Blood gas analysis of placental and uterine blood during cesarean delivery

Blood Gas Analysis of Placental and Uterine Blood During Cesarean Delivery TOSHIO FUJKURA,

MD, AND JOJI YOSHIDA,

Objective: To measure blood gases in uterine venous blood and maternal and fetal blood from the placenta, and to characterize gas exchange in the intervillous space. Methods: Blood gas measurements were performed immediately after collecting placental and uterine blood from the subchorial and marginal lakes, from the chorionic vein and artery in the placenta in utero, and from the uterine vein during 12 cesarean deliveries. Results: The mean oxygen pressure (PO,) values of the chorionic vein and subchorial lake were 28.7 f 6.0 and 29.9 f 7.5 mmHg, respectively, with a difference of 1.2 mmHg. The individual data for PO, of the chorionic vein exceeded those of the subchorial lake in five subjects and were almost equal in two of the 12 subjects. The mean values of carbon dioxide pressure (PCO,) and bicarbonate were greater in the chorionic vein than in the subchorial lake, but the mean pH values were the same in the two groups. The mean values of blood gas analysis were not different between subchorial and marginal lakes with similar blood composition. The mean PO, of the uterine vein in ten subjects was 45.9 mmHg, significantly higher than that of the subchorial lake. Conclusions: The human placenta may be defined as a multivillous model with a high degree of oxygen transfer. Arteriovenous anastomoses are suspected in the pregnant uterus beyond 37 weeks’ gestation. Subchorial and marginal lakes contain similar admixed blood, which circulates and performs gas exchange. (Obstet Gynecol 1996;87:133-6)

Gas analysis of blood from the intervillous space of the human placenta has been conducted by several investigators,lm5but the blood for these studies was collected through transabdominal or transuterine aspiration of the placental site. It is extremely difficult to ascertain the source of blood obtained by transuterine puncture of the placental site, even with the exposed uterus. The oxygen pressure (PO,), pH, and carbon dioxide pressure (PCO,) values often vary from one area to another,

From the Deprtment of Obstetrics ad Gynecology, Tokyo Dmtal College, Ichikawa General Hospital, lchikazua, Chiba: and the Depnrfmerit of Pathology, Keio Unizwrsity School of Medicine, Tokyo, Inpan.

VOL.

87,N0.

l,JANUARY

1996

MD even with the same subject. It has been suggested that the variations of these blood gas values might be attributable to the complexity of the intervillous space and the nonhomogeneity of the intervillous blood.’ Although experimental studies6r7of placental oxygen transfer have been reported, blood gas analysis of the maternal blood in the intervillous space has been neglected. The human placenta is known as a multivillous model becauseeach placental exchange unit comprises thousands of chorionic villi.’ This type of flow exchanger is less effective than a countercurrent exchanger, but more effective than a venous equilibrator. Previous studies7,8on placental oxygen exchange have suggested that the human placenta may function as a venous equilibrator. Venous drainage of the pregnant uterus is not well known in relation to placental circulation. Blood gas values of blood obtained from the uterine vein have been compared with those from umbilical vein blood during cesarean delivery.6-8 The umbilical venous PO, is lower than the uterine venous PO, and, conversely, umbilical venous PCO, is always higher than that of the uterine venous blood. Such a low degree of placental oxygen transfer has been defined as venous equilibrator.7,8 The purpose of this study was to measure blood gases in uterine venous blood and maternal and fetal blood from the placenta, and to characterize gas exchange in the intervillous space.

Materials and Methods This study included 12 pregnant women delivered by elective cesarean after 37 weeks’ gestation. All had appropriate for gestational age fetuses, and the indications for cesareanwere repeat cesareandelivery, pelvic contraction, or breech presentation. The subjects came from the obstetric service of Ichikawa General Hospital in 1992. In cases of onset of labor or maternal or fetal abnormal conditions, subjects were excluded before

