Development of baroreflex activity in unanesthetized fetal and neonatal lambs

Baroreflex activity was assessed in 10 fetal lambs at 85-145 days of gestation. ECG leads sewn to the fetal chest wall, vinyl catheters in brachial artery and femoral vein, and a balloon catheter in the descending aorta were exteriorized through the ewe's flank. After at least 36 hours, the reflex bradycardia, in response to increased blood pressure by balloon inflation, was measured. Baroreflex sensitivity was expressed as the slope of the beat-to-beat relationship between systolic (S.P.) or pulse (P.P.) pressure and the subsequent R-R interval or instantaneous heart rate (H.R.). Baroreflex activity was considered absent if the slope did not differ significantly from zero or if the correlation coefficient was less than 0.7. Throughout gestation baroreceptors could respond to pressure elevation, but the proportion of positive responses increased with age. Baroreflex sensitivity increased up to term when either S.P. or P.P. were plotted against the next R-R interval. Regression analysis of S.P. or P.P. vs. R-R interval, or of P.P. vs. H.R showed increasing baroreflex sensitivity with maturation Analysis of S.P. vs. H.R. showed no significant increase in response with advancing gestation; however, this type of analysis does not take into account the slower resting heart rates of older animals.