ABSTRACT. In the stage 24 chick embryo, a paced increase in heart rate reduces stroke volume, presumably by rate-dependent decrease in passive filling. We hypothesized that rate-dependent stroke volume reduction could be abolished by volume loading. Dorsal aortic blood velocity was measured with a 20 mHz pulsed-Doppler meter from a 0.75-mm piezoelectric crystal (eight embryos), and atrioventricular velocity was simultaneously measured from the ventricular apex (six embryos). Sinus venosus pacing (stimuli of 1 ms duration and <4 mA) was performed at intrinsic rate ( P I ) and at 150% of intrinsic rate (P:150%I). Volume loading was performed during P:150%I by intravenous injection of 7.5 pL of chick Ringer's solution. Using atrioventricular velocity profile, stroke volume was divided into the proportion due to passive (E-phase) and active (Aphase) filling. Stroke volume was compared during P I , P:150%I, immediately (P:150%If) and 30 s after (P:150%IU) volume loading. Data (mean + SEM) were compared by ANOVA. During pacing, stroke volume (mm2/cycle) decreased but increased after volume loading (I, 0.43 2 0.03; P I , 0.37 k 0.03; P:150%1, 0.19 f 0.03; P:150%If, 0.24 + 0.05; P:150%1", 0.28 f 0.04 (p < 0.005).During P:150%I, E-phase filling disappeared and was not restored by volume loading, whereas, A-phase filling diminished but was restored by volume loading. In stage 24 chick embryos, rate-dependent stroke volume decrease is reversed by volume loading that restores stroke volume due to an increase in active filling but not passive filling. Thus, even at rapid heart rate, the embryonic ventricle responds to volume loading, indicating that the FrankStarling relationship functions during tachycardia in the embryonic heart. (Pediatr Res 26: 438-441,1989) Abbreviations I, intrinsic heart rate P:I, pacing at rate slightly faster than I P:150%I, pacing at 150% of I P:150% If, P:150%1 immediately after volume loading P:150%1", P:150%1 15-30s after volume loading I", P:150%1 after cessation of pacing Received December 28, 1988; accepted June 28, 1989 Heart rate change is integrally involved in cardiovascular development, but little is known regarding adaptation of the developing cardiovascular system to heart rate change. In the stage 24 chick embryo, a paced increase in heart rate resulted in decreased cardiac output and stroke volume, presumably by altering the passive phase of ventricular filling (1).Inasmuch as the Frank-Starling relationship has been shown to function in the developing chick embryo (2), we hypothesized that volume loading would augment cardiac output and stroke volume during pacing-induced tachycardia. We found that in the stage 24 chick embryo, volume loading reversed the pacinginduced reduction in cardiac output and stroke volume. MATERIALS AND METHODSFertile White Leghorn chicken eggs were incubated to Hamburger-Hamilton stage 24 (3). The embryo was exposed by opening the shell and its membranes. Blood flow velocity was measured with a 20 mHz pulsed-Doppler meter from a 0.75-mm piezoelectric cr...

