228, no. 2, february u.s.a. development of autonomic ...€¦ · n. s. assail deuelopment of...

AMERICAN JOURNAL OF PHYSIOLOGY Vol 228, No. 2, February 1975. Printed in U.S.A.

Development of autonomic control of fetal circulation

B. NUWAYHlD, C. R. BRINKMAN III, Cm SU, J-. A. BEVAN, AND n-. S. ASSALI DeBartments of Obstetrics and Gvnecolopy and Pharmacology, UCLA School of Medicine, LOS Angeles, California 90024 * cl d UJ

NUWAYHID, B., C. R. BRINKMAN III, C. Su, J. A. BEVAN, AND

N. S. ASSAIL Deuelopment of autonomic contd of fetal circulation. Am. J. Physiol. 228(Z) : 337-344. 1975.-Development of parasympathetic and sympathetic reflexes controlling heart rate, vascular pressures, and blood flows was investigated in fetal lambs weighing 300-5,800 g (65-I 65 days’ gestation). Cardiovascular responses to veratridine injections, atria1 stretching, bilateral cervical vagotomy, and cholinergic blockade with atropine were used to test parasympathetic activities. Responses to propranolol and phenoxybenzamine were used to test beta- and alpha-adrenergic activities. Autonomic ganglionic blockade and stimulation provided additional information on both cholinergic and adrenergic systems, Fetal responses to various tests were compared to those of the mother. Results show: a) little parasympathetic tone on resting heart rate and other circulatory functions exists prior to fetal maturity; b) despite the feeble resting tone, the parasympathetic system is capable of exerting significant control when stimulated in both premature and mature fetuses, the capability increases as fetus approaches term; c) alpha- and beta-adrenergic tone in control of resting heart rate and peripheral circulation exists in early fetal life and increases as the fetus reaches maturity, and both adrenergic receptors respond strongly to stimuli in immature, premature, and mature fetuses; d> in immature fetuses, veratridine does not elicit a vagally mediated reflex; instead, it produces a centrally mediated alpha- and beta-adrenergic stimulation; e) the fetal cardiovascular response to any given test is dampened by the existence of the various vascular shunts, the umbilicoplacental circulation and, possibly, by incomplete maturation of vasomotor tone.

The present st d u ies were designed to investigate the autonomic reflexes controlling heart rate, pulmonary and systemic vascular pressures, and blood flows in fetal lamb at different stages of development. The effects of the type of

of the fetal cardio- anesthesi a given to the mother on some vascular reflexes were also assessed.

METHODS

A total of 57 ewes with dated pregnancies was used; con- firmation of the gestational length and fetal size was obtained radiologically. In Table 1 are listed data related to the nature of the experimental tests, the total number of tests and of fetuses in which a given test was performed, the fetal weight and gestational age, and the types of experimental preparation used in the various tests, We based fetal maturity primarily on fetal weight supplemented by gestational age and the presence or absence of hair on the skin. Fetuses weighing between 300 and 1,500 g were con- sidered immature and those weighing 1,600-2,600 g were classified as prema .ture; mature lambs were those wei ghing more than 2,700 g.

The studies were performed on acute and chronic experimental preparations as follows.

A) Acute Experiments

The ewe was given spinal anesthesia with an initial dose of

heart rate; pressure; flow; neural fetal maturity; Bezold-Jarisch reflex; adrenergic receptors

ALTHOUGH STUDIES have been performed on changes in fetal heart rate and arterial pressure at different gestational periods, the development of the autonomic control of the fetal cardiovascular functions has not been systemically investigated. Histochemical observations in sheep have indicated that on about 80-100 days of gestation, large nerve trunks begin to grow along the coronary arteries and, at term, they have involved most of the myocardial struc- tures; the growth of sympathetic innervation seems to continue for some time after birth (17). In this same animal species, the baroreceptor functions have been shown to mature progressively between 80 days and term gestation (2 I). In rabbits, cardiac concentrations of catecholamines have been found to be low in late gestation and to increase progressively after birth; the neurotransmitter concentrations correlated well with the extent of sympathetic innervation (1.3). In the rat, chick, and mouse, the adrenergic system seems to develop early during fetal life and to continue to grow for some time after birth (10, X8).

7-8 mg of pontocaine; additional doses were given as needed through an intrathecal catheter, One maternal carotid artery and vein were cannulated under local anesthesia. The artery served for monitoring maternal arterial pressure and heart rate and for collecting maternal ar terial blood samples anaerobicaIly. The vein served for drug injections (see below). An endo trache ostomy and th .e ewe’

1 tube was inser ted through a trache- respiration was supported with com-

pressed air or with a gas mixture containing sufficient oxygen to maintain a maternal blood Peg of about 100 mmHg. A small segment of the pregnant uterine horn was exposed and was marsupialized to the maternal abdominal walls to prevent evisceration. The acute studies were carried out on: a) 33 fetuses exteriorized and marsupialized to the uterine walls, and 6) 13 fetuses studied in utero.

