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Factors related to the placental transfer of thiopental in the hemochorial placenta
Factors related to the placental transfer of thiopental in the hemochorial placenta CHARLES E. FLOWERS, ]R., M.D. Chapel Hill, North Carolina
T H E purpose of this study was to determine some of the factors which are related to the placental transfer of analgesics and hypnotics that are used in obstetric anesthesia. Although thiopental sodium (Sodium Pentothal) is a useful obstetric hypnotic, when employed according to basic pharmacologic and anesthesiologic principles,H it was employed in this study as a tracer substance to study the movement of drugs across the placenta. 4 The explanation of a mother being anesthetized with thiopental but delivering an alert infant is of great importance in understanding basic concepts of obstetric anesthesia and analgesia and warrants continued investigation. Thiopental was selected as the agent of study since it is of low molecular weight, 221 could be used in sufficient doses to yield satisfactory tissue and plasma levels, and could be analyzed with satisfactory accuracy. Analytical recoveries range from 93 to 95 per cent. Meperidine, secobarbital, and pentobarbital arc more difficult to analyze and yield low plasma and tissue levels."· 5 • r. Thr rabbit and guinea pig were the sub-
jects of the experiments. Their placentas are hemochorial, by Grosser's 7 definition, and although they are labyrinthine, both have sufficient similarity to the human placenta in microscopic appearance and physiologic function to be suitable for this study. 8 • 9 Method and material
Domesticated pregnant does weighing 3 to 5 kilograms and guinea pigs weighing 1.3 to 1. 7 kilograms were used. The studies were made on the thirtieth gestation day in the rabbits and on the sixtieth to sixty-fifth day after conception in the guinea pigs. Two per cent thiopental was injected into the ear vein of the rabbits and into a mesenteric vein of the guinea pig after local abdominal or light ether anesthesia. Maternal blood for blood gas determination was removed from the left side of the heart by parasternal puncture and from the jugular and carotid vessels after partial decapitation of the rabbit and guinea pig fetuses. Maternal brains were removed after exsanguination by caval transection; fetal tissues were removed after exsanguination following decapitation. Because of the small amount of thiopental being recovered, particular care was taken to prevent contamination of the tissues with blood or thiopentaL All free blood was removed from the brain, muscle, and viscera with gauze. Time was recorded from the beginning of the injection until delivery. The injections were generally completed within 1 minute for the studies between 1 and 5 minutes and within 3 minutes in the other studies. In thr
From the Department of Obstetrics and Gynecology, University of North Carolina School of Medicine. Supported in part by a grant from the United Medical Research Foundation of North Carolina. Presented by invitation at the Seventy-third Annual Meeting of the American Association of Obstetricians and Gynecologists, Hot Springs, Virginia, Sept. 6 to 8, 1962.
646
Placental transfer of thiopental
Volume 85 Number!'>
647
the values were sufficiently close for validity. Duplicate determinations were made at periodic intervals to insure accuracy of determinations. Thiopental was analyzed by the method of Brodie 10 except that the first extraction was buffered to a pH of 8. Two milliliters 1 M. pHS phosphate buffer was used instead
intermittent injection experiments, one third of the thiopental was injected within 3 minutes; the remaining two thirds was given in 12 injections at 60 second intervals. Sixtyfive pregnant rabbits, 34 pregnant guinea pigs, and 3 dogs were used. Two to 4 animals were used for each time that was studied. The values were means provided
34 30
• • mattJrllt11 /Jroin o mattJrnal plasma
0
26 mgm I L Plasma mgm/K Brain
& felo/ brain
t. fetal ptasmo
22
•
16
0
14
•
•
0
0
10
• 0
6
i
t t
2 2
4
10 8 time minutes
6
12
14
16
Fig. 1. Thiopental levels in maternal and fetal plasma and brain in the rabbit after intravenous injection of 10 mg./kg.
56
•
•
48 40 mgm I L Plasma mgm/K Brain
• maternal bra1n o maternal plasma & felol brain t. ftJtal plasma
0
0
32
•
24
0
18
8
12 4
,.
t t 2
4 6 8 time minutes
,. 10
12
Fig. 2. Thiopental levels in maternal and fetal plasma and brain in the rabbit after intravenous injection of 20 mg./kg.
648
Flowers
\larch 1, .-\t~>.
Table I. Thiopental levels in maternal plasma and brain and in fetal plasma and ·--c-~·---·---····-----
l~lh:)
.) Oh
tissue~ HI
-------- ·-------- ----- ---·-·--- ------------ -----------·-·----
Maternal
Tunc (minutes)
30
10
-·-·-Plasma
Pla
28.5
35
16
20
'l
12
13
H
11
12
10
12
.52.3
63
34
44
31
H
25
~8
24 19
2·4 19 :lll 1:1
')
3H
:i :J.~)
12
2i
l
9 ]()
1'' L'i
8 ..'i
15
9
15.5
20
22
30
18.5
of 2 ml. 1.5 M. NaH 2 P0 4 in the plasma method. Tissues were homogenized in approximately an equal volume of pH8 buffer and 2 ml. of this homogenate was then carried through the same procedure as plasma. By buffering to a pH of 8 instead of a lower pH, the tissue blank was reduced virtually to zero without decreasing the extraction of thiopental. The determinations of lipid content of tissues were done with ethanol petroleum ether extraction based on the method of Ricr. 11 The analysis of pH, pO~, and pCO~ were performed by the use of the Clarkl 2 oxygen electrode and the Severinghaus 13 C02 e lectrodr. * Results
Comparison of thiopental in the maternal and fetal brains and plasma in the rabbit. Figs. 1, 2, and 3 and Table I give the analytical data of maternal and fetal plasma and brain according to various times after intravenous injections of 10, 20, and 30 mg. of thiopental. The curves of disappearance of thiopental in the maternal plasma and brain are similar to those described for animals and human beings.:', 14 • 15 Maximum concentrations appear in the brain and plasma almost in*Instrumentation chu~dts,
Laboratories Inc., Cambridge,
Massa-
stantaneously and are unhindered by the blood-brain barrier. The rapid decline of thiopental from the maternal plasma and brain is accounted for by its rapid movement into muscle and viscera and its subsequent slower deposition in fat. 14 While the maternal plasma and brain levels are limited only by the amount of thiopental injected and the cerebral blood flow, the concentration of thiopental in the fetal plasma and brain are controlled by other and more complex variables which stimulated these studies. The fetal values indicate that thiopental begins to cross the placenta within one or two circulation times. Initially the levels are quite low, 1 and 1.5 mg. per liter at 10 and 20 mg. per kilogram dosage and 2 and 3 mg. per liter at 30 mg. per kilogram dosage. These data seem to indicate that there is no immediate placental barrier to thiopental, but the placental transfer is in low concentrations. This is in agreement with the detailed studies of Baux and associates. 1"· 1 ' The values become level in about 5 minutes and the two circulations begin to approach equilibrium after 10 minutes. The maternal and fetal values which fall above or below the expected curve result from experimental error and the small amount of thiopental in the plasma and tissues. There was little difference between the
Placental transfer of thiopental
Volume 85 Numht'-'r 5
649
the rabbit after injections of 10, 20, and 30 mg. per kilogram
Plasma 1.5
1.5
3.5
3
6.5
2
3.6 3.5
3.2 3
6.5 8
3 3
·!.2
4
4
3.5
10
3
9
3
0.5
1.5
1.5
3
2
3
3.5
3
8
8
11
6
4.5 10.5
8
13
10
7
18
5
13.5 12.5 13 10
9
17
7
18 14 15
8 .:J" 7 9 7
4
10 and 20 mg. per kilogram dosage in the amount of thiopental in the fetal brain and plasma. This may have been caused by the low levels of thiopental which were involved in the determinations; there is significant difference, however, between the 10 and 30 mg. per kilogram studies. This seems to indicate that the drug passed by simple passive diffusion influenced by a higher concentration gradient in the maternal plasma. There are no fetal values which were higher than the maternal. However, the concentration of thiopental in the maternal
5
4
10.5 8.5
brain was uniformly higher than that in the plasma; the reverse was true in the fetus. These differences are probably related to the lipid content of the substances. There was a significant difference in the concentration of thiopental in the maternal and fetal brain at various times from injection to delivery. The concentration in the maternal brain was 30 times greater than that in the fetal brain after 1 minute of anesthesia, but becomes only 2 to 3 times higher after 10 minutes. The differences in the concentration of thiopental in the brain
•
64 60 56 52. 48 44 mgm/L Plasma 40 36 mgm/K Brain 32. 2.8 2.4 20 16 12 8 4 i
• maternal brain
o maternal plasma
• fetal brain A felal plasma
• 0
•
0
•0 ., A
A
•• • • • A
A
•
0
Cl
• A
0
•
• A
2. 4 6 8 10 12 14 16 18 20 22. 24 26 2.6 30 time minutes Fig. 3. Thiopental levels in maternal and fetal plasma and brain in the rabbit after intravenous injection of 30 mg./kg.
