Effects of contraction frequency and duty cycle on diaphragmatic blood flow

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Abstract
The effects of diaphragmatic contraction frequency (no. of intermittent tetanic contractions/min) at a given tension-time index and of duty cycle (contraction time/total cycle time) on diaphragmatic blood flow were measured in anesthetized mongrel dogs during bilateral supramaximal phrenic nerve stimulation. Diaphragmatic blood flow was measured by the radionuclide-labeled microsphere method. Contraction frequency was varied between 10 and 160/min at duty cycles of 0.25 and 0.75. Diaphragmatic blood flow increased with contraction frequency from 1.47 .+-. 0.13 ml/min per g (mean .+-. SE) at an average of 18/min to 2.65 .+-. 0.16 ml/min per g at 74/min (P < 0.01) with a duty cycle of 0.25 and from 1.32 .+-. 0.19 ml/min per g at an average of 15/min to 1.96 .+-. 0.15 ml/min per g at 80/min (P < 0.02) with a duty cycle of 0.75. At higher contraction frequencies diaphragmatic blood flow did not increase further at both duty cycles. Diaphragmatic blood flow was higher with a duty cycle of 0.25 than 0.75 at all contraction frequencies. Evidently, frequency of contraction is a major determinant of diaphragmatic blood flow and high duty cycle impedes diaphragmatic blood flow.

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