Effect of abdominal compression on maximum transdiaphragmatic pressure

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Abstract
Transdiaphragmatic pressure (Pdi) is lower during maximum inspiratory effort with the diaphragm alone than when maximum inspiratory and expulsive efforts are combined. The increase in Pdi with expulsive effort has been attributed to increased neural activation of the diaphragm. Alternatively, the increase could be due to stretching of the contracted diaphragm. If this were so, Pdi measured during a combined maximum effort would overestimate the capacity of the diaphragm to generate inspiratory force. This study determined the likely contribution of stretching of the contracted diaphragm to estimates of maximum Pdi (Pdimax) obtained during combined inspiratory and expulsive effort. Three healthy trained subjects were studied standing. Diaphragmatic Mueller maneuvers were performed at functional residual capacity and sustained during subsequent abdominal compression by either abdominal muscle expulsive effort or externally applied pressure. Measurements were made of changes in abdominal (Pab) and pleural (Ppl) pressure, Pdi, rib cage and abdominal dimensions and respiratory electromyograms. Three reproducible performances of each maneuver from each subject were analyzed. When expulsive effort was added to maximum diaphragmatic inspiratory effort, Pdimax increased from 86 .+-. 12 to 148 .+-. 14 (SD) cmH2O within the 1st s and was 128 .+-. 14 cmH2O 2 s later. When external compression was added to maximum diaphragmatic inspiratory effort, Pdimax increased from 87 .+-. 16 to 171 .+-. 19 cmH2O within the 1st s and was 152 .+-. 16 cmH2O 2 s later. In each subject abdominal compression increased Pdi by increasing pab (after a small and transient decrease, Ppl became less negative, representing a decrease in inspiratory force despite the large increase in Pdi) and decreased abdominal and increased rib cage dimensions, suggesting diaphragmatic lengthening. The changes in Pdimax were closely related to the degree of abdominal compression. We conclude that when maximum expulsive effort is superimposed on maximum diaphragmatic inspiratory effort the diaphragm is stretched with a resultant increase in its tension and Pdimax overestimates the capacity of the diaphragm to generate inspiratory force when measured in the presence of added abdominal expulsive effort.

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