1) Marsupialz’~ed fetus. The technique followed in the studies of the exteriorized fetus has been reported previously (2, 4, 8, 9, 22). I n essence, one jugular vein and one carotid artery were exposed in the neck and were cannulated with polyvinyl tubing, the diameter of which depended on the size of the fetus and its vessels. The jugular catheter was advanced to near the right atrium and served for drug injections. The carotid artery catheter served for recording arterial pressure and for collecting fetal blood samples

337

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338 NUWAYHID ET AL.

TABLE 1. Data on experimental tesis, number of lambs in which a armeights

given test was fierformed, and gestqtional ages, total

tests, and experimental preparation used

ExptI Test No. of No. of Exptl Prepn Lambs* Tests Chronic Acute

Gestational Fetal Wh g ke, Days

Veratridine alone

Veratridine after propranolol and phenoxybenzamine

Atrial stretching

Vagal section

A tropine

Propranolol

Phenoxybenzamine

Ganglionic blockade

Ganglionic stimulation

12

4

15

10

14

13

14

27

4

58 300-5000 60-160

11 2 2 300-1500 60-110

55

10

48

36

42

85

18

6 9 500~5,800

0 IO 1,800-5,000

5 9 300-5,800

6 7 300-5,000

6 8 300-5,800

7 20 l,OOO-5,800

3 1 l,lOO-3,700

70-160

120-160

60-160

60-160

60-160

105-160

110-150

* The number of fetuses listed exceeds the acrual number used because many animals w a-e used for more than one type of experimental procedure.

anaerobically. The chest was opened and electromagnetic flow transducers were placed around the ascending aorta, ductus arteriosus, and main pulmonary artery. In some animals, a Rochester-type cannula was inserted into the main pulmonary artery through a purse-string suture and served for recording pulmonary artery pressure.

2) In utero fetus. In the fetuses studied in utero, the procedure was as follows: after the ewe’s abdomen was opened, the fetal head was palpated inside the uterus and was brought out u rider the uterine segment that w as exteriorized. A purse-string suture (4 cm diam) was placed in the uterine walls including the membranes, the fetal head and neck were delivered through a small incision, and the purse-string suture was tightened to minimize the loss of amniotic fluid. The fetal head was covered with a saline- filled glove to prevent breathing. One fetal carotid artery and one jugular vein were cannulated and served for the same purpose as in the exteriorized fetus. The skin of the neck overlying the catheters was closed with interrupted sutures*

The purse-string suture was thereafter slightly loosened and the left fetal forelimb and the underlying parts of the chest were exposed. The chest was opened in the fourth intercostal space and the ascending aorta, main pulmonary artery, and ductus arteriosus were exposed and isolated as in the marsupiahzed fetus described above. The phrenic and vagus nerves in the left chest were preserved.

Because of technical difficulties encountered, particularly in the small fetuses, only one or two blood flows were monitored in the lambs studied in utero. After placing the flow transducers, the fetal chest was closed with interrupted sutures and the transducer cables were brought out through a small incision in the uterine walls. The fetal head, neck, and forearm were replaced inside the uterus and the uterine incision closed with the purse-string suture. The pregnant uterus was replaced inside the abdomen and the abdominal

incision was closed with Allis clamps; testing of the reflexes were begun immediately thereafter.

B) Chronic Experiments

The chronic experiments were carried out under sterile conditions and halothane-oxygen anesthesia given through an endotracheal tube. After opening the ewe’s abdomen, the fetus was manipulated inside the uterus so as to place it in the position appropriate for the neck and chest surgery as

described under the acute experiments. Placement of the carotid artery and jugular vein catheters, as well as of the flow transducers in the fetal chest followed the same technical steps used in the fetuses studied in utero as described above. After closing the uterine incisions, the flowmeter cables and the fetal catheters were brought out through a subcutaneous tunnel and were exteriorized through a stab wound in the back of the ewe. A mixture of streptomycin and penicillin was given daily to the mother. The ewe was allowed to recover from anesthesia and surgery for about 24 h. Thereafter, pharmacological testing on the fetus was done daily or every other day. Of the 11 lambs studied chronically, 2 survived for 4 wk, 4 for 3 wk, 3 for 2 wk and 2 for 1 wk after surgery. No less than four tests per day were carried out on each one of these animals. An appropriate interval was allowed between subsequent tests until all of the monitored circulatory functions had returned to c ontrol values. The same testing criteria were used in the acute experiments.

