Habituation to prolonged airflow obstruction.

The hypothesis that absence of dyspnea during airflow obstruction is caused by habituation to sensory information was tested. Adolescents without respiratory disease breathed through a mouthpiece interrupting airflow analogous to approximately 64% reduction in forced expiratory volume in 1 sec. They were randomly assigned to labored breathing for 1 min, 30 min, or twice 30 min (with a 30-min interval in between). Dyspnea was measured pretest without mouthpiece and posttest with mouthpiece in. Results showed no significant difference in posttest dyspnea between conditions. Dyspnea remained remarkably mild. Severe secondary symptoms (excessive saliva) during the experiment made habituation unlikely and the results were interpreted in favor of the alternative hypothesis: lack of or mild dyspnea is associated with a low-anxiety situation without connotation of suffocation, irrespective of level of airflow obstruction.