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Circulatory shock in pregnant sheep
Circulatory shock in pregnant sheep Ill. Effects of hemorrhage on uteroplacental and fetal circulation and oxygenation
CHARLES R. BRINKMAN III, M.D. MASSOUD MOFID, M.D. NICHOLAS S. ASSALI, M.D. Los Angeles, California Uteroplacental and fetal hemodynamics and oxygen transfer were studied in near-term pregnant sheep during progessively induced hemorrhagic shock and blood reinfusion. When the perfusing pressure fell to 50 or 60 mm. Hg, uteroplacental vascular resistance increased significantly and the blood flow fell more than the arterial pressure. These hemodynamic changes were probably related to the approaching of the critical closing pressure, although adrenergic stimulation cannot be ruled out. During maternal shock, uteroplacental oxygen transfer and fetal blood oxygen tension decreased markedly. Ductus arteriosus flow increased strikingly, most likely because of the fall in fetal blood Po,. This increase contributed to the maintenance of a normal fetal effective cardiac output during maternal shock. Despite an unaltered fetal effective cardiac output, umbilical blood flow decreased slightly and fetal oxygen consumption decreased significantly.
In his experiments, however, total uterine blood flow and oxygen transfer were not measured and the response of the fetus to maternal hemorrhage was not assessed. Since blood loss is a frequent obstetrical complication, we thought it relevant to investigate the behavior of the uteroplacental and fetal circulation and oxygen utilization during a period of standardized hypovolemic shock and blood reinfusion in the mother. The present report deals with the results of these studies.
A L T H o u a H the hemodynamic and metabolic effects of hypovolemic shock have been extensively studied in the nonpregnant animal, the alterations produced by this type of shock in the pregnant animal, particularly with respect to the uteroplacental and fetal circulation, have received little attention. Greiss1 investigated the effects of hemorrhage on the blood flow in one uterine artery in pregnant sheep and noted a slight increase in uterine vascular resistance after removal of 30 per cent of the blood volume. From the Department of Obstetrics and Gynecology, Center for the Health Sciences, University of California School of Medicine. This investigation was supported by United States Public Health Service Grants HL-13634 and HE-01755. Dr. Brinkman is the recipient of Career Development Award HL-70237. Received for publication May 11, 1973. Accepted june 28, 1973. Reprint requests: Charles R. Brinkman, III, M.D., Department of Obstetrics and Gynecology, University of California School of Medicine, Los Angeles, California 90024.
Materials and methods Studies were carried out on 12 near-term pregnant ewes and their fetuses (experiment 40 had a fetus weighing l.7 kilograms and probably was premature). Under local anesthesia, one carotid artery and jugular vein were cannulated. The carotid catheter served to monitor maternal arterial pressure and heart rate and to collect arterial blood samples anaerobically. Through the venous catheter, sodium pentobarbital (64 mg. per milliliter) was given in an initial dose of 10
77
78
Brinkman, Mofld, and Assoli
mg. per kilogram, supplemented by intermittent doses as needed. A tracheostomy was performed and the ewe's respiration was assisted with a Bird respirator, with the use of either compressed air or 95 per cent Oe, both with 5 per cent co~ added, depending on the arterial Po~ of the ewe during the procedure. The gas mixture used for lung ventilation was regulated so as to maintain a maternal arterial Po:; of approximately 100 mm. Hg throughout the experiment. A femoral artery and vein were also cannulated; the artery was used for gradual bleeding and the vein was used for blood reinfusion. The pregnant uterine horn was exposed through a midline incision and was marsupialized to the adbominal wall to prevent evisceration. The fetus was then delivered and marsupialized to the edges of the uterine incision to protect the umbilical circulation. The head was covered with a saline-filled glove to prevent respirations. An indwelling catheter was placed into an umbilical vein through an intercotyledonary branch and served for anaerobic collection of umbilical vein blood samples and for monitoring the umbilical vein pressure. The fetal left femoral artery and vein were cannulated; the femoral artery catheter served for collection of blood samples and for monitoring the fetal arterial pressure and heart rate. The venous catheter served for replacing blood collected for blood gases with like volumes of maternal blood. The total volume of maternal blood used did not exceed 5 ml. in any fetus: this quantity is insufficient to alter any of the biochemical characteristics of the fetal blood. In seven of the I 2 animals, the effects of maternal hemorrhagic shock on the uteroplacental and umbilical hemodynamics and oxygen transfer were investigated. In each animal the main right and left uterine arteries were exposed through bilateral inguinal incisions. The surrounding adventitia of a small segment of the artery was infiltrated with a mixture of cyclaine-dibenzylene to prevent spasm. Each artery was then fitted with a balanced field electromagnetic flow transducer. An indwelling catheter was
January 1, 19i4
Am.
J. Obstet. Gynccol.
placed into a major uterine vein via a tributary and served for anaerobic collection of uterine venous blood. The main umbilical vein was exposed in the fetal abdomen and was fitted with an electromagnetic flow transducer to measure total umbilical blood flow. In the remaining five animals, the effects of maternal hemorrhage on fetal effective cardiac output (the sum of ascending aortic and ductus arteriosus blood flows) and systemic vascular resistance were investigated. Each one of these fetuses was also delivered and was marsupialized to the uterine walls in the same manner as the other animals. The fetal chest was entered through the fourth intercostal space and the pericardium was opened. The phrenic and vagus nerves were kept intact. The ascending aorta and the ductus arteriosus were isolated and fitted with electromagnetic flow transducers. Technical details of all these procedures have been reported previously. 2 - 1 The experimental protocol comprised the following periods: (I ) A control period lasted for 30 to 60 minutes and during this time phasic and mean (electronically integrated) pressures and flows wt:-re continuously monitored. Maternal and fetal blood respiratory gases and pH were analyzed at least twice; the ewe was heparinized. ( 2) The control period was followed by a period of maternal hemorrhage during which maternal blood was removed from the femoral artery at a constant rate of 50 ml. per minute by a Harvard constant infusion-withdrawal pump. The onset of hypovolemic shock was arbitrarily defined vvhen the bleeding had reduced the maternal mean arterial blood pressure to 50 per cent of control values. At this level, the bleeding was stopped and the animal was allowed to remain in this shock state for 20 minutes. Throughout the period of blood withdrawal and shock, pressures and flows from mother and fetus were continuously recorded while blood respiratory gases and pH were analyzed at 10 minute intervals. (3) During a reinfusion period the blood was reinfused at the same rate at which it was removed.
Circulatory shock in pregnant sheep. Ill 79
Volume 118 Number 1
Table I. Data on maternal and fetal weights and estimated maternal blood volume
Ewe wt.
Estimated blood vol.
Exp. No.
(Kg.)
(L.)
32 33 36 37 38 39 40 41 43 44 64 65 N
62.7 54.1 73.6 69.5 61.8 60.9 58.6 65.8 41.7 50.4 61.7 64.6 12 60.45 ±0.7
5.7 4.8 6.9 6.5 5.6 5.5 5.3 6.1 3.5 4.4 5.6 5.9
X
S.E.
I:? 5.48 ±0.01
When all the blood removed had been replaced, the animal continued to be observed during a recovery period lasting 20 to 30 minutes. During all this time, flows and pressures were recorded continuously while blood respiratory gases and pH were analyzed at frequent intervals. At the termination of each experiment, the uterus, placenta, and fetus were weighed. Maternal and fetal arterial and venous blood samples were collected anaerobically in syringes in which the dead space was filled with heparin. Blood Po 2, Pco 2, pH, per cent saturation, and hemoglobin were analyzed by standard techniques used in our laboratories. 2 ' 5 Pressures were measured with Statham P-23 strain gauges calibrated to a common zero base line. Blood flows were measured with balanced-field type electromagnetic flowmeters previously described.n Each flow transducer was selected to fit the appropriate vessel snugly but without undue constriction. The fit between each vessel and its respective transducer was constantly observed through the open chest during the procedure and confirmed by stability of the flow signal on the dynagraphic recording. When a poor vessel-transducer contact was suspected, the transducer was changed. All flow transducers were calibrated in vitro, and a calibration factor was obtained to calculate mean flow as described elsewhere. 7
Blood removed Vol. (ml.) 1,000 1,300 900 1,400 800 950 850 1,150 850 1,425 900 900 12 1,035 :!:18.7
I
% of est. vol. 17.5 27.1 13.1
21.5 14.3 17.3 16.0 18.9 24.4 32.2 16.0 15.2 12 19.4 ±0.5
I
Fetal wt. (Kg.)
3.4 3.0 4.7 3.3 4.1 3.6 1.7 3.8 3.4 4.4 3.4 4.6 12 3.6 :!:0.1
This method has been checked against other calibration methods and has been used by us in the last 12 years; its error has been shown not to exceed ±5 per cent. 7 Heart rate ( HR) was determined from the arterial pressure pulse wave. Uterine vascular resistance ( UVR) was calculated from the ratio of arterial pressure and flow (uterine venous pressure was assumed to be very close to central venous pressure and its effects on the calculated resistance was negligible). Fetal systemic vascular resistance was estimated from the fetal pressure gradient divided by the fetal effective cardiac output; uteroplacental oxygen transfer (uvo2) was calculated from the product of the total uterine blood flow and the arteriovenous oxygen content difference across the uterus. Urnbilicoplacental vascular resistance (PVR) was calculated from the formula: fetal arterial pressure minus umbilical vein pressure divided by umbilical vem flow; fetal oxygen consumption (FV0 2 ) was calculated from the product of the umbilical vein blood flow and the umbilical vein-artery blood oxygen content difference. Details of these calculations have been published elsewhere. 2 • 4 The data obtained during the periods of blood removal (hemorrhage) and blood replacement were presented as a function of the per cent of the total amount of blood
80
January 1, 1974 Atn. J. Obstet. Gynecol.