The contributions of the early (passive) and late (active) components of ventricular filling have been reported to decrease and increase, respectively, during chick embryo cardiac development. We hypothesized that the observed changes in ventricular filling during early cardiac development results from a decrease in cycle length. We studied the effect of development and cycle length on atrioventricular inflow in 28 chick embryos, Hamilton-Hamburger stages 17, 24, and 26. Cycle length was perturbed (range 240-1040 ms) in ovo by transiently heating or cooling the sinus venosus. Atrioventricular inflow and dorsal aortic velocities were obtained by 20-MHz pulsed Doppler flowmeter and digitally recorded at 500 samples per second. Stroke volume was calculated from dorsal aortic velocity and crosssectional area. The atrioventricular inflow wave form was inte-The diastolic filling characteristics of the chick embryo ventricle change during developmental stages 12 through 27; the contributions of the early (passive) and late (active) components of ventricular filling have been reported to decrease and increase, respectively (1). Previous studies of altered embryonic heart rate using environmental temperature (I), cardiac pacing (2), and the thermal probe technique (3) have shown that early diastolic filling is heart rate dependent. This study further analyzes the differences in early ventricular filling velocities and flows resulting from heart rate perturbation. We found ventricular filling characteristics to be both developmentally determined and cycle length dependent. METHODSTwenty-eight fertile white Leghorn chicken eggs were incubated blunt end up at 38OC in a forced draft incubator to Hamburger-Hamilton stages 17 (n = 7, 3 d), 23-24 (n = 10, grated and partitioned by area and percentage of total into early (passive) and late (active) components using three methods. Regardless of method, the proportion of filling volume due to the early and late components was cycle length and stage dependent ( p < 0.05). The early and late filling volumes were large in the older embryos, and during cycle length decrease (heart rate increase) the early filling volume decrease was greater than the late filling volume decrease. When compared with the percentage of intrinsic heart rate, the percentage of stroke volume due to early filling decreased as heart rate increased and was greater in younger embryos at all heart rates. That due to late filling increased as the percentage of intrinsic heart rate increased. Ventricular filling characteristics are both developmentally determined and cycle length dependent. (Pediah. Res 37: 289-293, 1995) 4 d), and 25-26 (n = 11, 5 d) (4). Because of the similarity between adjacent stages, stage 23 (n = 1) and stage 24 (n = 9) embryos were consolidated into one group referred to as stage 24 (n = 10). Likewise, stages 25 (n = 2) and stage 26 (n = 9) were combined into stage 26 (n = 11). Eggs were placed in a heated sand bath (Bioengineering Department, Medical College of Georgia, Augusta, GA) under ...

ABSTRACT. We hypothesized that developmental in-(1) have shown that increases in both ventricular EDP and creases in both ventricular end-diastolic pressure (EDP) maximum dP/dt occur concurrently with CL decrease (heart and the maximum time derivative of pressure (dP/dt) ob-rate increase) from d 2 to 6 (Hamburger-Hamilton stages 12 to served in stage 12 to 29 chick embryos are the result of 29) of a 21-d incubation. observed cardiac cycle length (CL) decrease (heart rateWe wondered to what extent the developmental changes in increase). To test this hypothesis, we evaluated EDP and EDP and dP/dt might reflect observed changes in CL. The dP/dt changes that occur during acute CL alterations in purpose of this study, therefore, is to examine whether the the Hamburger-Hamilton stage 24 chick embryo (n = 18). observed increases in both EDP and dP/dt are a function of the Ventricular pressure measurements were obtained with a observed decrease in CL. We evaluated EDP and maximum dP/ servo-null pressure system and digitally recorded at 500 dt during acute CL alterations in the stage 24 (d 4) chick embryo. samples/s. A 1-mm steel probe, heated (decrease CL) or By studying these relationships in a single stage of development, cooled (increase CL), was applied to the sinus venosus. the effect of CL alteration can be assessed independently of other The average baseline CL was 454 ms. The heart rate effects secondary to changes in cardiac morphology. perturbation resulted in CL that varied over a range of 200-2966 ms, assimilating the range of CL change observed during development. Changes in EDP ranged from 0.014 to 0.130 kPa (baseline = 0.061 kPa) and maximum MATERIALS AND METHODS dP/dt ranged from 0.33 to 13.33 kPa/s (baseline = 5.99) Fertile White Leghorn chicken eggs were incubated to HamkPa/s). In each study, EDP varied directly with CL (R2 = burger-Hamilton stage 24 (4 d) at 38°C (2). An egg was removed 0.70). Conversely, maximum dP/dt changes were inversely from the incubator and placed in a heated sand bath (Bioengirelated to CL alterations (R2 = 0.54). Thus, we found that neering Department, Medical College of Georgia) under a radiant there is a direct relationship between changes in CL and warmer to maintain a constant egg temperature of 37-38°C EDP in the stage 24 chick embryo, whereas CL and dP/dt throughout the experiment. The embryo was exposed by removvary inversely. During cardiac development, observed in-ing a portion of the shell and the overlying membranes. The CL creases in maximum dP/dt may be attributed to CL de-was altered using a 1-mm stainless steel probe that was first creases. In contrast, developmental increases in EDP can-heated or cooled in a water bath. The probe was then placed on not be explained by CL decrease and must be accounted the sinus venosus for 10 s to shorten or lengthen the CL. An for by maturational changes in cardiac function in the chick additional 10 control studies were performed using a probe at embryo. (Pediatr Res 29: 338-341, 1991) 37-38°C to exclude pressure of the pr...