Pulmonary vascular response to prostacyclin in fetal lambs

PROSTAGLANDINS

PULMONARY VASCULAR RESPONSE

TO PROSTACYCLIN IN FETAL LAMBS1

Robert Green, Jorge Rojas, and Hakan Sundell'

Department of Pediatrics Vanderbilt University School of Medicine

Nashville, Tennessee 37232

ABSTRACT

Eighteen prostacyclin injections (19.4k1.5 ug/kg) were performed in five chronically instrumented, intact fetal lambs in order to study the effects on pulmonary blood flow. These resulted in a brief period of bradycardia followed by a more prolonged period of increased pulmonary blood flow. In this latter phase, pulmonary blood flow increased from a baseline valueof49*4 ml/(kg min) to 122+10 ml/ (kg min). Systolic/diastolic pulmonary arterial pressure simultan- eously fell from 73+2/48*1 to 68*2/42?1 mm Hg. Flow through the ductus arteriosus was unchanged and right ventricular output increased to account for the increased pulmonary blood flow. Thus, prostacyclin causes pulmonary vasodilation in intact fetal lambs and may partici- pate in the control of fetal pulmonary blood flow and the circulatory adjustments to extra-uterine life.

INTRODUCTION

Prostacyclin, or prostaglandin 12 (PGI2), was first identified in 1976 (1). It is synthesized from prostaglandin endoperoxides by enzymes which can be isolated in the microsomal fraction of a large variety of fetal and postnatal arteries (2-6). The ability of prostacyclin to inhibit platelet aggregation and relax arterial smooth muscle strips has now been well established (l-3,7,8). In addition, prostacyclin has been shown to be a

P otent vasodilator in a variety

of fetal (9-11) and postnatal (12-17 vascular beds in vivo. These observations have ledtothe hypothesis that prostacyclin participates in the control of vascular smooth muscle tone both before and after birth. Since pulmonary vasoconstriction is important in the patho-

1 This research was supported by grants #HL14214 and HL07256 from the National Institutes of Health, National Heart, Lung and Blood Institute, Bethesda, Md., and was presented in part at the Southern Society for Pediatric Research meeting in January, 1979.

2 Send reprint requests to Dr. Hakan Sundell at the Dept. of Pediat- rics, Vanderbilt Hospital, Nashville, Tennessee 37232.

DECEMBER 1979 VOL. 18 NO. 6 927

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genesisofa variety of neonatal cardiopulmonary disorders, particularly those related to the transition from a fetal toa neonatal circulatory pattern, knowledge of the effects of prostacyclin on fetal pulmonary blood flow might contribute to our understanding of such disorders. The present investigation was designed to study the effect of prostacyclin on fetal pulmonary flow in an intact, chronically instrumented unanesthetized fetal lamb preparation.

METHODS

Ewes with dated pregnancies were operated on at 120-125 days gestation under nitrous oxide-oxygen-halothane inhalation anesthesia. Left brachial arterial and cephalic venous catheters were placed into the fetal aorta and superior vena cava respectively. Electrocardio- graphic leads were placed on the lambs. Electromagnetic blood flow transducers (C and C Instruments,Culver City, California) were placed around the main pulmonary artery ofthe fetus both proximal and distal to the ductus arteriosus. The proximal cuff was located just above the pulmonic valve, while the distal cuff occupied the small segment of main pulmonary artery between the take-off of the ductus and the pulmonary arterial bifurcation (Figure 1).

Catheter in

Rlqht Pulmonary

Figure 1. Diagram of the Fetal Preparation

Thus, right ventricular output (RVO) and pulmonary blood flow (PBF) could be measured separately. Ductal blood flow (DBF) was cal- culated as the difference between these two values (DBF = RVO - PBF). A catheter was placed directly into the pulmonary artery through a small incision just distal to the proximal flow transducer but proximal to the ductus, A catheter was placed in the fetal chest to remove pleural fluid and to monitor fetal breathing. A balloon-tipped cath-

928 DECEMBER 1979 VOL. 18 NO. 6

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eter was placed in the amniotic cavity. Pressures were measured using Statham P23Gb and Gould-Statham P23ID transducers. Pressure readings were corrected for changes in amniotic fluid pressure during experiments. Flow transducers were connected to Narcomatic electromagnetic flowmeters, model RT510. Flows were normalized to body weight by use of birth weight and retroactive calculation according to the intra- uterine lamb growth charts of Barcroft (18). The ECG, pressure, and flow signals were recorded on a Sanborn 350 polygraph or a Hewlett- Packard 7758 recording system.

