Enhancement of signal quality in esophageal recordings of diaphragm EMG.

The crural diaphragm electromyogram (EMGdi) is recorded from a sheet of muscle, the fiber direction of which is mostly perpendicular to an esophageal bipolar electrode. The region from which the action potentials are elicited, the electrically active region of the diaphragm (EAR(di)) and the center of this region (EAR(di ctr)) may vary during voluntary contractions in terms of their position with respect to an esophageal electrode. Depending on the bipolar electrode's position with respect to the EAR(di ctr), the EMGdi is filtered to different degrees. The objectives of the present study were to reduce these filtering effects on the EMGdi by developing an analysis algorithm referred to as the "double-subtraction technique." The results showed that changes in the position of the EAR(di ctr) by +/- 5 mm with respect to the electrode pairs located 10 mm caudal and 10 mm cephalad provided a systematic variation in the EMG power spectrum center-frequency values by +/- 10%. The double-subtraction technique reduced the influence of movement of the EAR(di ctr) relative to the electrode array on EMG power spectrum center frequency and root mean square values, increased the signal-to-noise ratio by 2 dB, and increased the number of EMG samples that were accepted by the signal quality indexes by 50%.

Influence of the bipolar electrode transfer function on the electromyogram power spectrum.

This study demonstrates the impact of the bipolar electrode transfer function on the canine diaphragm electromyogram (EMG) power spectrum, as evaluated with a new electrode design and implantation technique. The results show that: (a) changes in interelectrode distance transformed single-peaked power spectrums into double-peaked spectrums; (b) the mean action potential conduction velocity (APCV), and power spectrum center frequency (CF) and median frequency (MF), are related for interelectrode distances of 5 and 10 mm, but not for 15 and 20 mm; and (C) CF, MF, -3-dB, and -6-dB bandwith values depend on interelectrode distance. We conclude that bipolar electrodes, with a nonfixed interelectrode distance, cannot be used for physiological interpretations of the EMG power spectrum. Nonetheless, power spectrums obtained with fixed and appropriate interelectrode distances can be trusted, if the electrodes are positioned in the direction of the muscle fibers and in regions with low densities of motor endplates.