We evaluated the effect of increased heart rate on cardiac output and stroke volume in the stage 24 chick embryo (day 4 of a 21-day incubation). Blood flow was measured with a 20 MHz pulsed-Doppler flowmeter. Heart rate was increased by pacing with square wave stimuli (1 ms duration, 3/s; P:150%I, 0.67 ± 0.07 mm3/s) (p p < 0.05). With rapid sinus venosus pacing, the atrioventricular blood flow velocity profile showed a rate-dependent decrease in passive ventricular filling while active filling remained the same or increased slightly. Thus, rate-dependent passive ventricular filling may be one reason for relatively slow heart rates during early embryonic development. During ventricular pacing at the intrinsic heart rate, mean dorsal aortic blood flow decreased to near zero presumably secondary to loss of normal atrioventricular synchrony. We speculate that atrial or ventricular tachycardia would be lethal to an embryo.
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