0029-7844/96/$15.00 SSDI 0029-7844(95)00300-2

133

Table

1. Blood

Gas Values

of Placental

and

Uterine

Blood

rco, Chorionic win Subchorial lake Marginal lake Chorionic artery Uterine vein PO, = oxygen Data

pressure;

are presented

,i

PH

12

7.34 2 0.07

28.7

2 6.0

45.3

12 10 Y

7.33 + 0.06 7 33 k 0.03 7.31 t 0.06

29.9 35.0 21.7

2 7.5 t 6.1 + 4.1

41.8 40.1 48.8

10

7.3h k 0.01

45.9

2 h.5

41.9

PCOz = carbon

as mean

+ standard

dioxide

(mmHg1

pressure;

HCO,

= bicarbonate;

Fujikura

and

Yoshida

Plimwtd

saturation

hlmol/L~

(lo)

k 10.0 i 10.3 2 7.3

24.6 22.0 21.4

i 2.1 i 3.3 i 3.4

50.4 52.1 62.4

-c 16.2 k 18.3 f 13.1

2 8.3 i 4.8

24.7 23.7

k 1.2 i 2.0

32.7 79.0

c 10.7 2 8.4

0, y oxygen.

deviation.

elective cesarean. Placental location and fetal size, compared with gestational age, were determined by serial ultrasonographic measurements. The subjects were delivered under epidural anesthesia. Arterial blood oxygen saturation was measured continuously by placing a pulse oximeter (Nippon Koden OLV-1200, Nippon Koden, Tokyo, Japan) on the tip of the index finger. This oxygen saturation was approximately 97% without oxygen administration. The infant was delivered by routine technique and, within 3 minutes after delivery, a 21-gauge needle was inserted into each of four placental sites-subchorial lake, marginal lake, chorionic artery, and chorionic vein-to collect 3 mL of blood in preheparinized syringes. To expose the uterine vein before the transverse uterine incision, we made an incision in the vesicouterine serosa, and uterine venous plexuses were identified. The same volume of blood sample was collected from the uterine vein before cesarean. The uterine vein was sampled on the side of placental location or on either side if the placenta was located in the mid-portion of the uterus. Blood pH, PO,, PCO,, bicarbonate, and oxygen saturation were immediately measured by a blood gas analyzer (CibaCorning 278 pH/blood gas analyzer; Ciba-Corning Diagnostics, Medfield, MA). To collect blood from the subchorial and marginal lakes, two needles attached to appropriate heparinized syringes were inserted tangentially into the amniotic surface of the placenta to a depth of about 0.5 cm. The central portion of the placenta, excluding the insertion area of the umbilical cord, was selected to draw blood from the subchorial lake. The marginal lake was located near the placental margin, and such localization was determined by needle tapping. The chorionic artery runs parallel to the chorionic vein on the fetal surface. The number of collected blood samples varied by placental sites and the uterine vein because of technical difficulty and inconsistent blood sampling. Paired f test and t test for small samples were used to calculate the significance of the differences (Table 1).

134

02 HCO,

Rloorl Gas

Results Table 1 shows the mean values of pH, PO,, PCO,, bicarbonate, and oxygen saturation of the blood samples obtained from the four placental sites and uterine vein. The mean POZ value of subchorial lake blood was not significantly different from that of chorionic vein blood (.3 < P < .4). The difference of mean PO, values between chorionic vein blood and subchorial lake blood was 1.2 mmHg. The PO, values of chorionic vein blood exceeded those of subchorial lake blood in five patients and were almost identical in two. The mean PO, value of marginal lake blood was not different from that of subchorial lake blood (.l < P < .2). The mean PO, value of uterine venous blood was significantly higher than that of the blood samples obtained from the subchorial lake (P < ,001). The difference of PO, between uterine venous blood and subchorial lake blood was 16.0 mmHg. The mean PO, value of chorionic vein blood was significantly higher than that observed in chorionic artery blood (.OOl < P < .Ol). The mean PCO, value of chorionic vein blood was significantly higher than that of subchorial lake blood, with a difference of 3.5 mmHg (.02 < P < .05), but the PCO, values of the chorionic vein in the individual data were almost identical to those of subchorial lake blood in two subjects. The mean PCO, value of chorionic vein blood was significantly lower than that of the PCO, of chorionic artery blood (.02 < P < .05). The mean value of PCO, of uterine venous blood was almost identical to that of the PCO, value of subchorial lake blood (P > .9). The mean PCO, value of chorionic vein blood was not higher than that of uterine venous blood t.2 < P < .3). The mean values of PCO,, bicarbonate, and oxygen saturation of subchorial lake blood were not different from those of marginal lake blood c.6 < P < .7, .6 < P < .7, and .1 < P < .2, respectively). The individual data of oxygen saturation rates were parallel to those of PO, values. The mean value of oxygen saturation of subchorial lake blood was not higher than that of chorionic vein blood (.l < P < .2). The mean value of bicarbonate