,\laH: h L 1'Jh >
650 Flowers
t\nL
were reflected in the clinical response of the mother and her young. During the first 10 to L'i minutes of anesthesia, the doe was asleep and her pups were awake. As the concentrations in the fetal and maternal brain become more equal so did the degree of narcotization. This was reflected in the reaction index of the fetuses. A reaction index was devised to give some indication of the effect of thiopental on the rabbit and guinea pig fetus; an Apgar index was impractical. Two points were given for respiration, crying, righting movements, and walking. An index of 8 indicates a normal and vigorous fetus. Some of the indices are
J, Obst.
& (~vncc
not ·whole numbers smce they represent means of the studies for a particular time. There is an inverse con·elation between the concentration of thiopental in the brain and the reaction of the fetus. Passage of thiopental into fetal tissues of the rabbit. Figs. 4 and 5 and Table I indicate the amount of thiopental which passed into the various fetal tissues. The curws are similar in shape to those for fetal plasma and brain. The greatest concentration of thiopental was found in the viscera which has the highest lipid content and consisted of the genitourinary, gastrointestinal i minus the stomach contents), and the canliopulA fetal brain
A fetal plasma
12
c fetal viscera • fetal muscle
10
c IJ
c
8 mgm /L Plasma mgm/K Tissues
0
0
A
A
6 4
2
•
• 2
A
t
•
IJ
.
4
i
10 8 time minutes
6
i
•
• 14
12
16
Fig. 4. Thiopental levels in fetal plasma and tissues in the rabbit after intravenous injection in the mother of 10 mg./kg.
20 18
0 IJ
16
0
I!J.
0
mgm/L Plasma 12 mgm/ K Tissues 10
0
8 6
z
brain p/asf11(J viscera muscle
0
14
4
.t. fetal A fetql 0 fetal • fetal
0
IZI.
I!J.
A
•• • • •
•
t;
•
t;
• •
t;
••
• •
2 4 6 8 10 12 14 16 18 20 22 a4 26 28 30 time minutes Fig. 5. Thiopental levels in fetal plasma and tissues in the rabbit after intravenous injection in the mother of 30 mg./kg.
Volume 85 ~1unber
Placental transfer of thiopental
5
monary system. The lowest concentration of the drug was in muscle. However, the majority of the fetal weight was distributed in viscera and muscle, thus the greater portion of the thiopental which reaches the fetal side of the placenta is stored in these tissues. In a typical litter, the total weight of the brains was 8 grams, viscera 17 grams, muscle 136 grams, and plasma 16 milliliters. Fat was not analyzed separately although some was present in the paranephritic tissues. Thiopental crosses the placenta in low concentrations with relatively greater amounts being stored in muscle and viscera than in brain. The fetal brain is therefore not generally subjected to the high plasma concentration of thiopental that is necessary for thioharbiturate anesthesia. Calculations were made to estimate the total amount of thiopental which passed into and was stored in fetal tissues. These calculations were made by multiplying the weight in grams of each tissue by the number of milligrams of thiopental per gram. Brain, placenta, plasma, viscera, and muscle were used. No studies were made on skin and bone because of the technical difficulties which were involved. Bollman18 has shown, however, that some thiopental passes into these tissues. These data appear in Table IV. The values are only relative but confirm a slow and continuous passive placental diffusion regulated by maternal and fetal tissue and plasma equilibrium. Comparison of thiopental in the maternal and fetal brain and plasma in the guinea pig. Figs. 6 and 7 and Table V give the analytical data for maternal and fetal brain and plasma concentrations of thiopental in the guinea pig after the maternal injection of lO and 30 mg. per kilogram. The curves are similar to the rabbit, but the levels of thiopental in the brain of the guinea pig fetus are higher than those in the plasma; the reverse was true in the rabbit. This probably related to the greater ma~ turity of the guinea pig at birth. At similar times the thiopental levels in the maternal and fetal guinea pig and the
651
Table II. Lipid content of maternal and fetal tissues and plasma Guinea pig
Rabbit
Maternal Fetal Maternal Fetal (mg./ (mg./ (mg./ (mg./ Gm.wet Gm. wet Gm. wet Gm. wet weight) weil[ht) weight) weight) Brain Viscera
98.5
107
15.3 41.2
45.5 77.5
Table III. Effect of thiopental upon the reaction index of rabbit and guinea pig fetuses after maternal injection of 10, 20, and 30 mg. per kilogram
Time (minutes)
Reaction index of guinea . pigs Reaction index of rabbit pups 10 mg./ kg.
1
8
3 3.5 4
8
5 8
7 8
9 10
7
120 mg./~ 30 kg. 7 7
8
8 7
8
7
7
5
6
.1 3
5
12 15 20 30
mg./ 10 mg.! kg. kg.
6 6
6
3
6 6
Table IV. Estimate of the total passage of thiopental into the products of conception in the rabbit after the maternal injection of 10 and 30 mg. per kilogram. thiopental in Time (minutes) 1 3 3.5 5
8 9 10 12 15 20 30
30 mg./kg. 0.7 1.5
1.2 1.4 1.6
1.7
2.1 2.0
2.7 3.1 3.3 3.9
652
~latch
Flowers
!Yt~,:;
l
\n1. .L OIN & U"n•·<..
Table V. Thiopental levels in maternal plasma and brain and in fetal plasma and tissues in the guinea pig after the injection of 10 and 30 mg./kg.
