Hemodynamic changes during pregnancy in the rabbit

II

Hemodynamic changes during pregnancy in the rabbit B. NUWAYHID, M.D., F.A.C.O.G. St. Louis, Missouri Blood volume (BV), plasma volume (PV), red cell volume (RCV), cardiac output (CO), and organ blood flows were measured in nonpregnant (Group 1), 10 to 19 days' pregnant (Group II), and 20 to 30 days' pregnant (Group Ill) rabbits. BV, PV, and RCV increased throughout pregnancy, with maximum increase during the second third of gestation. CO peaked during the second third of gestation and then decreased. Mean arterial pressure and systemic vascular resistance decreased. Total reproductive blood flow increased throughout pregnancy but when calculated per unit of weight it did not change. Redistribution of blood flow within the reproductive organs occurred, with ovarian and placental flow per unit of weight increasing and myometrial flow /unit of weight decreasing. Splenic flow decreased significantly; liver flow decrease.d. The flow to the lungs and kidneys remained unchanged. The data suggest that major hemodynamic changes occur during the first two thirds of gestation and this period appears to test the competency of the maternal organism to adapt to the needs of pregnancy. The last one third of gestation is characterized by the fetoplacental unit interaction. (AM. J. 0BSTET. GVNECOL. 135:590, 1979.)

THE H EMOD YN AMI c changes during pregnancy have been studied in many animal species. t-a There is a consensus of opinion that the cardiac output and blood volume increase by 40 to 50% toward the end of gestation. Uterine blood How increases concomitantly and the increment reaches several-fold the nonpregnant values; systemic and uterine vascular resistances fall considerably and the fall is largely related to the low resistance system of the uteroplacental vascular bed. It has been shown that the increment in uterine blood How during pregnancy is derived totally from the increased cardiac output and not from regional blood How redistributions. 4 The present study was undertaken in the rabbit to obtain information on the following points: (a) the extent of the changes in the cardiac output, blood volume, mean arterial pressure, and systemic vascular resistance at different periods of gestation; (b) the magnitude of changes in blood How to the reproductive and nonreproductive organs during the same gestational periods, and (c) the pattern of redistribution of blood

From the Department of Obstetrics and Gynecology, Wn1hington University School of Medicine, and Barne.1 Hospital. Receivedj(Jr publication November 21, 1977. Revi.1edjune6, 1978. Ampted December 14, 1978. Reprint requests: Dr. Bahij Nuwayhid, UCLA School of Medicine, Department of Obstetrics and Gynecology, Los Angeles, Califimtia 90024.

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How within the reproductive and nonreproductive organs.

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Material and methods

Thirty-three pregnant and nonpregnant rabbits of the New Zealand White breed were studied (term gestation of rabbits is 31 days). The animals were divided into three groups: eight that were nonpregnant (Group I); eight that were 10 to 19 days pregnant (Group II), and 17 that were 20 to 30 days pregnant (Group Ill). Although the animals were supplied from a local source, accurate dates of mating were kept (mating day was day 0 and the one afterward was day 1). Animals were kept in their cages in the animal facility for 2 to 3 days for acclimatization before the studies and were fed pellets and given water ad libitum. Animal preparation. Animals were weighed the morning of surgery and were anesthetized with 30 mg/kg intramuscularly of pentobarbital. A jugular vein, a carotid artery, and the femoral arteries and veins on both sides were catheterized with appropriate size polyvinyl catheters. The carotid catheter was advanced to the left atrium and its location was ascertained from the pressure wave form (Stathem 23 transducer). One femoral artery catheter was attached to a pressure transducer to measure the blood pressure and heart rate (on a Grass recorder) and for anaerobic sampling of arterial blood for analysis of blood gases. The other femoral artery catheter was attached to a withdrawal pump (Harvard Pump) for arterial sampling at the time of microsphere infusion. Oxygen by 0002-9378/79/210590+07$00.70/0

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1979 The C. V. Mosby Co.

