The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

Complications Resulting from Inadequate Humidification

Richard D. Branson, RRT Monday, November 4, 1996

During normal breathing the nose serves to condition and filter inspired gases. The exquisite architecture of the nasal turbinates allows inspired gases to come in contact with a tremendous surface area of moist mucous membrane. Under normal conditions, the nose increases the moisture content of inspired gases to 31 mg H_{2}O/L (32°C and 95% relative humidity) by the time gas reaches the trachea. Under extreme conditions, Cole has shown that the nose is capable of increasing gas by 60°C, from - 30°C to +30°C, prior to arriving in the pharynx.

On a typical day, the respiratory tract loses about 250 mL of water and 1470 J of heat per day. Loss of heat and moisture from the respiratory tree is, in fact, normal. This loss of heat and moisture occurs as room air travels to the alveolus. During the journey, temperature gradually increases to 37°C and the relative humidity to 100% (absolute humidity = 44 mg H_{2}O/L). The point in the respiratory tree where gas achieves 37°C and 100% relative humidity is known as the isothermic saturation boundary (ISB). The ISB may move up and down the respiratory tract as the result of altered environmental conditions or patient disease.

Following intubation, the form and function of the upper airway are bypassed. The burden of adding moisture then falls upon the lower respiratory tree, a job it is ill equipped to perform. As an added complication, gas from the ventilator is cold and dry, further taxing the lower respiratory tract. Gases from the ventilator are typically < 21°C and contain < 3 mg H_{2}O/L. The combination of intubation and dry gases forces the ISB down the respiratory tree resulting in tracheobronchial damage.

The adverse effects of dry anesthetic gases have been known for almost 40 years! From the initial work in 1961 by Burton to the clinical studies by Chalon in the 1970's, numerous authors have demonstrated the untoward effects of inadequate humidification. These studies have found a number of physiologic and structural alterations. A culmination of these findings is shown below.

Structural

Destruction of Cilia Damage to mucus glands Disorganization of columnar epithelium Reduction in the volume of cytoplasm Ulceration of the mucosa Loss of surfactant

Clinical

Retained secretions Mucus plugging Atelectasis Increased work of breathing Increased intrapulmonary shunting Hypothermia

Physiologic

Destruction of the mucociliary escalator Increase in sputum viscosity

Decreased pulmonary compliance Increased airway resistance

Reduced functional residual capacity

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