J'intt' (minutes)
25
24
'),j
I'>
19
+.'> !()
l''·'
1:2
f)
9
:l
:i
7
11
6.5
6
.,
+
11
.,
I.e,
.,
~l ':·,
2U
....
L'i
ti(l
ti
h
3.5 It!
:F,
'):)
ti.5
il
L:
F>
:!5
B
8.5
I '1.5
10
2(1
\0
II
1:.>
::t.:)
10
17
9
10
15
ti
)'j
16
Viscera i Musclr
,,
70
4.1 ·Ill
0
~.~
-----·~--~·~--~-·-----
Table VI. Contrast between single injection of thiopental and intermittent administration of 30 mg./kg. (delivery 15 minutes after initial injection) -
----
-~·-~---~--~·
--
Maternal
Plasma (mg./L.)
Brain (mg./ k!il..)
Intermittent injection 40 23 45 45 37 36 Mean :n 40
---
-·-~··-------~------
'
i
7.19 7.47 7.23 7.29
2~
17 19
I
I
po, 54.1 46.0 35.5 45.2
31.1
37.2 33.2 :13.8
16 19 16 17
14 16 15 15
35 33 30 33
10
8
12
7
'.!.7
15 1",)
6 7
pCO~
(minutes)
pH
I
7.39
60
7.36
3.5
7.39
61
U7
18
52.5
5
7.49 7.31 7.30 7.2H
60 65
7.38 7.27 7.29 7.29
19
43
18
47
8 ]II
15
6.)
67
16 13 U:l
6 5 3 4.7
f!.H
po,
7.17 7.21 7.16 7.18
14.1 14.0 17.5 15.2
Reac· tion pCO,i index 41.6 43.4 42.5
,,
2 2 :l 7 :)
:;
{)
~.5
6
after thiopental injection of
pCO,
pH
49.5
7.22 7.31
56 58
7.27 7.25
7.35
62
7.\8
16
!9.8
7.40
65.5
7.21
18.3
~17
3
+
Fetal
I
Table VII. Maternal and fetal pH, pO" and 10 and 20 mg./kg. in the rabbit
Time
------~-----
otic \Plasma Brain i Viscera' fluid I i (mg./ I (mg./ (mg./ (mg./ L.) I pco, I L.J I kg.) I kg.)
Single injection completed in 3 minufp, 16 22 jg 19 Mean 19
_____
'
1
i pH
....
__ i ------- ------
18 48.2
pCQ,
44
41)
Volume 85 .J
~umber
maternal and fetal rabbit were quite similar. The concentrations of thiopental in the brains of the guinea pigs at 30 mg. per kilogram dosage were slightly higher than those in the rabbits while at the 10 mg. dosage they were slightly lower. The differences in brain concentration of thiopental in the maternal guinea pig and the fetuses, for various times of anesthesia, were in the same order of magnitude as in the rabbits. However, the maternal guinea pigs were more depressed than the rabbits. In the 30 mg. per kilogram studies this could have been related to the light ether induction to obtain a mesenteric vein for intravenous injection. But the depression was greater than could be explained by the small amount of ether used. Consequently, local abdominal anesthesia was employed for the remainder of the guinea pig experiments. With the 10 mg. per kilogram dosage, without ether induction, the fetal guinea pigs were considerably more depressed than the rabbits after 5 minutes of anesthesia. The tissues of the guinea pig fetuses (Fig. 8) did not contain more thiopental than those of the fetal rabbits. Moroover, they were not acidotic; the lowest pH in the 10 mg. studies was 7.2. The explanation may lie in species sensitivity to thiopental anesthesia or other obstetric factors. These differences indicate the complexities of obstetric anesthesia and the difficulties of comparing experimental data in one experimental animal with another, despite the similarities of the placentas. Contrast between single and intermittent injection of thiopental. Evaluations of thiopental for obstetric delivery seem to indicate that the fetus is less depressed when a single injection rather than a number of intermittent injections of thiopental is used.H The intermittent injection of thiopental gives a number of high plasma concentrations of thiopental in the mother and allows greater amounts of the drug to cross the placenta. 3 This is substantiated by the data in Table VI. When 30 mg. per kilogram was injected intermittently, as previously described, the mean and individual levels of
Placental transfer of thiopental
653
thiopental in the plasma, brain, viscera, and amniotic fluid were higher than those in the animals that received a single injection. Moreover, the reaction indices were lower. Thiopental in the amniotic fluid and stomach contents. Attempts were made to determine the concentration of thiopental in amniotic fluid in the majority of the studies. In many instances, however, it was impossible to obtain amniotic fluid which was not contaminated with small amounts of blood. There is little amniotic fluid in the rabbit at term. Very little thiopental can be detected in the amniotic fluid before the 8 to 9 minute studies. Thereafter, it is easily recovered and the values are related to the concentration gradient in the maternal plasma and tissues as indicated in Table VI. The circulation of the amniotic fluid is complex and related to many variables. Both the rabbit and the guinea pig have small chorionic vessels which may facilitate the passage of thiopental into the amniotic fluid. However, thiopental has a low molecular weight and may diffuse into the amniotic fluid from the endometrium. McGaughey 19 has shown that creatine with a molecular weight of 113 can traverse the human chorionamnion. It is reasonable to suppose thiopental could pass similarly. Fifteen analyses for thiopental were made on the stomach contents of the rabbit. Drugs may pass from the plasma into the gastric juice according to their physical characteristics and the dissociation constant since the gastric mucosa behaves as an oily layer toward various drugs.~ 0 At the 20 and 30 mg. per kilogram dosage, the stomach contents contain as much as 4 mg. per liter. After the 8 to 9 minute studies the highest concentration of thiopental in the gastric juices was in the intermittent injection studies. These data further indicate that with increasing time greater amounts of thiopental pass into fetal plasma and tissues. Maternal and fetal pH, p02, and pC0 2 • Table VII contains the values of pH, pOz, and pC0 2 for the rabbit at 10 and 20 mg. per kilogram dosage and Table VI gives the
654
~1(irdt i, 196:~. Am . .1. Ohsr. & Gyrwc
Flowers
30
26 22 mgm I L Plasma mgm/K Brain
• matt~rnol brain o matemol plasma
.a. fetal brain
0
A ftlfol plosmo
• •
18
0
14
0
•
10
•
•
0
6 2
• t ...
