EFFECTS OF LUNG-VOLUME AND FATIGUE ON EVOKED DIAPHRAGMATIC PHONOMYOGRAM IN NORMAL SUBJECTS

Background - A diaphragmatic phonomyogram (PMG) evoked by maximal phre nic nerve stimulation at end expiratory lung volume (FRC) has been pre viously described as a good index of changes in diaphragmatic contract ility with fatigue. A study was undertaken to assess whether this conc lusion could be extended to different lung volumes. Methods - Diaphrag matic compound motor action potentials (CMAPs) were recorded on each s ide of the chest by the means of surface electrodes placed over the ei ght intercostal spaces in five healthy subjects. Diaphragmatic PMGs fr om both sides were recorded with condenser microphones fixed to the sk in close to the CMAP recording electrodes. Oesophageal and gastric bal loon tipped catheters were employed to measure transdiaphragmatic pres sure twitches (TwPDI) which served as the standard measure of changes in diaphragmatic contractility. PMG and TwPDI responses were compared at different lung volumes over inspiratory capacity both before and af ter fatiguing inspiratory resistive loading. Results - No consistent r elationship was found in different subjects or on different days in th e same subject between PMG and lung volume or between PMG and TwPDI. H owever, the PMG:CMAP ratio from both sides at any given lung volume de creased after fatigue in roughly the same proportion as the TwPDI. Con clusions - These results show that, although PMG can detect changes in diaphragmatic contractility caused by fatigue in normal subjects, lun g volume changes need to be controlled and each subject should serve a s his or her own control.