Brinkman, Mofid, and Assali
Table II. Data* on the effects of maternal hemorrhagic shock and blood reinfusion on uteroHemorrhage
100%
No.
Control
50%
MAP (mm. Hg)
12
97 ±3.7
83 :':3.0t
MHR (beats/min.)
12
141 :':8.8
QUA (mi./min./Kg.) UVR
(mm. Hg)
7 7
(mi./min./Kg.) UVO, (mi./min./Kg.)
7
50 :':2.0§ 1~8
!51 :':9.3
±!1.7
10.1 :':2.2t
4.7 ±Ut
8.5
10.0
14.6
:':1.7
:':1.9
±2.9t
15.2 ±3.3
4.1 :':0.9
*The control data are the mean values of the last 20 minutes of the control period. The reference weight for uterine blood flow description of per cent hemorrhage and reinfusion and for explanation of AP, HR, and so on.
t < 002. t < 0.01. §
<
0.00!.
Table III. Data* on the effects of maternal hemorrhagic shock on fetal cardiovascular functions Hemorrhage
I
No.
Control
50%
MAP (mm. Hg)
12
64 :':2.4
62 ±2.2
62 :t2.0
MHR (beats/min.)
12
210 ±5.9
203 ±8.6
176t ±11.2
QDA (mi./min./Kg.)
5
88.9 ±14.1
104 ±12.2
116 ±18.5
QAA (mi./min./Kg.)
5
72.6
66.4 ±10.1
56.4 ±14.1
168 ±21
186 :':24
±9.2 ECO (mi./min./Kg.)
5
161
±22 SVR
5
0.45 ±0.06
0.40 ±0.05
ZOO%
0.35t :':0.05
*The control data are the mean values of the last 20 minutes of the control period. The reference weight for the blood flows and
tP < :tP
0.02.
< 0.01.
removed or reinfused; in this way, the individual variation in the amount of blood that had to be removed to produce a 50 per cent decrease in the arterial pressure was circumvented. The shock and recovery periods were plotted as a function of time from the completion of blood withdrawal or infusion. Statistical significance was determined by an analysis of consecutive paired differences.8 This method allows testing signifi-
cance within each period since it minimizes the impact of individual differences among group members. Results Effects of the experimental procedure on the ewe and her fetus. The impact of the surgical procedure on the ewe and her fetus, including the anesthesia, was not different from that observed in previous studies using the same experimental preparation. 2 • 5 Con-
Volume 118 Number 1
Circulatory shock in pregnant sheep. Ill
81
placental circulation and oxygen transfer (figures represent mean ± 1 S.E.) Shock
±4.0§
125 ±6.1 3.2 ±0.6t
10 min.
15 min.
20 min.
47 ±4.6§
49 ±4.9§
51 ±5.0§
107 ±7.4t
110 ±7.2t
3.0 ±0.5t
3.5 ±0.6t
112
±4.7
120
127
-1:.5.7
±9.0
±7.9
±9.6
133 :.!:7.4
4.1 ±0.7t
8.6 ±1.Bt
12.6 ±2.9
12.6 ±3.5
17.6
18.1
15.7
14.8
10.8
9.3
9.4
±4.1
±4.4
±3.5t
±3.3t
±1.9
±1.7
±2.3
2.1
3.1
2.4
±0.9 is the total weight of the pregnant anima!; for uterine oxygen transfer it is the weight of the uterus, placenta, and fetus. See text for
(figures are mean ± 1 S.E.) Shock 50%
100%
Recovery 20 min.
57 ±2.7
57 ±3.6
59 ±3.3
62 ±3.0
187 ±6.6
195t
±6.2
193t ±4.4
191t ±6.4
190 ±6.!
120 ±11.8
128t ±16.3
125t :!:13.3
117 ±15.3
99 ±8.9
103 ±5.6
68.2 :!:4.5
69.1 ±5.0
68.9 ±6.0
86.8 ±6.7
98.7 ±9.7
±11.6
15 min.
5 min.
lO min.
61 :!:1.7
±2.0
58 ±2.8
163t ±11.6
179t ±9.1
129 ±5.5 68.8 ±6.7 199 ±13
62
188 ±16
198 ±20
0.31
0.36
0.34
±0.02
:!:0.05
±0.05
20 min.
194 ±24 0.34t ±0.06
203t
±n
198t :!:18
103.4 207t ±!9
0.31
0.33
0.3~
:!:0.03
±0.03
:!:0.02
cardiac output is the fetal weight. See text for description of per cent hemorrhage and reinfusion.
trol values for pressures, flows, blood respiratory gases, and pH prior to induction of hemorrhage were within the ranges previously published. In Table I are listed pertinent data on the weight of the ewe and of her fetus and on the maternal blood volume as estimated by a method previously published 5 ; also listed for each animal is the volume of blood (both absolute and as a fraction of the total estimated volume) which had to be removed in order
to produce hypovolemic shock as defined by the protocol. An average of 19 per cent of the estimated blood volume had to be removed to elicit the 50 per cent decrease in mean systemic arterial pressure. Effects of hemorrhagic shock on uteroplacental hemodynamics and oxygen transfer. The effects of hemorrhagic shock and blood replacement on maternal arterial pressure (MAP), heart rate (MHR), uteroplacental blood flow, and uteroplacental oxy-
82
January l, 1974 Am. J Obstet. Gynecol.
Brinkman, Mofld, and Assali
Hemorrhage
Infusion
% 25 _____50 0L- _L. __!_
100
MAP mmH
80
%
75
100 _,
0
25
50
75
100
'
J
60 40" 160.
MHR
140j
beots I mio
120j 100
'l
6.81
CVP mm Hg
6.0 6 5.6 .
52~-j 48:J !
l
_L
10001
QUA ml/m"
800'
600~ 400 200
UVR Hg .iii!~ rm
i
"j
0 22 0 0.18 0.14 0 10 20
T
16
0 2 transfer ml/min
8 4 0
I
SE
I
I I
12.
0
5
/0
15
20
mm
min
Shock
Recovery
Fig. 1. Changes in MAP, MHR, CVP, QUA, UVR, and uterine O, transfer during progressively induced hemorrhagic shock and blood reinfusion. Note that the major increase in UVR occurred only after the perfusing pressure had fallen to below 60 mm. Hg. Note also the marked fall in oxygen transfer during shock. (Figures represent mean ± 1 S.E. and are plotted as a function of the amount of blood withdrawn or reinfused before and after the establishment of the shock state.)
gen transfer are presented in Table II and Fig. 1. In Table II are listed the mean values ± 1 standard error of the maternal arterial pressure and heart rate for the entire experimental group of 12 animals: the
values for uterine blood flow (QUA), UVR, and uvo2 listed in this table are for the seven animals included in this portion of the protocol (see Methods). Fig. I illustrates the progressive changes that occurred
Volume 118 Number 1
Circulatory shock in pregnant sheep. Ill 83
Infusion
Hemorrhage
%
%
0
25
50
75
100
0
25
50
75
100
I,.._..._...L_.-.. ~---.l
"l
FAP
66
mrn Hg
62
5A
540 500
QDA
460
ml/mm
420 380 340 420 380 340 300 260 220 860 l 820 780
co
740 700 660 620 580
SVR
0 14l 0 I2 0 10 0 08
0 06 I~
SE
0
5
10
15
20
0
5
10
15
min
min
Shock
Recovery
20
Fig. 2. Changes in fetal cardiovascular parameters during maternal hemorrhagic shock and blood reinfusion. Note the minor changes in FAP and the marked increase in QDA which contributed to the increase in the fetal effective CO during maternal hemorrhage. QAA and fetal SVR did not change greatly during maternal shock; aortic flow, however, increased during reinfusion and recovery. (Figures represent mean ± 1 S.E. and are plotted in a manner similar to those in Fig. 1.)
during hemorrhage, shock, reinfusion, and recovery in these seven animals. The systemic arterial pressure progressively decreased during bleeding until it reached the level established for the onset of the shock state (50 per cent of control) ; it remained relatively stable at this shock level for the
period of 20 minutes. Maternal heart rate initially increased at the onset of bleeding but, as bleeding progressed and hemorrhagic shock became established, it fell significantly and remained low through the entire reinfusion period (Table II; Fig. 1). Central venous pressure (CVP) decreased progres-
84
January 1, 1974 Am. J. Obstet. Gynecol.
Brinkman, Mofid, and Assali
Table IV. Data* on the effects of maternal hemorrhagic shock on fetal umbilical hemodynamics Hemorrhage
No.
Control
50%
UVP (mm. Hg)
7
26 ±2.9
20 ±3.5
QUV (ml./min./Kg.)
7
UVR
7
(mm. Hg) (mi./min./Kg.)
FVO, (mi./min./Kg.)
