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Autonomic Adjustments to Severe Hypotension in Fetal and Neonatal Sheep

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Autonomic Adjustments to Severe Hypotension in Fetal and Neonatal Sheep 0031-3998/01/4901-0056 PEDIATRIC RESEARCH Vol. 49, No. 1, 2001 Copyright © 2001 International Pediatric Research Foundation, Inc. Printed in U.S.A. Autonomic Adjustments to Severe Hypotension in Fetal and Neonatal Sheep ANNE MONIQUE NUYT, JEFFREY L. SEGAR, AARON T. HOLLEY, AND JEAN E. ROBILLARD The Research Center Hôpital Sainte-Justine, Department of Pediatrics, University of Montreal, Montreal, Québec, Canada [A.M.N.]; Departments of Pediatrics and the Cardiovascular Center, College of Medicine, University of Iowa, Iowa City, Iowa, U.S.A. [J.L.S., A.T.H.]; and Departments of Pediatrics, School of Medicine, University of Michigan, Ann Harbor, Michigan, U.S.A. [J.E.R.] ABSTRACT In fetal sheep, severe hypotension causes heart rate (HR) which a depressor reflex was triggered was significantly (p Ͻ 0.05) slowing. Studies during development have also shown that a lower in fetal than newborn sheep (35 Ϯ 2 versus 53 Ϯ 3mmHg reflex bradycardia and hypotension can be elicited after chemo- for HR and 35 Ϯ 2 versus 57 Ϯ 2 mm Hg for RSNA). The rates of stimulation with veratridine and is dependent on the age of the fall (slopes) for both HR and RSNA were also significantly (p Ͻ animal. In adults, a vagally mediated depressor reflex character- 0.05) more pronounced in fetuses (1.85 Ϯ 0.27 and 6.08 Ϯ ized by bradycardia, hypotension, and withdrawal of efferent 2.45%/mm Hg) than in newborns (1.21 Ϯ 0.16 and 1.97 Ϯ sympathetic activity can be observed after stimulation of chemo- 0.32%/mm Hg). Bilateral vagotomy significantly increased the pla- sensitive or mechanosensitive cardiac receptors with veratridine teau of mean arterial blood pressure over which maximum RSNA or in circumstances of reduced cardiac filling. This reflex, known and HR were maintained constant. Vagotomy also decreased the as the Bezold-Jarisch reflex, plays a role in disease states such as threshold for both RSNA and HR and the slope of the RSNA myocardial ischemia and hemorrhage. The objectives of our response to the nitroprusside infusion in newborn lambs. Results study were to determine whether a sympathoinhibitor depressor from this study show that activation of the arterial baroreflex during reflex, along with the bradycardia, is observed during pharmacolog- nitroprusside-induced hypotension is followed by withdrawal of ically induced hypotension in fetal and newborn lambs. In both fetal sympathetic tone and bradycardia and that this depressor reflex is and newborn lambs, HR and renal sympathetic nerve activity more pronounced in late-gestation fetuses than newborn lambs and (RSNA) initially increased (p Ͻ 0.05) in response to nitroprusside is significantly attenuated after bilateral vagotomy in newborn infusion to reach a maximum value. The range (or “plateau”) of lambs. (Pediatr Res 49: 56–62, 2001) mean arterial blood pressure over which maximum RSNA was maintained constant before withdrawal of sympathetic tone started Abbreviations: to be observed was significantly (p Ͻ 0.05) smaller in fetuses (0.3 AP, amniotic pressure Ϯ 0.3 mm Hg) than newborn (6 Ϯ 1 mm Hg) lambs. Similarly, the BJ, Bezold-Jarisch plateau over which maximum HR was maintained before onset of HR, heart rate bradycardia was significantly smaller in fetuses (4 Ϯ 1 versus 11 Ϯ MABP, mean arterial blood pressure 2 mm Hg). The mean arterial blood pressure level (“threshold”) at RSNA, renal sympathetic nerve activity In fetal sheep, in contrast to the tachycardic response to of major increase in left ventricular end-diastolic pressure but also moderate hypotension (1, 2), severe hypotension causes HR of reduced cardiac filling and increased inotropic activity (8–12), slowing (2–4). This reflex bradycardia can be observed in and after the administration of veratrum alkaloids (13, 14). When response to hemorrhage (2), vena caval obstruction (5, 6), and bradycardia and hypotension are observed after administration of vasodilator infusion (7). In adults, hypotension and bradycardia veratrum alkaloids, the phenomenon is classically termed BJ can be elicited after stimulation of mechanical or chemosensi- reflex. This response is reflex in origin and mediated by stimula- tive left ventricular receptors, such as in circumstances not only tion of cardiac chemo- or mechanoreceptors with nonmyelinated afferent fibers traveling within the vagus [reviewed by Minisi and Thames (13) and Nganele and Hintze (14)]. The efferent part of Received August 5, 1999; accepted August 9, 2000. the reflex appears to involve withdrawal of peripheral sympathetic Correspondence and reprint requests: Jean E. Robillard, M.D., Department of Pediat- rics, University of Michigan Hospital, 1500 Simpson Drive, MBP-D3202, Ann Arbor, MI outflow and stimulation of vagal efferent activity. In adults, this 48109-0718, U.S.A. vagally mediated depressor reflex plays a role in hemodynamic Supported by the National Institutes of Health grants HL-14388, DK-38302, and DK-43961. J.L. Segar is supported by Clinical Investigator Development Award HL- responses to disease states (such as myocardial ischemia, aortic 02865. A.M. Nuyt was supported by the Medical Research Council of Canada. stenosis, and hemorrhage) (12, 15) and in vasovagal syncope (15). 