The interaction between the diaphragm, intercostal/accessory muscles of inspiration and the rib cage

During Mueller maneuvers (MM), the volume change of rib cage, delta Vrc, and abdomen, delta Vab, are equal and opposite. Thus delta Vrc = -delta Vab. Substituting delta Prc.Crc for delta Vrc and delta Pab.Cab for delta Vab yields: delta Prc = - delta Pab.Cab/Crc, where delta Prc, delta Pab, Crc and Cab are applied pressures and compliances of rib cage and abdomen respectively. MM performed solely with the diaphragm permits calculations of Prc in terms of observed changes in Pab and pleural pressure, Ppl. Three trained subjects performed MM with no evidence of inspiratory intercostal or abdominal muscle contraction. During the diaphragmatic MM delta Pab was positive and delta Prc negative. The magnitude of delta Prc/delta Pab was 2-6 times greater than that of delta Ppl/delta Pab. We conclude that neither Pab nor Ppl by themselves displace the relaxed rib cage during Mueller maneuvers. A model in which the diaphragm acts both in parallel and in series with the rib cage, and in which Prc is the sum of Pab and a pressure lying between Pab and Ppl explains these results as well as the hypothesis that Pab displaces the relaxed rib cage during quiet breathing.