THE CONCENTRATION OF ACID AND BASE IN THE SERUM IN NORMAL PREGNANCY.

BY HARRY C. OARD AND JOHN P. PETERS.

(From the Department of Internal Medicine of Yale University and ths Medical Service of the New Haven Hospital, New Haven.)

(Received for publication, September 15, 1928.)

INTRODUCTION.

In 1912 Hasselbalch and Lundsgaard (28) found a lowering of the alveolar carbon dioxide tension in the later stages of pregnancy. This finding was quickly confirmed by others, notably Leimdorfer, Novak, and Porges (35) and Hasselbalch and Gammeltoft (27). That there is a reduction of the alkaline reserve in pregnancy as evinced by lowered alveolar carbon dioxide tension and lowered plasma or serum carbon dioxide tension, content, and combining power has been established by a large number of observers (12, 27, 25, 30, 36, 37, 39, 51, 58, 68). This change is demonstrable as early as the 2nd month of pregnancy (27), and Cook (11) detected a lowered plasma carbon dioxide-combining power as early as 3 days follow- ing the first missed menstrual period.

A reduction of the blood alkaline reserve may be the result of one or the other or of a combination of both of two factors: namely, an unusual collection of acid in the blood, or a reduction of the fixed base of the blood.

Along with the lowered alveolar carbon dioxide tension, Hasselbalch and Gammeltoft (27) found an increased ammonia nitrogen to total nitro- gen ratio in the urine. They considered that these reactions served to maintain the pH of the blood constant in spite of increased acid production in the pregnant organism. This belief that there is in pregnancy an in- crease of acid in the blood has been embodied in the term “acidosis of pregnancy.” The increased ammonia coefficient and the ease of production of ketonuria in pregnancy have frequently been advanced in support of that theory (49,50,56). Bokelmann and Bock (7) believe they have proved this by a study in which they found that the blood acetone of pregnant individuals on a high fat diet increased 63 per cent while that of normal individuals increased only 10 per cent. Bokelmann (6) has also found an increase of 2.7 mg. per cent of blood lactic acid in pregnancy. Stander and Radelet (59), on the other hand, have found no change in blood lactic acid or uric acid in normal pregnancy. Because on a given carbohydrate- free diet, pregnant individuals developed ketonuria, which could be checked by additional carbohydrate, while non-pregnant women on the

9

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Acid and Base of Serum in Pregnancy

same diet did not develop ketonuria, Kleesatel (31) concludes that in preg- nancy an unusually large quantity of carbohydrate is required for the combustion of a given amount of fat. He ascribes this to a change in the secretory function of the liver cells.

On the other hand, Harding and Allin (26) have pointed out that the mere production of ketonuria by high fat or by carbohydrate-poor diets is not satisfactory evidence of a lowered ketogenic threshold, especially if careful quantitative checks are not made. They found not only no de- crease of the “threshold of ketonuria” in pregnancy, but also that, “Diets which theoretically should produce a large excretion of acetone may only produce a ketonuria slightly above the threshold value.” They find no evidence that the lowered alkaline reserve of pregnancy is the result of unusual ketogenesis.

Against the theory of increased acid, Marrack and Boone (39) in 1923 wrote as follows: “If appreciable amounts of abnormal acid were present in the plasma of pregnant women this should reveal itself by an excess of kations; this we have not found. . Such lowering of the dissociation curve as we have found appears to be due to deficiency of alkali rather than accumulation of abnormal acids in the blood.”

It is apparent from the foregoing that no adequate solution of the problem is yet available. The chief difficulty lies in the fact that all approaches have been by more or less indirect methods. Peters, Bulger, Eisenman, and Lee (43) have described a more direct method for determining the total acid-base equilibrium of serum. The results obtained by the application of that method to the serum of normal pregnant individuals are here presented.

Methods Employed.