7
132 ±17
1l.'i ±13
I
100% 20 ±2.6 116 ±15
0.31
0.38
0.38
±0.05
±0.04
±0.04
3.7 ±0.3
are the mean values of the last 20 minutes of the control period. The reference weight for the umbilical blood
tP :j:P
< <
0.02. O.ot.
sively during blood withdrawal and the shock period (Fig. 1). Uteroplacental blood flow also decreased progressively during blood withdrawal (Table II; Fig. 1). The decrease in flow was relatively proportional to the decrease in pressure until the latter had reached a level of about 75 mm. Hg; during this time the uteroplacental vascular resistance did not change significantly (Fig. 1). But as the perfusing pressure fell to about 50 mm. Hg, the uteroplacental blood flow decreased precipitously, reaching values of about 25 per cent of control levels (Table II; Fig. 1). This greater decrease in flow than in pressure persisted throughout the shock period and was accompanied by a significant increase in uteroplacental vascular resistance (Table II; Fig. 1). Uterine oxygen transfer decreased by less than 50 per cent during the shock period (Fig. l; Table II) ; the decrease was related to the marked fall in uterine blood flow. Reinfusion of the blood progressively restored the maternal systemic arterial and central venous pressures to control values; they remained at such levels throughout the recovery period (Table II; Fig. 1). Heart rate did not return to control values until the end of the recovery period (Fig. 1). Although uterine blood flow increased during blood reinfusion, it remained somewhat below control values until the end of recovery period; uterine vascular resistance progres-
sively decreased during blood reinfusion and returned to control values during the recovery period. Uterine oxygen transfer also returned to near-control levels in the recovery period (Table II; Fig. 1). Effects of maternal shock on the fetal circulation. The impact of maternal hemorrhage and shock on the cardiovascular system of the fetus is illustrated in Figs. 2, 3, and 4; Table III presents the data on the effects of maternal hemorrhage on fetal arterial pressure (FAP), heart rate, ductus and ascending aortic flows, and the sum of the two (the effective cardiac output [CO] ) and systemic vascular resistance; Table IV presents the data on the changes in umbilicoplacental hemodynamics. Progressive removal of approximately 20 per cent of maternal blood volume had insignificant effects on the fetal arterial pressure; only after 15 minutes of established shock did the arterial pressure show some fall, but it returned to normal during recovery (Figs. 2, 3, and 4; Table III). Fetal heart rate decreased somewhat during the bleeding and shock periods and returned to near-control values during recovery (Figs. 3 and 4; Table III). Ductus arteriosus blood flow ( QDA) increased progressively during bleeding, reaching significantly higher levels than control during the shock period; during blood reinfusion and recovery, ductus flow progressively decreased toward control values (Figs. 2 and 3; Table
Circulatory shock in pregnant sheep. Ill
Volume 118 Number I
and oxygen consumption (figures represent mean ±
S.E.) Reinfusion
Shock 5 min.
10 min.
15 min.
20 min.
50%
100%
Recovery 20 min.
23 ±3.6
25 ±4.3
17 ±4.3
18 ±4.4
17 ±3.3
17 ±3.7
18 ±3.6
105 ±10
108 ±14
95t ±19
112 ±15
107 ±19
105 ±18
98 ±14
0.38
0.36
0.34
0.30
0.36
0.37
0.42
±0.05
±0.06
±0.07
±0.08
±o.os
±o.os
±0.03
1.9t ±0.6
85
3.0 ±0.4
2.0t ±0.4
flow and oxygen consumption is the fetal weight. See text for description of per cent hemorrhage and reinfusion.
III). Ascending aortic flow ( QAA) did not change significantly during maternal hemorrhagic shock; it increased somewhat during recovery (Figs. 2 and 3; Table III). Because of the marked rise in ductus flow and the lack of changes in the ascending aortic flow, the fetal effective cardiac output (ECO) increased during maternal hemorrhagic shock; it remained above control values during recovery owing to the increase in ascending aortic :flow and the near-control ductus flow (Figs. 2 and 3; Table III). Fetal systemic vascular resistance (SVR) tended to decrease slightly throughout maternal shock and recovery (Fig. 2). Total umbilical blood :flow and umbilical vein pressure (UVP) decreased slightly during maternal hemorrhagic shock and remained somewhat below control values during recovery (Table IV; Fig. 4). Umbilical vascular resistance tended to increase during maternal hemorrhagic shock but the changes were not significant (Fig. 4; Table IV). Fetal oxygen consumption (FV0 2 ) decreased by about 50 per cent during maternal shock but returned to near-control values during recovery (Fig. 4; Table IV). The decrement was related to the decrease in umbilical vein flow (QUV) as well as in the arteriovenous oxygen content difference. Effects on maternal and fetal blood gases, hemoglobin, and pH. In Table V are listed the values on maternal and fetal Po 2 , Pco 2 , pH, per cent saturation, and hemoglobin
(Hgb.) during control, shock, and recovery. Maternal Po 2 and oxyhemoglobin saturation did not change significantly, probably because of the supported respiration. Maternal blood Pco 2 increased and pH decreased slightly. A minor increase in maternal hemoglobin concentration occurred during recovery (Table V). On the fetal side, blood Po2 and oxygen saturation in both the umbilical vein and artery fell considerably. Fetal blood Pco 2 increased and pH decreased during maternal shock; neither parameter returned to control values during recovery Table V). Some increase in fetal hemoglobin concentration occurred. Comment
The classically described circulatory events that are triggered by blood loss in the nonpregnant animals are: ( 1) a decrease in the effective circulating blood volume and cardiac output; ( 2) a decrease in the arterial pressure including that perfusing the baroreceptor centers; ( 3) inhibition and decreased firing of baroreceptor reflexes; ( 4) vagal inhibition leading to tachycardia which is thought to compensate for the decrease in stroke volume; ( 5) stimulation of the medullary centers and of the thoracolumbar sympathetic chains leading to generalized vasoconstriction and to increased systemic peripheral resistance. These events are observed in unanesthetized as well as anesthetized animals. 9 • 10
86 Brinkman, Mofld, and Assali
CONTROL
MAT~"jlfA!,·,,W. ~
12.0.·J~
60]
~T. ~~, mmttt
January I, 1974 Am. ]. Obstet. Gynecol.
BLEEOING I'ERIOO
AT 20min OF SHOCK
~
I'
li M I,. J
tl'lli
-~
RECOVERY
~
. ,• •,,••• ,• •
30 0
~ ~~l
,
~
FLCIW
mt/min
~l
Fig. 3. Segments of a dynagraphic record illustrating changes in phasic and integrated rna· ternal and fetal systemic pressures and in fetal ascending aortic and ductus arteriosus flows during control, bleeding, shock, and recovery periods. Note the insignificant changes in FAP despite the marked fall in fetal heart rate and maternal pressure during shock. Note also tht' striking increase in ductus flow during bleeding and shock; at the same time, ascending aortic flow decreased only slightly. During recovery, ductus flow returned to control values while ascending aortic flow rebounded to levels higher than control.
The behavior of the regional circulation during hemorrhagic shock depends on the vascular bed under study. The splanchnic, renal, muscular, and cutaneous vascular beds undergo active vasoconstriction, leading to an increase in their respective vascular resistances and a decrease in their blood flows. The circulation to other organs such as the heart and brain does not seem to be affected until the final irreversible stage of shock. 9 • 10 The present data obtained from pregnant sheep studied under pentobarbital anesthesia suggest that the maternal circulatory response to blood loss differs somewhat from the classical picture outlined above. The main difference consists in the absence of any clear evidence for adrenergic stimulation. Neither the progressive loss of blood nor established shock caused the maternal heart rate to increase significantly, which might be anticipated to compensate for the decrease in stroke volume and the cardiac
output. Also, the progressive decrease in central venous pressure speaks against venomotor constriction caused by sympathetic stimulation. Likewise, the behavior of uterine vascular resistance and uterine blood flow during blood withdrawal does not seem to favor adrenergic stimulation, at least in the early phase of blood loss. Our previous studies have shown that the uterine vascular bed in both the pregnant and the nonpregnant sheep is under a strong adrenergic control; the uterine vascular resistance increases markedly with minor alpha-adrenergic stimulation.11 But the present data show that the uterine vascular resistance did not change significantly even with a substantial blood loss sufficient to decrease the perfusing pressure to about 75 mm. Hg. During this bleeding period the uterine blood flow kept pace with the perfusing pressure. Had there been any adrenergic stimulation during this period, the uterine vascular resistance
Volume 118 Number 1
Circulatory shock in pregnant sheep. Ill
Infusion
Hemorrhage
%
%
0
25
50
87
0
75 100
25
50
75
100
FAP mm Hg
2204
FHR
200j
beats I min
!80~ ! 60j
OUmV ml/mh
4401 400 360
320 '
PVR
280
j
0 17
j
0.\51 0 13
i
0.1 I e
009j
I I
i I
SE
0
0
15
5
20
min
Shock
Recovery
Fig. 4. Changes in umbilical hemodynamics during maternal hemorrhagic shock and blood reinfusion. In this series, FAP and FHR decreased slightly; the fall in QUV and the increase in PVR was also minor. FVO,, however, decreased markedly during maternal shock. (Figures represent mean :t 1 S.E. and are plotted as in Fig. 1.)
should have increased markedly and the uterine blood flow decreased much more than the perfusing pressure. Hence, the alpha~adrenergic receptors controlling the uterine vasculature were not stimulated by this degree of blood loss. On the other hand, the present data show that the uterine vascular resistance increased significantly during hemorrhagic shock when the perfusing pressure had faiJen to about 50 mm. Hg. At this pressure level, the uterine
blood flow decreased more than the perfusing pressure. Although these latter changes may suggest alpha-adrenergic stimulation caused by the severe hemorrhage, they are probably related to collapse of the uterine vessels caused by the fall in the perfusing pressure to levels approaching the critical closing pressure for the uterine vascular bed. This latter hypothesis receives support from our previous studies in the pregnant and the nonpregnant sheep which showed
88
January 1, 1974 Am. J. Obstet. Gynecol.
