BASIC SCIENCE SECTION Autonomic nervous system regulation of baseline heart rate in the fetal lamb Akihiko Wakatsuki, MD, Yuji Murata, MD, Yuko Ninomiya, MD, Naoki Masaoka, MD, James G. Tyner, MD, and Krishna K. Kutty Orange, California OBJECTIVE: We examined 29 chronically instrumented fetal lambs from 125 to 143 days' gestation to investigate the effects of fetal behavioral states and autonomic nervous system maturation on baseline fetal heart rate. STUDY DESIGN: Behavioral states were defined from electrocorticographic analysis as low-voltage fast activity or high-voltage slow activity. Decrease and increase in baseline fetal heart rate subsequent to administration of propranolol and methylatropine represented l3-sympathetic and parasympathetic activity. RESULTS: Baseline fetal heart rate decreased with gestation in both states, with steeper regression in low-voltage fast activity (p < 0.001). Positive correlation was noted between gestational age and percent decrease baseline fetal heart rate in both states with steeper regression in high-voltage slow activity (p < 0.001), and between gestational age and percent increase baseline fetal heart rate with steeper regression in low-voltage fast activity (p < 0.001). Fetal heart rate l3-sympathetic and parasympathetic tones increased with age in both states, with elevation of l3-sympathetic tone in high-voltage slow activity and parasympathetic tone in low-voltage fast activity. CONCLUSION: Sympathetic and parasympathetic systems influence baseline fetal heart rate in these behavioral states and with age. (AM J OBSTET GYNECOL 1992;167:519-23.)

Key words: Baseline fetal heart rate, behavioral state, gestational age

~-sympathetic tone,

Baseline fetal heart rate (FHR) declines with gestational age in the human fetus':" and fetal lamb," Schifferli and Caldeyro-Barcia' attribute this decline to maturing vagal parasympathetic functional control. Others report that baseline FHR is consistently higher in the high-voltage slow activity behavioral state than in the low-voltage fast activity state."? Using a-sympathetic and ~-sympathetic blockades, Zhu and Szeto" concluded that the changes in baseline FHR relative to behavioral states were determined uniquely by sympathetic activity. B The regulation mechanisms of these fetal cardiovascular changes, especially the interaction of fetal behavioral states and autonomic nervous maturation on baseline FHR, remains unidentified. Our study investigated these control mechanisms at different gestational ages and behavioral states.

From the Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, University of California, Irvine. Received for publication July 9, 1991; revised March 9, 1992; accepted March 18, 1992. Reprint requests: Yuji Murata, MD, Department of Obstetrics and Gynecology, University of California, Irvine, UCI Medical Center, P.O. Box 14091, Orange, CA 92613-1491.

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parasympathetic tone, fetal lamb,

Material and methods Surgical preparation. Twenty-nine chronically instrumented fetal lambs between 125 and 143 days' gestation from time-mated, cross-bred Columbia-Rambouillet ewes were used in this study. Sheep gestation averages 145 days. All animals were maintained in a facility approved by the American Association for the Accreditation of Laboratory Animal Care and in accordance with Guide for the Care and Use of Laboratory Animals and met the standards of care of the U.S. Department of Agriculture. Before implementation research protocols were approved by the Animal Research Committee at the University of California, Irvine. After 24 hours of food and water withdrawal anesthesia was induced with xylazine (Rompun) 12 to 14 mg administered intramuscularly. Lidocaine 1% was used locally as needed. The uterus was exposed with a midline abdominal incision. The fetal head was delivered through a uterine myometrial incision and covered with a surgical glove filled with warm saline solution. Bilateral electrodes were sutured under the skin in the fetal chest wall for electrocardiography. Paired stainless steel electrodes (ethylcyanoacrilate, Loctite

