useful in decreasing the viscosity of respiratory secretions in those suffering from conditions such as

cystic fibrosis Cystic fibrosis (CF) is a genetic disorder inherited in an autosomal recessive manner that impairs the normal clearance of Sputum, mucus from the lungs, which facilitates the colonization and infection of the lungs by bacteria, notably ''Staphy ...

croup Croup ( ), also known as croupy cough, is a type of respiratory infection that is usually caused by a virus. The infection leads to swelling inside the trachea, which interferes with normal breathing and produces the classic symptoms of "bar ...

, epiglottitis, and

bronchiectasis Bronchiectasis is a disease in which there is permanent enlargement of parts of the bronchi, airways of the lung. Symptoms typically include a chronic cough with sputum, mucus production. Other symptoms include shortness of breath, hemoptysis, co ...

Evaporators

Evaporators enrich the inspired respiratory gas with

water vapor Water vapor, water vapour, or aqueous vapor is the gaseous phase of Properties of water, water. It is one Phase (matter), state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from th ...

. In a throughflow evaporator, the inspiration flow is led through a warmed up water bath, in case of a surface evaporator however the inspiration flow is guided along the surface of the water level. Consequently, a surface evaporator transports only water vapor and no water droplets into the patient. Therefore, the risk of passing on germs by surface evaporators is minimal.

Bubble humidifiers

In a bubble humidifier, or bubble bottle as they are affectionately known by respiratory therapists, the inspiration flow is guided through a capillary system. In this capillary system warmed up water is circulating. Although the humidifying capacity of a bubble respiratory gas humidifier is rather low, it may be improved by increasing the water temperature. A bubble bottle is mostly used in oxygen therapy with high flow rates via a mask, nasopharyngeal catheter, or nasal cannula in order to prevent drying of the mucous membranes in the nose and mouth.

Passive respiratory gas humidifiers

Passive respiratory gas humidifiers are independent from any external energy source or external water supply. They function as heat and moisture exchangers (HMEs) and are placed like an artificial nose between a tube and Y piece. Here they withdraw heat and moisture from expirations, which they resupply to the inspired gas during the following inspiration. As there are significant functional differences among the various HMEs on the market, respiratory therapists should test the efficacy of each individual model. The ideal HME has high reversible water retention capacity, small internal volume, and low flow resistance. To enable the absorption of sufficient amounts of water and heat, the expiratory stream of respiratory gas must be fully filtered through the HME. Leakages in the system, such as may be caused by bronchial fistulae, will render this system less effective. Other negative effects of this technology include increased secretions (i.e. mucus) and nosebleeds, either or which may clog an HME. In such cases, the application of active respiratory gas humidifiers is recommended.

References

* W. Oczenski, H. Andel und A. Werba: "Atmen - Atemhilfen." Thieme, Stuttgart 2003, * J. Rathgeber: "Grundlagen der maschinellen Beatmung." Aktiv Druck, Ebelsbach 1999, * S. Schäfer, F. Kirsch, G. Scheuermann und R. Wagner: "Fachpflege Beatmung." Elsevier, 2005, * A. Schulze: "Respiratory Gas Conditioning and Humidification." In: Clin Perinatol, 2007; 34: 19–33, * M.P. Shelly, G.M. Lloyd und G.R. Park: "A review of the mechanism and methods of humidification of inspired gases." In: Intens Care Med, 1988; 14:1, * F. Kapadia, M. Shelly, J.M. Anthony, et al.: "An active heat and moisture exchanger." In: Br. J. Anaest. 1992; 69: 640–642, cx {{DEFAULTSORT:Respiratory Gas Humidification Intensive care medicine Respiratory system procedures