Effect of heart rate increase on dorsal aortic flow in the stage 24 chick embryo

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, <4 mA). The sinus venosus was paced from bipolar Teflon-coated silver electrodes in eight embryos and the ventricular apex was paced in three embryos. The pacing rates were at the intrinsic heart rate (P:I); 125% of intrinsic heart rate (P:125%I); and 150% of intrinsic heart rate (P:150%I). Physiologic measurements during pacing were compared to those obtained at the control intrinsic rate (I). We also evaluated the velocity profile of atrioventricular inflow and conotruncal outflow at intrinsic rate and during sinus venosus and ventricular pacing. With sinus venosus pacing, mean dorsal aortic blood flow was similar at control (1.07 ± 0.05 mm3/s) and P:l (1.06 ± 0.06 mm3/s) (x ± SEM). However, at P:125%I and P:150%I, mean dorsal aortic blood flow decreased significantly (P:125%1,0.88 ± 0.05 mm3/s; P:150%I, 0.67 ± 0.07 mm3/s) (p < 0.05). Stroke volume per beat also decreased with increasing heart rates (I, 0.41 ± 0.02 mm3; P:I, 0.39 ± 0.02 mm3; P:125%I, 0.28 ± 0.02 mm3; P:150%I, 0.18 ± 0.02 mm3) {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.