Uterine pressure-flow relationships during early gestation

Uterine pressure-flow relationships during early gestation FRANK

C.

STEPHEN J.

GORDON

WinstomS&n,

JR., M.D.

GREISS, G.

ANDERSON, STILL.

Iv’orth

M.D. B.S.

Caroliwz

Placental and myoendometrial f.wessure$ow relationships were determined in 17 gravid ewes betuleen 29 and 110 gestational days with th use of radiowtiue-labeled mtirosphew injections under jtou~ meter guidance. Myoendometrial relationships were consistent& curz~ihnear ulith conz~exz’ty toulard the jlou~ axis. From 29 to 40 da?, caruncnlar-cotvledonary relationships were similar to myoendometrral ones. After 50 days, the relationship was slightly but signficantly curvilinear with convexity toward the @sure axis. From 42 to 50 days, responses ulere intermediate. The changes in rarurlcle-coty~l~(lo?l vascular reactivity correlated with the onset and maturation oj interdigitation of j&al and matrrnal tissues during de$nitiue placentation. A hypothesis jar thtw rhm,p~ it! ~wwuku rmctivity is presented. (AM. J. OBSTET. GYNEWL. 126:

799, 1976.)

PREVIOUS STUDIES OF the pressure-flow (P/F) relationship in the uterine vascular bed have shown an autoregulatory curve in nonpregnant ewes and a linear relationship during late ovine pregnancy.4s ’ These observations suggested the possibility that the nonplacental vasculature might maintain an autoregulatnry P/F relationship throughout pregnancy while the placental vasculature behaves as a passive widely dilated bed. If so, as the behavior of the placental vasculature becomes increasingly dominant in late pregnancy, a resultant linear P/F relationship of the total uterine circulation would be understandable. Indeed, observations of differential flow patterns at 110 days of gestation in a single experiment were consistent with this hypothesis. However, a simiIar single experiment at 50 days of gestation showed autoregulatory curves in both the placental and nonplacental vasculatures.’ This observation suggested that the placental vasculature might undergo a transistion in its inherent From the Department of Obstetrics Bowman Gray School of Medicine l~tzizmity,

reactivity as pregnancy progresses and that this transistion may be related to event of placentation. The purpose of the experiments reported herein was to confirm differential P/F relationships in the placental and nonplacental vasculatures and to determine if reactivity of the placental vasculature does change as pregnancy progresses.

Methods Gravid Western ewes between 29 and 110 days of gestation were fasted for 48 hours, placed in the right lateral decubitus position, anesthetized with nitrous oxide-oxygen and curare, and ventilated via an endotracheal tube from a respiration pump. Respiratory rate, tidal volume, and concentrations of inhaled oxygen were adjusted to maintain arterial blood gases within physiologic ranges. Cathethers were positioned in the common femoral artery, in the descending aorta just above its trifurcation, in a left uterine collecting vein, and in the left ventricle. An open-ended catheter was placed in the allantoic sac of the left uterine born, and an electromagnetic flow probe was placed around the left middle uterine artery or one of its ma-jar branches. In four ewes, flow probes had been implanted around both middle uterine arteries prior to conception. Finally, a plastic occluding device designed to permit graduated aortic occlusion was placed around the abdominal aorta proximal to the origin of the ovarian arteries. The aortic, venous, and intrauterine catheters were connected to precalibrated

and Gynecology, of Wake Forest

Suppor&ed by lIni&ed States Public Health SerzCe Grant )Vo. HL-03941-16 from the National Heart and Lung tmtitute. Presented at the Ninety-ninth Annual ‘i’merican Gynecological Society, Hot May 25-29, 1976.

Meeting Springs,

of the Virginia,

Reprint requests: Dr. Frank C. Greks, Jr., Department Obstetrtis and Gynecology, Bowman Gray School of I%fedkine, Winston-Salem, North Carolina 2710?,

