Respiratory muscle length and strength in patients with chronic abdominal distension

Starling's law (the energy of muscle contraction is proportional to the initial fiber length) has been applied to contraction of inspiratory muscles. Its application to the expiratory muscles is difficult because both maximal length and maximal pressure development occur at total lung capacity (TLC). We hypothesize that decrease of both inspiratory (Pimax) and expiratory (Pemax) muscle strength in chronic ascites (CA) will reflect generalized muscle weakness and stretching of both the diaphragm and abdominal wall as well. To test this hypothesis, we evaluated Pimax and diaphragm length (at functional residual capacity) in 22 patients. Pemax, external oblique and transversus abdominus muscle lengths, and anterior abdominal wall muscle thickness were measured at TLC. We found Pimax (78 +/- 19% predicted), Pemax (61 +/- 17%), and--as an index of general muscle strength--handgrip strength (75 +/- 22%) all to be minimally reduced. Respiratory muscle strength did not correlate with any measurement of inspiratory/expiratory muscle length or thickness. With fluid removal, abdominal muscles shortened; diaphragmatic curvature decreased although diaphragm length was unchanged. Nevertheless, neither Pimax nor Pemax increased. Respiratory muscle strength depends upon generalized muscle strength more so than upon muscle length in CA patients.