Obstetrics

& Gynecology

of chorionic vein blood was significantly higher than that of subchorial lake blood t.02 < P < .05). The mean pH value of the blood obtained from the subchorial lake was 7.33 ? 0.06 and was not different from that of chorionic vein blood (.3 < P < .4).

Discussion The difference in mean PO, values between chorionic vein blood and subchorial lake blood was 1.2 mmHg, indicating that the human placenta may function as a multivillous mode17,’ with a high degree of oxygen transfer. The PO, values of chorionic vein blood exceeded those of subchorial lake blood in five patients, and the mean PO, value of subchorial lake blood was not significantly different from that of chorionic vein blood. This indicates that the human placenta is more effective than a venous equilibrator and less effective than a countercurrent exchanger.7 According to Wilkening and Meschia7 and Pardi et a1,8the human placenta is defined as a venous equilibrator with a low degree of oxygen transfer. In their studies,8 uterine venous PO, (44.2 mmHg) obtained from the uterine vein was compared with umbilical venous PO, (27.6 mmHg), and the PO, gradient was 16.6 mmHg. In the present study, uterine venous PO, was significantly higher than the PO, of subchorial lake blood. Because subchorial lake blood circulates in the intervillous space and drains into the uterine vein, uterine venous PO, is expected to be lower than that of subchorial lake blood. The contradictory finding of uterine venous POZ indicates that arteriovenous anastomoses (shunts) are present in the decidua” and the myometrium.“~” The uterine venous blood is not a suitable indicator for the assessmentof placental oxygen exchange. Because the mean values of the blood gas analysis were not different in the subchorial and marginal lakes, the composition of the blood in these regions is not different. It is quite possible that the subchorial and marginal lakes function as a kind of admixed blood reservoir. During uterine contraction, the admixed blood of these lakes may be used to minimize the asphyxiating effects of contraction on the fetus. The marginal lake reported by Spanner’” is probably of clinical importance becauseof the admixed blood flow from rupture of the so-called “marginal sinus.” According to direct perfusion theory made by Freese,i4 Wigglesworth,‘” and Schuhmann and Wehler,lh villous trees have a hollow central cavity with a very loose structure. The orifice of the spiral artery is located near the center of the villous trees, which direct the maternal arterial blood into the cavity. They have suggested that the maternal blood flows into this cavity first, then moves outward through the narrow intervil-

VOL.

87, NO.

7, JANUARY

1996

lous channels. Although placental oxygen consumption and umbilical circulation shunts are considered factors of oxygen mismatch,6*7the oxygen gradient between maternal arterial blood (SO-100 PO, mmHg) and chorionic vein blood (28.7 mmHg) is too high to support direct perfusion theory. Low oxygen concentration has been reported to stimulate trophoblastic proliferation and villous sprouting in the central portion of the fetal lobule.i7,is If the arterial blood flows from the center to the periphery of the lobule, according to direct perfusion theory, then the site of trophoblastic proliferation may be contradictory to each other in relation to oxygen concentration. Becausefree flow of maternal placental blood exists even in early placental development,” it is difficult to believe that villous growth would impede the maternal blood flow in the term placenta, according to direct perfusion theory. The mean values of P02, oxygen saturation, and pH of chorionic vein blood were comparable with those of subchorial lake blood, without any significant difference, but the mean values of PCO, and bicarbonate of chorionic vein blood were significantly higher than those of subchorial lake blood. According to Prystowsky et al,3 the concentrations of PCO, and bicarbonate are greater in umbilical vein blood than in intervillous spaceblood, but the pH of umbilical vein blood is the same asor minimally lower than that of intervillous space blood. These findings do not agree with the concept that a considerable pH gradient is present between the fetal and maternal circulation.3,5

References 1. Fuchs

F, Spackman

of the intervillous 2. I’rystowsky H. gradient between normal

and

‘I, Assali

NS. Complexity

and

nonhomogeneity

space. Am J Obstet Gynecol 1963;86:226-33. Fetal blood studies. VII. The oxygen pressure the maternal and fetal bloods of the human in

abnormal

pregnancy.

