Effect of tension and timing of contraction on the blood flow of the diaphragm.

An open-chest animal model was developed to study the diaphragmatic blood flow (Qdi) during bilateral electrophrenic stimulation. Two patterns of stimulation were used, continuous and intermittent, and both patterns were held at various transdiaphragmatic pressures (Pdi). The contractions were nearly isometric and held at an initial length of supine functional residual capacity (FRC). Qdi was measured in six dogs by catheterizing a branch of the diaphragmatic vein and by counting the blood drops with an infrared cell. During continuous contractions Qdi increased as a function of Pdi up to 70 +/- 12 ml . 100 g-1 . min-1 at 20% Pdimax. At higher levels Qdi decreased progressively and approached zero at 75% of Pdimax. A postcontraction hyperemia occurred at Pdi values greater than 20% Pdimax and increased as a function of Pdi. During intermittent contractions Qdi was a unique function of the diaphragmatic tension-time index (TTdi), a product of Pdi times the duty cycle (contraction time/total cycle time). Qdi increased progressively up to a TTdi of 25% Pdimax and decreased above that point toward zero at TTdi of 80% Pdimax. The postcontraction hyperemia appeared at a TTdi of about 15% of Pdimax and increased as a function of TTdi. It is concluded that Qdi is limited beyond a TTdi of about 20% Pdimax, as indicated by the increase in postcontraction hyperamia, and that Qdi is a function of both Pdi and the timing of contraction.