56 AUTONOMIC RESPONSE TO HYPOTENSION 57 Studies performed during early development have shown secured in the paravertebral muscle. Fetal skin incisions were that the responses elicited after chemostimulation with veratri- closed in separate layers, and the fetus was returned to the dine are dependent on the age of the animal (16–18), resulting uterus. Maternal incisions were closed in separate layers. All in a paradoxical increase in HR and blood pressure in preterm catheters and wires were exteriorized through a s.c. tunnel and fetal lambs and eliciting the classic depressor reflex in term placed in a cloth pouch on the ewe’s flank. lambs. Anesthesia and surgery in the newborn lambs (n ϭ 21) as The paradoxical slowing in HR observed in hypotensive well as the placement of the femoral catheters and electrodes fetal sheep mimics the vagally mediated depressor reflex re- on renal nerves were conducted in a similar manner (1). sponse to rapid hemorrhage in adult. It is not known, however, Catheters and electrode wires were tunneled s.c. and secured to whether in developing animals withdrawal of sympathetic tone the lamb’s back using porous elastic bandages. is associated with the bradycardia and whether this reflex is In a second group of newborn lambs (n ϭ 7), in addition to vagally mediated. Therefore the objectives of our study were the procedures described above, the vagus was identified bi- first to determine whether a BJ-like depressor reflex, in partic- laterally though a midline anterior cervical incision. A cotton ular the sympathoinhibition component, accompanies severe umbilical tape was carefully placed around each nerve, tied hypotension during development. Second, we sought to char- loosely, and exteriorized. The skin incision was closed and the acterize this reflex by comparing the hypotension-mediated umbilical tape was sutured to the skin to prevent pulling and bradycardia and sympathoinhibition observed in fetal and new- damage to the nerves until the experiment. born lambs. The third objective was to evaluate whether this Ampicillin sodium (Wyeth-Ayerst Laboratories, Philadel- depressor reflex is vagally mediated early in life. phia, PA, U.S.A.) was administered intramuscularly before surgery to the newborn lamb (0.5 g) and the ewe (2 g) and into METHODS the amniotic cavity after surgery (2 g). After surgery, the pregnant ewes were returned to individual pens and allowed Animals. Studies were performed in chronically instru- free access to food and water. The lambs were housed with mented near-term fetal sheep (131–137 d, term 145 d) and their mothers and nursed adequately. The animals were al- newborn lambs (3–7 d of age). Pregnant ewes and newborn lowed at least 24 h of recovery before the experiment. lambs of Dorset and Suffolk mixed breeding were obtained Physiologic studies. Before the start of the experiments, the from a local source. The pregnant ewes and the newborn lambs ewes were transferred to the laboratory in a small cart and with their mother were housed in individual pens. Gestational placed inside a Faraday cage. The newborn lambs were placed ages were based on the induced ovulation technique as previ- in a supportive sling-frame assembly inside a Faraday cage. ously described (19). The animals were sedated with an i.v. bolus of diazepam (ewe, All surgical and experimental procedures were performed 10 mg; lamb, 0.3 mg/kg; Valium, Elkins-Sinn, Cherry Hill, NJ, following the Guiding Principles in the Care and Use of U.S.A.), then given an i.v. bolus injection of the paralytic agent Animals approved by the Council of the American Physiologic vecuronium bromide (0.1 mg/kg, Norcuron, Organon, West Society and governed by the Animal Care and Use Committee Orange, NJ, U.S.A.), intubated, and ventilated to maintain of the University of Iowa. arterial blood gas values similar to those obtained during Surgical preparation. Anesthesia and surgery of the ewe spontaneous respiration as previously described (22). Supple- ϭ Ͼ and the fetus (n 14) were conducted as previously described mental oxygen was administered to maintain Pao2 100 mm (20, 21). Briefly, the ewe was fasted for 24 h before surgery; Hg to eliminate any potential stimulation of chemoreceptors. anesthesia was induced with 12 mg/kg of thiopental sodium Vecuronium bromide (0.1 mg/kg estimated weight) was also (Pentothal Sodium, Abbott Laboratories, North Chicago, IL, administered to the fetus. Muscular paralysis was necessary to U.S.A.) and maintained with a mixture of halothane (1%), eliminate muscle movements that interfere with nerve record- oxygen (33%), and nitrous oxide (66%). Under sterile condi- ing. Additional doses of vecuronium bromide (0.05 mg/kg) tions, the uterus was opened over the fetal hindlimbs, and were administered when muscle movements were detected.
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