... 2
4
l
... 8 10 12 time minutes
6
14
16
Fig. 6. Thiopental levels in maternal and fetal plasma and brain in the guinea pig after intravenous injection of 10 mg./kg.
data for the intermittent injection studies at 30 mg. per kilogram. The C0 2 and 02 electrodes for analysis of blood gases .were not available for the guinea pig studies. The maternal blood was obtained anaerobically from the left side of the heart just prior to delivery. Fetal blood \vas not obtained anaerobically but was collected by allowing jugular and carotid blood to flow into a 20 c.c. syringe. The collection was made as quickly as possible, but this technique possibly created some error by elevating the p0 2 and lowering the pC02. When the many variables of anesthesia and homeostasis are considered, these values indicate that the doe and fetuses were slightly hypoxic but in a reasonable physiologic state. The does which received thiopental intermittently at the 30 mg. per kilogram dosage were more depressed than the animals which received the thiopental by single injection. This is reflected in the maternal and fetal p0 2 • The fetuses were more acidotic than their mothers, but this is due to the delay in killing the doe by cervical fracture before obtaining fetal blood at delivery. The data from animals which responded poorly: to anesthesia or became hypoxic and depressed were not used in these studies. InYariably, the fetal values of thiopental were
lower in these fetuses than in the animals in good condition. A limited number of studies were done with the mothers receiving oxygen during anesthesia. Sufficient data were not available for statistical analysis, but preliminary data did not indicate that pH, p0 2 , and pCO:: influenced the placental transfer of thiopental provided the mother responded normally to a carefully administered anesthetic and that uterine and fetal circulations were not impaired. Effect of the change in pH and pC0 2 upon the level of thiopental. Wad dell and Butler21 have reported that a rise in pH and a fall in pC0 2 reduces the level of phenobarbital in tissues and brain and raises the concentration in plasma, thereby reducing the level of anesthesia. In previous stndies:J it was postulated that thiopental would respond similarly to phenobarbital since the pK of thiopental is very close to that of phenobarbital. Fig. 9 gives the experimental data to indicate that thiopental does respond similarly to phenobarbital with changes in pH and pC02. The data in this illustration is from one dog; however, three experiments rielded similar values. Thiopental equilibrium was established after 2 Gm. of thiopental was given intra-
venously over 1Y2 hours. The first three values indicate this equilibrium. The 30 per cent C0 2 caused the thiopental to leave the plasma and accumulate in the tissues; hyper~ ventilation and lowering of the pC02 caused the reverse to occur. At birth there is a fall in pC0 2 and a rise in pH. This could theoretically be of benefit to the fetus in lowering the concentration of thiopental and other related drugs in the brain. However, this shift is slight and re~ quires 15 or more minutes to be completed. It is felt that the pH and the pC0 2 shift have a limited influence on the anesthetic level in the fetus.
verse 2 and explained Cohen and his coworkers'1 results which indicated a single injection of thiopental for cesarean section caused little fetal depression and allowed no correlation between the time of administration of the drug and the delivery of the infant. The blood gas determinations were made to assess the degree of homeostasis in the
80
10 60 50
• mtt!er!ICI lm;m o moterool plasma braih p/asmQ
•
&!~!ttl
A''"''
...•
mgm I L Plasma 40 30 mgm/ K Brain
20 10
••
Comment
It is important to ponder the factors which allow an infant to be born alert when its mother is anesthetized with ether, cyclopropane, or a barbiturate. It was hoped that the analysis of thiopental in the rabbit and guinea pig tissues as well as plasma would yield a tenable explanation for what is often clinically apparent in the human being. Both the rabbit and guinea pig were used as experimental animals in the hope that each would offer slightly different data to aid in the study of placental transfer. The analysis of thiopental in both plasma and tissues were made to study the movement of the drug into and storage in fetal tissues. These studies indicate that thiopental enters fetal tissues in the same manner that it enters adult tissues. The real difference lies in the placenta protecting the fetus from the high plasma concentration which is necessary for thiobarbiturate anesthesia. By the time equilibrium is approached in maternal and fetal plasma, the thiopental has passed into maternal tissues and the concentration gradients between the two circulations are low. Essentially, the mother has received a single, rapid, concentrated dose of thiopental while the fetus has received a very slow intravenous injection containing a low concentration of the hypnotic. The intermittent injection studies con· firmed the ideas of McKechnie and Con·
655
Placental transfer of thiopental
Volume 85 5
~umber
•
s
e
~
m
~
~
~
w
lime minutes
Fig. 7. Thiopental levels in maternal and fetal plasma and brain in the guinea pig after intravenous injection of 30 mg./kg.
eo
A I1Jtt! bMt/1 A f#IIJI /!JfXff>IJ !lftta/ V!Sf:ell1 • few! ~rd~Ufe
70 60
mgm I L Plosmo mgm/ K Tissues
50
40
30
ro
10
0
2
i • 24
8101214161620 11me minutes
Fig. 8. Thiopental levels in fetal plasma and tissues in the guinea pig after intravenous injection in the mother of 30 mg./kg.
mgt~~IL
40
Tliopet1lri
30 2Q
Fig. 9. Effect of the change in pH and pCO. upon the levels of thiopental in the dog.
656
M~1dJ
Flowers
mother and fetus. Data from animals in acidosis were not used. When homeostasis was apparent, the administration of 0 2 did not ~eem to influenCf~ the placental transfer of thiopental. The studies on the effect of the change of pH and pC0 2 on the plasma level of thiopental were performed to challenge statements made in earlier thiopental studies. 3 It is believed that these changes are not of clinical importance at birth. Summary The~e
data may be summarized by outlining the pharmacologic effect of thiopental on the mother and fetus. 1. Thiopental produces anesthesia in the mother by a high plasma concentration of thiopental immediately entering the brain caused by the absence of a blood-brain barncr. 2. Thiopental in the maternal circulation is quickly bound to plasma protein and rapidly enters muscle, viscera, and body water. This materially reduces the amount of thiopental which is available to the fetus.
1~63
Am. ,l. Obst. & Gynec
3. There is no immediate placental barrier to the passage of thiopental to the fetu~. hut it crosses the placenta slowlv and in low concentrations. 4. As the time from injection increases and materHal and fetal equilibrium are approached. the maternal plasma level is low and the differences in the gradients between the two circulations is small. 5. As thiopental enters the fetal circulation it is concentrated in greater amounts in viscera and muscle than in brain. 6. When thiopental is injected intermittently. peak plasma concentration gradients occur that assist in a greater placental transfer and accumulation in the fetus. 7. Fetal depression may result from tillcompensated fetal or maternal homeostasis, excessive concentrations of drug in the fetal brain, or other obstetric factors. Appreciation is expressed to Dr. \Villiam Waddell of the Department of Pharmacology for his assistance in developing the method of analysis and t:n Mrs. Lynda Jensen for her invaluable technical assistance.
REFERENCES
1. Cohen, E. N., Paulson, W. J., Wall, ]., and Elert, B.: Surg. Gynec. & Obst. 97: 456, 195:). ·> McKechnie, F. B., and Converse, ]. G.: A.M. J. Oasr. & GYNEC. 70: 639, 1955. :l. Flowers. C. E.: AM. J. 0BsT. & GYNEC. 78: 730, 1959. -+. Hagerman, D. D., and Viller, C. A.: Physiol. Rr,:. 40: 313, 1960. 5. Burns, J. J.: ]. Pharmacol. & Exper. Therap. 114: 289, 1955. 6. Root, B., Eichner, E., and Sunshine, I.: AM. .J. 0BST. & GYNEC. 81:948,1961. i. Grosser, L.: Zentralbl. Physiol. 22: 198, 1908. 8. Amoroso, E. C.: Brit. M. Bull. 17: 81, 1961. 9. Mossman, H. W.: Am. J. Anat. 37: 433, 192b. I 0. Brodie, B. B., et al.: J. Pharmacol. & Exper. Therap. 98: 85, 1950. II. Rice. L. I., !'t al.: .J. Bioi. Chern. 201: 867, 19'):>
1'l
13.
1+. 15. 16. 17.
18. 19. ~0.
21.