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Hemodynamic changes during pregnancy 591

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Table I. Plasma volume, total blood volume, and red blood cell volume as determined by Evans blue dye Croup I Nonpregnant

Changes in

Plasma-volume (ml) Total blood volume (ml)

Red blood cell volume (ml)

116.00 ± 8.50 (6) 35.00 ± 2.10 (6) 151.00 ± 11.60 (6) 55.78 ± 2.60 (6) 63.46 ± 5.40 (6) 20.13 ± 0.89 (6) 3.36 ± 0.13 (8)

Total Per kg body weight TotalPer kg body weight Total Per kg body weight

Animal weight/kg Fetal litter weight/gm

Croup II Pregnant

(;roup III Pregnant

(10-19 days)

(20-30 days)

165.50 ± 16.:10*

(6) 48.00 ± 4.90:f: (6) 266.00 ± 24.60§ (5) 75.00 ± 8.20:f: (5)

90.00 ± 9.30:f: (5) 25.40 ± 2.90 (5) 3.42 ± 0.13 (6) 6.15 ± 0.50 (5)

lHH.OO ± 4.90t (16) :>o.oo ± l.ti4·r (!6)

284.00 ± 7.30t !16)

7'1.00 ± 2.40t (16) 96.00 ± 3.40t (16) 25.00 ± 0.98§ (16) ~-~ 80 ± 0.07§ ( 15) 335.00 ± 15.00 (15)

Also shown are P values when Group II and Group III animals are compared to Group I animals. ± SE of mean: ( ) ber of tests. *P <0.02. tP <0.001. :f:P
=

num-

Table II. Cardiovascular parameters of the three animal groups Blood flow

Heart rate (beats/min) Systolic pressure (mm Hg) Diastolic pressure (mmHg) Mean arterial pressure (mm Hg) Cardiac output (mil min) Systemic vascular resistance mmHg mil min./kg

Croup I Nonpregnant

Group II

Group III

(10-19 days)

(20-30 days)

227.00 ± 4.60

230.00 ± 10.20

240.00 ± 4.90 (15) 96.80 ± :l.90* (15) 72.60 ± ~j.80t (15) 88.60 ± 3. 70* (15) 861.50 ± 113.60

(8)

(7)

(6) 110.80 ± 5.80 (5)

96.00 ± 2.40

90.40 ± 4.90

ll5.43 ± 5.10 (7)

108.71 ± 3.80 (7)

516.20 ± 27.90 (5)

(5) 103.80 ± 5.20 (5)

1,006.00 ± 307.00:f: (5)

0.65 ± 0.06

0.41 ± 0.09:f:

(5)

(5)

(14)

0.46 ± O.Ol:f: (14)

P values when Group II and Group III animals are compared to Group I animals. ± SE of mean; ( ) number of tests. The number of tests is put between parenthesis, since in some cases the sample size was small and care should be taken in interpreting the data. *P
mask was given to keep the arterial Po 2 in the range of IOO to 120 mm Hg. All animals were observed for a control period, during which arterial pressure, heart rate, and blood gases were monitored. Blood volume was measured by means of Evans blue dye. 3 • -' A 3 cc sample of whole blood was taken from the jugular vein for measuring the hematocrit, and the rest was spun to obtain "blank" serum for the standards. A 0.5 cc amount of Evans blue dye (5 mg/cc) was injected into the jugular vein and the syringe flushed several times

until no color remained. An equilibration period of 10 minutes was allowed after which a :~ cc blood sample was taken from the femoral vein. Dye concentration in the sample was compared with standards made up in the "blank serum" of each animal, by measuring optical density at a wave length of 620 mu in a spectrophotometer. Cardiac output and its regional distributions were measured with the microsphere method. Radioactive carbonized microspheres (3M Co., St. Paul. Minnesota)

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Table III. Summary of the data obtained from the three animal groups as to reproductive organs' weight total flow and flow/ 10 gm of wet tissue weight Group II Pregnant rabbits (10-19 days)

Group I Nonpregnant rabbits

Ovaries Tubes Vagina Myometrium

Weight (gm)

Total blood flow (mil min)

Blood flow 10 gm (mil min)

0.38 ± 0.12 (7) 1.09 ± 0.20 (7) 2.68 ± 0.63 (7) 5.20 ± 0.38 (7)

0.68 ± 0.30 (6) 1.20 ± 0.28 (5) 1.03 ± 0.28 (6) 3.40 ± 0.71 (6)

26.4 ± 10.00 (5) 14.00 ± 2.00 (5) 6.30 ± 2.00 (5) 10.50 ± 3.00 (5)