Brinkman, Mofld, and Assali
Table V. Maternal arterial and fetal umbilical vein and arterial blood gases, pH, and hemoglobin concentration during maternal hemorrhagic shock and recovery
Maternal arterial: Po, (mrn. Hg) 12 O, sat. ( o/o) 12 Pco, (mm. Hg) 12 12 pH Hgb. (Gm./ 100 mi.) 12 Fetal umbilical Po, (mm. Hg) Q, sat. ( o/o) Pco, (mm. Hg) pH Hgb. (Gm./ 100 mi.) Fetal arterial: Po, (mm. Hg) O, sat. ( o/c) Pco, (mm. Hg) pH Hgb. (Gm./ !00 mL
137 96
± 30 ±
113 93
:± 3Dt
±
1.5t
126 93
± 36 ± 1.4
136 95
± 31 ±
± 32
130 95
±
± 1.1 33 7.49 ± 0.02
:!: 1.1 35 7.48 ± 0.02
± 1.5t 38 7.45± O.Dlt
39 ± 1.3§ 7.45 ± 0.02§
± 1.8§ 39 7.44± 0.02t
10.5 ± 0.3
10.4 ± 0.3
10.3 ± 0.4
11.0
±
0.4t
11.2 ± 0.3t
vein:
20
± 2.4§
41
±
3.0
36
± 2.5
57
± 6§
90
± 2t
90
±
11
41
± 1.8
17
±
11
94
±
48
± 6§
11 11
± 1.2 40 7.42 ± 0.02
± 2.9§ 52 7.32 ± 0.03§
11
13.4 ± 0.5
13.6
±
12
36
± 1.3
12
± 1.6§
15
± 1.3§
25
:!: 1.4
23
:!:
12
79
± 2
39
:!:: 4§
42
± 5§
73
± 3
70
+ 4t
12 12
± 1 41 7.40 :± 0.02
± 2.3§ 54 7.29:!: 0.03§
12
12.7 ± 0.4
13.4 ±
2.9§
0.5
2t
55 ± 3.6t 7.24 ± 0.03t
± 2.3t 45 7.33 ± 0.03t
± 2.7t 47 7.33 :': O.D2t
13.2 ± 0.6
13.1 ± 0.4
13.2 ± 0.4
:!:: 4.1 § 62 7.17 ± 0.04§
1.2
± 2.9t 50 7.28± 0.03§
± 2.7t 51 7.28 ± O.o3t
12.4 ± 0.3
12.7 ± 0.3
*The control data are the average of at least two samples taken during the last 20 minutes of the control period.
tP
< O.D2.
tP §P
<
<
0.01. 0.001.
a critical closing pressure for the uterine vascular network in the range of 40 to 50 mm. Hg. 11 The question arises as to why blood loss and hypovolemic shock in the pregnant sheep did not elicit adrenergic stimulation. We believe that the reason is probably the type of anesthesia used in these experiments. Although some reports 9 • 10 seem to suggest negligible influence of anesthetic agents on the cardiovascular response to hemorrhage, preliminary experiments in our laboratory seem to indicate that in the pregnant sheep pentobarbital anesthesia partially inhibits adrenergic stimulation.
The decrease in uterine oxygen transfer that occurred during hemorrhage is qualitatively and quantitatively similar to that observed by us in endotoxin and spinal shock and during maternal hypotension induced by hydralazine. 12 - 14 In all these hypotensive states maternal oxygenation was maintained at normal levels and the only variable was the marked decrease in uterine blood flow. These observations demonstrate the dependence of uteroplacental oxygen transfer on uterine blood flow. The remarkable stability of the fetal circulation in the face of such a profound maternal hypotension, uterine ischemia, and
Volume 118 Number 1
depressed uteroplacental oxygen transfer is also in agreement with previous data obtained in a variety of hypotensive states imposted on the mother. 12 - 15 We believe that the absence of any major change in the fetal arterial pressure, despite the fall in fetal blood oxygen levels, is due to the fact that the fetal effective cardiac output remained normal or increased slight· ly during the periods of maternal shock and blood reinfusion. Such an increase in fetal effective cardiac output was largely related to the striking increase in ductus arteriosus blood flow which coincided with the fall in fetal blood Po2. This increase was probably accompanied by a shift of blood toward the ductus from the pulmonary circulation secondary to pulmonary vasoconstriction. This reciprocal behavior of the ductus arteriosus and pulmonary circulations has been observed by us and by others during states of fetal hypoxia and hyperoxia. 2 - 4 • 16 When hypoxia is imposed on the fetus through ventilation of either the maternal or fetal lung with a gas mixture low in oxygen, ductus flow increases and net pulmonary flow decreases.3• 4 • 1 " The opposite is observed during hyperoxia. 2 • 16 The present observations add further evidence of the sensitivity of the ductus arteriosus to changes in blood oxygen tension and the important role that this vessel plays in fetal homeostases. By shifting blood from the lungs toward the systemic circulation during hypoxia, the fetal systemic arterial pressure and the effective cardiac output may be maintained at near normal levels. But if a shift of blood from the lungs toward the descending aorta through the ductus had occurred during maternal shock, the blood return to the left side of the heart should have decreased, and this should have been reflected in a fall in the ascending aortic flow. Yet the ascending aortic flow did not change significantly during maternal hemorrhagic shock and fetal hypoxia. This can be explained only on the basis of an increase in foramen ovale flow. Dawes 16 had suggested increased foramen ovale flow during hypoxia. This increase can be favored by
Circulatory shock in pregnant sheep. Ill 89
a fall in the left atrial pressure caused by the diminished return of blood from the lung. Such an increase in foramen ovale flow may also be construed as a protective mechanism to maintain an adequate fetal cardiac output during fetal hypoxic conditions. The changes in umbilicoplacental circulation observed in the present studies, even though minor, are interesting. The decrease in umbilical blood flow and umbilical vein pressure in the face of unaltered fetal effective cardiac output and systemic vascular resistance is probably related to a shift of blood from the umbilical circuit to other vascular beds. The most likely site to which the blood is shifted is the cephalic circulation. Several reports have shown that, as in the adult, the fetal cephalic circulation is sensitive to changes in blood Pco 2 .H- 19 The increase in fetal blood Pco 2 during the periods of shock and recovery could have led to cephalic vasodilatation and a shift of blood from the umbilical circulation to the head. Another interesting finding in the present set of experiments is the increase in hemoglobin concentration in the fetal blood during maternal shock. Such an increase is possibly related to disturbance in water exchange across the placenta. Faber and Green 20 have shown that in the sheep placenta there is osmotic and hydrostatic equilibration of water. With the fall of maternal arterial pressure to levels below the fetal arterial pressure, this hydrostatic equilibrium is altered and passage of water from the fetus toward the mother may occur, and this may account for the increased hemoglobin concentration in the fetal arterial blood. The decrease in fetal oxygen consumption observed in the present studies is similar in magnitude to that observed in other types of maternal hypotensive states. 12 - 14 It is related to the marked decrease in uterine oxygen transfer as well as the slight fall in umbilical blood flow. The fall in fetal heart rate observed in the present studies is consistent with our previous observations in other types of ma-
90 Brinkman, Mofld, and Assali
ternal hypotensive conditions with fetal hypoxia.12-15 Since this fall did not affect the arterial pressure or the effective cardiac output, the fetal stroke volume must have compensated for the bradycardia. These findings are also in agreement with our previous
January 1, 1974 Am. J. Obstet. Gynerol.
studies on the relationship between fetal heart rate and stroke volume 21 ; they further point out the unreliability of fetal heart rate as a diagnostic sign of hemodynamic disturbances.
REFERENCES
Jr.: Obstet. GynecoL 27: 549, 1966. Assali, N. S., Kirschbaum, T. H., and Dilts, P. V., Jr.: Circ. Res. 22: 573, 1968. Dilts, P. V., Jr., Brinkman, C. R., III, Kirschbaum, T. H .. and Assali, N. S.: AM. J. 0BSTET. GYNECOL. 103: 138, 1969. Brinkman, C. R., III, Weston, P., Kirschbaum, T. H., and Assali, N. S.: AM. J. 0BSTET. GYNECOL. 108: 288, 1970. Kirschbaum, T. H., Brinkman, C. R., III, and Assali, N. S.: AM. J. 0BSTET. GYNECOL. 110: !90, 1971. Westersten, A., Rice, E., Brinkman, C. R., III, and Assali, N. S.: J. Appl. Physiol. 26: 497, 1969. Beck, R., Morris, J. A., and Assali, N. S.: Am. J. Med. Elect. 4: 87, 1965. Steel, R. G. D., and Torrie, J. H. Principles and Procedures of Statistics, New York, 1960, McGraw-Hill Book Company, Inc. Bock, K. D., editor: Shock: Pathogenesis and Therapy, New York, Academic Press, Inc. Fine, J.: In Field, J., editor: Handbook of Physiology, Baltimore, 1965, The Williams & Wilkins Company, Inc., Sect. 2, VoL III. Ladner, C., Brinkman, C. R., III, 'Weston, P., and Assali, N. S.: Am. J. Physiol. 218: 257, 1970.
!. Greiss, F. C.,
2.
3. 4.
5. 6. 7. 8. 9. 10.
11.
12. Lucas, W. E., Kirschbaum, T. H., and Assali, N. S.: Bioi. Neonat. 10: 166, 1966. 13. Bech-Jansen, P., Brinkman, C. R. III, Johnson, G. H., and Assali, N. S.: AM. J. 0BsTET. GYNECOL 112: 1084, 1972. 14. Ladner, C. N., Weston, P. V., Brinkman, C. R., III, and Assali, N. S.: AM. J. 0BSTET. GYNECOL. 108: 375, 1970. 15. Bech-Jansen, P., Brinkman, C. R. III, Johnson, G. H., and Assali, N. S.: AM. J. 0BSTET. GYNECOL. 113: 37, 1972. 16. Dawes, G. S: Foetal and Neonatal Physiology, Chicago, 1968, Year Book Medical Publishers, Inc. 17. Lucas, W. E., Kirschbaum, T. H., and Assali, N. S.: Am. J. Physiol. 210: 287, 1966. 18. Makowski, E. L., Schneider, J. M., Tsoulos, N. G., Colwell, J. R., Battaglia, F. C., and Meschia, G.: AM. J. 0BSTET. GYNECOL. 114: 29:!, 1972. 19. Quilligan, E. L Hon, E. H., Anderson, G. G., and Yeh, S. Y.: AM. J. 0BSTET. GYNECOL. 102: 716, 1968. 20. Faber, J. J., and Green, T. J.: J. Physiol. 223: 375, 1972. 21. Brinkman, C. R. III, Johnson, G. H., and Assali, N. S.: Am. J. Physiol. 223: 1465, 1972.