The following procedures were used to test the development of the reflexes controlling the fetal circulatory functions: 1) stimulation of the aRerent vagal pathways by veratridine (Bezold-Jarisch reflex) or by right atria1 stretching, 2) bilateral cervical vagotomy, 3) blockade of the autonomic control at the ganglionic level with trimetba- phan or mecamylamine and ganglionic stimulation with I,1 -dimethyl-4-phenylpiperazinium iodide (DMPP) ; 4) alpha-adrenergic blockade with phenoxybenzamine and beta blockade with propranolol, 5) muscarinic cholinergic blockade with atropine.

The protocol of the study in both the acute and chronic experiments comprised the following periods: a) A control period lasting about 30 min was observed during which flows and pressures were recorded every 3-5 min; maternal and fetal blood respiratory gases and pH were analyzed 2-3 times. b) A testing period then fallowed during which the test selected for assessing a given reflex was performed (see specific details under RESULTS). The pharmacological agents used for the tests were in.jetted through the fetal jugular catheter in varying doses, starting with the dose usually employed in adult animals and taking into con- sideration an assumed fetal weight. At the end of the acute experiments, the fetus was weighed and the doses used were converted to weight units. In the chronic experiments, an assumed fetal weight was taken from Barcroft’s data on the relationship between gestational age and fetal weight (5). Phasic and integrated flow and pressure signals were recorded continuously on an Offner Dynograph after each test until these signals returned to control values. During the testing period, maternal and fetal blood respiratory gases and pH were analyzed period ically.

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CONTROL OF FETAL CIRCULATION 339

We gauged the fetal lamb response to a given pharmacological agent in terms of adult response by performing the same tests on the mother, using intravenous doses based on the total weight of the ewe (includes fetus and placenta). The effects of each test on maternal arterial pressure and heart rate were monitored continuously.

23) Stimulatiun uf Aferent Vagal Pathways in Heart

We assessed the influence of maternal anesthesia on maternal and fetal responses to veratridine, phenoxybenzamine, and DMPP in 6 of the 32 acute experiments near term. We first tested the fetal and maternal responses with the ewe under spinal anesthesia as described above. There- after we gave to the ewe one of the following three general anesthetics : halothane-oxygen mixture, nitrous oxide-oxygen mixture, and pentobarbital. The same tests were again repeated after full anesthesia had been accomplished.

I) Ba+tpld-Jarisch rq7ex. A total of 58 tests of the Bezold- Jarisch reflex was performed on 12 fetal lambs weighing between 300 and 5,000 g. Veratridine dissolved in normal saline (pH lowered to 3.00 for complete alkaloid dissolu- tion) was used in doses of 3 to 40 pg/kg given as single intravenous or intra-atria1 injection.

Figure 1 presents the average changes in heart rate, arterial pressure and ascending aortic flow in response to injections of 10 pg/kg of veratridine in the immature, premature, and mature fetuses. Table 2 lists data obtained from a group of immature fetuses which were given veratridine before and after alpha- and beta-adrenergic blockade.

Blood flows, pressures, heart rate, and blood respiratory gases and pH were measured by techniques previously described (4, 8, 9, 22).

RESULTS

A) Control Data From Exteriorized and In Utero Fetuses

The circulatory response to veratridine depended on the gestational age and fetal maturity. In immature fetuses, veratridine injections invariably produced a significant increase in heart rate and arterial pressure regardless of the dose employed (Fig- 1). The increase occurred about 15 s after the injection and lasted for about 5-10 min, Ascending aortic and main pulmonary artery flows did not change appreciably (Fig. I and Table 2).

In the acute experiments, whether the fetus was exteriorized or remained in utero, the values for maternal arterial pressure, heart rate, and blood respiratory gases and pH were similar because all ewes received the same type of anesthesia and their respiration was assisted. Likewise, average figures for fetal arterial pressure, heart rate, ascending aortic, pulmonary and ductus flows, blood Paz, Pcoz, and pH were not significantly different in the exteriorized and in utero studied fetuses; both maternal and fetal values were within the range of data reported before from our laboratories (2, 4, 8, 9, 22).

In the chronic experiment, with the ewe standing, maternal blood Paz averaged 75 & 6 mmHg; these values were significantly lower than those of the acute experiments (average 110 & 10 mmHg) because the respiration in the chronic animals was not supported. Blood Pcoz and pH however were within the range of values of the acute experiments. Mean maternal arterial pressure in the chronic animals in the standing position averaged 75 mmHg while the average in the acute experiments was 92 mmHg. Fetal arterial pressure, main pulmonary artery and ascending aortic flows and blood Paz, Pcog, and pH were within the range of values observed in the acute experiments. Details of the data comparing the materna1 and fetal cardiovascular and metabolic status of chronically instrumented and acutely studied animals under different types of anesthesia are reported elsewhere (3).