of

799

pressure transducers, and their outputs as well as that of the flow probe were connected to individual channels of a dynograph. After stabilization of all parameters, progressive one-minute partial occlusions of the aorta were performed until complete occlusion was obtained. Details of these procedures have been reported previously.‘, ‘. ’ When the above observations were completed, 1 .O to 1.5 ml. of ‘%e-, “Cr-, “St-, or ‘%c-labeled microspheres (15 * 5 rnp) diluted to 10 ml. with 0.9 per cent sodium chloride followed by 10 ml. of 0.9 per cent a sodium chloride flush was injected into the left ventricle over 15 seconds during a control period. Sixty seconds after selected reductions in perfusion pressure, a similar injection was made over 10 seconds while the reduction was still being maintained. The physiologic parameters were recorded simultaneously. For the 15 seconds before, during, and for 30 seconds after each i?jection, blood was withdrawn from the femoral artery and the uterine vein at a constant rate of 3.88 or 19.1 ml. per minute to permit determination of total uterine blood flow (UBF\,) and to ensure that the microspheres were trapped in the uterine vasculatures. Then the ewes lvere killed, and the uteri were grossl) dissected into right and left caruncles and/or cotyledons, right and left myoendometrium, and cervix. Particular attention was directed to the location of the fetus or fetuses, the location of the fetal membranes, and the gross degree of attachment between the developing cotyledons and the fetal components of the placenta. Each uterine component was weighed and counted for gamma emissions as \vas each arterial and venous blood sample. In selected cases, the fetal membranes were also counted. UBF and distributional UBF were determined by the methods of Makowski and his associates.’ Details of the gross experimental procedures and the techniques employed for isotope counting have been presented previously.’ Control and response values of perfusion pressure (AP = aortic blood pressure [ABP] minus venous blood pressure [VBP]), intrauterine pressure (IUP), and flow-meter uterine blood flow (UBFfiJ were determined for ,each occhlsive episode. The average of three 10 second observations immediately preceding occlusion was considered the control value. Response values were taken as those immediately preceding aortic occlusion release, i.e., after 60 seconds of reduced AP. For microsphere injections, the averages of five second observations during the injection were considered as either control or response values. Proportionate changes in UBFb; and AP \vere evaluated by multiple linear regression and were tested to determine whether a quadratic coefficient was significantly different

(P < 0.05) from zero. If so, the relationship was called curvilinear. If not, it was called linear. To evaluate individual tissue flo- responses. control and response UBFbZ were multiplied by the proportion of blood flowing to the specific tissue being analyzed in the uterine horn ipsilateral to the Ho!\ probe. Thus, relative rather than absolute caruncular or (,otylcdonary and myoendometrial blood How were determined. Proportionate changes in tissue Holes and Al’ were then plotted against a straight line drawn betwren the control origins of both parameters aml the awrap. point of observed zero UBFb, on the AP axis. Responst, patterns were compared by the sign test (above 1~ below the straight line) and by the Mann-Whitney L test which is based on the magnitudes of differences of individual observations frorn the line.‘3

Rssll~ts Seventeen ewes between 29 and 1 10 gestational days were tested. Duration of pregnancy was co&n-mecl b!j personal observations of conceptual estrus under How-meter guidance in ewes in which How probes had been implanted and by correlation of fetal crol\‘nrump lengths with dates of conception supplied by our breeder. Ten single and seven twin pregnancies were tested, but results were similar in both groups. Data were analyzed in three groups determined by phases of placental development. Phase 1 was characterized bl absent or minimal attachment between the uterim caruncles and the fetal membranes. Prior to 36 gestational days, the nonpregnant, sessile, nipplelike caruncles enlarge and become more discrete abot,e the endometrial surface with an increase in the diameter of their central core. The fetal membranes overlie the caruncles without attachment. Between 36 and 40 days. fetal placental tissue develops from the membranes over the caruncular cores and begins interdigitation with maternal tissues. Upon gross dissection. it leas evident that the contact is slight since the fetal tissues fall away from the caruncles, leaving scattered blood? punctation in the cores. Phase 2 was characterized b+ conversion of caruncles to typical cotyledons and increased but incomplete fetomaternal attachment. Between 42 and 50 days, the caruncles become cupshaped and thicker, attenuating the endometrium above its normal surface level. Usually the fetal placenta could still be torn from the cotyledonary core without gross disruption of the cotyledon. In Ewe 74-4, at 42 days, with a single fetus in the left uterine horn, placental development was much more advanced on the left with firm but separable attachment while that in the right horn was more characteristic of 36 day development. Phase 3 was characterized by inseparable

Uterine

Table

I.

Clontrol

parameters

of pregnant

day

.4BP (nun. Hg)

29

105

74-101 74-105 74- 102 74-l

33 36 37 40

128 107 114 98

742

40

] 9‘1 --

i4--4 7%14 73-g

42 48 -49

129 11.5 109

73-8 72-l 1

49 ,50

98

73-15 73-6 i3-2 1 7:3-12 7:s5 72.12

56 60 63 68 7s

i4-

h’o, 103

Table xcording

(mm. Hg) 8.5 3.5 7.0

8.0

113 11-l 107 106

99

74- 103

Gcstahn

(dayx)

74-105

29 33 36

71-l

40

74-2

40

74-4

42 48 49 49 50

74-101

i3-l-% 73-Y

73-8 72-11 73-15 73-t?

73-21 73-12 72-]2

56 60 63 68 7.5

110

PIF

E

.5!)