CHARLES R. BRINKMAN III, M.D. MASSOUD MOFID, M.D. NICHOLAS S. ASSALI, M.D. Los Angeles, California Uteroplacental and fetal hemodynamics and oxygen transfer were studied in near-term pregnant sheep during progessively induced hemorrhagic shock and blood reinfusion. When the perfusing pressure fell to 50 or 60 mm. Hg, uteroplacental vascular resistance increased significantly and the blood flow fell more than the arterial pressure. These hemodynamic changes were probably related to the approaching of the critical closing pressure, although adrenergic stimulation cannot be ruled out. During maternal shock, uteroplacental oxygen transfer and fetal blood oxygen tension decreased markedly. Ductus arteriosus flow increased strikingly, most likely because of the fall in fetal blood Po,. This increase contributed to the maintenance of a normal fetal effective cardiac output during maternal shock. Despite an unaltered fetal effective cardiac output, umbilical blood flow decreased slightly and fetal oxygen consumption decreased significantly.
In his experiments, however, total uterine blood flow and oxygen transfer were not measured and the response of the fetus to maternal hemorrhage was not assessed. Since blood loss is a frequent obstetrical complication, we thought it relevant to investigate the behavior of the uteroplacental and fetal circulation and oxygen utilization during a period of standardized hypovolemic shock and blood reinfusion in the mother. The present report deals with the results of these studies.
A L T H o u a H the hemodynamic and metabolic effects of hypovolemic shock have been extensively studied in the nonpregnant animal, the alterations produced by this type of shock in the pregnant animal, particularly with respect to the uteroplacental and fetal circulation, have received little attention. Greiss1 investigated the effects of hemorrhage on the blood flow in one uterine artery in pregnant sheep and noted a slight increase in uterine vascular resistance after removal of 30 per cent of the blood volume. From the Department of Obstetrics and Gynecology, Center for the Health Sciences, University of California School of Medicine. This investigation was supported by United States Public Health Service Grants HL-13634 and HE-01755. Dr. Brinkman is the recipient of Career Development Award HL-70237. Received for publication May 11, 1973. Accepted june 28, 1973. Reprint requests: Charles R. Brinkman, III, M.D., Department of Obstetrics and Gynecology, University of California School of Medicine, Los Angeles, California 90024.
Materials and methods Studies were carried out on 12 near-term pregnant ewes and their fetuses (experiment 40 had a fetus weighing l.7 kilograms and probably was premature). Under local anesthesia, one carotid artery and jugular vein were cannulated. The carotid catheter served to monitor maternal arterial pressure and heart rate and to collect arterial blood samples anaerobically. Through the venous catheter, sodium pentobarbital (64 mg. per milliliter) was given in an initial dose of 10
77
78
Brinkman, Mofld, and Assoli
mg. per kilogram, supplemented by intermittent doses as needed. A tracheostomy was performed and the ewe's respiration was assisted with a Bird respirator, with the use of either compressed air or 95 per cent Oe, both with 5 per cent co~ added, depending on the arterial Po~ of the ewe during the procedure. The gas mixture used for lung ventilation was regulated so as to maintain a maternal arterial Po:; of approximately 100 mm. Hg throughout the experiment. A femoral artery and vein were also cannulated; the artery was used for gradual bleeding and the vein was used for blood reinfusion. The pregnant uterine horn was exposed through a midline incision and was marsupialized to the adbominal wall to prevent evisceration. The fetus was then delivered and marsupialized to the edges of the uterine incision to protect the umbilical circulation. The head was covered with a saline-filled glove to prevent respirations. An indwelling catheter was placed into an umbilical vein through an intercotyledonary branch and served for anaerobic collection of umbilical vein blood samples and for monitoring the umbilical vein pressure. The fetal left femoral artery and vein were cannulated; the femoral artery catheter served for collection of blood samples and for monitoring the fetal arterial pressure and heart rate. The venous catheter served for replacing blood collected for blood gases with like volumes of maternal blood. The total volume of maternal blood used did not exceed 5 ml. in any fetus: this quantity is insufficient to alter any of the biochemical characteristics of the fetal blood. In seven of the I 2 animals, the effects of maternal hemorrhagic shock on the uteroplacental and umbilical hemodynamics and oxygen transfer were investigated. In each animal the main right and left uterine arteries were exposed through bilateral inguinal incisions. The surrounding adventitia of a small segment of the artery was infiltrated with a mixture of cyclaine-dibenzylene to prevent spasm. Each artery was then fitted with a balanced field electromagnetic flow transducer. An indwelling catheter was
January 1, 19i4
Am.
J. Obstet. Gynccol.
placed into a major uterine vein via a tributary and served for anaerobic collection of uterine venous blood. The main umbilical vein was exposed in the fetal abdomen and was fitted with an electromagnetic flow transducer to measure total umbilical blood flow. In the remaining five animals, the effects of maternal hemorrhage on fetal effective cardiac output (the sum of ascending aortic and ductus arteriosus blood flows) and systemic vascular resistance were investigated. Each one of these fetuses was also delivered and was marsupialized to the uterine walls in the same manner as the other animals. The fetal chest was entered through the fourth intercostal space and the pericardium was opened. The phrenic and vagus nerves were kept intact. The ascending aorta and the ductus arteriosus were isolated and fitted with electromagnetic flow transducers. Technical details of all these procedures have been reported previously. 2 - 1 The experimental protocol comprised the following periods: (I ) A control period lasted for 30 to 60 minutes and during this time phasic and mean (electronically integrated) pressures and flows wt:-re continuously monitored. Maternal and fetal blood respiratory gases and pH were analyzed at least twice; the ewe was heparinized. ( 2) The control period was followed by a period of maternal hemorrhage during which maternal blood was removed from the femoral artery at a constant rate of 50 ml. per minute by a Harvard constant infusion-withdrawal pump. The onset of hypovolemic shock was arbitrarily defined vvhen the bleeding had reduced the maternal mean arterial blood pressure to 50 per cent of control values. At this level, the bleeding was stopped and the animal was allowed to remain in this shock state for 20 minutes. Throughout the period of blood withdrawal and shock, pressures and flows from mother and fetus were continuously recorded while blood respiratory gases and pH were analyzed at 10 minute intervals. (3) During a reinfusion period the blood was reinfused at the same rate at which it was removed.
Circulatory shock in pregnant sheep. Ill 79
Volume 118 Number 1
Table I. Data on maternal and fetal weights and estimated maternal blood volume
Ewe wt.
Estimated blood vol.
Exp. No.
(Kg.)
(L.)
32 33 36 37 38 39 40 41 43 44 64 65 N
62.7 54.1 73.6 69.5 61.8 60.9 58.6 65.8 41.7 50.4 61.7 64.6 12 60.45 ±0.7
5.7 4.8 6.9 6.5 5.6 5.5 5.3 6.1 3.5 4.4 5.6 5.9
X
S.E.
I:? 5.48 ±0.01
When all the blood removed had been replaced, the animal continued to be observed during a recovery period lasting 20 to 30 minutes. During all this time, flows and pressures were recorded continuously while blood respiratory gases and pH were analyzed at frequent intervals. At the termination of each experiment, the uterus, placenta, and fetus were weighed. Maternal and fetal arterial and venous blood samples were collected anaerobically in syringes in which the dead space was filled with heparin. Blood Po 2, Pco 2, pH, per cent saturation, and hemoglobin were analyzed by standard techniques used in our laboratories. 2 ' 5 Pressures were measured with Statham P-23 strain gauges calibrated to a common zero base line. Blood flows were measured with balanced-field type electromagnetic flowmeters previously described.n Each flow transducer was selected to fit the appropriate vessel snugly but without undue constriction. The fit between each vessel and its respective transducer was constantly observed through the open chest during the procedure and confirmed by stability of the flow signal on the dynagraphic recording. When a poor vessel-transducer contact was suspected, the transducer was changed. All flow transducers were calibrated in vitro, and a calibration factor was obtained to calculate mean flow as described elsewhere. 7
Blood removed Vol. (ml.) 1,000 1,300 900 1,400 800 950 850 1,150 850 1,425 900 900 12 1,035 :!:18.7
I
% of est. vol. 17.5 27.1 13.1
21.5 14.3 17.3 16.0 18.9 24.4 32.2 16.0 15.2 12 19.4 ±0.5
I
Fetal wt. (Kg.)
3.4 3.0 4.7 3.3 4.1 3.6 1.7 3.8 3.4 4.4 3.4 4.6 12 3.6 :!:0.1
This method has been checked against other calibration methods and has been used by us in the last 12 years; its error has been shown not to exceed ±5 per cent. 7 Heart rate ( HR) was determined from the arterial pressure pulse wave. Uterine vascular resistance ( UVR) was calculated from the ratio of arterial pressure and flow (uterine venous pressure was assumed to be very close to central venous pressure and its effects on the calculated resistance was negligible). Fetal systemic vascular resistance was estimated from the fetal pressure gradient divided by the fetal effective cardiac output; uteroplacental oxygen transfer (uvo2) was calculated from the product of the total uterine blood flow and the arteriovenous oxygen content difference across the uterus. Urnbilicoplacental vascular resistance (PVR) was calculated from the formula: fetal arterial pressure minus umbilical vein pressure divided by umbilical vem flow; fetal oxygen consumption (FV0 2 ) was calculated from the product of the umbilical vein blood flow and the umbilical vein-artery blood oxygen content difference. Details of these calculations have been published elsewhere. 2 • 4 The data obtained during the periods of blood removal (hemorrhage) and blood replacement were presented as a function of the per cent of the total amount of blood
80
January 1, 1974 Atn. J. Obstet. Gynecol.
Brinkman, Mofid, and Assali
Table II. Data* on the effects of maternal hemorrhagic shock and blood reinfusion on uteroHemorrhage
100%
No.