Blockade of the beta-adrenergic receptors with propranolol (0.75 pg/kg) abolished the veratridine-induced tachycardia in the immature fetus without altering the pressor response to any appreciable degree (Table 2). Ad- ninistration of phenoxybenzamine (200 pg/kg), however,

abolished the veratridine pressor response without altering the tachycardia; main pulmonary artery and ascending aortic flows were not altered (Table 2).

Because of the 15 s delay between the intravenous veratridine administration and the appearance of tachycardia and hypertension, small doses of veratridine were injected into the carotid artery of several immature lambs. This route of administration elicited a response similar to that produced by intravenous injections.

The injections of the same doses of veratridine in premature fetuses elicited either a slight increase or decrease in

P*OOl I = *I SE

+20 -

+I0 -

0

AP BAA HU AP &A

The response to a given test of autonomic reflexes at a given fetal age was not appreciably different in the three types of experimental preparations; for instance, in the immature fetuses studied acutely, the increase in heart rate and arterial pressure following veratridine injections (see below) averaged 27 and 20 %, respectively; in the chronically instrumented animals, the values were 28 and 18 70. Because of the insignificant differences in the responses in the three types of experimental preparations, the results for all the animals studied were grouped together as a function of fetal maturity.

! HR AP CIAA NS

P CO.01

-10 NS

NS

-20 P*O.OOl

300 - 1500 gm 1600 -2600gm 2700 - 5000gm

FIG. 1. 14verage changes from control values of heart rate (FIR), arterial pressure (AP), and ascending aortic flow (&AA) in response to veratridine in immature, premature, and mature fetal lambs. Note strikingly different responses in immature and term fetuses and transitional response in premature Iambs.

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340 NUWAYHID ET AL.

TABLE 2. Effects of veratdine on fetal ch-culation in immature lambs before and after firopranolol or Dibenzyline

n Heart Rate, beats/min

Arterial Pressure, l-Kg

Main PuImon Art Ascend Aort Flow, ml/min per kg Pa Flow, ml/min per kg

Control 11 175 It 10 11 45 zt 4 6 125 Lt 20 5 99 & 15

Veratridine response 11 216 zt 18* 11 51 zt 6* 6 128 zt 16 5 95 zt 10

Veratridine after pro- 6 132 zt 10* 6 50 * 5* 4 122 l 15 3 96 pranolol

Veratridine after diben- 5 210 zfz 12* 5 42 Liz 3 3 120 2 92 zyline

Values are averages &I SE. Control figures represent average of several readings prior to veratridine administration; response figures represent maximum changes after drug injections. n = number of tests in which a given parameter was measured. * P < 0.01 in relation to control values.

the heart rate, but the overall changes were not significant (Fig. 1). Likewise, the changes in arterial pressure and in aortic and pulmonary (not shown) flows were not significant (Fig. 1).

In the mature fetuses, veratridine administration in the same doses produced a significant bradycardia and a slight hypotension (Fig. 1); blood flows in the ascending aorta, ductus arteriosus, and main pulmonary artery (not pre- sented) did nut change significantly (Fig. l), The veratridine-induced bradycardia was totally abolished by atropine.

At any given fetal age, a pattern of dose-response relationship existed for veratridine. The response increased linearly with doses between 3 and 10 pg/kg; thereafter, increasing the dose produced no further increase in response.

The effects of the solvent used in the veratridine experiments were tested in the three groups of fetal lambs, using the same volume and the same route as those used for veratridine administration. No circulatory changes were observed in any of the animals after the solvent injections.

Administration of veratridine to the ewe in doses of 3 pg/kg invariably produced bradycardia and hypotension.

2) stretch receptors. The influence of stretch receptors on the fetal circulation was tested in 15 lambs (500-5,800 g). Stretching of the receptors in the right atrium and possibly in other areas of the fetal vasculature was accomplished by a rapid bolus injection of 4-5 ml of maternal blood or saline solution. This procedure resulted in a prompt bradycardia which often was severe and was accompanied by arrhythmia (Fig. 2). Arterial pressure and blood flows in the great vessels fell slightly only during the arrhythmia (Fig. 2). The response to this procedure was more frequently observed in the fetuses weighing greater than 1,600 g than in the smaller fetuses; it was abolished by atropine or bilateral cervical vagotomy.

C) Efects of Autonomic Inhibition on Fetal Circulution

1) Section of vugus nerves. Bilateral cervical vagotomy was carried out in 9 pregnant ewes and their 10 fetuses as described elsewhere (3). The fetal weight in this series ranged from 1,800 to 5,000 g. Section of both vagi had no significant effects on the resting heart rate, systemic arterial and pulmonary artery pressures, nor on the blood flows in the ascending aorta, ductus arteriosus, and main pulmonary artery (Table 3).

Bilateral vagal section in the mother resulted in a prompt but mild tachycardia which was of short duration.