9.33 ± 0.82 (7)

5.32 ± 1.05 (6)

8.60 ± 2.00 (5)

Placenta Total reproduction

Weight (gm)

Total blood flow (mllmin)

Blood flow 10 gm (mllmin)

0.72 ± 0.05 (8) 1.70 ± 0.31 (6) 4.17 ± 1.08 (8) 30.35 ± 2.67 * (8) 17.95 ± 3.60 (5) 67.00 ± 5.00* (5)

7.89 ± l.IO* (5) 1.70 ± 0.29 (5) 1.08 ± 0.26 (5) 8.00 ± 1.40t (5) 3.71 ± 1.30 (5) 21.70 ± 2.10* (5)

108.00 ± 17.00t (5) 8.30 ± 0.80 (5) 5.10 ± 1.50 (5) 2.60 ± 0.10§ (5) 2.70 ± 0.90 (5) 5.40 ± 0.70 (5)

P values when Group 11 and Group Ill animals are compared to Group I animals ± SE of mean; ( ) number of tests. *P
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Table IV. Summary of the data obtained from the three animal groups for nonreproductive organs' weight, total flow, and flow/10 gm of wet tissue weight Group I Nonpregnant rabbits

Organ

Kidneys Liver Lungs Spleen Heart

Group II (I 0-19 days)

Weight (gm)

Total blood flow (mllmin)

Blood flow 10 gm (mllmin)

Weight (gm)

Total blood flow (mllmin)

Blood flow IOgm (mllmin)

20.12 ± 0.50 (8) 114.10 ± 5.76 (6) 10.95 ± 0.41 (6) 0.97 ± 0.08 (6) 7.58 ± 0.09 (6)

95.00 ± 13.10 (6) 6.20 ± 1.70 (5) 13.30 ± 2.80 (6) 10.20 ± 1.00 (6) 21.20 ± 3.00 (6)

43.70 ± 7.00 (6) 0.50 ± 0.10 (5) 12.00 ± 3.00 (6) 108.50 ± 13.00 (6) 27.90 ± 3.00 (6)

19.64 ± 1.37 (6) 153.20 ± 11.80* (6) 12.20 ± 2.24 (6) 2.30 ± 0.40t (6) 7.48 ± 0.43 (6)

84.30 ± 6.40 (5) 6.78 ± 1.90 (5) 20.70 ± 6.30 (4) 6.60 ± 1.10t (4) 37.20 ± 11.20 (4)

42.90 ± 3.00 (5) 0.40 ± 0.10 (5) 14.00 ± 2.70 (4) 44.80 ± 14.00t (4) 45.30 ± 13.00 (4)

P values are shown when Group 11 and Group III animals are compared to Group I animals. When Group III animals were compared to Group 11 animals, no significant changes were observed ± SE of mean; ( ) number of tests. *P <0.02. tP <0.01. :j:P <0.05. §P <0.001.

with a specific activity of 10 mCi/gm and a mean diameter of 15 um ± 5 were used. Strontium 85label was chosen because it has a pure gamma emission that is relatively strong (514 Kev) and has a long half-life (65 days). The microspheres were suspended in 10% solution of dextran. Although the amount of microspheres injected varied somewhat from one animal to another

because of decay with time, the amount injected was sufficient to give tissue count several-fold the background. The amount of radioactivity injected was calculated by means of the following procedure: A preweighed syringe was used and 1.5 to 2 cc of the suspension was withdrawn. The syringe and the suspension were weighed again. After the desired amount

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Hemodynamic changes during pregnancy

Group III Pregnant rabbits (20- 30 days) Total blood

Weight (gm)

flow

Blood flow JOgm

(mllmin)

(mllmin)

0.77 ± 0.88* (17) 1.67 ± 0.27 (10) 8.92 ± 0.62* (17) 33.00 ± 6.40* (17) 31.80 ± 2.30 (15) 75.68 ± 3.40* (15)

5.85 ± 0.88* ( 14) 1.32 ± 0.13 (13) 4.58 ± 0.64t (14) 10.71 ± 1.10* (13) 25.00 ± 3.20 (13) 46.90 ± 3.00* (12)