8

‘48

20

ll.S.0

6 12 5

141 3i 120

26 6. 9y ZLJ

9.5 8.0 li.0

2.3 97 45

2‘4 27 .54

7.5 1:<.0

7!1 23 I

102 90 Ill

5.5 5.0 9.0 5.0 7.0

113.5 108

9.0

8

112 35

.va. oj 0, dlL.wJ?l.s

91.5

106

fetomaternal attachment and maturation of cotyledonary development. After 50 days, the cotyledons enlarged very rapidly in all dimensions, approaching term siLe by 70 days. Six ewes between 29 and 40 days of gestation, live het\+een 42 and 50 days, and six hetweet .56 and 110 days were tested. (:ontrol parameters of the tested ewes are shown in Table I. ABP levels were similar to those ohserved in long-term preparations. Perfusion pressures at zero UBFE varied from 5 to 13 mm. Hg, averaging 9.0 mm. Hg. UBFb, levels in the animals in which flow prohes had been implanted (series of 100) were similar to those ohserved during conscious observations on the day preceding short-term experimentation. UBFk;

~ml.lnt~r~~

100.0 106X)

7.0 8.0 7.0

*Convexity toward flcn\- axis in all cases.

UBFE

24 2.‘3 $31 -

9.0

Curvilinear* Curvilinear Linear Curvilinear Curvilinear Curvilinear Curvilinear Linear Curvlinear Curvilinear Lineal Curvilinear Linear Linear Linear Linear

AP at zero fh (mm. Hg)

13 9 9

96.5 124.5

127

curw

801

AP (ABP-VBPj (mm. Hg)

6.5 8.0 2.0

II. Type of composite P/F relationship t0 gestational age

Ew

73-5

VBP

118 1 l
110

relationships

ewes

GP,statlfJna~ Ew

pressure-flow

i,i

IT

105

9.0

205

102

I I.0

234

99

12.0

167

27 23 37 32

90

i.0

15‘)h

:?A

varied widely in thk short-term preparations. l’hese variations were thought to reflect implantations on uterine artery branches and the site and development of the placenta rather than experimental stress. No significant differences between control and response IUP were ohserved in any of the ewes tested. The types of P/F curves determined from UBF,.; and AP data are shown in Table II. Technical difficulties precluded such observations in Ewe 74-102. Up to 50 days. curvilinear relationships with convexity to the flow axis predominated. Thereafter, the rel&mship was primarily linear. In Ewe 74-105 at 36 davs and Ewe 73-6 at 60 days. the relationship described met our statistical criteria. hut only one or tt+o of !Zt! to 32 experimental observations accounted for most of the variation in each case. Difficulties with the microsphere te( hnique f\.ere typical of those previously encountered with rapid injec:tions. Grreiations between UBFkZ and L!BF&, within animals varied from 17 to 128 per cent clespite the fact that arterial blood was sampled rapidly during most experiments, uterine venous microsphere content was less than 1 per cent of that in arterial samples, and fetal membranes contained no significant radioactivity. Retognizing these limitations, we still calculated L’ BFb, per kilogram of uterine ancl fetal weight. These observetions are shown in Fig. 1 with respect to the curve determined by Huckabee and his associates.’ Distribution of uterine hlood flow (VBF) between caruncle-cotyledon and myoendometrium is shown in Fig. 2 with respect to pregnancy durSion. L’p to 40 days, myoendometrial flow exceeded 70 per cent of total UBF while caruncles and/or cotyledons received less than 30 per cent of total UBF. Aftcar 60 days.

802

Greiss,

0’

Anderson,

’

IO

”

and

30

”

SMI

50 Gestation

Fig. 1. Uterine blood flow during posed on a graph adapted from

”

”

70

90

”

II0

’

Day ovine

pregnancy

superim-

Huckabee and associates. (Huckabee, W. E., Metcalfe, J., Prystowsky, H., and Barron, D. H.: Blood flow and oxygen consumption of the pregnant uterus, Am. J. Physiol. 200: 274, 1961.) The values determined by the microsphere technique (solid symbols) generally exceeded those determined by the aminoantipyrine method (solid line). l’he two values over 1,000 ml. per kilogram per minute probably reHect methodologic errors. (Solid circles and squares indicate single and twin pregnancies, re-

cent of the original control conductances. In ~hrc~~ Vperiments, where repeat control microsphere injcc tions were made after +28 to -9 per cent conductancc~ changes, distributional flows varied only 1.5 to 3 per cent of the original values. Myoendometrial responses according to phases 01 placental development are shown in Fig. 3. From 2Y to 40 gestational days, 13 of 14 responses (P = 0.001) csceeded the linear line. From 42 to 50 days. eight of’ nine responses (P = 0.02) exceeded linearity, and aftet 55 days, all responses (15 of 15, P < 0.001) cxcecdcd linearity. According to the Mann-N’hitney test. myoendometrial responses between Phases 2 and 3 were not different. Between Phases I and 2 and Phases I and 3, responses were increasingly curvilinear% P < 0.05 and <0.02, respectively, with increasing gesta tional age. Cotyledonary responses according to phases of placental development are shown in Fig. 4. From 2Y to 40 gestational days, 12 of 14 responses (P < 0.0 1) cxceeded the linear line. From 42 to 50 days, no difk.l.ences from linearity could be discerned. After .50 da!s. 12 of 15 responses were below the linear line (P < 0.02). Significant cotyledonary response dif’fi>rences were present only between the Phase 1 and Phase 3 groups (P < 0.002). Myoendometrial and caruncle-cotyledonq responses are shown in Fig. 5 according to the phase of placental development. During Phase I, responses of’ both vasculatures were similar. Thereafter, during Phase 2, myoendometrial flows exceeded those of the placenta (P < 0.02), and during Phase 3, ai1 myoendometrial flows exceeded linearity while cotyledonq blood flows were below the linear line (P < O.OOJ).

spectively-.)

