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%.