79.00 ± 10.001" (14) 9.80 ± 1.40 (10) 5.20 ± 0.50 (14) 3.30 ± 0.40t (13) 8.80 ± 1.60 (12) 6.40 ± 0.50 (II)

Group III (20-30 days)

Weight (gm)

19.83 ± 0.91 (15) 126.80 ± 6.50 (14) 10.54 ± 0.80 (14) 1.78 ± 0.17 (13) 7.57 ± 0.26 (14)

Total blood flow

Blood flow JOgm

(mllmin)

(mllmin)

85.40 ± 6.70 (15) 4.90 ± 0.98 (9) 13.85 ± 2.70 (8) 5.00 ± 0.80t (12) 31.11 ± 5.50

43.10 ± 3.30 (15) 0.40 ± 0.10 (9) 14.20 ± 3.40 (8) 28.80 ± 4.60§ (12) 42.00 ± 8.22 (10)

(10)

of microsphere suspension was infused, a quantity of 0.2 to 0.5 cc of the suspension was left in the syringe. It was weighed and diluted in 500 cc of water. A magnetic stirrer was held in this solution for 30 minutes and five 2 cc samples were taken for measuring the radioactivity. The amount of radioactivity injected was calculated hy means of the Fick equation. Amount injected

=

Total radioactivity of the remaining suspension Weight in grams of remaining suspension x weight in grams of infused suspension

593

Microsphere infusion. The microsphcrc suspension was stirred for five minutes prior to being infused into the left atrium. Ten seconds before the infusion. blood withdrawal from the femoral artery \\
=

Total radioactivity infused . . . . . x rate f>f withdrawal Radwactlvlty m artenal sample . ) Organ rddioactivity CO 0 rgan fl ow ( m I/ mm = Total radioacti\·itv x '

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:-lovembt·r I. 197'l Am . .J. Obstet. Gvneml.

Ovaries

MyoVag ina metrium Placenta

!Zi Gil

10-19D

Totol Reproductive

ttt

.Gill20-30D

I ±ISE

P
Fig. l. A graphic representation of the reproductive flow in

Group II and Group III animals; also shown is the P value when Group III animals are compared to Group II animals. Ovaries

MyoVagina metrium Placenta

Wet tissue wt =10gm ~

G II 10-190

•

Gill 20-300

I± 1 SE 20 Itt

10

P
ttt

Fig. 2. A graphic representation of the reproductive flow I I 0

gm of wet tissue weight in Group II and Group III animals; also shown is the P value when Group III animals were compared to Group II animals. Blood How to the liver was determined from the samples' radioactivity and total organ weight. Blood How per I 0 gm of wet tissue was calculated for all organs by dividing organ How/organ weight in grams and multiplying by 10. Statistical significance was determined by means of the t-test for unpaired groups, and P values greater than 0.05 were considered not significant. Experiments in which the pulmonary tissue radioactivity exceeded 5% of the total injected dose were rejected.

Results

Blood volume. Table I depicts the changes in blood volume, plasma volume, and red cell mass in absolute values and as per kilogram of body weight in the three animal groups. A significant increase in all of these parameters occurred in the pregnant rabbits when compared to nonpregnant animals. The rate of increase in these parameters was much greater during the second third of gestation (Group II) than during the last one third (Group Ill). When the same data were calculated per kilogram of body weight, there was no difference between Group II and Group III. Cardiovascular parameters. Table II shows the changes in the cardiac output, arterial pressure, heart rate, and vascular resistance in the three groups of animals. Heart rate increased by 3 and 13 beats per minute in the pregnant animals (Groups II and Ill, respectively) but the changes were not significant. Systolic, diastolic, and mean arterial pressure fell in Group II and Group III but only the changes in the latter group were significant. Cardiac output increased in both Group II and Group Ill but reached significant levels in Group II only. In the latter part of gestation there was some decline in CO in relation to midgestation. The systemic vascular resistance decreases significantly in Group II and Group Ill. Reproductive Row. In Table III the reproductive blood How for the three groups of animals is listed as a function of different parts of the reproductive tract and also per unit of weight. The weights of the different components of the reproductive tract is also tabulated. When compared to the nonpregnant animals (Group 1), the pregnant animals (Group II and Group III) showed: (a) an increase in tissue weight of the ovaries, tubes, vagina, myometrium, and total reproductive tract; (b) a significant increase in blood How to the ovaries, vagina, myometrium, and total reproductive tract; (c) when the How was calculated as per unit of weight, the ovarian How /unit weight increased and the myometrial flow /unit of weight decreased significantly and the placental How/unit of weight increased. In order to shed more light on the changes in the reproductive blood How during the last one third of gestation, we compared Group Ill and Group II in relation to total blood How and How per unit of weight. The findings were as follows: (a) a further significant increase in the total reproductive How in Group III occurred. The major increase was due to a change in the placental How although there was a significant increase in the vaginal How (Fig. 1); (b) Fig. 2 shows that when flow/unit of weight was calculated, there was no significant change in the ovarian, vaginal, and myometrial