Comment cotyledonary flow exceeded 80 per cent of total UBF and myoendometrial flow was below 20 per cent of total UBF. Between 42 and 60 days, distributional flows were transitional and very variable on a given gestation day, probably related to the degree of definitive placentation. Generally, UBF distribution during control periods was determined only once in a single experiment. If subsequent control uterine conductances (UBFK/AP) varied markedly from the original control value, a second control microsphere injection was made. Thus, 20 control and 40 response injections were made. Control conductances varied * IO per cent of the original control 28 times. Before six microsphere-measured occlusions, control conductances were + 11 to + 15 per cent of the original control value, and before 4 occlusions, control conductances were between - I I and - 19 per

Previous experience with the microsphere method indicates that when rapid injections are necessary distributional UBF is highly reproducible while considerable differences may occur in determinations of UBF$,, presumably because of nonrepresentative sampling ot arterial microsphere content.’ For these reasons% wc felt

that

e xpresslng ’

mdlvldual

uterine

tissue

fiords

in

relative rather than absolute terms would be mow accurate. Despite such potential limitations in determining UBFM, it is of interest that values presently observed consistently exceeded those reports by Huckabee and his associates. Such results suggest that our control UBF&, values were reasonably accurate and that early in gestation UBF per unit weight of the uterus and its contents may be higher than that previously reported. The present observations provide further evidence

Uterine

pressure-fbw

relationships

603

l l .

l

l

l

l l

- Caruncles-cotyledons

x - Myoendometrtum

X

X X

X x

I 40

I 50 Gestation

Fig. 2. Distribution preplacentaf-placental live plxentation

of uterine blood flow during sites and to the nonplacental occurs.

X

l l

29-40 days 42 - 50 days X~~-II~~CWS

x

x

l

l

l e /

x

l

‘5575 ii0

Day ovine pregnancy. The tissue (myoendometrium)

2 2 z b %

x

I 70

I 60

proportions of blood to the are reversed as defink

I40 I20

l 29-40 8 42-50 x 56-110

days days days

X

x

20 AP

40

60

80

(%

of control

)

I00

Fig. 3. Myoendometriai pressure-how responses at progressive phases of placentation shown with respect to a linear relationship. Proportionate flow responses were consistently higher than the decreases in perfusion pressure (P < 0.02 to
I 20 fiP

I i 40 60 f% of control

I 80 ;

1 IO0

Fig. 4. Caruncle-cotyledon pressure-flow responses at progressive phases of placentation shown with respect to a linear relationship. From 29 to 40 days, proportionate flow responses were higher than the decreases in perfusion pressure (P < 0.01). After 55 days, flow responses were lower than the decreases in AP (P < 0.02). Between 42 and SO days, rcsponses were intermediate.

804

Greiss,

Anderson,

and Still

I40

29-40

I20

X = Myoendometrium l =Coruncle-cotyledon

56-110

42- 50 do@

doys

doys X

20

1 40

1 60

t 60

1 Ia3

20 4 P

40 1%

60 of

Fig. 5. Comparison sive phases were similar, nonplacental

of caruncle-cotyledon and myoendometrial of placentation shown with respect to a linear Thereafter. placental responses approximated responses were unchanged or accentuated.

for the concept that the linear relationship between total UBF and perfusion pressure observed after 50 days of ovine gestation is really the resultant of differential P/F relationships in the placental and nonplacental circulations. The placental curve is curvilinear with subtle convexity toward the pressure axis. The nonplacental (myoendometrial) curve is more markedly curvilinear, but with convexity toward the flow axis similar to the autoregulatory P/F curve observed in the nongravid and early pregnant uterus. As pregnancy progresses, the proportion of UBF perfusing the placenta progressively increases. Therefore, the characteristics of the placental vasculature become increasingly dominant. The reactivity of the nonplacental bed unbends the placental curve to a straight line. Although the curvilinearity of the nonplacental curve increased significantly as pregnancy progressed in these experiments, we have no explanation for such accentuated reactivity unless it would be to facilitate the maintenance of myoendometrial blood flow during the increased myometrial activity of prelabor and labor. The changes in vascular reactivity and distributional blood flow occurred primarily between 40 and 50 days of ovine gestation, the time definitive interdigitation of fetal and maternal tissues occurs. Between 50 and 70 days, Barcroft described a mushroom growth of placental tissue resulting from the development of shafts of fetus-derived Wharton’s jelly which form a scaffold on which fetal villi are built up. The rapid enlargement of cotyledons observed after 40 days suggests that ear-