Clark, L. C.: Tr. Am. Soc. Art. Int. Oregon 2: 41, 1956. Severinghaus,]. W., and Bradley. A. F.: J. Appl. Physiol. 13: 515, 1958. Mark, L. C., et a!.: J. Pharmacol. & Exper. Therap. 119: 35, 1957. Goldstein, A .. and Aronow, L.: J. Pharmacol. & Exper. Therap. 128: I, 1960. Baux, R., et al.: Ann. chir. 12: 1275, 1958. Baux, R., et a!.: Bull. Fed. soc. obst. et gynec 11: 200, 1959. Bollman, J. L., Brooks, L. M., Flock, E. V., and Lundy, J. S.: Anesthesiology 11: 1, 1950. McGaughey, H. S., ct al.: A.M. J. 0BsT. & GYNEC. 80: 108, 1960. Brodie, B. B., and Hogen, A. M.: ]. Pharm. & Pharmacol. 9: 345, 1957. Waddell, W. J., and Butler, T. C.: J. Clin. Invest. 36: 121 7, 195 7.
Discussion DR. BEN PEcKaA~t, Madison, Wisconsin. In his paper, Dr. Flowf'rs has presented a mass of
interesting data, painstakingly acquired, primarily in the rabbit and the guinea pig, from which he
T H E purpose of this study was to determine some of the factors which are related to the placental transfer of analgesics and hypnotics that are used in obstetric anesthesia. Although thiopental sodium (Sodium Pentothal) is a useful obstetric hypnotic, when employed according to basic pharmacologic and anesthesiologic principles,H it was employed in this study as a tracer substance to study the movement of drugs across the placenta. 4 The explanation of a mother being anesthetized with thiopental but delivering an alert infant is of great importance in understanding basic concepts of obstetric anesthesia and analgesia and warrants continued investigation. Thiopental was selected as the agent of study since it is of low molecular weight, 221 could be used in sufficient doses to yield satisfactory tissue and plasma levels, and could be analyzed with satisfactory accuracy. Analytical recoveries range from 93 to 95 per cent. Meperidine, secobarbital, and pentobarbital arc more difficult to analyze and yield low plasma and tissue levels."· 5 • r. Thr rabbit and guinea pig were the sub-
jects of the experiments. Their placentas are hemochorial, by Grosser's 7 definition, and although they are labyrinthine, both have sufficient similarity to the human placenta in microscopic appearance and physiologic function to be suitable for this study. 8 • 9 Method and material
Domesticated pregnant does weighing 3 to 5 kilograms and guinea pigs weighing 1.3 to 1. 7 kilograms were used. The studies were made on the thirtieth gestation day in the rabbits and on the sixtieth to sixty-fifth day after conception in the guinea pigs. Two per cent thiopental was injected into the ear vein of the rabbits and into a mesenteric vein of the guinea pig after local abdominal or light ether anesthesia. Maternal blood for blood gas determination was removed from the left side of the heart by parasternal puncture and from the jugular and carotid vessels after partial decapitation of the rabbit and guinea pig fetuses. Maternal brains were removed after exsanguination by caval transection; fetal tissues were removed after exsanguination following decapitation. Because of the small amount of thiopental being recovered, particular care was taken to prevent contamination of the tissues with blood or thiopentaL All free blood was removed from the brain, muscle, and viscera with gauze. Time was recorded from the beginning of the injection until delivery. The injections were generally completed within 1 minute for the studies between 1 and 5 minutes and within 3 minutes in the other studies. In thr
From the Department of Obstetrics and Gynecology, University of North Carolina School of Medicine. Supported in part by a grant from the United Medical Research Foundation of North Carolina. Presented by invitation at the Seventy-third Annual Meeting of the American Association of Obstetricians and Gynecologists, Hot Springs, Virginia, Sept. 6 to 8, 1962.
646
Placental transfer of thiopental
Volume 85 Number!'>
647
the values were sufficiently close for validity. Duplicate determinations were made at periodic intervals to insure accuracy of determinations. Thiopental was analyzed by the method of Brodie 10 except that the first extraction was buffered to a pH of 8. Two milliliters 1 M. pHS phosphate buffer was used instead
intermittent injection experiments, one third of the thiopental was injected within 3 minutes; the remaining two thirds was given in 12 injections at 60 second intervals. Sixtyfive pregnant rabbits, 34 pregnant guinea pigs, and 3 dogs were used. Two to 4 animals were used for each time that was studied. The values were means provided
34 30
• • mattJrllt11 /Jroin o mattJrnal plasma
0
26 mgm I L Plasma mgm/K Brain
& felo/ brain
t. fetal ptasmo
22
•
16
0
14
•
•
0
0
10
• 0
6
i
t t
2 2
4
10 8 time minutes
6
12
14
16
Fig. 1. Thiopental levels in maternal and fetal plasma and brain in the rabbit after intravenous injection of 10 mg./kg.
56
•
•
48 40 mgm I L Plasma mgm/K Brain
• maternal bra1n o maternal plasma & felol brain t. ftJtal plasma
0
0
32
•
24
0
18
8
12 4
,.
t t 2
4 6 8 time minutes
,. 10
12
Fig. 2. Thiopental levels in maternal and fetal plasma and brain in the rabbit after intravenous injection of 20 mg./kg.
648
Flowers
\larch 1, .-\t~>.