EXPERIMENT # 17 Body Weight 2000 g

Right A triul Stretching

HEART RATE beots / mm

MEAN

ART. PRESS.

mm Hg

ART.

lz7* I- --.. - .._-------I -+ i-j 150 t

i

t -> 'I I- I- x. --. ' L

,/- ,><.- ‘_ 1; "-> r? 30

120 r 60 ! [ ,- .- __ .--- ..- _-- -- -- --

0 120 r

ASC. AORT. FLOW ml/min OL

430 - DUCTUS FLOW ml / min 0 L-

SPEED 5 mm/set

FIG. 2. Example of effects of atria1 stretching Note marked bradycardia and arrythmia.

TABLE 3. Effects of bilateral uugal section on fetal circuhtion

b! Parameter

Arterial pressure, mmHg

AT

10

Control

60+2

After Section

61 x2

Heart rate, beats/min 10 195zt8 196+7

Pulmonary arterial pressure, mmHg

4 64+3 64&4

Ascending aorta flow, m1/ kg per min

5 95*15 96&14

Main pulmonary artery flow, ml/kg per min

4 120+ 16 124zt12

Ductus arteriosus flow, ml/kg per min

9 86It 14 82&W

Figures represent averages +l SE of several readings taken before and after vagal sections. n = number of fetuses in which a given parameter was measured.

Traction of the fetal right or left vagal nerve produced a 20-30 % decrease in heart rate in all of the fetuses included in this series; systemic arterial pressure and blood flows did not change appreciably (Fig. 3). The bradycardia caused by vagal traction could be abolished by atropine.

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CONTROL OF FETAL CIRCULATION 341

2) Gholinergic blockade with atro@e. A total of 48 tests of cholinergic blockade were performed in 14 fetuses (300- 5,800 g) using OS-O.3 mg/kg of atropine intravenously; the completeness of the blockade was assured by lack of response to acetylcholine (1-l 0 pg/kg). Xo appreciable changes in the resting heart rate, arterial pressure and blood flows were observed in the immat Le fetuses (Table 4). In premature fetuses, a slight increase in heart rate (4 %) occurred with no changes in pressures and flows (Table 4). Mature fetuses had an average of 10 7; increase in heart rate without significant alterations in the pressures and flows (Table 4).

When the fetal heart rate was depressed by events such as vagal stimulation, veratridine injections, atria1 stretching, etc., atropine administration promptly restored the heart rate to control levels irrespective of the fetal age.

Injections of 0.2 mg/kg of atropine into the mother produced a prompt tachycardia which was much greater than that observed in the fetus.

3) Blockade of the beta- and alpha-ndrenergic receptors. Adren- ergic beta receptors were blocked with injections of 0.75- 100 pg/kg of propranolol in 13 fetuses varying in weight

from 300 to 5,000 g, Completeness of the beta blockade was assessed by the absence of cardiac acceleration to injections of isoproterenol and norepinephrine (l-l 0 pg/kg) l

In all fetuses regardless of the age, beta-receptor blockade

ml / min OL A

Traction on left vugus nerve

FIG. 3. Example of effects of vagal traction on fetal circulation (near term). Note bradycardia and insignificant changes in pressure and flows.

produced a consistent decrease in heart rate averaging lo-14 7; ; changes in systemic arterial and pulmonary artery pressures and blood flows were of borderline significance only in the mature fetuses (Table 4).

,4 total of 42 tests of alpha-adrenergic blockade was performed on fetuses using phenoxybenzamine (100-200 pg/ kg). The completeness of the blockade was ascertained by the lack of pressor response to norepinephrine (l-10 pg/kg).

Blockade of the alpha-adrenergic receptors with phenoxybenzamine in fetuses weighing less than 1,600 g resulted in an average of a 10 % decrease in systemic arterial pressure with little change in the heart rate and blood flows (Table 4). In fetuses weighing more than 1,600 g, alpha-adrenergic blockade produced a significant decrease in the systemic arterial (26-30 %) and pulmonary artery pressures (ZO- 27 %) (Table 4) ; an average of 10 9; decrease in main pulmonary artery and ascending aortic flows occurred which was of borderline significance.

Injections of 100 pg/kg of phenoxybenzamine into the ewe produced an average of 40 % decrease in the systemic arterial pressure and 10 % fall in heart rate.

4) Ganglionic blockade. Interruption of the autonomic im- pulses at the ganglionic level was performed in a total of 27 experiments. The fetal weights in this series ranged from 1,000 to 5,800 g. The ganglionic blocking agents were ad- ministered by : a) single-bolus intravenous injections of trimethaphan in doses ranging from 16 to 2,000 pg/kg or mecamylamine hydrochloride in doses of 1-2 mg/kg, and b) intravenous infusion of trimethaphan in doses of lo-150 pg/kg per min.

