Influence of bipolar esophageal electrode positioning on measurements of human crural diaphragm electromyogram.

We aimed to describe how the human diaphragm electromyogram (EMGdi) is filtered at different positions within the esophageal hiatus, i.e., in the close proximity of the diaphragm, when obtained with an array of bipolar electrode pairs mounted on an esophageal catheter. We defined the electrically active crural diaphragm musculature that covers the esophagus as the electrically active region of the diaphragm (EARdi) and its center as the EARdi center. EMGdi signals were obtained via a multiple-array esophageal catheter consisting of seven sequential electrode pairs with three different electrode configurations. Subjects (n = 5) performed voluntary contractions of the diaphragm at functional residual capacity. Visual inspection of the signals revealed reversal of signal polarity on either side of the EARdi center. Extreme correlation values (r values close to -1 at 0-ms time offset) were observed for the correlation of signals on either side of the EARdi center. The root mean square (RMS) was reduced at the EARdi center; moving peripherally from the EARdi center (caudally and cephalad), the RMS increased to a peak (range of 2.1-4.1 dB for the different electrode configurations) and then decreased for the most peripheral electrode pairs. From a position where the RMS values peaked, center frequency values increased at the EARdi center (range of 26-29 Hz for the different electrode configurations). Computer simulation yielded similar data to the experimental results. We conclude that electrode positioning within the EARdi severely influences center frequency and RMS values and that the center of the EARdi can be identified via cross-correlation analysis.