Table I. Thiopental levels in maternal plasma and brain and in fetal plasma and ·--c-~·---·---····-----
l~lh:)
.) Oh
tissue~ HI
-------- ·-------- ----- ---·-·--- ------------ -----------·-·----
Maternal
Tunc (minutes)
30
10
-·-·-Plasma
Pla
28.5
35
16
20
'l
12
13
H
11
12
10
12
.52.3
63
34
44
31
H
25
~8
24 19
2·4 19 :lll 1:1
')
3H
:i :J.~)
12
2i
l
9 ]()
1'' L'i
8 ..'i
15
9
15.5
20
22
30
18.5
of 2 ml. 1.5 M. NaH 2 P0 4 in the plasma method. Tissues were homogenized in approximately an equal volume of pH8 buffer and 2 ml. of this homogenate was then carried through the same procedure as plasma. By buffering to a pH of 8 instead of a lower pH, the tissue blank was reduced virtually to zero without decreasing the extraction of thiopental. The determinations of lipid content of tissues were done with ethanol petroleum ether extraction based on the method of Ricr. 11 The analysis of pH, pO~, and pCO~ were performed by the use of the Clarkl 2 oxygen electrode and the Severinghaus 13 C02 e lectrodr. * Results
Comparison of thiopental in the maternal and fetal brains and plasma in the rabbit. Figs. 1, 2, and 3 and Table I give the analytical data of maternal and fetal plasma and brain according to various times after intravenous injections of 10, 20, and 30 mg. of thiopental. The curves of disappearance of thiopental in the maternal plasma and brain are similar to those described for animals and human beings.:', 14 • 15 Maximum concentrations appear in the brain and plasma almost in*Instrumentation chu~dts,
Laboratories Inc., Cambridge,
Massa-
stantaneously and are unhindered by the blood-brain barrier. The rapid decline of thiopental from the maternal plasma and brain is accounted for by its rapid movement into muscle and viscera and its subsequent slower deposition in fat. 14 While the maternal plasma and brain levels are limited only by the amount of thiopental injected and the cerebral blood flow, the concentration of thiopental in the fetal plasma and brain are controlled by other and more complex variables which stimulated these studies. The fetal values indicate that thiopental begins to cross the placenta within one or two circulation times. Initially the levels are quite low, 1 and 1.5 mg. per liter at 10 and 20 mg. per kilogram dosage and 2 and 3 mg. per liter at 30 mg. per kilogram dosage. These data seem to indicate that there is no immediate placental barrier to thiopental, but the placental transfer is in low concentrations. This is in agreement with the detailed studies of Baux and associates. 1"· 1 ' The values become level in about 5 minutes and the two circulations begin to approach equilibrium after 10 minutes. The maternal and fetal values which fall above or below the expected curve result from experimental error and the small amount of thiopental in the plasma and tissues. There was little difference between the
Placental transfer of thiopental
Volume 85 Numht'-'r 5
649
the rabbit after injections of 10, 20, and 30 mg. per kilogram
Plasma 1.5
1.5
3.5
3
6.5
2
3.6 3.5
3.2 3
6.5 8
3 3
·!.2
4
4
3.5
10
3
9
3
0.5
1.5
1.5
3
2
3
3.5
3
8
8
11
6
4.5 10.5
8
13
10
7
18
5
13.5 12.5 13 10
9
17
7
18 14 15
8 .:J" 7 9 7
4
10 and 20 mg. per kilogram dosage in the amount of thiopental in the fetal brain and plasma. This may have been caused by the low levels of thiopental which were involved in the determinations; there is significant difference, however, between the 10 and 30 mg. per kilogram studies. This seems to indicate that the drug passed by simple passive diffusion influenced by a higher concentration gradient in the maternal plasma. There are no fetal values which were higher than the maternal. However, the concentration of thiopental in the maternal
5
4
10.5 8.5
brain was uniformly higher than that in the plasma; the reverse was true in the fetus. These differences are probably related to the lipid content of the substances. There was a significant difference in the concentration of thiopental in the maternal and fetal brain at various times from injection to delivery. The concentration in the maternal brain was 30 times greater than that in the fetal brain after 1 minute of anesthesia, but becomes only 2 to 3 times higher after 10 minutes. The differences in the concentration of thiopental in the brain
•
64 60 56 52. 48 44 mgm/L Plasma 40 36 mgm/K Brain 32. 2.8 2.4 20 16 12 8 4 i
• maternal brain
o maternal plasma
• fetal brain A felal plasma
• 0
•
0
•0 ., A
A
•• • • • A
A
•
0
Cl
• A
0
•
• A
2. 4 6 8 10 12 14 16 18 20 22. 24 26 2.6 30 time minutes Fig. 3. Thiopental levels in maternal and fetal plasma and brain in the rabbit after intravenous injection of 30 mg./kg.
,\laH: h L 1'Jh >
650 Flowers
t\nL
were reflected in the clinical response of the mother and her young. During the first 10 to L'i minutes of anesthesia, the doe was asleep and her pups were awake. As the concentrations in the fetal and maternal brain become more equal so did the degree of narcotization. This was reflected in the reaction index of the fetuses. A reaction index was devised to give some indication of the effect of thiopental on the rabbit and guinea pig fetus; an Apgar index was impractical. Two points were given for respiration, crying, righting movements, and walking. An index of 8 indicates a normal and vigorous fetus. Some of the indices are
J, Obst.
& (~vncc
not ·whole numbers smce they represent means of the studies for a particular time. There is an inverse con·elation between the concentration of thiopental in the brain and the reaction of the fetus. Passage of thiopental into fetal tissues of the rabbit. Figs. 4 and 5 and Table I indicate the amount of thiopental which passed into the various fetal tissues. The curws are similar in shape to those for fetal plasma and brain. The greatest concentration of thiopental was found in the viscera which has the highest lipid content and consisted of the genitourinary, gastrointestinal i minus the stomach contents), and the canliopulA fetal brain
A fetal plasma
12
c fetal viscera • fetal muscle
10
c IJ
c
8 mgm /L Plasma mgm/K Tissues
0
0
A
A
6 4
2
•
• 2
A
t
•
IJ
.
4
i
10 8 time minutes
6
i
•
• 14
12
16
Fig. 4. Thiopental levels in fetal plasma and tissues in the rabbit after intravenous injection in the mother of 10 mg./kg.
20 18
0 IJ
16
0
I!J.
0
mgm/L Plasma 12 mgm/ K Tissues 10
0
8 6
z
brain p/asf11(J viscera muscle
0
14
4
.t. fetal A fetql 0 fetal • fetal
0
IZI.
I!J.
A
•• • • •
•
t;
•
t;
• •
t;
••
• •
2 4 6 8 10 12 14 16 18 20 22 a4 26 28 30 time minutes Fig. 5. Thiopental levels in fetal plasma and tissues in the rabbit after intravenous injection in the mother of 30 mg./kg.
Volume 85 ~1unber
Placental transfer of thiopental
5
monary system. The lowest concentration of the drug was in muscle. However, the majority of the fetal weight was distributed in viscera and muscle, thus the greater portion of the thiopental which reaches the fetal side of the placenta is stored in these tissues. In a typical litter, the total weight of the brains was 8 grams, viscera 17 grams, muscle 136 grams, and plasma 16 milliliters. Fat was not analyzed separately although some was present in the paranephritic tissues. Thiopental crosses the placenta in low concentrations with relatively greater amounts being stored in muscle and viscera than in brain. The fetal brain is therefore not generally subjected to the high plasma concentration of thiopental that is necessary for thioharbiturate anesthesia. Calculations were made to estimate the total amount of thiopental which passed into and was stored in fetal tissues. These calculations were made by multiplying the weight in grams of each tissue by the number of milligrams of thiopental per gram. Brain, placenta, plasma, viscera, and muscle were used. No studies were made on skin and bone because of the technical difficulties which were involved. Bollman18 has shown, however, that some thiopental passes into these tissues. These data appear in Table IV. The values are only relative but confirm a slow and continuous passive placental diffusion regulated by maternal and fetal tissue and plasma equilibrium. Comparison of thiopental in the maternal and fetal brain and plasma in the guinea pig. Figs. 6 and 7 and Table V give the analytical data for maternal and fetal brain and plasma concentrations of thiopental in the guinea pig after the maternal injection of lO and 30 mg. per kilogram. The curves are similar to the rabbit, but the levels of thiopental in the brain of the guinea pig fetus are higher than those in the plasma; the reverse was true in the rabbit. This probably related to the greater ma~ turity of the guinea pig at birth. At similar times the thiopental levels in the maternal and fetal guinea pig and the
651
Table II. Lipid content of maternal and fetal tissues and plasma Guinea pig
Rabbit
Maternal Fetal Maternal Fetal (mg./ (mg./ (mg./ (mg./ Gm.wet Gm. wet Gm. wet Gm. wet weight) weil[ht) weight) weight) Brain Viscera
98.5
107
15.3 41.2
45.5 77.5
Table III. Effect of thiopental upon the reaction index of rabbit and guinea pig fetuses after maternal injection of 10, 20, and 30 mg. per kilogram
Time (minutes)
Reaction index of guinea . pigs Reaction index of rabbit pups 10 mg./ kg.