From the material in the dispensary and in the wards of the New Haven Hospital gravid patients were selected who through- out their pregnancy showed no signs of abnormality. The criteria were : absence of obvious disease conditions, blood pressure below 130 systolic, negative urine (except for some cases exhibit- ing a minimal albuminuria without other signs), normal eye grounds, absence of edema, absence of vomiting except for the usual morning sickness, and uncomplicated delivery and con- valescence. All patients received ordinary mixed diets. Blood was drawn anaerobically and without stasis from an arm vein, usually 3 or 4 hours after breakfast.

The techniques used in handling the blood have been described in detail elsewhere (46).

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H. C. Oard and J. P. Peters

One fraction was treated with a few crystals of sodium oxalate. This whole blood was used for the non-protein nitrogen which was determined by subjecting a protein-free filtrate obtained by trichloroacetic acid precipitation to micro digestion by the usual procedure. The ammonia was distilled into 0.02 N acid and was estimated by titration with 0.02 N alkali.

A second fraction of the blood was defibrinated without contact with air by the method of Eisenman (15). Cell volumes were determined in duplicate on the defibrinated blood with a Daland hematocrit of the type manufactured by the International Equip ment Company for their centrifuge. This was rotated at about 2000 R.P.M. until the cells were homogeneous and translucent and showed no further tendency to reduction of volume. Dup- licate estimations usually differ by considerably less than 1 volume per cent, and it is probably justifiable to assume that for com- parative purposes this represents the upper limit of error in the actual determination.

This serum of the defibrinated blood was then separated from the cells with precaution against contact with air (2).’

Serum carbon dioxide content was determined by the method of Van Slyke and Neil1 (65) in a carefully calibrated, water-jacketed, Van Slyke constant volume pipette.

A third fraction of the blood was allowed to clot without contact with air in a centrifuge tube, and the serum was separated by the same technique employed with the second fraction. This serum and the remainder of that from the defibrinated blood were then used for chloride, inorganic phosphorus, serum protein, and total base determinations.

Chlorides were determined by the latest procedure of Van Slyke (63); inorganic phosphorus by the method of Benedict and

1 The technique for stoppering the centrifuge tubes used in this opera- tion has been improved in the following manner: a No-Air rubber stopper of the type used for vaccine bottles, through which a No. 28 hypodermic needle has been introduced, is inserted into the tube so that the excess of oil at the top is forced out through the needle. The cap part of the stopper is then turned down and the needle is withdrawn. In this way the tube is perfectly sealed and the danger of the cork flying out during centrifugation is obviated. To prevent exposure of the serum after centrifugation, oil is injected with a fine hypodermic needle through the stopper as the stopper is withdrawn.

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12 Acid and Base of Serum in Pregnancy

Theis (5); total base by a modification (46) of Stadie and Ross’ (57) adaptation of the Fiske (19) urine method.

For the determination of serum proteins about 0.5 cc. of serum was subjected to the macro-Kjeldahl procedure with an extra digestion for 3 hour following the addition of 0.5 cc. of superoxol. From the total nitrogen the non-protein nitrogen was subtracted. The remainder was multiplied by the usual factor, 6.25. Since the serum non-protein nitrogen is ordinarily about three-fourths that of the whole blood, the value of the serum proteins is about 0.05 per cent low when the whole blood non-protein nitrogen is subtracted from the total serum nitrogen.

Calculations Employed.

Volumes per cent of COz were converted into milli-equivalents present as bicarbonate in 1000 cc. of serum by the following equation,:

B (as BHCO,) = 0.4225 COn

This was derived from the Henderson-Hasselbalch equation as follows :

BHCOS pH = pK + log H

2 3

where

(2) BHCO~ = CO1 - H&O*; COO = volumes per cent CO*; and H&OS = dissolved CO,.

Therefore

pH = p& + log CO, - H&03

H&OS >

(3) Or H&Os = co* antilog (pH - PK) + 1

From (2) and (3)

(4) BHCO, = CO2 - co* antilog (pH - pIL) + 1

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H. C. Oard and J. P. Peters 13

Assuming pH = 7.35 and pK1 = 6.10

(5) B (as BHCOS) =

co2 - g$ 22.4

x 10

where 22.4 = gas constant. Solving (5) we find

B (as BHCOJ = CO2 X 0.4225

Serum proteins were converted into milli-equivalents per liter by the following equation (64):

BP = 1.072 X gm. protein per cent X (pH - 5.04)

This becomes BP = 2.476 X gm. protein per cent, when pH = 7.35.