519

520 Wakatsuki at al.

Corp., Newington, Conn.) were placed and cemented bilaterally in the fetal parietal skull for e1ectrocorticography. The instantaneous FHR was obtained from the R-R interval of the electrocardiogram. Polyvinyl catheters were inserted into the fetal carotid artery,jugular vein, and trachea. The procedures were performed carefully to avoid disturbing the fetal vagal nerve. An open-end catheter was retained in the amniotic cavity, and additional catheters were placed in the maternal femoral artery and vein. All electrodes and catheters were exteriorized to the flank of the ewe. Experimental protocol. The animals were maintained in an air-conditioned room and given free access to food and water. For 3 to 4 days after surgery the mother and fetus received ampicillin (200 mg/kg/ day intravenously) and gentamicin (6 mg /kg/day intravenously), and ampicillin (1 gm/day) was administered to the amniotic cavity. Maternal and fetal arterial pH, blood gases, and hematocrits were checked daily. For pH and gas determinations approximately 0.6 ml arterial blood was collected anaerobically and measured (Corning pH/Blood Gas Analyzer, Model 168 BGA, Ciba Corning Diagnostic Corp., Medfield, Mass.) at 37° C. Fetal well-being was determined by the following criteria: pH > 7.30. Po. > 16 mm Hg, hematocrit >30%. Fetal arterial blood pressure, FHR, electrocorticogram, tracheal pressure, intrauterine pressure, and maternal blood pressure were continuously monitored on an 8channel rectilinear recorder (Beckman Model R612, Sensor Medics Corp. , Anaheim, Calif.). Animals that demonstrated interpretable electrocorticogram were selected for this study. Fetal behavioral states were analyzed from the electrocorticogram for low-voltage fast activity and high-voltage slow activity. Experiments were performed after allowing ~5 days of postoperative recovery. All experiments were carried out between 9 AM and 5 PM with the ewe standing or lying quietly. In 10 fetal lambs between 125 and 143 da ys' gestation baseline FHR was assessed under normal physiologic conditions in the low-voltage fast activit y and high-voltage slow activity behavioral states. Baseline FHR was measured after FHR was stable for at least 10 minutes in which uterine contractions were absent. A set of observations during adjacent low-voltage fast activity and high-voltage slow activit y was obtained every day after fetal well-being was confirmed. The average duration of observation per fetus was 6.3 da ys and the average number of observations per animal was 12.6. In 10 fetal lambs between 126 and 143 days ' gestation saline solution was infused via the fetal jugular vein at 4.0 ml per hour for a 2-hour control period. The fetus then received an intravenous priming dose of propranolol (1.0 mg, 1 ml) followed by propranolol infusion (2.0 mg/hr, 4.0 ml/hr) for 2 hours to block ~­ sympathetic control of the heart rate. Baseline FHR

August 1992 Am J Obstet Gynecol

was obtained in low-voltage fast activit y and high-voltage slow activity states during control and propranolol periods. The percent decrease in baseline FHR between control and propranolol periods was calculated as an index of FHR ~-sympathetictone. Completeness of the ~-blockade was indicated by the absence of cardiac acceleration response to an injected ~-agonist (isoproterenol 2.0 IJ.g/kg estimated fetal weight) . The experiment was repeated on the same fetuses in 2 to 5 da ys, confirming complete recovery from the blockade. The average number of experiments per fetus was 4.5. In nine fetal lambs between 127 and 143 days' gestation a 2-hour control period was followed by methylatropine (200 IJ.g/kg estimated fetal weight intravenously) for blockage of the parasympathetic control of the heart rate. The methylatropine dose was determined in preliminary experiments. Baseline FHR was measured in the low-voltage fast activit y and high-voltage slow activit y states during the control period and subsequent to methylatropine administration. The percent increase of baseline FHR from control to methylatropine periods was taken as representing FHR parasympathetic tone. Completeness of methylatropine blockade was evidenced by the absence of deceleration response to norepinephrine (4 IJ.g/kg estimated fetal weight). The effect of the blocking dose in our study lasted for ~3 hours. The experiment was repeated on the same fetuses in 2 to 5 day s, confirming complete recovery from the blockade. The average number of experiments per fetus was 4.5 . Data analysis. Regression lines were determined by the least squares method. The regression slopes were compared by t test regression analysis . to Statistical significance was accepted at p < 0.05. Results

Figure I shows the normal physiologic relationship of fetal gestational age and baseline FHR in low-voltage fast activity (LVFA) and high-voltage slow activity (HVSA). Baseline FHR in both behavioral states decreased with gestational age . The regression equations for baseline FHR (BFHR) versus gestational age (GA) are as follows: LVFA: HVSA:

BFHR

= -4.47(GA) + 745.97

n = 126, P < 0.001) BFHR = - 2.44(GA) + 488.84 n = 126, P < 0.001)

(r

= 0.79 ,

(r

= 0.47,

The low-voltage fast activity slope (- 4.4 7) was significantly less (p < 0.001) than that of the high-voltage slow activity slope (- 2.44). Figure 2 shows the influence of gestational age on baseline FHR response to [3-sympathetic blockade with propranolol infusion in the low-voltage fast activity and high-voltage slow activity states. A positive correlation was found between gestational age and percent de-

Baseline heart rate in fetal lamb 521

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250

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130

140

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Gestational

Age

145

(Days)

Fig. 1. Relationship between baseline FHR (BFHR) and gestational age in low-voltage fast activity (LVFA) and high-voltage slow activity (HVSA) states.

