Respiratory Pressure Partitioning during Quiet Inspiration in Unilateral and Bilateral Diaphragmatic Weakness

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Abstract
To compensate for diaphragmatic weakness, intercostal/accessory muscles may be recruited in inspiration and/or abdominal muscles in expiration with relaxation during subsequent inspiration. As a consequence, for a given decrease in pleural pressure (Ppl) during quiet inspiration (qi), abdominal pressure (Pab) should either undergo a smaller increase than normal or, in severe cases, decrease. If so, the ratio of change in Pab to Ppl during qi (.DELTA.Pab/.DELTA.pl(qi)), which is normally < -1 when upright, should increase, approaching +1 in profound diaphgragmatic weakness. To examine the relationship between degree of diaphragmatic weakness and .DELTA.Pab/.DELTA.Ppl(qi), we measured (erect and supine) anteroposterior rib cage and abdominal motion, Pab, Ppl, and transdiaphragmatic pressure (Pdi) during qi, maximal inspiration (Pdi(max)mi) and maximal inspiratory effect at FRC (Pdi(max)FRC) in 10 patients with bilateral and 8 with unilateral diagphragmatic weakness. Pdi(max)mi and Pdi(max)FRC were low in all patients. .DELTA.Pab/.DELTA.Ppl(qi) (erect) was increased in all patients (0.28 .+-. 0.7; mean .+-. SD) and correlated closely with both Pdi(max)mi (r = -0.89, p < 0.001) and Pdi(max)FRC (r = -0.76, p < 0.001). There was extensive overlap in the data between unilateral and bilateral diaphragmatic weakness. The ratio of .DELTA.Pdi during qi to Pdi(max)FRC was less than 0.31 in all patients. The results suggest that .DELTA.Pab/.DELTA.Ppl(qi) is a useful index of the degree of diaphragmatic weakness and that the functional consequences of unilateral and bilateral weakness are not rigidly separable. The finding that the ratio of .DELTA.Pdi(qi) to Pdi(max)FRC was always less than the threshold for fatigue (0.4) suggests that recruitment of intercostal/accessory and abdominal muscles allows ventilation to be maintained without risk of development of diaphragmatic fatigue.