TONIC INSPIRATORY MUSCLE-ACTIVITY AS A CAUSE OF HYPER-INFLATION IN ASTHMA

Tonic activity of the inspiratory muscles during exacerbation of asthma was studied in 5 female and 2 male patients. Exacerbation was provoked by withholding bronchodilatory medication for 12 h prior to the study. Thoracic gas volume (TGV) at the end of resting expiration was determined before and after albuterol (salbutamol) inhalation with a body plethysmograph. Intercostal muscle electromyogram (EMG) was recorded with surface electrodes and diaphragmatic EMG with esophageal electrodes. Tonic activity was defined as electrical activity in the EMG present throughout expiration. After salbutamol the TGV decreased 13.4 .+-. 2.9% (mean .+-. SE) (P < 0.01). This decrease in TGV was accompanied by a proportional reduction in tonic intercostal (r = 0.78, P < 0.05) and diaphragmatic activity (r = 0.84, P < 0.05). The hyperinflation present during exacerbation of asthma is partially due to active inspiratory muscle activity present throughout expiration.