Measurement of diaphragm myoelectric activity in humans

This thesis is about the development and evaluation of a standardized method to measure the crural diaphragm electromyogram (EMG) in humans, with an esophageal electrode. In order for the diaphragm EMG to be physiologically relevant, its measurement and analysis require objective control of the disturbances and filter effects which can influence the sigma. One issue of importance is the maintenance of diaphragm-to-electrode positioning throughout the inspiration. In the present work, we describe a cross-correlation algorithm by which the position of the diaphragm along a multiple army esophageal electrode can be determined at any instant during a breath, and a second algorithm, the "double subtraction technique", which further minimizes the detrimental effects of diaphragm movement. The double subtraction technique also results in the improvement of the diaphragm EMG signal to noise ratio by 2 dB. By implementing these algorithms, we demonstrate in healthy subjects that there is no artifactual influence of lung volume/chest wall configuration on the diaphragm EMG frequency content nor on the diaphragm EMG signal strength during voluntary isometric contractions of the diaphragm. This is in contrast to the electrically elicited diaphragm compound muscle action potentials which are severely influenced by changes in lung volume. We also show that the volume-activation relationship of the diaphragm (required change in activation for changes in lung volume at a given tension) is directly related to the length-tension properties of the muscle. During dynamic, voluntary breathing maneuvers, we could find no evidence for an increase in diaphragm EMG signal strength when inspirations from functional residual capacity to total lung capacity (TLC) were performed at increasing inspiratory flow rates (velocities of shortening) up to 1.4 l/s. To account for anatomical and physiological differences between subjects, we demonstrate that the diaphragm EMG signal strength can be n