Vascular volume expansion and thermally induced asthma

To determine whether a relationship exists between intravenous infusion of isotonic fluid and reactivity to hyperpnoea, eight normal and eight asthmatic subjects underwent rapid intravascular administration of approximately 2 l of warm normal saline, by itself and before and after hyperventilation of cold air. In the asthmatic subjects, saline infusion mirrored the obstruction seen with hyperventilation; whereas, in normal subjects saline produced more bronchial narrowing than hyperventilation. When the two stimuli were given together, the timing of the infusion altered the asthmatic subjects' responses. Giving fluid early in the hyperventilation challenge blunted obstruction, whereas giving it later amplified the airway narrowing. Similar findings, but on a smaller scale, occurred in normal subjects. These data demonstrate that sudden elevations in intrathoracic vascular volume with warm saline produce airway obstruction that is quantitatively similar to that seen with hyperventilation in asthmatic individuals. They also demonstrate that these two stimuli interact together in such a manner that a common mechanism may exist to account for the decrease in airflow.