Single injections of trimethaphan or mecamylamine produced insignificant changes in the fetal heart rate, pressure, and flows, irrespective of the fetal weight or the dose used.

Intravenous infusion of trimethaphan for 15-20 min, however, produced a response which depended on the fetal age. Fetuses weighing between 1,000~2,500 g exhibited a decreased in the systemic and pulmonary artery pressures which was of borderline significance; the changes in heart rate and blood flows were not significant (Table 5). More mature fetuses had a significant decrease in heart rate and

in the systemic arterial and pulmonary artery pressures (Table 5). Main pulmonary artery and ascending aortic

TABLE 4. E$ects of &opine (A), jm$~anoh~ (P), nnd phenoxybenzamine (D) on fetal circulation .-

30&1,500 g* 1,60&2,500 g* Parameter

A P D A P D A

Aortic pressure, mmHg 0 -1 -1o*zt 0 6~3-F -26&4$ -2

Pulmonary artery pressure, -2 0 7*3t -20&3$ -3 mmHg

----- 2,60&5,800 g”

P D

- 10*3t -3OG$

- 10*4 -27+6$

Heart rate, beats/min -t-z - 10+4t -4+2 +4 - 12%2t -4&Z + 10*2t - 14+4$ =-4&Z

Pulmonary artery flow. ml/kg 0 0 0 0 -8*6 -IO+4 -2 - 12&6 -7+3 per rni~~

Ascending aorta flow, ml/kg 0 -2 0 +8+6 -l&4 +2 - 10&6 -9*3 per min

Ductus arteriosus flow, mI/kg 0 0 -4 0 -2 -6zt4 +2 t-4 -2 per min

Values are average (& 1 SE) percent changes from control. * Fetal weight. t P< l 05* $P< .Ol.

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342

TABLE 5. Effects of ganglionic blockade with constant infusion of trimethuphan on fetal circuhtion

l,OOO-2,300 g* Parameter n IS

c R

Heart rate, beats/min 12 182 zt 9 180 + 10 33

Arterial mmHg pressure, 12 60 =t: 4 56 It 5 33

Pulmonary artery pressure, mmHg 8 66 k 6 62 + 5 12

Main pulmonary artery flow, ml/min per kg 8 110 * 15 105 zt 16 12

Ductus arteriosus flow, ml/min kg per 6 98 + 18 108 zt 17 10

Ascending aorta flow, ml/min per kg 8 100 zt 14 96 + 15 12

NUWAYHID ET AL.

2,6OQ-5,800 g*

C R

176 zk 8 160 + lO*

65 III 4 54 -4 61

71 zt 6 59 * 71

135 =I= 14 105 + 1st

105 zt 16 112 + 18

110 It 17 97 h 16t

Control figures (C) represent averages (&l SE) of readings prior to drug administration. Response figures (R) represent averages (+I SE) of 34 readings at peak of drug action; n = number of tests in which a given parameter was measured. * Fetal weight. t P < .05. f P < al.

DMPP EXPERIMENT # 24 175 Jig/kg Body Weight 3630 g

240 I

HEART i

_,_-~-- --------- --- -- ------~--- RATE 120 beats /min

0 L 120 r

ART. PRESS. m m Hg

MEAN 0 L

120 r

ART, W------l

PRESS. ” --

m m Hg 0’

PULM. 3620 r

ART. FLOW -- ml / min

ASC. 2090 p

SPEED 5 mm/set

FIG. 4, Example of effects of ganglionic stimulation on fetal circulation. Note slight and transient fall in heart rate and arterial pressure after injection and marked rise that occurred thereafter. Note also striking increase in phasic pressure and flows.

flows decreased by an average of 22 and 12 $I, respectively; the changes in ductus flow were not significant (Table 5). The results obtained in mature fetuses were similar to those observed by us previously (22).

Administration of trimethaphan to the ewe in doses of

l-1bgkp er min produced a profound fall (average 40 %) in systemic arterial pressure and heart rate which lasted for the duration of the infusion.

D) Chnglionic Stimulation

Ganglionic stimulation was performed on four fetuses (18 tests), weighing between 1,100 and 3,700 g, using DMPP (100-400 lug/kg). Immediately after the injection a transient but insignificant fall (4 %) in the fetal arterial pressure and heart rate occurred; this was followed by a striking rise in the systemic arterial pressure (both mean and phasic) and

TABLE 6. Response to ganglionic stimulation with DMPP

Parameter

Arterial pressure, mmHg

Heart rate, beats/min

Ascending aorta flow, ml/kg per min

?i? Control

18 64&3

18 186zt8

15 118dA2

Response

79*4*

215+10*

12432:lO

Main pulmonary artery flow, ml/ 15 135+15 145zt12 kg per min

-_-~

Control values represent average +I SE of several readings prior to injection. Response values represent average of 3 readings during maximum response. *P-c .Ol.

heart rate (Fig. 4 and Table 6). Phasic ascending aortic and main pulmonary artery blood flows increased but the mean flows did not change appreciably (Fig. 4 and Table 6). Although this response pattern was observed in all the fetuses, the magnitude of the tachycardic and pressor response was lo-20 YC less in the premature and immature than in the mature lambs.