Bull

Johns

Hopk

Hosp

1957;lOl:

48-55. 3. Prystowsky carbon maternal

H, Hellegers dioxide blood

4. Quill&an Partial

AE, Bruns

concentration of humans.

EJ, Vasicka pressure of

I? Fetal

gradient Am J Obstet

blood

studies.

between Gynecol

XV. The

the fetal 1961;81:372-6.

A, Aznar R, Lipsitz I’J, Moore T, Bloor oxygen in the intervillous space and

umbilical vessels. Am J Obstet Gynecol 1960;79:1048-52. 5. Rooth G, Sjoestedt S, Caligara F. Hydrogen concentration, dioxide tension and acid base balance in blood of human cord

and

intervillous

and

space

of placenta.

Arch

Dis

BM. the

carbon umbilical

Child

1961;36:

278-85. 6. Faber

JJ, Thornburg

tion of fetomaternal 119-21.

KL.

Placental

exchange.

physiology New

York:

structure Raven

Press,

7 Wilkening RB, Meschia G. Current topic: Comparative of placental oxygen transport. Placenta 1992;13:1-15. 8. Pardi G, Cetin I, Marconi AM, et al. Venous drainage uterus: retarded

Respiratory pregnancies.

gas studies Am J Obstet

Fujikura and Yoshida

in normal Gynecol

and

func-

1983:63-78, physiology of the human

and fetal 1992;166:699-706.

growth-

Placental Blood Gas

135

9. Benirschke K, Kaufmann P. Pathology of the human placenta 2nd ed. New York: Springer-Verlag, 1990:8 -11. 10. Boyd JD, Hamilton WJ. The human placenta. Cambridge, England: Heffer, 1970;64:254-256. 11. Heckel GE’, Tobin CE. Arteriovenous shunt in the myometrium. Am J Obstet Gynecol 1956;71:199-205. 12. Kormano M, Timonen H, Luukkainen T. Microangiographic observations on the uterine and maternal placental vasculature in early pregnancy. Am J Obstet Gynecol 1974;120:8-13. 13. Spanner R. Der Kreislauf im intervilloesen Raum des Menschen: Untersuchungen an den Uteroplacentargefaessen schwangercr Uteri. Anat Anz 1934;78:127-9. 14. Freese VF. The uteroplacental vascular relationship in the placenta Am J Obstet Gynecol 1968;101:8-16. 15. Wigglesworth JS. Vascular anatomy of the human placenta and its significance for placental pathology. J Obstet Gynaecol Br Commonw 1969;76:979-89. 16. Schuhmann R, Wehler V. Histologische Lnterschiede an Placentazotten innerhalb der materno-fetalen Stroemungseinheit: Ein Beitrag zur Funktionellen Morphologie der Placenta. Arch Gpnaekol 1971;210:425-39. 17. Alvarez H, Benedetti WL, Morel RL, Scavarelli M. Trophoblast

136

Fujikura

and

Yoshida

Placental

Blood

Gas

development gradient and its relationship to placental hemodynamics. Am J Obstet Gynecol 1970;106:416-20. 18. Fox H. Effect of hypoxia on trophoblast in organ culture. Am J Obstet Gynecol 1970;107:1058-64.

Address

reprint

requests

to:

Toshio Fujikura, MD Sho Hospkal l-41-14 ltabnshi ltabashi-ku Tokyo 173 ]47pan

ReceivedApril 28, 1995. Rrcebed in revised form A14g14sf10, 1995 Accepted Az4gust 25, 1995. Copyright 0 1996 by The Gynecologists.

American

College

of Obstetricians

and

Obstetrics & Gynecology