1
8
3 3.5 4
8
5 8
7 8
9 10
7
120 mg./~ 30 kg. 7 7
8
8 7
8
7
7
5
6
.1 3
5
12 15 20 30
mg./ 10 mg.! kg. kg.
6 6
6
3
6 6
Table IV. Estimate of the total passage of thiopental into the products of conception in the rabbit after the maternal injection of 10 and 30 mg. per kilogram. thiopental in Time (minutes) 1 3 3.5 5
8 9 10 12 15 20 30
30 mg./kg. 0.7 1.5
1.2 1.4 1.6
1.7
2.1 2.0
2.7 3.1 3.3 3.9
652
~latch
Flowers
!Yt~,:;
l
\n1. .L OIN & U"n•·<..
Table V. Thiopental levels in maternal plasma and brain and in fetal plasma and tissues in the guinea pig after the injection of 10 and 30 mg./kg.
J'intt' (minutes)
25
24
'),j
I'>
19
+.'> !()
l''·'
1:2
f)
9
:l
:i
7
11
6.5
6
.,
+
11
.,
I.e,
.,
~l ':·,
2U
....
L'i
ti(l
ti
h
3.5 It!
:F,
'):)
ti.5
il
L:
F>
:!5
B
8.5
I '1.5
10
2(1
\0
II
1:.>
::t.:)
10
17
9
10
15
ti
)'j
16
Viscera i Musclr
,,
70
4.1 ·Ill
0
~.~
-----·~--~·~--~-·-----
Table VI. Contrast between single injection of thiopental and intermittent administration of 30 mg./kg. (delivery 15 minutes after initial injection) -
----
-~·-~---~--~·
--
Maternal
Plasma (mg./L.)
Brain (mg./ k!il..)
Intermittent injection 40 23 45 45 37 36 Mean :n 40
---
-·-~··-------~------
'
i
7.19 7.47 7.23 7.29
2~
17 19
I
I
po, 54.1 46.0 35.5 45.2
31.1
37.2 33.2 :13.8
16 19 16 17
14 16 15 15
35 33 30 33
10
8
12
7
'.!.7
15 1",)
6 7
pCO~
(minutes)
pH
I
7.39
60
7.36
3.5
7.39
61
U7
18
52.5
5
7.49 7.31 7.30 7.2H
60 65
7.38 7.27 7.29 7.29
19
43
18
47
8 ]II
15
6.)
67
16 13 U:l
6 5 3 4.7
f!.H
po,
7.17 7.21 7.16 7.18
14.1 14.0 17.5 15.2
Reac· tion pCO,i index 41.6 43.4 42.5
,,
2 2 :l 7 :)
:;
{)
~.5
6
after thiopental injection of
pCO,
pH
49.5
7.22 7.31
56 58
7.27 7.25
7.35
62
7.\8
16
!9.8
7.40
65.5
7.21
18.3
~17
3
+
Fetal
I
Table VII. Maternal and fetal pH, pO" and 10 and 20 mg./kg. in the rabbit
Time
------~-----
otic \Plasma Brain i Viscera' fluid I i (mg./ I (mg./ (mg./ (mg./ L.) I pco, I L.J I kg.) I kg.)
Single injection completed in 3 minufp, 16 22 jg 19 Mean 19
_____
'
1
i pH
....
__ i ------- ------
18 48.2
pCQ,
44
41)
Volume 85 .J
~umber
maternal and fetal rabbit were quite similar. The concentrations of thiopental in the brains of the guinea pigs at 30 mg. per kilogram dosage were slightly higher than those in the rabbits while at the 10 mg. dosage they were slightly lower. The differences in brain concentration of thiopental in the maternal guinea pig and the fetuses, for various times of anesthesia, were in the same order of magnitude as in the rabbits. However, the maternal guinea pigs were more depressed than the rabbits. In the 30 mg. per kilogram studies this could have been related to the light ether induction to obtain a mesenteric vein for intravenous injection. But the depression was greater than could be explained by the small amount of ether used. Consequently, local abdominal anesthesia was employed for the remainder of the guinea pig experiments. With the 10 mg. per kilogram dosage, without ether induction, the fetal guinea pigs were considerably more depressed than the rabbits after 5 minutes of anesthesia. The tissues of the guinea pig fetuses (Fig. 8) did not contain more thiopental than those of the fetal rabbits. Moroover, they were not acidotic; the lowest pH in the 10 mg. studies was 7.2. The explanation may lie in species sensitivity to thiopental anesthesia or other obstetric factors. These differences indicate the complexities of obstetric anesthesia and the difficulties of comparing experimental data in one experimental animal with another, despite the similarities of the placentas. Contrast between single and intermittent injection of thiopental. Evaluations of thiopental for obstetric delivery seem to indicate that the fetus is less depressed when a single injection rather than a number of intermittent injections of thiopental is used.H The intermittent injection of thiopental gives a number of high plasma concentrations of thiopental in the mother and allows greater amounts of the drug to cross the placenta. 3 This is substantiated by the data in Table VI. When 30 mg. per kilogram was injected intermittently, as previously described, the mean and individual levels of
Placental transfer of thiopental
653
thiopental in the plasma, brain, viscera, and amniotic fluid were higher than those in the animals that received a single injection. Moreover, the reaction indices were lower. Thiopental in the amniotic fluid and stomach contents. Attempts were made to determine the concentration of thiopental in amniotic fluid in the majority of the studies. In many instances, however, it was impossible to obtain amniotic fluid which was not contaminated with small amounts of blood. There is little amniotic fluid in the rabbit at term. Very little thiopental can be detected in the amniotic fluid before the 8 to 9 minute studies. Thereafter, it is easily recovered and the values are related to the concentration gradient in the maternal plasma and tissues as indicated in Table VI. The circulation of the amniotic fluid is complex and related to many variables. Both the rabbit and the guinea pig have small chorionic vessels which may facilitate the passage of thiopental into the amniotic fluid. However, thiopental has a low molecular weight and may diffuse into the amniotic fluid from the endometrium. McGaughey 19 has shown that creatine with a molecular weight of 113 can traverse the human chorionamnion. It is reasonable to suppose thiopental could pass similarly. Fifteen analyses for thiopental were made on the stomach contents of the rabbit. Drugs may pass from the plasma into the gastric juice according to their physical characteristics and the dissociation constant since the gastric mucosa behaves as an oily layer toward various drugs.~ 0 At the 20 and 30 mg. per kilogram dosage, the stomach contents contain as much as 4 mg. per liter. After the 8 to 9 minute studies the highest concentration of thiopental in the gastric juices was in the intermittent injection studies. These data further indicate that with increasing time greater amounts of thiopental pass into fetal plasma and tissues. Maternal and fetal pH, p02, and pC0 2 • Table VII contains the values of pH, pOz, and pC0 2 for the rabbit at 10 and 20 mg. per kilogram dosage and Table VI gives the
654
~1(irdt i, 196:~. Am . .1. Ohsr. & Gyrwc
Flowers
30
26 22 mgm I L Plasma mgm/K Brain
• matt~rnol brain o matemol plasma
.a. fetal brain
0
A ftlfol plosmo
• •
18
0
14
0
•
10
•
•
0
6 2
• t ...