Administration of atropine to the fetus prior to DMPP injections abolished the transient bradycardia and hypotension but did not alter the pressor and cardioaccelerator responses. The pressor response to DMPP was abolished by phenoxvbenzamine, whereas the tachycardia was abolished by pr’opranolol.

Intravenous injections of 100 pg/kg of DMPP to the ewe produced circulatory changes which were greater than those observed in the fetus.

E> i?$cts of Maternal Anesthesia on Cardiowascular &flexes

When the ewe was anesthetized with pentobarbital or halothane, both maternal and fetal responses to verat.ridine, phenoxybenzamine, and DMPP were abolished or considerably reduced. Figure 5 illustrates an example of the effects of halothane anesthesia on the veratridine-induced bradycardia in a mature fetal lamb.

Maternal anesthesia with nitrous oxide-oxygen mixture diminished but did not abolish totally the fetal cardiovascular response to the tests employed. Details cf the influence of the various types of anesthetic agents on maternal

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CONTROL OF FETAL CIRCULATION

EXPERIMENT # 4 UNANESTHETIZED Body Weight 4358 g

VERATRIDINE 4.5 pg/kg

240 HEART -RATE 120 beats/min o E

c

p-7. --- lr-

ASC.

AORT. FLOW

ml/min

DUCTUS FLOW ml/min

HALOTHANE ANESTHESIA

VERATRIOINE 13.0 ,ug/kg

I

SPEED 5 mm /set

FIG. 5. Example of effects of halothane anesthesia given to ewe on fetal response to veratridine. Note absence of bradycardia during anesthesia even when dose was considerably higher than in unanesthetized condition.

and fetal cardiovascular and metabolic functions are reported elsewhere (3).

F) Efects of Tests on Fetal Blood Respiratory Gases and PH

None of the tests employed had any significant effect on letal blood respiratory gases and pH.

DISCUSSION

The present data obtained from unanesthetized fetal lambs show that the parasympathetic and sympathetic nervous systems begin to exert control on fetal cardiovascular functions at different periods of development.

A) Parasympathetic Control

Our observations indicate that prior to fetal maturity (before 130 days’ gestation), the parasympathetic system does not seem to exert an appreciable control on the various cardiovascular functions in the resting condition. This is evidenced by the lack of any significant alterations in the resting heart rate, pulmonary and systemic pressures, and ventricular outputs following ganglionic and peripheral cholinergic blockade as well as bilateral cervical vagotomy. This view receives further support from the absence of a clear-cut Bezold-Jarisch reflex in the immature and premature animals, as well as from the available literature on the effects of atropine and vagotomy on heart rate of pup- pies, kittens, and rabbits (1, 2, 7, 11, 16, 19, 20, 23).

When maturity of the fetal lamb (greater than 2,500 g) is reached, however, a certain degree of chronotropic parasympathetic tone takes place as evidenced by the increase in heart rate following cholinergic inhibition with atropine and the adultlike response to the different modes of vagal sensory stimulation. The absence of circulatory changes following bilateral cervical vagotomy even in term fetuses is not surprising, since the effects of this procedure are in- consistent even in the adult animals (1, 8, 11, 15, 19, 20, 23).

The present data further indicate that, although the fetal vagal tone is minor in the resting condition, the parasympathetic system is present and, when stimulated, is capable of exerting influence on the circulation even before fetal maturity. This view receives support from reports which show that direct cervical vagal stimulation or atria1

343

stretching elicit bradycardia in, premature as well as mature fetuses (1, 2, 5, 7-9, 11, 16, 20, 23). The administration of the cholinergic neurotransmitter, acetylcholine, produces profound fetal circulatory alterations which are again more pronounced at term (2, 11, 16). All these observations seem to suggest a progressive maturation of the parasympathetic control on the cardiovascular functions during fetal development and possibly through a certain period of neonatal life. This hypothesis receives further support from the circulatory response to veratridine which begins to resemble the adult pattern only at term. It is also supported by the response to ganglionic stimulation or blockade which, although evident at term, was still considerably less striking than that of the mother. The available literature seems to support progressive maturation of neural control of the circulation in other species (1, 2, 7, 10, 11, 13, 17, 18, 2 1).