Prostacyclin was generously supplied by Dr. John Pike of the Upjohn Company. The chemical was stored in a stock solution of tris- buffered ethanol at -4OC. Dilution with 0.9% NaCl was performed im- mediately prior to injection. The prostacyclin was injected per the fetal venous catheter into the superior vena cava. Periods of fetal breathing were avoided for the injections, and there were no uterine contractions during the experiments. The vehicle for the stock solution (tris-buffered ethanol) was diluted and injected in a manner similar to the prostacyclin. The paired two-tailed t-test was used to compare pre- and post-injection values of all variables.

RESULTS

Ewes were allowed to recover for at least three days prior to experiments. Fetal and maternal arterial pH and blood gases were within normal limits. Eighteen bolus injections of prostacyclin into the superior vena cava were performed on five fetal lambs. A fifty microgram dose was injected. The dose per kilogram varied due to variation in weight of the fetal lambs, which could not be determined until the lambs had been delivered. The average dose was 19.4i1.5 (SEM) ug/kg. The response to these injections (Table 1) can be arbi- trarily divided into two phases: (A) a transient period of bradycardia in the first minute followed by (B) a longer period of increased pulmonary flow beginninginthe second minute and ending within twenty minutes.

Phase A was characterized by a short period of bradycardia which reached a nadir at 25 seconds after the prostacyclin injection. The average heart rate (HR) fell to 91 beats/minute from a baseline value of 186 beats/minute (p<O.OOl). The lambs' electrocardiograms demonstrated first and second degree AV block with prolonged PR in- tervals and non-conducted atria1 beats. Associated with the bradycardia, there were transient falls in right ventricular output (RVO) and ductal blood flow (DBF). However, despite the bradycardia and decreased RVO, pulmonary blood flow (PBF) was preserved and, in fact, increased from a baseline value of 49 ml/(kg min) to 61 ml/(kg min) during this phase, a non-significant change. Right ventricular stroke index (RVSI) increased, and aortic (Pao) and pulmonary arterial (Ppa) pressures fell during the bradycardia. This phase was short-lived, ending within the first minute after the beginning of the injection.


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Table 1. Prostacyclin Injections in Fetal Lambsa

Baseline Phase A Phase B

Pa0 sysc Pao dia Et:; :;@I

63(I),

Ppa sys 73(2) 63(2 b 1 Z#b Ppa dia RVOd

48(I) 36(2)bb 42(Ubb

PBF 3;;W:j) 2;;w;l

172(23 b E:I:60;b

DBF HRe ! RVSIf

:::I:;) 9I(9) :E[:P) 1.9(.1) 2.6( .2)b 2.2( .l)b

aValues shown are mean (standard error of the mean) for eighteen injections in five lambs.

bStatistically significantly different from baseline at p~O.001 by paired t-test.

cpressures in mm Hg; sys=systolic; dia=diastolic.

dFlows in ml/(kg min).

eHeart rate in beats/minute.

fStroke index in ml/kg.

Phase B was characterized by a continuing increase in PBF, which reached a maximum value 93 seconds after injection. The average PBF increased from the baselineof ml/(kg min) to a peak of 122 ml/ (kg min) (p<O.OOl). This was accomplished by an increase in the mean RVO from 354 ml/(kg min) to427 ml/(kg min) (p<O.OOl) with no significant change in ductal blood flow from baseline. The average right ventricular stroke index increased from 1.9 ml/kg to 2.2 ml/kg (p<O.OOl). Although the increase in mean heart rate from 186 to 197 beats/minute was not statistically significant, in some lambs there was a marked tachycardia. Pulmonary arterial systolic/diastolic pressures fell from an average baseline of 73/48 to 68/42 (both p<O.OOl). Aortic pressure also fell. After the peak effect during the second minute, hemodynamic values slowly returned to pre-injection levels; by an average of 22 minutes all values had returned to baseline.

In addition to the eighteen injections described above, a dose-response curve of seven injections was performed in one of the fetal lambs. The results are shown in Figure 2. Response is defined as the increase in PBF during phase 8; baseline PBF was similar for all seven injections (33?3 ml/(kg min)).

Three injections of the vehicle (tris-buffered ethanol) in which prostacyclin was dissolved, diluted in the same manner as the prostacyclin, caused no changes in the parameters measured.