Control
50%
MAP (mm. Hg)
12
97 ±3.7
83 :':3.0t
MHR (beats/min.)
12
141 :':8.8
QUA (mi./min./Kg.) UVR
(mm. Hg)
7 7
(mi./min./Kg.) UVO, (mi./min./Kg.)
7
50 :':2.0§ 1~8
!51 :':9.3
±!1.7
10.1 :':2.2t
4.7 ±Ut
8.5
10.0
14.6
:':1.7
:':1.9
±2.9t
15.2 ±3.3
4.1 :':0.9
*The control data are the mean values of the last 20 minutes of the control period. The reference weight for uterine blood flow description of per cent hemorrhage and reinfusion and for explanation of AP, HR, and so on.
t < 002. t < 0.01. §
<
0.00!.
Table III. Data* on the effects of maternal hemorrhagic shock on fetal cardiovascular functions Hemorrhage
I
No.
Control
50%
MAP (mm. Hg)
12
64 :':2.4
62 ±2.2
62 :t2.0
MHR (beats/min.)
12
210 ±5.9
203 ±8.6
176t ±11.2
QDA (mi./min./Kg.)
5
88.9 ±14.1
104 ±12.2
116 ±18.5
QAA (mi./min./Kg.)
5
72.6
66.4 ±10.1
56.4 ±14.1
168 ±21
186 :':24
±9.2 ECO (mi./min./Kg.)
5
161
±22 SVR
5
0.45 ±0.06
0.40 ±0.05
ZOO%
0.35t :':0.05
*The control data are the mean values of the last 20 minutes of the control period. The reference weight for the blood flows and
tP < :tP
0.02.
< 0.01.
removed or reinfused; in this way, the individual variation in the amount of blood that had to be removed to produce a 50 per cent decrease in the arterial pressure was circumvented. The shock and recovery periods were plotted as a function of time from the completion of blood withdrawal or infusion. Statistical significance was determined by an analysis of consecutive paired differences.8 This method allows testing signifi-
cance within each period since it minimizes the impact of individual differences among group members. Results Effects of the experimental procedure on the ewe and her fetus. The impact of the surgical procedure on the ewe and her fetus, including the anesthesia, was not different from that observed in previous studies using the same experimental preparation. 2 • 5 Con-
Volume 118 Number 1
Circulatory shock in pregnant sheep. Ill
81
placental circulation and oxygen transfer (figures represent mean ± 1 S.E.) Shock
±4.0§
125 ±6.1 3.2 ±0.6t
10 min.
15 min.
20 min.
47 ±4.6§
49 ±4.9§
51 ±5.0§
107 ±7.4t
110 ±7.2t
3.0 ±0.5t
3.5 ±0.6t
112
±4.7
120
127
-1:.5.7
±9.0
±7.9
±9.6
133 :.!:7.4
4.1 ±0.7t
8.6 ±1.Bt
12.6 ±2.9
12.6 ±3.5
17.6
18.1
15.7
14.8
10.8
9.3
9.4
±4.1
±4.4
±3.5t
±3.3t
±1.9
±1.7
±2.3
2.1
3.1
2.4
±0.9 is the total weight of the pregnant anima!; for uterine oxygen transfer it is the weight of the uterus, placenta, and fetus. See text for
(figures are mean ± 1 S.E.) Shock 50%
100%
Recovery 20 min.
57 ±2.7
57 ±3.6
59 ±3.3
62 ±3.0
187 ±6.6
195t
±6.2
193t ±4.4
191t ±6.4
190 ±6.!
120 ±11.8
128t ±16.3
125t :!:13.3
117 ±15.3
99 ±8.9
103 ±5.6
68.2 :!:4.5
69.1 ±5.0
68.9 ±6.0
86.8 ±6.7
98.7 ±9.7
±11.6
15 min.
5 min.
lO min.
61 :!:1.7
±2.0
58 ±2.8
163t ±11.6
179t ±9.1
129 ±5.5 68.8 ±6.7 199 ±13
62
188 ±16
198 ±20
0.31
0.36
0.34
±0.02
:!:0.05
±0.05
20 min.
194 ±24 0.34t ±0.06
203t
±n
198t :!:18
103.4 207t ±!9
0.31
0.33
0.3~
:!:0.03
±0.03
:!:0.02
cardiac output is the fetal weight. See text for description of per cent hemorrhage and reinfusion.
trol values for pressures, flows, blood respiratory gases, and pH prior to induction of hemorrhage were within the ranges previously published. In Table I are listed pertinent data on the weight of the ewe and of her fetus and on the maternal blood volume as estimated by a method previously published 5 ; also listed for each animal is the volume of blood (both absolute and as a fraction of the total estimated volume) which had to be removed in order
to produce hypovolemic shock as defined by the protocol. An average of 19 per cent of the estimated blood volume had to be removed to elicit the 50 per cent decrease in mean systemic arterial pressure. Effects of hemorrhagic shock on uteroplacental hemodynamics and oxygen transfer. The effects of hemorrhagic shock and blood replacement on maternal arterial pressure (MAP), heart rate (MHR), uteroplacental blood flow, and uteroplacental oxy-
82
January l, 1974 Am. J Obstet. Gynecol.
Brinkman, Mofld, and Assali
Hemorrhage
Infusion
% 25 _____50 0L- _L. __!_
100
MAP mmH
80
%
75
100 _,
0
25
50
75
100
'
J
60 40" 160.
MHR
140j
beots I mio
120j 100
'l
6.81
CVP mm Hg
6.0 6 5.6 .
52~-j 48:J !
l
_L
10001
QUA ml/m"
800'
600~ 400 200
UVR Hg .iii!~ rm
i
"j
0 22 0 0.18 0.14 0 10 20
T
16
0 2 transfer ml/min
8 4 0
I
SE
I
I I
12.
0
5
/0
15
20
mm
min
Shock
Recovery
Fig. 1. Changes in MAP, MHR, CVP, QUA, UVR, and uterine O, transfer during progressively induced hemorrhagic shock and blood reinfusion. Note that the major increase in UVR occurred only after the perfusing pressure had fallen to below 60 mm. Hg. Note also the marked fall in oxygen transfer during shock. (Figures represent mean ± 1 S.E. and are plotted as a function of the amount of blood withdrawn or reinfused before and after the establishment of the shock state.)
gen transfer are presented in Table II and Fig. 1. In Table II are listed the mean values ± 1 standard error of the maternal arterial pressure and heart rate for the entire experimental group of 12 animals: the
values for uterine blood flow (QUA), UVR, and uvo2 listed in this table are for the seven animals included in this portion of the protocol (see Methods). Fig. I illustrates the progressive changes that occurred
Volume 118 Number 1
Circulatory shock in pregnant sheep. Ill 83
Infusion
Hemorrhage
%
%
0
25
50
75
100
0
25
50
75
100
I,.._..._...L_.-.. ~---.l
"l
FAP
66
mrn Hg
62
5A
540 500
QDA
460
ml/mm
420 380 340 420 380 340 300 260 220 860 l 820 780
co
740 700 660 620 580
SVR
0 14l 0 I2 0 10 0 08
0 06 I~
SE
0
5
10
15
20
0
5
10
15
min
min
Shock
Recovery
20
Fig. 2. Changes in fetal cardiovascular parameters during maternal hemorrhagic shock and blood reinfusion. Note the minor changes in FAP and the marked increase in QDA which contributed to the increase in the fetal effective CO during maternal hemorrhage. QAA and fetal SVR did not change greatly during maternal shock; aortic flow, however, increased during reinfusion and recovery. (Figures represent mean ± 1 S.E. and are plotted in a manner similar to those in Fig. 1.)
during hemorrhage, shock, reinfusion, and recovery in these seven animals. The systemic arterial pressure progressively decreased during bleeding until it reached the level established for the onset of the shock state (50 per cent of control) ; it remained relatively stable at this shock level for the
period of 20 minutes. Maternal heart rate initially increased at the onset of bleeding but, as bleeding progressed and hemorrhagic shock became established, it fell significantly and remained low through the entire reinfusion period (Table II; Fig. 1). Central venous pressure (CVP) decreased progres-
84
January 1, 1974 Am. J. Obstet. Gynecol.
Brinkman, Mofid, and Assali
Table IV. Data* on the effects of maternal hemorrhagic shock on fetal umbilical hemodynamics Hemorrhage
No.
Control
50%
UVP (mm. Hg)
7
26 ±2.9
20 ±3.5
QUV (ml./min./Kg.)
7
UVR
7
(mm. Hg) (mi./min./Kg.)
FVO, (mi./min./Kg.)