60

I00

20

40

60

80

loo

control 1 pressure flow responses at progresrelationship. BeFore 41 days, responses and then subtended the linear line while

liest ingrowth of such fetal tissue has already begw. Initially the fetal masses contain only a few axial vessels. The maternal vessels, on the other hand, permeate the whole tissue. 0ne would like to explain the changes in vascular reactivity from the caruncular to the cotyledonary phase on the basis of these morphologic changes. In addition, an estimation of the timing of analogous changes during human placentation would be desirable. The recent observations of Ramsey and associates” in the primate indicate that vascular morphologic changes may alter intervillous space circulation. Cytotrophoblastic cells begin to migrate into the lumina of maternal blood vessels as soon as the invading blastocyst taps the subepithelial capillary network. In man, these cells migrate along the walls of the spiral arterioles, even involving some myometrial segments. and accumulate almost to the point of occluding the lumina. Thereafter, cytotrophoblast destroys the vascular endothelium and invades and replaces tissues of’ the vessel wall, eventually forming the primary element of its structure. Subsequently, endothelium regenerates over the trophoblast wall4 and the lumina are no longer clogged. Such a vessel wall, devoid of smooth muscle elements, would act as a passiire tube and would be compatible with the placental P/F relationship determined in the ewe. Given these observations, one must actually

then

ask in what occurs

for

vessels spiral

placental

muscle cannot actively constrict

vasoconstriction

of smooth any more than they can

arterioles

devoid

Uterine pressure-flow

actively dilate. Available data suggest that such reactivity occurs in blood vessels of a generally larger caliber than those which are usually the primary site of vascular resistance. For example, nonplacental vascular reactivity to alpha-adrenergic stimulation is greater than that of placental vessels.‘. ” Were the vasoactive placental vessels of a larger circumference than the nonplacental ones, a greater degree of vascular smooth muscle contraction would be necessary to cause resistance increases comparable to those in smaller vessels. This would explain observed differential vascular reactivity. Similarly, during resting tonus, placental vessels would cause less resistance than smaller nonplacental ones and would be compatible with the concept of a widely dilated placental vascular bed. These considerations suggest that a function of the placentation process may be to displace the site of active placental vascular reactivity from blood vessels CJ~’a small caliber to those of an increasinglv larger cross-sectional area. In human subjects, it would appear that effective although not yet maximal intervillous space circulation is operating by 8 to 10 weeks after conception.1o Such timing is relatively very similar to that of definitive placentation during ovine gestation. However, ovine placentation is quite different from that in the primate, and it is dif’ficult to correlate morphologic changes with alterations in vascular reactivity. In preliminary studies in ewes after i0 days’ gestational age, RamseyI’ could find no evidence of trophoblastic replacement of maternal vessel walls lrithin the cotyledons. There also is 110 evidence of smooth muscle in the \vaIls of the vessels which radially penetrate the cotyledon. In injection studies, I4akowski described these vessels as dilated. Proximal to the dilated vessels, he observed focal

relationships

805

areas of constriction associatecl \\ith sphincterlike thickenings of the muscular bayer of the radial arteries at the base of the cotyledons. Ramse~z also observed that smooth muscle is present in thc*s<, vessels It WJdd a[JpC!x fIxH11 but only in average amounts. these observations that with mature placcntai d~velopment functional vascular anatomy and physiolog? may be very similar in the ewe and primate. However, the mechanism of these developmentat changes is unknown, and further morphologic studies must bc performed during early phases of ovine ptacentation. The present observations also provide a possible explanation for the high UBF rates per unit L%eight of the uterus observed during the first halfofovinc gestation. Before cotyledonary development, thcrc ih II<) intimate contact between maternat and l’etal ve\scls, and fetomaternal exchange is inefficient, necessitating high UBF rates. When definitive placelltation begil~s, a more efficient exchange system develops. and LIBF is appropriately shifted to these areas. .l’hcs ~II~IOUII~ of blood necessary for adequate fetal nutrition progressively decreases as the placenta c~~~~I~J~Js until a (‘onstant flow rate is reached when the placenta matures midway in gestation. Bv these changes, thts &&tory burden imposed upon the mother t~j the tc,rns is minimized in anticipation of the marl&h increaseci nutritionat demands of the fetus during its phase f)f rapid absolute growth in the last halI’ of gcstatic,t,. The authors wish to thank Dr. Elizabeth ht. for her assistance and guidance during prepal the manuscript and Dr. ‘4. Leonard IUl~we th vice and assistance in interpreting the st‘ttistic

Damsel ation oi his adal data.