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flows. The placental flow per unit of weight increased significantly. Nonreproductive flow. Table IV depicts the changes in weight. total organ blood flow, and flow/unit weight of the kidneys, liver, lungs, spleen, and heart. When the pregnant animals (Group II and Group III) were compared with the nonpregnant animals, we observed the following: (a) there was no change in the weights of the kidneys, lungs, and heart. The weight of the liver and spleen increased in Group II and this increase was significant; (b) the relative total organ flows to the kidneys, liver, lungs, and heart did not change significantly but the splenic flow total and per weight unit decreased significantly. Comment

The present data obtained from groups of nonpregnant and pregnant rabbits that served for the study of the circulatory changes of pregnancy shed additional light on the systemic and regional hemodynamic alterations brought about by the gestational process. The reader should be reminded that these were acute experiments and the rabbits were not fully recovered from anesthesia. Extrapolating such data to chronic experimental results, other animal species, or humans might not be appropriate. Blood, plasma, and red cell volumes. The present studies show that a progressive increase in total blood volume (45%), plasma volume (48%), and red cell volume (3~(/c·) occurs during term pregnancy in the rabbit. These values including the disproportional increase between plasma and red cell mass compare favorably with published data 9 using Cr 51 -labeled red cells in pregnant women. It is evident from the present observations that the plasma volume continues to increase up until delivery, although the rate of maximum increase occurs during tlw second third of gestation. Such an increase continues to be evident even when the data are calculated on the basis of body weight. The increase in maternal weight during the last one third of gestation in the rabbit is mainly due to the enlargement of the fetoplacental unit (Table 1). When this weight adjustment is taken into consideration, the progressive increase in blood and plasma volume remains even though it is much less. Thus, the statement often observed in the literature that the blood and plasma volumes per unit of weight remain constant during pregnancy is misleading if not incorrect. Hemodynamic changes. The present data show a definite decrease in the mean arterial pressure which is primarily related to a fall in the diastolic arterial pressure. This latter is in turn related to a fall in the total

Hemodynamic changes during pregnancy

595

systemic resistance brought about by the implantation of the placenta which acts as a low-1·esistance vascular structure in parallel with the other vascular beds. Evidence to this concept is obtained fi·om 1 hcse data which show that during the last third of gestation, when the placental blood flow in Group II and (~roup Ill animals is compared, a significant increase uccurs; this attests to the impact of the placental circulation 011 the systemic circulation and confirms information obtained from other animal species. 2 · 1 Our data further show that an increase in cardiac output occurs during pregnancy in tlw rabbit accompanied by minimal changes in heart rate: thus the increase must be due largely to changes in stroke \Olume. The changes in CO observed by us were greater than those reported by other investigators 6 • 11 ), 11 from rabbit gestation. Obviously, methodology and differences in experimental approach could account for the differences. At any rate our results seem to 'uggest that the maximum increase in cardiac output occurs during the second third of gestation with a slight fall occurring during the last third of gestation. Reproductive flow. Since the presence o! the ovaries is essential to the maintenance of pregnancy in the rabbits, 12 we have included the ovarian flow as part of the total reproductive flow. In this study the mammary glands' weight and flows were not measured. Total reproductive flow and total reproductiw weight in this discussion is taken as the sum of flows or weights to the following structures: ovaries, tubes. vagina, myometrium, and placenta in pregnant animals (Table Ill). The suggestion that the microsphere method, when used for regional circulation, gives relative organ llow' rather than absolute values, is not valid.''· '" Compared to the nonpregnant state, both tissue weight (ovaries, tubes, vagina, myometrium) and tissue How increased. Due to a disproportionate increase in flow. the ovarian flow per unit of weight increased and myometrial flow per unit of weight decreased (Table Ill). These changes occurred during the second part of gestation, and there were no further significant changes (except for the vagina) (Fig. I). During the last one third of gestation, the placental How total and per unit of weight increased (Figs. l and 2). The disllibution of flow in the reproductive system favored 1 he ovaries and placenta; both are sites of hormone production in the rabbit. The placenta also supplies the respiratory and nutritional requirements of the rapidly growing fetuses (Table I). Nonreproductive flow. Of the nonreproductive organs studied, the spleen showed the greatest decrease in blood flow per unit of weight. Although the hepatic

