Interaction between transient metabolically mediated dilatation and pressure induced constriction in the canine...

This study explored the interaction between metabolically mediated vasodilatation (ventricular extra-activation) and pressure induced vasoconstriction (transient augmentation of aortic diastolic pressure). Eight dogs having formalin-induced heart block were chronically instrumented with aortic and left ventricular catheters and an electromagnetic flow probe on the left circumflex coronary artery. At a heart rate of 60 beats/min a single ventricular extra-activation introduced at 200 ms after the normal paced beat resulted in a 13 +/- 1% decrease in diastolic coronary vascular resistance index (DCVRI) in the first response beat (D1) and a persistent vasodilatation lasting for five beats (D1-D5). An increase in aortic diastolic pressure (32 +/- 3% for 520 +/- 15 ms) resulted in 13 +/- 2% increase in DCVRI in the D1 which was not evident in subsequent beats. Following a combined intervention, DCVRI in D1 was not significantly different from control but DCVRI did decrease to a greater degree in the subsequent response beats (D2-D7). These data indicate that the responses of two opposing vasoactive stimuli, i.e., pressure induced vasoconstriction and metabolic vasodilatation, were negated in the first response beat. The metabolically mediated vasodilatation was unaltered in the subsequent response beats.