Humidification of the Airways Adequate for Function and Integrity?

Abstract

The upper respiratory tract serves as an air conditioning system; particulate matter is entrapped while inspired air is warmed and humidified prior to entry into the major airways. Turbulent flow through the nose and pharynx propels solid particles towards the surface, enhances convective transfer of heat from the mucosa, and increases the apparent surface area of exposure for evaporation of water vapor into the air stream. Because the nasal mucosa becomes cooled in the process, exhaled gas, which is fully humidified at body temperature, gives up water vapor to the cooler surface and the effluent gas temperature is reduced to 32-34°C.1Liese W Joshi R Cumming G Humidification of respired gas by nasal mucosa.Ann Otol. 1973; 82: 32-330Google Scholar Fully saturated air at this temperature contains 10.2 mg/L less water vapor than alveolar gas. Water and heat recapture by this mechanism conserves 20-25 percent of insensible losses during normal breathing in temperate atmospheric conditions.2Walker JEC Wells RE Jr Merrill EW Heat and water exchange in the respiratory tract.Am J Med. 1961; 30: 67-259Abstract Full Text PDF Scopus (151) Google Scholar The administration of relatively dry anesthetic gases and oxygen-enriched mixtures via an endotracheal tube obliges the lower respiratory tract to perform these functions. The potential hazard of greater heat and water losses, as well as structural damage was appreciated early in tracheotomized patients, and external application of humidity was demonstrated to alleviate excess mucus production and encrustation of secretions.3Tovell RM Little DM Jr The utilization of fog as a therapeutic agent.Anesthesiology. 1957; 18: 79-470Crossref Scopus (3) Google Scholar Much subsequent study has revealed the sensitivity of the tracheobronchial tree to incompletely humidified gas. Cessation of ciliary beating occurs within ten minutes of exposure of the trachea to 50 percent relative humidity.4Dalhamn T Mucous flow and ciliary activity in the trachea of healthy rats and rats exposed to respiratory irritant gases. (SO2, H3N, HCHO).Acta Physiol Scand (suppl). 1956; 123: 5-161Google Scholar Tracheal mucus velocity, studied in intact dogs,5Hirsch JA Tokayer JL Robinson MJ Sackner MA Effects of dry air and subsequent humidification on tracheal mucous velocity in dogs.J Appl Physiol. 1975; 39: 242-246Crossref PubMed Scopus (82) Google Scholar was reduced by half after one hour of breathing dessicated room air and completely ceased after three hours. Subsequent rehumidification of the airway rapidly restored tracheal mucus velocity, but histologic evidence demonstrated that extensive inflammation and sloughing of ciliated epithelium in the trachea and bronchi were present despite the functional restoration observed. Cytologic studies in anesthetized human subjects confirmed that damage to the tracheal epithelium occurs within two hours of administration of dry gases through an endotracheal tube, whereas gases at 60 percent relative humidity produce no damage.6Chalon J Loew DAY Malbranche J Effects of dry anesthetic gases on tracheobronchial ciliated epithelium.Anesthesiology. 1972; 37: 43-338Google Scholar However, air at 60 percent relative humidity delivered through an endotracheal tube reaches only 83 percent of full humidification at body temperature at the carina, and is not fully saturated until it reaches 10 cm further below this point.7Dery R The evolution of heat and moisture in the respiratory tract during anaesthesia with a non-rebreathing system.Canad Anaesth Soc J. 1973; 20: 296-309Crossref PubMed Scopus (80) Google Scholar Thus, although confirmation is uniform that relative humidities of 60-70 percent do not affect ciliary function for up to three hours, and that the lower airways continue to warm and humidify inspired gas, the structural response of the peripheral airways during prolonged periods of incomplete humidification remains unknown. The hygroscopic condenser humidifier reproduces the action of the nasopharynx to partially recapture exhaled heat and humidity and donates this retained moisture to the next inspired tidal volume. The nasopharynx increases its efficiency under colder and drier conditions, maintaining full humidification of inspired gas despite ambient temperatures down to −100°C.2Walker JEC Wells RE Jr Merrill EW Heat and water exchange in the respiratory tract.Am J Med. 1961; 30: 67-259Abstract Full Text PDF Scopus (151) Google Scholar By contrast, delivery of larger tidal volumes and higher inspired oxygen concentrations through the hygroscopic condenser humidifier reduces inspired humidity.8Hay R Miller WC Efficacy of a new hygroscopic condenser humidifier.Crit Care Med. 1982; 10: 49-51Crossref PubMed Scopus (14) Google Scholar The study presented by MacIntyre and co-authors (see page 560) is a valuable clinical evaluation of this device and demonstrates that mucociliary activity is preserved for periods longer than have been evaluated previously. Further histologic investigation must ensure that the potential for histologic damage to subcarinal airways is not present.