Consideration of the International Standard for Airway Humidification Using Simulated Secretions in An Artificial Airway

Abstract

BACKGROUND: The use of humidifiers with a heating wire incorporated into the inspiratory limb of the ventilator circuit has become commonplace in mechanical ventilatory support. The heating wire in the circuit, however, may represent a potential hazard with regard to inspissation of secretions in the upper airway, especially in an artificial airway, due to reduced relative humidity. The current International Standard for airway humidification specifies only absolute humidity (AH) without regard for relative humidity (RH) or temperature. We studied whether either RH or the temperature range and AH should be specified in the Standard to maintain adequate RH. METHODS & MATERIALS: Five kinds of gases with various humidities were passed through an endotracheal-tube model, in which a brown water-soluble gel on dry filter paper was used as a substitute for secretions. Because the gel was hydrated by moisture from the delivered gas, the gel melted and a brown stain spread over the dry filter paper to the extent that the gas could hydrate the gel. The stains were measured and the stained area calculated by computer. The extent of each stained area was assessed with respect to RH, AH, and temperature of the gas. RESULTS: Gases satisfying the recommendations of the International Standard (ie, AH > 30 mg H2O/L), but with low RH, were no more advantageous for hydrating the gel than a gas (AH < 30 mg H2O/L) saturated with water vapor (ie, RH = 100%). The extent to which the brown water-soluble gel stained an increasing area of filter paper, which we assessed as the ability to provide humidification, was almost directly proportional to RH rather than AH or temperature. CONCLUSION: The current International Standard for humidification of bypassed airways should specify either RH or the temperature range with AH of the delivered gas.