The results on the Bezold-Jarisch reflex provide additional information on the pattern of parasympathetic maturation during fetal development. In the adult dog or cat, this reflex produces bradycardia, hypotension, and apnea and is stimulated by intravenous or intra-atria1 injections of veratridine; the afferent pathways of this reflex are through the vagus nerve (6, 12, 14, 15). The present data show that, in the immature fetus, veratridine does not elicit a reflex mediated by the afferent vagal pathways. Although this lack of response may be at least in part due to relative insensitivity of the afferent vagal sensory receptors, the possibility exists that these receptors are still undeveloped in the immature fetus. Neither one of these hypotheses can be ruled out by the transitional response to veratridine stimulation observed during prematurity and the obvious adultlike response depicted near term.

The cardioaccelerator and pressor response to veratridine in the immature fetus indicates a centrally mediated alpha- and beta-adrenergic stimulation. Evidence in support of this conclusion was obtained by: a) mimicking this circulatory response by intracarotid injections of veratridine, 6) abolishing the cardioacceleration by beta-adrenergic blockers, and c) abolishing the pressor action by alpha-adrenergic blockers.

All these findings point out clearly that the sympathetic system appears to begin to exert a more prominent control on the fetal cardiovascular functions earlier than the parasympathetic system.

B) Sympathetic Control

The histochemical observations of Lebowitz and his co- workers (17) in the fetal lamb show that, although dop- amine-containing cells are present in the 75-day heart, no true sympathetic fibers can be detected. In the rat, sympathetic fibers were observed histochemically by the 2nd wk of gestation (10). The present data indicate the existence of functional cardiovascular adrenergic control as early as the 60th day of gestation. This is evidenced by the bradycardia that follows beta-adrenergic blockade with propranolol, as well as by the hypotension following alpha-adrenergic blockade with phenoxybenzamine. Although the adrenergic tone in the resting heart rate and systemic arterial pressure is relatively small at that period of gestation, clear chronotropic and inotropic responses can be elicited by a variety of adrenergic stimuli. This is shown by the striking increase

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344 NUWAYHID ET AL.

vessels possess no neural control, although they react to neurotransmitters (2, 5, 11). Since the umbilicoplacental

deal to the overall fetal circulatory response of the

in heart rate and in the pulsatile pressures and flows in response to veratridine and to sympathetic stimulation with

responses to circulation contributes systemic vascular resista

a great .nce, the

fetus to autonomic blockade or to other neural stimuli would

DMPP in the immature fetus. Such a tone and both beta- and alpha-adrenergic stimulation increase in magnitude

It should of the age,

card given

as the fetus be pointed

the magni iovascular

tude of the stimulus required to response is much greater than

the adult. In all the pharmacological tests employed, the dose required to stimulate or block either the sympathetic or

approaches out that in

maturity. the fetal lamb, be expected to be much less than that of the adult or even

that of the neonate. Third, there is the possibility that the overall vasomotor tone of the fetus does not reach full maturity until some time after birth. All these factors tend to ne

,ive to make the fetal cardiovascular functions less respons ural and humoral stimuli than those of the adult.

regardless elicit a that of

parasympathetic system was much greater than that used in the mother. And even with these relatively large doses, the fetal circulatory response was considerably less than that of the mother. For instance, the ewe suffered a near circulatory shock with a relatively small dose of a ganglionic blocker such as trimethaphan; yet the fetus required considerably larger doses and more prolonged infusion to show even a small fall in blood pressure. In fact, in this and in previous series of observations (ZZ), no fetal response of any

The present series of experiments show that anesthetic agents such as halothane, barbiturates, and to a certain extent nitrous oxide impair some of the neural reflexes controlling both maternal and fetal cardiovascular functions. These findings are in agreement with those of others (20). These results which will be reported in detail elsewhere (3)

significance was observed to amine or trimethaphan. We

single large believe that

doses of mecamyl- this apparent fetal

resistance is related to the following three factors which tend

should be kept in mind when with and without anesthesia.

reporting experimen tal results

We are indebted to Dr. Robert Bauer, Helena Martinek, David

Zive, and Cliff Tsai for their technical assistance.

This study was supported by grants from the National Institutes of

Health : HE-01 755 and HE- 13634.

C. R. Brinkman III is the recipient of Career Development Award

HL-70237.

Address reprint requests to N. S. Assali, Department of Obstetrics and Gynecology, UCLA School of Medicine, Los Angeles, Calif. 90024.

to dampen the response of the fetal circulation to vaso- pressor or depressor stimuli. The first factor is related to the presence of various circulatory shunts such as the ductus arteriosus, ductus venosus, and foramen ovale. Second is the presence of the placental vascular bed which acts as a low- resistance network in parallel with the fetal vascular system. The available evidence indicates that the umbilicoplacental Received for publication 7 November 1973.

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14. GOODMAN, L. S., AND A. GXLMAN. The Pharmacokgical Basis of Therapeutics (3rd ed.). New York : hrIacmillan, 1955, p. 7 18.

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