REFERENCES

Anderson. S. G., and Hackshaw, B. T.: The effect of estrogen on uterine blood flow and its distribution in nonpregnant ewes, A~I. J. OBSTET. GYNECOL. 119: 589, 1974. 2. Anderson, S. G., Still, J. G., and Greiss, F. C., Jr.: Unpublished observations. 3. Barcroft, J,: Researches on Prenatal Life, Springfield, IIlinois. 1947, Charles C Thomas, Publisher. 4. Greiss, F. C., Jr.: Pressure-flow relationship in the gravid uterine vascular bed, AM. J. OBsrrr. GYNECOL. 96: 41. 1966.

Greiss, F. C.. Jr., and Anderson, S. G.: Pressure-flow relationship in the nonpregnant uterine vascular bed, AM. J. OBSTET. GYNECOL. 118: 763, 1974. 6. Huckabee, W. E., Metcalfe, J., Prystowsky, H., and Barran, D. H.: Blood flow and oxygen consumption of the pregnant uterus, Am. J. Physiol. 200: 274, 1961. 7. James. F. M., III, Greiss, F. C., Jr., and Kemp, R. A.: An evaluation of vasopressor therapy for maternal hypoten-

8.

9.

10. 11.

5.

12. 13.

sion during spinal anesthesia, Anesthesiolqv 33: 25* 1970. Makowski, E. L.: Maternal and fetal vasc.uiar nets in placentas of sheep and goats, AM. J. OBSTY~. GYNECOL. 100: 283, 1968, Makowski, E. L., Meschia, G., Droegemueller. W.. and Banaglia, F. G.: Distribution of uterine blood flow in pregnant sheep, AX J. OBSTET. GYNEWL 101: 409, 1968. Ramsey. E. M.: Personal communication. Ramsey, E. %I., Houston, M. L., and Harris, J. W. S.: Interactions of. the trophoblast and maternal tissues in three closely related irimate species, AM. J. OLDSTER. GYNECOL. 124: 647. 1976. Rosenfeld, C. R., B&on, M. D., and Meschia, G.: Effects of epinephrine on distribution of blood fl~n\ in the pregnant ewe, AM. J. OBSTET. GYNECOL. 124: 1.56, 1976. Siegel. S.: Nonparametric Statistics, iYe\\ York, 1956. McGraw-Hill Book Company. Inc.

806

Greiss, Anderson, and SM

Discussion Denver, Colorado. Dr. Greiss and his co-workers have provided us today with additional information on uterine hemodynamics by extending their basic observations on uterine pressure-flow relationships in the sheep to include the first two thirds of gestation. Previously, Dr. Greiss had demonstrated that the composite uterine pressure-flow relationship in nonpregnant sheep was curvilinear with its convexity toward the Row axis. Such a response is indicative of the presence of autoregulation. However, similar studies at 110 days of gestation demonstrated a linear uterine pressure-flow relationship. This observation implies that the pregnant uterus at 110 days of gestation is unable to maintain its blood flow during changes in perfusion pressure. Since the pregnant uterus contains placental, endometrial, and myometrial vascular beds, the authors estimated not only the composite uterine but also the platen tal and myoendometrial pressure-ffow relationships in ewes between 29 and 110 days of gestation. Up to and including 50 days of gestation, they predominantly observed a curvilinear uterine pressure-flow relationship with the convexity toward the flow axis. Thereafter, the response was primarily linear. On the other hand, the combined myoendometrial vascular beds responded in early pregnancy with a curvilinear pattern, and the convexity toward the flow axis became progressively more accentuated with the duration of pregnancy. However, it should be noted that the vascular beds of the myoendometrium were not studied separately and that the response of these DR.

EDGAR

L.

MAKOWSKI,

two vascular beds could be dissimilar. Moreover, the caruncular-cotyledonary or placental vasculm~ bed showed a similar pattern to that of the myoenclom~trium up to 50 days of gestation, After 50 days. the placental pressure-flow relationship became rc~rscd so that the convexity was toward the pressure axis. This latter pattern would be consistent with a passi1.t. pressure-flabrelationship and, according to the authors, it correlated with the gross degree of fetal villous attachment to the caruncles. only relative tissue flows were estimated in this stud! because the investigators noted a “17 to 128 per cent” variation between uterine blood flow determinations by the electromagnetic flow probe technique and the microsphere tnethod. In 61 uterine blood flop measurements we have not experienced a discrepancy of such magnitude.’ Fig. 1 shows a comparison of uteri~~~ blood flows measured simultaneously by the electromagnetic flow probe technique and the microsphere method. There was exceIlent correlation betlceen the two techniques (R = 0.94). The variations bet%veen the two methods in their study were most likely dw to difficulties with arterial sampling, as already noted b! the authors, or to defective flow probes. During ovine pregnancy. the trophoblastic cells, as evidenced by electron microscopic studies. do not destroy the endometrial surface epithelium at sites of implantation.2 Furthermore, it was suggested rhat changes which the investigators observed in placental vascular reactivitv tnay have been due to the composition of the vessel walls. Apparently no stnooth muscle elements were demonstrated histologically in these in-

