Eucapnic voluntary hyperventilation of compressed gas mixture. A simple system for bronchial challenge by respiratory heat loss

Eucapnic voluntary hyperventilation (EVH) of cold, dry air has been shown to be an effective stimulus for bronchoconstriction in people with reactive airways. The system for respiratory heat exchange (RHE) challenge can be greatly simplified from what is presently used. A relationship was derived which predicts that a single fraction of inspired CO₂ (0.0489) will produce near normal alveolar CO₂ over a wide range of voluntary hyperventilation. This relationship was verified in 19 normal subjects who performed a total of 110 periods of hyperventilation with minute ventilation (V̇E) randomly distributed between 40 and 105 L/min. The experimentally determined CO₂ production of the voluntary hyperventilation was found to be 3.72 ml/min per L/min over a range of V̇E from 40 to 105 L/min. Next, a group of 10 patients with exercise-induced asthma (EIA) were challenged with a standard exercise protocol, ventilating ad libitum from a source of dry air at room temperature. On another day, the same pattern of V̇E, and hence RHE, was required of them using the simplified EVH scheme. The average decreases in forced expiratory volume in one second and specific airway conductance were 32 ± 10% and 66 ± 13%, respectively, after the exercise challenge, and 33 ± 12% and 73 ± 12% after EVH. The difference between corresponding mean values was not significant. We conclude that a simplified EVH challenge can be done using a single dry gas mixture without the need for cooling inspired gas or monitoring end-tidal fraction of CO₂. This test can be used to identify and study patients with EIA without the requirement for an exercise challenge or the need for elaborate gas conditioning and monitoring equipment.