Effects of Fentanyl on Baroreceptor Reflex Control of Heart Rate in Newborn Infants

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Abstract
Baroreceptor reflex control of heart rate was studied in ten neonates and young infants before and after intravenous fentanyl (10 .mu.g/kg). All infants were in stable condition while being mechanically ventilated. Mean (.+-. SD) corrected gestational age was 40.1 .+-. 3.7 weeks, mean weight 3120 .+-. 700 g. The pressor response was tested using phenylephrine and the depressor response using nitroglycerin. Changes in heart rate (R-R interval) were plotted against changes in systolic arterial pressure, and the slope of the linear portion of this relationship expresses the baroreflex sensitivity. No significant changes in systolic arterial pressure, heart rate, and blood gas values were observed after fentanyl injection when compared to control values. Mean (.+-. SEM) control phenylephrine slope was 8.44 .+-. 2.05 msec/mmHg, and mean nitroglycerin slope was 2.54 .+-. 0.37 msec/mmHg. Both slopes decreased significantly by 48% and 42%, respectively, after fentanyl injection (P < 0.02). Mean plasma fentanyl concentrations measured at the end of each test were not statistically different (5.11 .+-. 0.65 ng/ml and 4.28 .+-. 0.58 ng/ml, respectively). This suggests that the baroreflex control of heart rate is present in term neonates and markedly depressed during fentanyl anesthesia. Changes in blood pressure occurring during fentanyl anesthesia have to be carefully considered, because cardiac output is principally rate-dependent in newborns.