Inorganic phosphorus in mg. per 100 cc. was reduced to milli-

equivalents per liter by the factor, &, according to L. J.

Henderson’s (29) estimate of the proportions of primary and secondary phosphate in the blood.

Total acid is calculated as the sum:

TA = BP + BHCOa + BCI + BP04

where BP, BHCOI, BCl, and BPO( represent milli-equivalents of base combined with protein, COz, chlorides, and inorganic phos- phorus respectively, and TA, therefore, the total base-combining power of the inorganic acids and of the protein of the serum.

The difference between this and the total base, which in the tables has been termed “undetermined acid,” represents the amount of base present as sulfates and the salts of organic acids. Failure to determine the sulfur makes the total sum of the acids incomplete, and to that extent any conclusions concerning the acid-base balance must be considered tentative. Under ordinary circum- stances the concentration of sulfur is negligible. Denis and King (13) have shown that the blood sulfate is not altered in pregnancy.

Results.

Tables I and II present the results. In Case 2933 there was present a very early decompensation on the basis of an old rheu-

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TABL

E I.

Acid

-Bas

e Eq

uilib

rium

in

No

rmal

Pr

egna

ncy.

6152

8 61

492

6567

2 31

392

2932

49

43c

6789

Z 67

86i

6792

1 66

51i

6651

; 66

43t

6643

t 67

35!

6735

! 61

52:

- $ - yrs.

41

23

22

24

20

35

25

2.5

24

26

26

23

23

23

23

16 _

-

, , , , , 1 i -

- k - 1 38

1

39

1 28

5

18

2 40

8

32

3 27

3

24

1 16

1

40

1 0

1 37

1

0 2

31

2 0

0 0

- -

--

MI/7

8 f

110/

65

0 12

0/78

0

110/

80

f X0

/68

+ 12

0/75

0

110/

60

0 11

0/50

0

105/

70

0 12

0/60

0

115/

65

0 W

/70

zlz

130/

70

0 12

0/70

0

125/

70

0 12

0/65

0

- !i - 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -

- - $ < 0 - - 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

- -

Max

imum

...

......

......

......

......

......

M

inim

um

......

......

......

......

......

...

Aver

age

......

......

......

......

......

.....

* m

y =

milli

equi

vale

nts

per

liter

32

5.70

14.0

21.8

101.

8 2.

9140

.514

6.:

20

5.81

14.7

22.7

100.

8 2.

7140

.914

0.!

25

6.17

15.3

22.9

103.

2 2.

0143

.414

7.4

12

5.93

14.7

22.8

103.

8 2.

2143

.514

5.f

27

6.09

15.7

20.5

105.

8 2.

5143

.914

5.C

23

5.97

14.8

21.9

103.

3 1.

9141

.914

9.:

21

6.14

15.2

22.7

103.

2 1.

6142

.714

7.t

14

6.95

17.2

20.6

104.

8 1.

9144

.514

6.:

23

6.82

16.9

22.2

104.

3 1.

7145

.114

5.!

28

5.80

14.4

23.4

103.

3 2.

7143

.814

5.f

33

6.84

16.9

25.7

102.

8 2.

3147

.714

7.1

21

5.72

14.2

21.8

105.

3 2.

8144

.1

144.

t 25

6.

7316

.627

.610

0.4

2.51

47.1

15

6.t

26

6.59

16.5

22.5

105.

4 2.

3146

.414

8.:

32

7.36

18.2

24.7

104.

6 1.

9149

.415

0.(

23

7.22

17.8

27.0

100.

2 2.

4147

.415

2.t

------

--

32

6.95

17.2

23.4

105.

8 2.

9146

.414

9.:

14

5.70

14.0

24I.6

100.

8 1.

6140

.514

2.1

23

6.18

15.2

22.1

10

3.7

2.31

43.4

146.

