EFFECTS OF MUSCLE-TO-ELECTRODE DISTANCE ON THE HUMAN DIAPHRAGM ELECTROMYOGRAM

It has been suggested that esophageal recordings of the diaphragm elec tromyogram (EMGdi) are influenced by changes in chest wall configurati on. Whether the changes are of physiological or artifactual origin is unclear. For example, the distance between the esophageal electrode an d the diaphragm is likely to alter with chest wall configuration and m ay lead to misinterpretations of EMGdi. The aims of this study were 1) to evaluate and quantify the effect of the muscle-to-electrode (ME) d istance filter on EMGdi, as obtained with a multiple-array esophageal electrode, 2) to take advantage of the ME distance filter to locate th e position of the diaphragm with respect to the electrode, and 3) to e valuate the influence of lung volume and chest wall configuration on E MGdi center frequency (CF) while controlling for the ME distance filte r and signal quality. Five subjects performed six static contractions of the diaphragm at each of seven chest wall configurations, as evalua ted by the method of K. Konno and J. Mead (J. Appl. Physiol. 22: 407-4 22, 1967). EMGdi was measured with seven pairs of electrodes mounted o n an esophageal catheter. The pair of electrodes whose EMGdi power spe ctra were the least filtered by the ME distance was assumed to be clos est to the diaphragm. The results of the study indicated that 1) EMGdi power spectra were strongly affected by the distance between the diap hragm and the electrodes. CF decreased by similar to 1 Hz/mm displacem ent away from the electrode pair closest to the diaphragm; and 2) no s ystematic relationship was found between changes in chest wall configu ration and CF, when CF was measured from the electrode pair closest to the diaphragm. We conclude that the EMGdi CF can be reliably measured with a multiple-array esophageal electrode that covers the span of di aphragmatic excursion and by selecting the pair of electrodes that is the closest to the diaphragm.