Pattern of diaphragmatic activity during forced expiratory vital capacity

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Abstract
Transdiaphragmatic pressure (Pdi) was measured during forced expiratory vital capacity (FVC) maneuvers in 13 normal subjects and electromyographic activity of the diaphragm (Edi) in 8 of these subjects. In all subjects, Pdi increased at the initiation of the FVC. In most, this increase lasted 30-50 ms and reached levels well above the Pdi observed at total lung capacity (TLC). After the initial transient increase, approximately half of the subjects demonstrated a substantial fall in Pdi to values near the relaxation level in the mid-vital capacity (VC) volume range, while half showed a 2nd large increase in Pdi in this volume range. Of 8 subjects tested 7 showed a rapid increase in Edi at the onset of the FVC, reaching a minimum in 30-50 ms. After this initial transient decrease, Edi increased in 6 subjects in the mid-VC volume range, in association with secondary rises in Pdi. In 2 subjects, Edi remained low throughout the remainder of the FVC, and Pdi in the mid VC range was generally lower. Results are consistent with the conclusion that the diaphragm is neither electromyographically silent nor mechanically unimportant during the FVC. Changes in abdominothoracic configuration, superimposed upon antagonistic activity of the diaphragm, result in substantial reductions in pleural (esophageal) pressure that may influence reginal lung empyting during the FVC.

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