The effect of tubing length, gas flow, and number of heaters on maximum gas temperature for aerosol circuits used for cold water near-drowning or hypothermia.

Background: Clinicians who treat patients suffering from cold water near-drowning or hypothermia routinely warm inspire gases greater than body temperature in accordance with care guidelines promulgated by the various organizations. However, humidifiers are designed to prevent heating gases beyond 41 C (assuming the use of a standard six foot aerosol circuit) in order to meet International Standards Organization regulations (ISO). Clinicians must modify equipment in order to deliver care. There are several factors, which can effect the highest temperature that a particular circuit will achieve. Among the factors that are considered most important for maximum circuit are tubing length, gas flow, and the number of heaters (heat source).

Methods: The maximum temperature that a circuit could achieve was measured after varying tubing lengths (1.5 feet, 3 feet, and 6 feet), gas flow (opening or closing a venturi receiving a fixed flow rate of 10 L/min), and the size of the heat source (one or two heated humidifiers in aerosol circuit). A total of ten runs were made in each of the possible twelve combinations.

Results: Univariate statistics showed significant differences for Venturi open/close (p = .0001) and the number of heaters (p = .0001) but not the tubing length (p = 0.19). However, the multivariate analysis revealed significance for tubing length, number of heaters, and venturi open/closed (p = .01).

Conclusion: All factors (tubing length, number of heaters, and tubing length) were important determinants of maximum gas temperature. The effect of tubing length can be overwhelmed by higher gas flows.