crease (% dec) in baseline FHR in both states. The regression equations follow: 0.54(GA) - 62.86 (r = 0.50, P < 0.001) HVSA: % dec BFHR = 0.97(GA) - 115.42 (r = 0.65, n = 45, P < 0.001)

LVFA: % dec BFHR

=

n = 45,

The high-voltage slow activity slope (0.97) was significantly steeper than the low-voltage fast activity (0.54) (P < 0.001). Figure 3 shows the relationship of gestational age and FHR response to parasympathetic blockade with methylatropine. The percent increase (% inc) in baseline FHR from control to methylatropine periods during low-voltage fast activity and high-voltage slow activity is illustrated. In both behavioral states the response increased with gestational age. The regression equations follow: LVFA: % inc BFHR = 1.94(GA) - 243.28 (r n = 30, P < 0.001) HVSA: % inc BFHR = 1.01(GA) - 129.59 (r n = 30, P < 0.001)

=

0.73,

= 0.64,

The low-voltage fast activity slope (1.94) was significantly steeper (p < 0.001) than the high-voltage slow activity (1.01).

Comment Baseline FHR is regulated by the combined efforts of both intrinsic and extrinsic factors. The extrinsic

factors are primarily the sympathetic and parasympathetic limbs of the autonomic nervous system. The effects of the autonomic nervous system on baseline FHR in different behavioral states and gestational ages were assessed. Most previous studies used atropine sulfate to evaluate parasympathetic tone on FHR I I - I " ; however, atropine sulfate has been reported to alter sleep behavior. Animals that received atropine sulfate did not show desynchronized sleep for several hours, and the desynchronized sleep that ultimately appeared was markedly modified. 14 For our study methylatropine was used as a parasympathetic blockade. This substance is much less lipid soluble and does not appear to act centrally. 15, 16 According to Guazzi et aI., 16 methylatropine (1 mg/kg) could be administered intravenously without any apparent sleep disturbance. 17 In our study methylatropine 200 I-lg/kg estimated fetal weight was determined by prior experiments to be the appropriate dose. Complete parasympathetic block was obtained for :::0:3 hours with no effect upon low-voltage fast activity- and high-voltage slow activity-cycling behavior. For the evaluation of autonomic nervous system effects on FHR, the sympathetic and parasympathetic blocking agents were not studied together. Twenty-four hours were allowed between tests for recovery of homeostasis. The percent decrease in baseline FHR subsequent to propranolol infusion and the percent increase in baseline FHR subsequent to methylatropine injection were calculated as representing FHR l3-sympathetic and parasympathetic tone, re-

522 Wakatsuki et al.

August 1992

Am

J Obstet Gynecol

40 o LVFA



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O'---...&...(l_--'---()oI ....--.L-- ........- - - L - - L . - - - - - - I

130

125

135

140

145

Gestational Age (Days) Fig. 2. Relationship between percent decrease in baseline FHR (BFHR) and in low-voltagefast activity (LVFA) and high-voltage slow activity (HVSA) states after propranolol.

50 o LVFA



40

o

o

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........----L--"O'--+--'----'I....--.......135 130 140

OL..oCOO~

125

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145

Gestational Age (Days) Fig. 3. Relationship between percent increase in baseline FHR (BFHR) and gestational age in lowvoltage fast activity (LVFA) and high-voltage slow activity (HVSA) states after atropine.

spectively.":" The percent decrease in baseline FHR increased with gestational age, and the slope of the regression line was steeper in the high-voltage slow activity state. These data indicate that FHR l3-sympathetic tone increases with gestational age and is greater in the high-voltage slow activity state. The difference be-

tween low-voltage fast activity and high-voltage slow activity sympathetic tone also increased with gestational age. The percent increase in baseline FHR rose with gestational age, and the regression line slope was steeper during the low-voltage fast activity state. These results

Baseline heart rate in fetal lamb 523

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indicate that FHR parasympathetic tone increases with gestational age. FHR parasympathetic tone was higher in low-voltage fast activity, and the difference between states increased with gestational age. A number of studies have reported conflicting results about the autonomic system influence on FHR. The existence of adrenergic cardiovascular function in the fetal lamb as early as 85 days' gestation was demonstrated by bradycardia response to propranolol. 13 Parasympathetic cardiac function in the fetal lamb at 60 days' gestation was shown by vagally driven heart rate slowing." Nuwayhid et al." reported that parasympathetic tone on resting heart rate in the fetal lamb was feeble at

Autonomic nervous system regulation of baseline heart rate in the fetal lamb.

We examined 29 chronically instrumented fetal lambs from 125 to 143 days' gestation to investigate the effects of fetal behavioral states and autonomi...
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