The effects of airway impedance on work of breathing during halothane anesthesia

Humidifiers and small diameter endotracheal tubes placed in the airway circuit increase the impedance to breathing. The effect of such impedances on the work of breathing and respiratory patterns was studied in eight healthy adult patients (60-80 kg) anesthetized with 1 and 2 MAC halothane in oxygen. A Cascade Humidifier and Portex Humid-Vent (dry and water saturated) were evaluated while patients breathed through an 8.0-mm endotracheal tube. A 6.0-mm endotracheal tube was also assessed without the humidifiers. At 1 MAC the Cascade Humidifier and the wet Humid-Vent when used with the 8.0-mm tube increased the work of breathing to 86.8 ml and 76.8 ml, 77% and 70% above baseline levels of 48.1 ml, whereas the 6.0-mm tube without the humidifiers increased work 89% to 78.9 ml. Tidal volume and respiratory frequency were unchanged throughout the study, although inspiratory time was prolonged. Lightly to moderately anesthetized healthy adult patients are able to maintain minute ventilation despite the impedance associated with commonly used humidifiers by significantly increasing work of breathing.