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b k zi 75-

A

& ", 50- 0 3 2

z 25- Y

ii 8 z 0 1

0 5 IO 15 20

DOSE (MICROGRAMSIKILOGRAM)

Figure 2. Prostacyclin Dose-Response Curve in a Fetal Lamb

DISCUSSION

An important occurrence in the transition from fetal to neonatal life is .the reduction in the pulmonary vascular resistance that occurs with the onset of air ventilation after birth. Both the ex- pansion of alveoli with gas and oxygenation have been shown to contribute to this change (19). Prostaglandins may play a role in the pulmonary vascular relaxation that characterizes thenormal transition from fetal to neonatal life. The pulmonary vasodilation seen withthe onset on ventilation of fetal goat lungs was reduced after inhibition of prostaglandin synthetase activity with indomethacin (20). The role of these substances in the physiologic or pharmacologic control of postnatal pulmonary vascular resistance and their relationship to pathologic neonatal states characterized by persistent pulmonary hypertension remains speculative.

There is, however,evidence from animal experiments that points to the possible role of prostaglandins in general and prostacyclin in particular as regulators of pulmonary blood flow (8,9,10,13,15,21,22). A recent report has related maternal ingestion of prostaglandin synthetase inhibitors to persistent pulmonary hypertension in human newborns (23). The importance of prostacyclin in the fetal circulation is suggested by its abundance in fetal blood vessels (4-6). Thus, the present investigation was designed to observe the effects


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of prostacyclin on the intact pulmonary circulation in unanesthetized fetal lambs.

This study demonstrates that bolus injections of prostacyclin into unstressed, fetal lambs cause an immediate increase in pulmonary blood flow, even though right ventricular output is decreased due to transient bradycardia. With disappearance of the bradycardia, PBF increases to more than twice the baseline value. This increased flow is accompanied by a decrease in pulmonary arterial pressure. Although left atria1 pressure was not measured in these studies for technical reasons,the most likely cause of these changes is a fall in pulmonary vascular resistance. This is consistent with earlier studies which describe the pulmonary vasodilating effect of PG12 inthe exteriorized fetal lamb (10) and goat (9).

A generalized vasodilating effect of PG12 is suggested by the decrease in systemic arterial pressure concomitant with an increase in RVO and preservation of DBF. The increase in RVO was accomplished both through an increase in stroke volume and heart rate, although the latter did not achieve statistical significance. The decrease in systemic arterial pressure observed in the fetal lambs was modest, in spite of a high dose of PGI

5’. when compared to the large fall in

diastolic blood pressure observe rabbits (17).

in anesthetized dogs (16), rats and The 13% decrease in systolic and 24% decrease in dias-

tolic systemic arterial pressure observed in phase A from the 20 ug/kg PG12 used in these fetal lambs is, however,similar to the decrease in pressure reported in exteriorized fetal lambs infused with a compar- able PG12 concentration of 200 ng/ml (10) A much higher dose of PG12 (180 ug/kg) resulted in a 30% decrease in mean systemic arterial pressure in unanesthetized fetal lambs (11). The modest effect of PG12 on systemic arterial pressure in the fetal lamb may be explained by the observation that the fetal ovine placental circulatory bed, which receives 40% of the combined ventricular output (24), has been found to be unresponsive to exogenous PG12 (11). The ductal flow re- mained unchanged as might be expected in the intact fetus where this blood vessel is already maximally dilated (25,26).

In summary, the present work demonstrates that prostacyclin causes a significant augmentation of pulmonary blood flow with a decrease in pulmonary arterial pressure in intact fetal lambs. A possible role for prostacyclin in the control of the pulmonary circulation, the transition from intra-uterine to extra-uterine life, and pathologic alterations of neonatal pulmonary vascular resistance is thus suggested and merits further study.

ACKNOWLEDGEMENTS

We wish to thank Dr. Mildred Stahlman and Dr. John Oates for their support and encouragement. Appreciation is also expressed to Patricia Minton, R.N., Linda Jones, M.A., David Oliver, and Rao Gaddipati, Research Assistants,for help in carrying out these experiments. We wish to thank Richard Whorton, Ph.D. and Alan Brash, Ph.D. for preparation of the prostacyclin stock solution.


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REFERENCES

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Pulmonary vascular response to prostacyclin in fetal lambs