Respiratory effects of transient axial acceleration

Whereas gravity has an inspiratory effect in upright subjects, transient up ward acceleration is reported to have an expiratory effect. To explore the respiratory effects of transient axial accelerations, we measured axial acc eleration at the head and transrespiratory pressure or airflow in five subj ects as they were dropped or lifted on a platform. For the first 100 ms, up ward acceleration caused a decrease in mouth pressure and inspiratory flow, and downward acceleration caused the opposite. We also simulated these exp erimental observations by using a computational model of a passive respirat ory system based on anatomical data and normal respiratory characteristics. After 100 ms, respiratory airflow in our subjects became highly variable, no longer varying with acceleration. Electromyograms of thoracic and abdomi nal respiratory muscles showed bursts of activity beginning 40-125 ms after acceleration, suggesting reflex responses responsible for subsequent flow variability. We conclude that, in relaxed subjects, transient upward axial acceleration causes inspiratory airflow and downward acceleration causes ex piratory airflow, but that after similar to 100 ms, reflex activation of re spiratory musculature largely determines airflow.