500

0 Blood

flow

ml/min

Fig. 1. Comparison of uterine blood flow measured probe technique (ordinate) and by the microsphere

simultaneously method (abscissa).

by the electromagnetic

flOW-

Uterine

tracotyledonary vessels, and the authors conjectured that this may &count for the passive behavior of the placental vascular bed. Kecent studies by Rosenfeld, and colleagues,’ in our laboratory, showed that the infusion of systemic doses of epinephrihe without alterations of systemic pressure in pregnant sheep during the last 60 days of gestation did indeed produce a differential reactivity of the various vascular beds. The reduction in endometrial flow was 59 per cent whereas the decreases in the myometrial and placental flows \t.c’re quite similar, 37 and 35 per cent, respectively. Therefore, all three vascular beds are sensitive to the vasoconstrictive effect of a catecholamine. As pregnancy progresses, there is a rapid shift of uterine bloocl flow to the placenta which receives about 27 per cent of the total flow prior to 50 days of gestation and approximately 82 per cent from 100 to 140 days. When blood flow per gram of uterine mass is examined, there is a rapid decline from 40 to 80 days of gestation. This reflects a decrease in the flow per gram of tissue to the endometrium: myometrium, and placenta. However, the absolute flows, in milliliters per minute, to the endometrium and placenta increase substantially during this interval. These changes in flow may be the result of different growth patterns of blood flow and tissue weight. REFERENCES

Rosenfeld, C. R., Morriss, F. H., Makowski, E. L., Meschia, G., and Battaglia, F. C.: Circulatory changes in the reproductive tissues of ewes during pregnancy, Gynecol. Invest. 5: 252, 1974. Ludwig, K. S.: Zur Feinstrukur der materno-fetalen Verbindung im Placentom des Schages, Experientia 18: 212, 1962.

Rosenfeld. (:. U., Barton, M. D., and Meschia, G.: Effects of epinephrine on distribution of blood flow in the pregnant ewe, AM. J. OBSTET. GYNECOL. 124 156, 1976. DR. HARRY PRYSTOWSKY, Hershey, Pennsylvania, The data in this report by Dr. Greiss appear to have been obtained with his usual skill and high standards of technique. Investigative work such as this one provides new support for the old adage that progress in science is dependent upon the development of new techniques. His point that some of the estimates of the flow rates which were made by Huckabee, Metcalfe, Barron, and myself early in gestation with the antipyrine technique may have been too low is one I would agree with; indeed, I think we indicated previously that might be the case. The demonstration that the fraction of the total volume of blood reaching the uterus that perfuses the myometrium and endometrium, respectively, changes as gestation advances and that the new fractions are about equal around the fiftieth day of gestation is very nice. I have never really understood what to do with

pressure-flow

relationships

807

pressure-flow curves: they have always puz&d me. Accordingly, I do not undqrstand the significance of a change in the curvilinearity. But the change appears to be well documented, and it is of interest that it takes place around the fiftieth day of gestation. .l’he correlation in time with the “maturation” of thy placenta is interesting. But the case that the events are causal11 related will require further evidenc.e for iIs cstablisliment. As I reviewed this report, a series of yuestions cams to mind, and I pose them to Dr. Greiss. Specific answers may not be at hand, but I feel that intelligent speculation can, and frequently does, go bevond tc&nical advances and awaits their development. What causes the development of the uterine circulation during gestation? What is responsible for vessel g:owth? What regulates the rate of uterine blood flow? What are the factors involved in the relationship between fetal weight and the development of the uterine circulation? Stated in other words, does the fetus only grow to the point that its metabolic needs <‘an he handled by a given state of development of the uterine vessels? 0r does the fetus grow at its ow11 rate at all times, then inducing changes necessarv for the uterine vessels to satisfy its needs? DR. RONALD A. CHEZ, Bethesda, .~aryland. Dr. Greiss has consistently provided cornerstones to the foundation of our knowledge about o\+ne uterine blood floti. Today. he once more offer3 important conclusions as well as hypotheses li)r future Investigation. Dr. Greiss has repeatedly emphasized in all of his published work the induced experimental variables, the opportunities for methodologic error>, in the assays used, and the difficulty in achieving both reproducibility and correlation between different methods measuring the same variable (such as uterine hl<>od flow via electromagnetic ancl microsphere approaches). It is necessary to recall that his data arr from fisted anesthetized animals lying on their right side l+ith respirations controlled. Also, the flow probe is on the middle uterine artery. This means that uterine blood flow data from the m&n uterine artery to the dorsal uterine artery. This artery can have longitudinal branches which variably anastomose with branches of-the micldle uterine artery. This defined and specific set of applied experimental conditions is acceptable because the same set is repeatedly recognized and defined hv the investigator. However, this set may not lend itself to ready comparison with the work of the referenced investigators. Namely, there is a minimal 10 per cent error for the 4-aminoantipyrine technique in Huckabee’s amsthetized goats; there is a wide range of the per cent distributions of uterine blood 80~ in the last month ot gestation as determined by the microsphere t.echnique in Makowski’s awake nonstressed standing ewes, and there is a need for exogenous estradiol benzoate and progesterone t.o enhance flow in Greiss’s nonpregnant