... 2
4
l
... 8 10 12 time minutes
6
14
16
Fig. 6. Thiopental levels in maternal and fetal plasma and brain in the guinea pig after intravenous injection of 10 mg./kg.
data for the intermittent injection studies at 30 mg. per kilogram. The C0 2 and 02 electrodes for analysis of blood gases .were not available for the guinea pig studies. The maternal blood was obtained anaerobically from the left side of the heart just prior to delivery. Fetal blood \vas not obtained anaerobically but was collected by allowing jugular and carotid blood to flow into a 20 c.c. syringe. The collection was made as quickly as possible, but this technique possibly created some error by elevating the p0 2 and lowering the pC02. When the many variables of anesthesia and homeostasis are considered, these values indicate that the doe and fetuses were slightly hypoxic but in a reasonable physiologic state. The does which received thiopental intermittently at the 30 mg. per kilogram dosage were more depressed than the animals which received the thiopental by single injection. This is reflected in the maternal and fetal p0 2 • The fetuses were more acidotic than their mothers, but this is due to the delay in killing the doe by cervical fracture before obtaining fetal blood at delivery. The data from animals which responded poorly: to anesthesia or became hypoxic and depressed were not used in these studies. InYariably, the fetal values of thiopental were
lower in these fetuses than in the animals in good condition. A limited number of studies were done with the mothers receiving oxygen during anesthesia. Sufficient data were not available for statistical analysis, but preliminary data did not indicate that pH, p0 2 , and pCO:: influenced the placental transfer of thiopental provided the mother responded normally to a carefully administered anesthetic and that uterine and fetal circulations were not impaired. Effect of the change in pH and pC0 2 upon the level of thiopental. Wad dell and Butler21 have reported that a rise in pH and a fall in pC0 2 reduces the level of phenobarbital in tissues and brain and raises the concentration in plasma, thereby reducing the level of anesthesia. In previous stndies:J it was postulated that thiopental would respond similarly to phenobarbital since the pK of thiopental is very close to that of phenobarbital. Fig. 9 gives the experimental data to indicate that thiopental does respond similarly to phenobarbital with changes in pH and pC02. The data in this illustration is from one dog; however, three experiments rielded similar values. Thiopental equilibrium was established after 2 Gm. of thiopental was given intra-
venously over 1Y2 hours. The first three values indicate this equilibrium. The 30 per cent C0 2 caused the thiopental to leave the plasma and accumulate in the tissues; hyper~ ventilation and lowering of the pC02 caused the reverse to occur. At birth there is a fall in pC0 2 and a rise in pH. This could theoretically be of benefit to the fetus in lowering the concentration of thiopental and other related drugs in the brain. However, this shift is slight and re~ quires 15 or more minutes to be completed. It is felt that the pH and the pC0 2 shift have a limited influence on the anesthetic level in the fetus.
verse 2 and explained Cohen and his coworkers'1 results which indicated a single injection of thiopental for cesarean section caused little fetal depression and allowed no correlation between the time of administration of the drug and the delivery of the infant. The blood gas determinations were made to assess the degree of homeostasis in the
80
10 60 50
• mtt!er!ICI lm;m o moterool plasma braih p/asmQ
•
&!~!ttl
A''"''
...•
mgm I L Plasma 40 30 mgm/ K Brain
20 10
••
Comment
It is important to ponder the factors which allow an infant to be born alert when its mother is anesthetized with ether, cyclopropane, or a barbiturate. It was hoped that the analysis of thiopental in the rabbit and guinea pig tissues as well as plasma would yield a tenable explanation for what is often clinically apparent in the human being. Both the rabbit and guinea pig were used as experimental animals in the hope that each would offer slightly different data to aid in the study of placental transfer. The analysis of thiopental in both plasma and tissues were made to study the movement of the drug into and storage in fetal tissues. These studies indicate that thiopental enters fetal tissues in the same manner that it enters adult tissues. The real difference lies in the placenta protecting the fetus from the high plasma concentration which is necessary for thiobarbiturate anesthesia. By the time equilibrium is approached in maternal and fetal plasma, the thiopental has passed into maternal tissues and the concentration gradients between the two circulations are low. Essentially, the mother has received a single, rapid, concentrated dose of thiopental while the fetus has received a very slow intravenous injection containing a low concentration of the hypnotic. The intermittent injection studies con· firmed the ideas of McKechnie and Con·
655
Placental transfer of thiopental
Volume 85 5
~umber
•
s
e
~
m
~
~
~
w
lime minutes
Fig. 7. Thiopental levels in maternal and fetal plasma and brain in the guinea pig after intravenous injection of 30 mg./kg.
eo
A I1Jtt! bMt/1 A f#IIJI /!JfXff>IJ !lftta/ V!Sf:ell1 • few! ~rd~Ufe
70 60
mgm I L Plosmo mgm/ K Tissues
50
40
30
ro
10
0
2
i • 24
8101214161620 11me minutes
Fig. 8. Thiopental levels in fetal plasma and tissues in the guinea pig after intravenous injection in the mother of 30 mg./kg.
mgt~~IL
40
Tliopet1lri
30 2Q
Fig. 9. Effect of the change in pH and pCO. upon the levels of thiopental in the dog.
656
M~1dJ
Flowers
mother and fetus. Data from animals in acidosis were not used. When homeostasis was apparent, the administration of 0 2 did not ~eem to influenCf~ the placental transfer of thiopental. The studies on the effect of the change of pH and pC0 2 on the plasma level of thiopental were performed to challenge statements made in earlier thiopental studies. 3 It is believed that these changes are not of clinical importance at birth. Summary The~e
data may be summarized by outlining the pharmacologic effect of thiopental on the mother and fetus. 1. Thiopental produces anesthesia in the mother by a high plasma concentration of thiopental immediately entering the brain caused by the absence of a blood-brain barncr. 2. Thiopental in the maternal circulation is quickly bound to plasma protein and rapidly enters muscle, viscera, and body water. This materially reduces the amount of thiopental which is available to the fetus.
1~63
Am. ,l. Obst. & Gynec
3. There is no immediate placental barrier to the passage of thiopental to the fetu~. hut it crosses the placenta slowlv and in low concentrations. 4. As the time from injection increases and materHal and fetal equilibrium are approached. the maternal plasma level is low and the differences in the gradients between the two circulations is small. 5. As thiopental enters the fetal circulation it is concentrated in greater amounts in viscera and muscle than in brain. 6. When thiopental is injected intermittently. peak plasma concentration gradients occur that assist in a greater placental transfer and accumulation in the fetus. 7. Fetal depression may result from tillcompensated fetal or maternal homeostasis, excessive concentrations of drug in the fetal brain, or other obstetric factors. Appreciation is expressed to Dr. \Villiam Waddell of the Department of Pharmacology for his assistance in developing the method of analysis and t:n Mrs. Lynda Jensen for her invaluable technical assistance.
REFERENCES
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Discussion DR. BEN PEcKaA~t, Madison, Wisconsin. In his paper, Dr. Flowf'rs has presented a mass of
interesting data, painstakingly acquired, primarily in the rabbit and the guinea pig, from which he