:

- 6.t 1.i

4.f

2.:

2.t

7.1

4.! 1’

0::

1.:

0.:

0.1

S.!

2.:

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5.t

- 7.3

0.8

2.:

-

- _- e

1 1 3 4 3:

1

3,

73

4 31

2

3,

1 3:

5

3,

9 3,

3 63

D

38

_-

438

43

83

-

l!i

9 P 3 V -

Rem

arks

.

4.9

4.6

1.7

2.8

4.2

3.5

4.0

4.1

Mul

tiple

pr

egna

ncy.

2 da

ys a

nte

partu

m.

9 “

post

“

25

“ an

te

“ 18

“

post

“

9 wk

s.

ante

“

11 d

ays

post

“

Hem

orrh

oide

ctom

y.

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H. C. Oard and J. P. Peters 1.5

matic heart lesion, but this was successfully relieved by digitaliza- tion and the patient had an uncomplicated delivery of normal twins 48 hours after the blood specimen was taken. Her con- valescence was uneventful. Two studies, one before and one after delivery, were obtained on each of three individuals (Cases 66517, 66436, and 67359).

Determinations of total base were made on ten normal adults, all of whom were engaged in their ordinary pursuits and were free from known disease process (Table II).

TABLE II.

Total Base Concentration in Normal Sera.

1

2 3 4 5 6 7 8 9

10

Maximum.. Minimum. Average

Subject. Date.

-,-

D.M. ‘I “

H.C.O. F.S.B. L.D. M.W. E.D. A. J.E. R.S.I.

19.27

Nov. 15 (‘ 21 “ 28 “ 10 “ 12 “ 18 “ 20 “ 20 “ 10 Female.

Dec. 2 “

S0X.

Male. ‘I ,‘ ‘I ,‘ I‘ ‘L “

Ape. Total base --

yrs. n&M*

29 154.4 29 157.2 29 155.7 27 153.0 30 153.6 27 152.5 30 155.0 26 153.0 28 153.0 31 152.5 15 158.0 16 152.0

. . . . .._...._._....._................_........

-

158.0 152.5 153.8

* mid = milli-equivalents per liter.

One complete study was made on serum from a young, non- pregnant female 10 days convalescent from an operation for hemorrhoids. Her hemoglobin and blood cell counts were normal.

DISCUSSION.

Analysis of Results.-Table III presents the values obtained for the various ions in both pregnant and non-pregnant individuals by the authors and by others who hate published studies on elec-

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TABL

E III

.

No.

of -8

.

Prea

nant.

- I Se

rum

W

ectro

lvter

in

Pr

e

Seru

m

prot

eins

, gm

. pe

r ce

nt.

BHCC

h, rn

~.

- -

4 - --

.9! j-

5 - .z # - 1.7(

---

12

.18

23.4

20.E

17

17

27

.524

.C

16

25.4

17.4

5

- 5 4 - 2.

0.

1.

-

32

88

7.78

5.70

6.50

84

7.

826.

067.

02**

6 6.

925.

856.

32

6 6.

885.

996.

49

18

7.06

6.00

6.54

- I -- --

11 1

51

81

-

BCl.

mu.

d 05.1

06

.: 06

.f 03

.f

.I!

fi z

4 -- DO

.SlO

ti.7

59.7

99

.9

95.0

100

.9

92.4

96

.9

-

mt

and

Non-

Preg

nant

No

rmal

s.

BP&,

m

y.

Tota

l ba

se.

mu.

- d - 2.9 2.2

2.f

1.7

2.2 -

-- . i

2 z3 2 I

1 d

---

1.6

2.3t

14

9.:

0.6

1.5Q

157.

E 147.

4 1.

6 1.

8s 1

69.2

1.

3 1.

65

1.3

1.70

i .E

! z

3 -- 42

.5

146.

1 37

.1

145.

911

41.4

14

4.51

51

.3

159.

5

- I - I -

Obse

rver.

Oar

d (p

rese

nt

stud

y).

Kreb

s an

d Br

iggs

(3

3).

Mar

rack

an

d Bo

one

(39)

. De

nis

and

King

(1

3).