anesthetized ewes. What happens though, and it happens to all of us in each of our areas of endeavor, is that only the conclusions of each of these studies are used conveniently as a shorthand which, however, subsumes the cited variables. There has to be caution. All of LIS who work with animal models of human pregnancy have persistent and unresolved concerns, if not disillusionment, about the variables that we introduce. Certainly, anesthesia, the one variable that is crucial to allow work in some species and that certainly provides a convenient working environment in others, induces changes that are very difficult to quantify. Kecent work of Assali and Brinkman highlights this problem in sheep. Therefore, I ask Dr. Greiss for his view as to how the data might change if it were possible to perform this same work in unanesthetized postoperative animals? The possibility exists that uterine vascular dilatation is not an active response, perhaps to estrogenprostaglandin stimulation but rather reflects a passive response secondary to a lessened vessel capacity to constrict. Vascular constriction tnay be associated with hypertensive states. Since pressure and flow are directly related, would Dr. Greiss speculate as to what happens to nonplacental and placental curvilinearity in the presence of maternal hypertension? Dr. Greiss posits that the efficiency of exchange is enhanced by the anatotnic changes that occur at the end of the first and the beginning of the second trimester in the sheep. This results in or allows decreased uterine blood flow. But what is the signal that indicates that this enhancement of efficiency has occurred? IS it repetitive throughout gestation? The latter has clinical implications. Perhaps acute episodic inefficiency of exchange stimulates increased uterine blood flow. When this signal or mechanism is inoperative, a chronic inefficiency state then eventuates in intrauterine growth retardation. Perhaps there is a clue for therapy of this prevalent obstetric problem. DR. K. WULF, WurtzburgY Germany. I would like to ask Dr. Greiss what are the limiting factors in fetal growth or fetal nutrition. If you have a curvilinear relationship between pressure and flow, it could be the obtained uptake, but. if it is linear, is it altered flow or altered glucose or protein uptake? DR. ELIZABETH RAMSEY, Washington, D. C. Like all good investigators, Dr. Greiss, while answering certain questions. has raised even more profound ones. He suggests, or throws out for discussion, the possibility that the change in structure of the uterine vessels in the ewe is similar to that which occurs in the primate; in the latter instance by virtue of trophoblastic action, which has not as yet been found to be duplicated in the ewe. This directs our attention to the marked difference between the hemochorial placenta and the epithe-

liochorial placenta and leads us into consideration of certain profound aspects of trophoblastic nature and action. This could, in the long I-LI~I, have important clinical applications. DR. GREISS (Glosing). I should like to thank the discussants for thinking I would be able to even begin to answer any of the questions they posed to me. Wc mmt remetnber that the present observations represent .just a small piece of a large puzzle. If we get enough pieces of that puzzle, the big picture may become clearer. Drs. Chez, Prystowsky, and Wulf asked about limiting factors with respect to fetal growth. ~4Ithotigh delivery of oxygen to the fetus is flow-limited9 I think we tend to forget at times that blood flow may not be 11~ end-all. That is hard for me to say since I have spent so tnuch time in this area but there are other limiting factors for transfer of other nutrients and, when one thinks of intrauterine growth-retarding factors. the composite effects of all of them, tnany as vet unrecognized, must be considered. In response to Dr. Ghez’s questions, although we did use estrogen in the nonpregnant ewes prior to obtaining pressure flow curves, control flows were within the ranges observed during the normal estrous cycle rather than at the maximal values observed after acute in travascular estrogen administration so we think that the resting levels of vascular tonus approxitnated normal conditions. Anesthesia certainly may be a f’actor in such experiments. We were concerned that vve got such high proportionate distribution to the placenta so earl? in pregnancy as compared to the 80 per cent values Dr. Makowski found only after 110 days of gestation. If we accept differential reactivities of’ the two vasctdatures to stress, such distribution might indicate increased adrenergic stimulation. However, these experiments could not be done with awake animals, and we believe the high slopes observed suggest minimal adrenergic. stress. I completely agree with Dr. Chez that the degree oi observed dilatation of the placental vasculature is not an active process. Rather the vascular changes probably follow some other process. I was trying to hypothesize about that process. With respect to hypertensive complications of pregnancy, since we do not know the specific stimulus causing the hypertension, we do not know whether it would stimulate similar responses in the uterus as in the pcriphery. Therefore, speculation is difficult except to say that we tnust be careful when we give antihypertensive medications since UBF may follow the linear curve presently described. In conclusion, I would like to acknowledge my coauthors, Dr. Stephen Anderson and Mr. Gordon Still. In particular, 1 would like to thank Dr. Ramsey who has given us so much advice about the speculations that evolved after the data were accumulated.