7
132 ±17
1l.'i ±13
I
100% 20 ±2.6 116 ±15
0.31
0.38
0.38
±0.05
±0.04
±0.04
3.7 ±0.3
are the mean values of the last 20 minutes of the control period. The reference weight for the umbilical blood
tP :j:P
< <
0.02. O.ot.
sively during blood withdrawal and the shock period (Fig. 1). Uteroplacental blood flow also decreased progressively during blood withdrawal (Table II; Fig. 1). The decrease in flow was relatively proportional to the decrease in pressure until the latter had reached a level of about 75 mm. Hg; during this time the uteroplacental vascular resistance did not change significantly (Fig. 1). But as the perfusing pressure fell to about 50 mm. Hg, the uteroplacental blood flow decreased precipitously, reaching values of about 25 per cent of control levels (Table II; Fig. 1). This greater decrease in flow than in pressure persisted throughout the shock period and was accompanied by a significant increase in uteroplacental vascular resistance (Table II; Fig. 1). Uterine oxygen transfer decreased by less than 50 per cent during the shock period (Fig. l; Table II) ; the decrease was related to the marked fall in uterine blood flow. Reinfusion of the blood progressively restored the maternal systemic arterial and central venous pressures to control values; they remained at such levels throughout the recovery period (Table II; Fig. 1). Heart rate did not return to control values until the end of the recovery period (Fig. 1). Although uterine blood flow increased during blood reinfusion, it remained somewhat below control values until the end of recovery period; uterine vascular resistance progres-
sively decreased during blood reinfusion and returned to control values during the recovery period. Uterine oxygen transfer also returned to near-control levels in the recovery period (Table II; Fig. 1). Effects of maternal shock on the fetal circulation. The impact of maternal hemorrhage and shock on the cardiovascular system of the fetus is illustrated in Figs. 2, 3, and 4; Table III presents the data on the effects of maternal hemorrhage on fetal arterial pressure (FAP), heart rate, ductus and ascending aortic flows, and the sum of the two (the effective cardiac output [CO] ) and systemic vascular resistance; Table IV presents the data on the changes in umbilicoplacental hemodynamics. Progressive removal of approximately 20 per cent of maternal blood volume had insignificant effects on the fetal arterial pressure; only after 15 minutes of established shock did the arterial pressure show some fall, but it returned to normal during recovery (Figs. 2, 3, and 4; Table III). Fetal heart rate decreased somewhat during the bleeding and shock periods and returned to near-control values during recovery (Figs. 3 and 4; Table III). Ductus arteriosus blood flow ( QDA) increased progressively during bleeding, reaching significantly higher levels than control during the shock period; during blood reinfusion and recovery, ductus flow progressively decreased toward control values (Figs. 2 and 3; Table
Circulatory shock in pregnant sheep. Ill
Volume 118 Number I
and oxygen consumption (figures represent mean ±
S.E.) Reinfusion
Shock 5 min.
10 min.
15 min.
20 min.
50%
100%
Recovery 20 min.
23 ±3.6
25 ±4.3
17 ±4.3
18 ±4.4
17 ±3.3
17 ±3.7
18 ±3.6
105 ±10
108 ±14
95t ±19
112 ±15
107 ±19
105 ±18
98 ±14
0.38
0.36
0.34
0.30
0.36
0.37
0.42
±0.05
±0.06
±0.07
±0.08
±o.os
±o.os
±0.03
1.9t ±0.6
85
3.0 ±0.4
2.0t ±0.4
flow and oxygen consumption is the fetal weight. See text for description of per cent hemorrhage and reinfusion.
III). Ascending aortic flow ( QAA) did not change significantly during maternal hemorrhagic shock; it increased somewhat during recovery (Figs. 2 and 3; Table III). Because of the marked rise in ductus flow and the lack of changes in the ascending aortic flow, the fetal effective cardiac output (ECO) increased during maternal hemorrhagic shock; it remained above control values during recovery owing to the increase in ascending aortic :flow and the near-control ductus flow (Figs. 2 and 3; Table III). Fetal systemic vascular resistance (SVR) tended to decrease slightly throughout maternal shock and recovery (Fig. 2). Total umbilical blood :flow and umbilical vein pressure (UVP) decreased slightly during maternal hemorrhagic shock and remained somewhat below control values during recovery (Table IV; Fig. 4). Umbilical vascular resistance tended to increase during maternal hemorrhagic shock but the changes were not significant (Fig. 4; Table IV). Fetal oxygen consumption (FV0 2 ) decreased by about 50 per cent during maternal shock but returned to near-control values during recovery (Fig. 4; Table IV). The decrement was related to the decrease in umbilical vein flow (QUV) as well as in the arteriovenous oxygen content difference. Effects on maternal and fetal blood gases, hemoglobin, and pH. In Table V are listed the values on maternal and fetal Po 2 , Pco 2 , pH, per cent saturation, and hemoglobin
(Hgb.) during control, shock, and recovery. Maternal Po 2 and oxyhemoglobin saturation did not change significantly, probably because of the supported respiration. Maternal blood Pco 2 increased and pH decreased slightly. A minor increase in maternal hemoglobin concentration occurred during recovery (Table V). On the fetal side, blood Po2 and oxygen saturation in both the umbilical vein and artery fell considerably. Fetal blood Pco 2 increased and pH decreased during maternal shock; neither parameter returned to control values during recovery Table V). Some increase in fetal hemoglobin concentration occurred. Comment
The classically described circulatory events that are triggered by blood loss in the nonpregnant animals are: ( 1) a decrease in the effective circulating blood volume and cardiac output; ( 2) a decrease in the arterial pressure including that perfusing the baroreceptor centers; ( 3) inhibition and decreased firing of baroreceptor reflexes; ( 4) vagal inhibition leading to tachycardia which is thought to compensate for the decrease in stroke volume; ( 5) stimulation of the medullary centers and of the thoracolumbar sympathetic chains leading to generalized vasoconstriction and to increased systemic peripheral resistance. These events are observed in unanesthetized as well as anesthetized animals. 9 • 10
86 Brinkman, Mofld, and Assali
CONTROL
MAT~"jlfA!,·,,W. ~
12.0.·J~
60]
~T. ~~, mmttt
January I, 1974 Am. ]. Obstet. Gynecol.
BLEEOING I'ERIOO
AT 20min OF SHOCK
~
I'
li M I,. J
tl'lli
-~
RECOVERY
~
. ,• •,,••• ,• •
30 0
~ ~~l
,
~
FLCIW
mt/min
~l
Fig. 3. Segments of a dynagraphic record illustrating changes in phasic and integrated rna· ternal and fetal systemic pressures and in fetal ascending aortic and ductus arteriosus flows during control, bleeding, shock, and recovery periods. Note the insignificant changes in FAP despite the marked fall in fetal heart rate and maternal pressure during shock. Note also tht' striking increase in ductus flow during bleeding and shock; at the same time, ascending aortic flow decreased only slightly. During recovery, ductus flow returned to control values while ascending aortic flow rebounded to levels higher than control.
The behavior of the regional circulation during hemorrhagic shock depends on the vascular bed under study. The splanchnic, renal, muscular, and cutaneous vascular beds undergo active vasoconstriction, leading to an increase in their respective vascular resistances and a decrease in their blood flows. The circulation to other organs such as the heart and brain does not seem to be affected until the final irreversible stage of shock. 9 • 10 The present data obtained from pregnant sheep studied under pentobarbital anesthesia suggest that the maternal circulatory response to blood loss differs somewhat from the classical picture outlined above. The main difference consists in the absence of any clear evidence for adrenergic stimulation. Neither the progressive loss of blood nor established shock caused the maternal heart rate to increase significantly, which might be anticipated to compensate for the decrease in stroke volume and the cardiac
output. Also, the progressive decrease in central venous pressure speaks against venomotor constriction caused by sympathetic stimulation. Likewise, the behavior of uterine vascular resistance and uterine blood flow during blood withdrawal does not seem to favor adrenergic stimulation, at least in the early phase of blood loss. Our previous studies have shown that the uterine vascular bed in both the pregnant and the nonpregnant sheep is under a strong adrenergic control; the uterine vascular resistance increases markedly with minor alpha-adrenergic stimulation.11 But the present data show that the uterine vascular resistance did not change significantly even with a substantial blood loss sufficient to decrease the perfusing pressure to about 75 mm. Hg. During this bleeding period the uterine blood flow kept pace with the perfusing pressure. Had there been any adrenergic stimulation during this period, the uterine vascular resistance
Volume 118 Number 1
Circulatory shock in pregnant sheep. Ill
Infusion
Hemorrhage
%
%
0
25
50
87
0
75 100
25
50
75
100
FAP mm Hg
2204
FHR
200j
beats I min
!80~ ! 60j
OUmV ml/mh
4401 400 360
320 '
PVR
280
j
0 17
j
0.\51 0 13
i
0.1 I e
009j
I I
i I
SE
0
0
15
5
20
min
Shock
Recovery
Fig. 4. Changes in umbilical hemodynamics during maternal hemorrhagic shock and blood reinfusion. In this series, FAP and FHR decreased slightly; the fall in QUV and the increase in PVR was also minor. FVO,, however, decreased markedly during maternal shock. (Figures represent mean :t 1 S.E. and are plotted as in Fig. 1.)
should have increased markedly and the uterine blood flow decreased much more than the perfusing pressure. Hence, the alpha~adrenergic receptors controlling the uterine vasculature were not stimulated by this degree of blood loss. On the other hand, the present data show that the uterine vascular resistance increased significantly during hemorrhagic shock when the perfusing pressure had faiJen to about 50 mm. Hg. At this pressure level, the uterine
blood flow decreased more than the perfusing pressure. Although these latter changes may suggest alpha-adrenergic stimulation caused by the severe hemorrhage, they are probably related to collapse of the uterine vessels caused by the fall in the perfusing pressure to levels approaching the critical closing pressure for the uterine vascular bed. This latter hypothesis receives support from our previous studies in the pregnant and the nonpregnant sheep which showed
88
January 1, 1974 Am. J. Obstet. Gynecol.
