Diaphragmatic blood flow in the dog

In anesthetized mongrel dogs we measured the blood flow in the left phrenic artery (Q̇di), using an electromagnetic flow probe, before and during supramaximal phrenic nerve stimulation (pacing). This was done at constant respiratory rate (24/min) but at three different stimulation frequencies at a duty cycle of 0.4 (20, 50, and 100 Hz) and at three different duty cycles at a stimulation frequency of 50 Hz (duty cycle = 0.2, 0.4, and 0.8). Q̇di was unchanged during diaphragm contraction until transdiaphragmatic pressure (Pdi) was greater than ~11 cmH₂O, whereafter it began to decrease, reaching zero at Pdi ~20 cmH₂O. Thus, when Pdi was >21 cmH₂O, all flow occurred during relaxation. Q̇di averaged over the entire respiratory cycle (Q̇t) was less at duty cycle = 0.8 than under the other conditions. This was because of decreasing length of relaxation phase rather than a difference of relaxation phase flow (Q̇r), which was maximal during all conditions of phrenic stimulation. During pacing-induced fatigue, Q̇t actually rose slightly as Pdi fell. This was due to an increase in contraction phase flow while Q̇r remained constant. The relationship between Q̇t and tension-time index was not unique but varied according to the different combinations of duty cycle and stimulus frequency.