Ill

596

Nuwayhid Am.

blood flow per unit of weight also tended to decrease, the changes did not seem to be significant. Thus the decrease in the splenic and liver blood flows per unit of weight could be viewed as representing redistribution of regional circulation during the rabbit gestation in favor of the reproductive system. 15

The author gratefully acknowledges the technical assistance of R. Trollard, B.S.

REFERENCES 1. Hoversland, A. S., Parer,]. T., and Metcalfe, J.: Hemodynamic adjustments in the pygmy goat during pregnancy and early postpartum, Bioi. Reprod. 10:578, 197 4. 2. Metcalfe, J., and Parer, ]. T.: Cardiovascular changes during pregnancy in ewes, Am.J. Physiol. 210:821, 1966. 3. Hytten, F. E., and Paintin, D. B.: Increase in plasma volume during normal pregnancy, Br.]. Obstet. Gynaecol. 70:402, 1963. 4. Dilts, P. V., Brinkman, C. R., III, Kirschbaum, T. H., and Assali, N. S.: Uterine and systemic hemodynamic interrelationships and their response to hypoxia, AM. J. 0BSTET. GYNECOL. 103:135, 1969. 5. Adams,]. G.: Cardiovascular physiology in normal pregnancy: Studies with the dye dilution technique, AM. ]. 0BSTET. GYNECOL. 67:741, 1954. 6. Bruce, N. W., and Karim Abdul, R. W.: Relationship between fetal weight, placental weight and maternal placental circulation in the rabbit at different stages of gestation, ]. Reprod. Fertil. 32:15, 1973. 7. Tsuchiya, M., Walsh, G. M., and Frohlich, E. D.: Systemic hemodynamic effects of microspheres in conscious rats, Am. J. Physiol. 233:617, 1977. 8. Kaihara, S., Von Herrden, P. D., Migita, T., and Wagner,

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J.

9. 10. 11.

12.

13. 14. 15.

H. N .,Jr.: Measurement of distribution of cardiac output, J. Appl. Physiol. 25:696, 1968. Pritchard, T.: Changes in blood volume during pregnancy and delivery, Anesthesiology 26:393, 1965. Venuto, R. C., and Cerra, F.: Aortic constriction and uteroplacental blood flow in gravid rabbits, Clin. Res. 25:209, 1977. (Abst.) Ferris, T. F., Venuto, R. C., and Bay, W. H.: Studies of the uterine circulation in the pregnant rabbit, Lindheimer, M., editor:. Hypertension in Pregnancy, New York, 1976, John Wiley & Sons, Inc., pp. 351-361. Csapo, A.: The regulatory interplay of progesterone and prostaglandin F2a in the control of the pregnant uterus, in Jasimovich, J. B., editor: Uterine Contraction-Side Effects of Steroidal Contraceptives, New York, 1973, Wiley-Interscience, chap. 15, pp. 223-255. Greiss, F. C.,J r., Anderson, S. G., and Still,]. G.: Uterine pressure-flow relationships during early gestation, AM. J. 0BSTET. GYNECOL. 126:799, 1976. Makowski, E. L., Meschia, G., Droegemveller, W., and Battaglia, F. L.: Distribution of uterine blood flow in the pregnant sheep, AM. J. 0BSTET. GYNECOL. 101:409, 1968. Rosenfeld, C.: Distribution of cardiac output in ovine pregancy, Am.]. Physiol. 232:231, 1977.

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