Brinkman, Mofld, and Assali
Table V. Maternal arterial and fetal umbilical vein and arterial blood gases, pH, and hemoglobin concentration during maternal hemorrhagic shock and recovery
Maternal arterial: Po, (mrn. Hg) 12 O, sat. ( o/o) 12 Pco, (mm. Hg) 12 12 pH Hgb. (Gm./ 100 mi.) 12 Fetal umbilical Po, (mm. Hg) Q, sat. ( o/o) Pco, (mm. Hg) pH Hgb. (Gm./ 100 mi.) Fetal arterial: Po, (mm. Hg) O, sat. ( o/c) Pco, (mm. Hg) pH Hgb. (Gm./ !00 mL
137 96
± 30 ±
113 93
:± 3Dt
±
1.5t
126 93
± 36 ± 1.4
136 95
± 31 ±
± 32
130 95
±
± 1.1 33 7.49 ± 0.02
:!: 1.1 35 7.48 ± 0.02
± 1.5t 38 7.45± O.Dlt
39 ± 1.3§ 7.45 ± 0.02§
± 1.8§ 39 7.44± 0.02t
10.5 ± 0.3
10.4 ± 0.3
10.3 ± 0.4
11.0
±
0.4t
11.2 ± 0.3t
vein:
20
± 2.4§
41
±
3.0
36
± 2.5
57
± 6§
90
± 2t
90
±
11
41
± 1.8
17
±
11
94
±
48
± 6§
11 11
± 1.2 40 7.42 ± 0.02
± 2.9§ 52 7.32 ± 0.03§
11
13.4 ± 0.5
13.6
±
12
36
± 1.3
12
± 1.6§
15
± 1.3§
25
:!: 1.4
23
:!:
12
79
± 2
39
:!:: 4§
42
± 5§
73
± 3
70
+ 4t
12 12
± 1 41 7.40 :± 0.02
± 2.3§ 54 7.29:!: 0.03§
12
12.7 ± 0.4
13.4 ±
2.9§
0.5
2t
55 ± 3.6t 7.24 ± 0.03t
± 2.3t 45 7.33 ± 0.03t
± 2.7t 47 7.33 :': O.D2t
13.2 ± 0.6
13.1 ± 0.4
13.2 ± 0.4
:!:: 4.1 § 62 7.17 ± 0.04§
1.2
± 2.9t 50 7.28± 0.03§
± 2.7t 51 7.28 ± O.o3t
12.4 ± 0.3
12.7 ± 0.3
*The control data are the average of at least two samples taken during the last 20 minutes of the control period.
tP
< O.D2.
tP §P
<
<
0.01. 0.001.
a critical closing pressure for the uterine vascular network in the range of 40 to 50 mm. Hg. 11 The question arises as to why blood loss and hypovolemic shock in the pregnant sheep did not elicit adrenergic stimulation. We believe that the reason is probably the type of anesthesia used in these experiments. Although some reports 9 • 10 seem to suggest negligible influence of anesthetic agents on the cardiovascular response to hemorrhage, preliminary experiments in our laboratory seem to indicate that in the pregnant sheep pentobarbital anesthesia partially inhibits adrenergic stimulation.
The decrease in uterine oxygen transfer that occurred during hemorrhage is qualitatively and quantitatively similar to that observed by us in endotoxin and spinal shock and during maternal hypotension induced by hydralazine. 12 - 14 In all these hypotensive states maternal oxygenation was maintained at normal levels and the only variable was the marked decrease in uterine blood flow. These observations demonstrate the dependence of uteroplacental oxygen transfer on uterine blood flow. The remarkable stability of the fetal circulation in the face of such a profound maternal hypotension, uterine ischemia, and
Volume 118 Number 1
depressed uteroplacental oxygen transfer is also in agreement with previous data obtained in a variety of hypotensive states imposted on the mother. 12 - 15 We believe that the absence of any major change in the fetal arterial pressure, despite the fall in fetal blood oxygen levels, is due to the fact that the fetal effective cardiac output remained normal or increased slight· ly during the periods of maternal shock and blood reinfusion. Such an increase in fetal effective cardiac output was largely related to the striking increase in ductus arteriosus blood flow which coincided with the fall in fetal blood Po2. This increase was probably accompanied by a shift of blood toward the ductus from the pulmonary circulation secondary to pulmonary vasoconstriction. This reciprocal behavior of the ductus arteriosus and pulmonary circulations has been observed by us and by others during states of fetal hypoxia and hyperoxia. 2 - 4 • 16 When hypoxia is imposed on the fetus through ventilation of either the maternal or fetal lung with a gas mixture low in oxygen, ductus flow increases and net pulmonary flow decreases.3• 4 • 1 " The opposite is observed during hyperoxia. 2 • 16 The present observations add further evidence of the sensitivity of the ductus arteriosus to changes in blood oxygen tension and the important role that this vessel plays in fetal homeostases. By shifting blood from the lungs toward the systemic circulation during hypoxia, the fetal systemic arterial pressure and the effective cardiac output may be maintained at near normal levels. But if a shift of blood from the lungs toward the descending aorta through the ductus had occurred during maternal shock, the blood return to the left side of the heart should have decreased, and this should have been reflected in a fall in the ascending aortic flow. Yet the ascending aortic flow did not change significantly during maternal hemorrhagic shock and fetal hypoxia. This can be explained only on the basis of an increase in foramen ovale flow. Dawes 16 had suggested increased foramen ovale flow during hypoxia. This increase can be favored by
Circulatory shock in pregnant sheep. Ill 89
a fall in the left atrial pressure caused by the diminished return of blood from the lung. Such an increase in foramen ovale flow may also be construed as a protective mechanism to maintain an adequate fetal cardiac output during fetal hypoxic conditions. The changes in umbilicoplacental circulation observed in the present studies, even though minor, are interesting. The decrease in umbilical blood flow and umbilical vein pressure in the face of unaltered fetal effective cardiac output and systemic vascular resistance is probably related to a shift of blood from the umbilical circuit to other vascular beds. The most likely site to which the blood is shifted is the cephalic circulation. Several reports have shown that, as in the adult, the fetal cephalic circulation is sensitive to changes in blood Pco 2 .H- 19 The increase in fetal blood Pco 2 during the periods of shock and recovery could have led to cephalic vasodilatation and a shift of blood from the umbilical circulation to the head. Another interesting finding in the present set of experiments is the increase in hemoglobin concentration in the fetal blood during maternal shock. Such an increase is possibly related to disturbance in water exchange across the placenta. Faber and Green 20 have shown that in the sheep placenta there is osmotic and hydrostatic equilibration of water. With the fall of maternal arterial pressure to levels below the fetal arterial pressure, this hydrostatic equilibrium is altered and passage of water from the fetus toward the mother may occur, and this may account for the increased hemoglobin concentration in the fetal arterial blood. The decrease in fetal oxygen consumption observed in the present studies is similar in magnitude to that observed in other types of maternal hypotensive states. 12 - 14 It is related to the marked decrease in uterine oxygen transfer as well as the slight fall in umbilical blood flow. The fall in fetal heart rate observed in the present studies is consistent with our previous observations in other types of ma-
90 Brinkman, Mofld, and Assali
ternal hypotensive conditions with fetal hypoxia.12-15 Since this fall did not affect the arterial pressure or the effective cardiac output, the fetal stroke volume must have compensated for the bradycardia. These findings are also in agreement with our previous
January 1, 1974 Am. J. Obstet. Gynerol.
studies on the relationship between fetal heart rate and stroke volume 21 ; they further point out the unreliability of fetal heart rate as a diagnostic sign of hemodynamic disturbances.
REFERENCES
Jr.: Obstet. GynecoL 27: 549, 1966. Assali, N. S., Kirschbaum, T. H., and Dilts, P. V., Jr.: Circ. Res. 22: 573, 1968. Dilts, P. V., Jr., Brinkman, C. R., III, Kirschbaum, T. H .. and Assali, N. S.: AM. J. 0BSTET. GYNECOL. 103: 138, 1969. Brinkman, C. R., III, Weston, P., Kirschbaum, T. H., and Assali, N. S.: AM. J. 0BSTET. GYNECOL. 108: 288, 1970. Kirschbaum, T. H., Brinkman, C. R., III, and Assali, N. S.: AM. J. 0BSTET. GYNECOL. 110: !90, 1971. Westersten, A., Rice, E., Brinkman, C. R., III, and Assali, N. S.: J. Appl. Physiol. 26: 497, 1969. Beck, R., Morris, J. A., and Assali, N. S.: Am. J. Med. Elect. 4: 87, 1965. Steel, R. G. D., and Torrie, J. H. Principles and Procedures of Statistics, New York, 1960, McGraw-Hill Book Company, Inc. Bock, K. D., editor: Shock: Pathogenesis and Therapy, New York, Academic Press, Inc. Fine, J.: In Field, J., editor: Handbook of Physiology, Baltimore, 1965, The Williams & Wilkins Company, Inc., Sect. 2, VoL III. Ladner, C., Brinkman, C. R., III, 'Weston, P., and Assali, N. S.: Am. J. Physiol. 218: 257, 1970.
!. Greiss, F. C.,
2.
3. 4.
5. 6. 7. 8. 9. 10.
11.
12. Lucas, W. E., Kirschbaum, T. H., and Assali, N. S.: Bioi. Neonat. 10: 166, 1966. 13. Bech-Jansen, P., Brinkman, C. R. III, Johnson, G. H., and Assali, N. S.: AM. J. 0BsTET. GYNECOL 112: 1084, 1972. 14. Ladner, C. N., Weston, P. V., Brinkman, C. R., III, and Assali, N. S.: AM. J. 0BSTET. GYNECOL. 108: 375, 1970. 15. Bech-Jansen, P., Brinkman, C. R. III, Johnson, G. H., and Assali, N. S.: AM. J. 0BSTET. GYNECOL. 113: 37, 1972. 16. Dawes, G. S: Foetal and Neonatal Physiology, Chicago, 1968, Year Book Medical Publishers, Inc. 17. Lucas, W. E., Kirschbaum, T. H., and Assali, N. S.: Am. J. Physiol. 210: 287, 1966. 18. Makowski, E. L., Schneider, J. M., Tsoulos, N. G., Colwell, J. R., Battaglia, F. C., and Meschia, G.: AM. J. 0BSTET. GYNECOL. 114: 29:!, 1972. 19. Quilligan, E. L Hon, E. H., Anderson, G. G., and Yeh, S. Y.: AM. J. 0BSTET. GYNECOL. 102: 716, 1968. 20. Faber, J. J., and Green, T. J.: J. Physiol. 223: 375, 1972. 21. Brinkman, C. R. III, Johnson, G. H., and Assali, N. S.: Am. J. Physiol. 223: 1465, 1972.