Stan

der,

Dunc

an,

Siss

on

(58)

. de

Wes

selo

w (6

6).

Plas

s an

d Bo

gert

(47)

. ‘<

“

Mat

thew

(4

8).

Zang

emei

ster

(6

9).

Dien

st

(14)

. La

ndsb

erg

(34)

.

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91

17

NOW

NO

W

preg

nant

. pr

egna

nt.

15

15

7.75

5.7M

3.59

ti

10

10

12

12 21

21

: 5 5 91

17

7.83

6.45

7.04

15

7.

78 6

.96

7.42

10

8.

396.

637.

45

9 9 7.

916.

947.

17

7.91

6.94

7.17

8 8

7.25

6.7

8 7.

01

7.25

6.7

8 7.

01

2 2 8.

367.

94

8.36

7.94

1 1

7.22

7.

22

15

10

O.!

-

I1 .I

5.t

06.:

00.1

95.:

02.5

:

100.

2

2.7

1.7

2.3t

161 15

8 15

8 1.

8 1.

4 1.

50

2.0

1.5

2.3

1.3

1.70

158

II I

2.4t

47.0

15

5.7

158.

0 52

.5

153.

8 46

.6

153.

6

47.4

15

3.5

152.

0

Pete

rs,

Bulg

er,

Eise

nman

, Le

e (4

3).

Kram

er

and

Tisd

all

(32)

. Pr

esen

t st

udy.

Br

iggs

(8

). x

Stan

der,

Dunc

an,

Siss

on

9 W

I. To

lstoi

(6

2).

9 de

Wes

selo

w (6

6).

Plas

s an

d Bo

gert

(47)

. &

‘I “

Mat

thew

(4

8).

$ Pe

ters

et

al.

(46)

. e

Salve

sen

and

Linde

r (5

4).

r-(

Dien

st

(14)

. La

ndsb

erg

(34)

. ‘d

Zang

emei

ster

(6

9).

cd

Oar

d (p

rese

nt

stud

y).

%

g *V

an

Slyk

e m

etho

d.

tBen

edict

-The

is m

etho

d.

jwhit

ehor

n (6

7)

met

hod.

$B

ell-D

oisy

m

etho

d.

/IEig

ht

CJJ

ca

ses.

nN

a in

m

M +

12

.4 m

M.

See

text

. **

Aver

age

of c

ases

from

3

to 6

mon

ths

grav

id.

ttPro

babl

y in

com

plet

e di

gest

ion.

Se

e te

xt.

#3el

ecte

d aa

pro

bable

no

rmal

s.

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18 Acid and Base of Serum in Pregnancy

trolytes concerned in the present work. Results by different workers using similar methods are in close agreement. With one exception, discrepancies can be explained by the differences in techniques employed; the apparent differences in BPO, are ex- plained by the fact that the Bell-Doisy (4) method gives lower values than the’Benedict-Theis (5). By the addition of superoxol, it has been found in this laboratory that more complete digestion is effected in the Kjeldahl procedure for serum proteins. Failure to obtain complete digestion probably accounts for the lower values obtained by most workers as compared to those obtained by Plass and Matthew (48), and recent determinations from this laboratory (46). The literature contains no data for direct total base determinations in pregnancy. Where the values listed have not been obtained from direct determinations of sodium, calcium, magnesium, and potassium (as was the case with Krebs and Briggs (33) and Denis and King (13)) but from determinations of sodium alone (Marrack and Boone (39)), the value for total base has been calculated by adding 12.4 mM to the value for sodium. That this is legitimate has been proved by Briggs (8), Krebs and Briggs (33), and Denis and King (13), who have shown that the fraction of total base present as calcium, mag- nesium, and potassium varies insignificantly in health, in disease, or in normal or abnormal pregnancy.

It is evident from Table III that the values for the various ions in pregnancy found in the authors’ studies are closely in agreement with those of many other workers. The one exception, total base by Denis and King (13), will be discussed later. A large number of independent observers are likewise in close agreement as to the range of fluctuation and the average values of the individual ions in non-pregnant normals. Table III, therefore, affords a basis for comparing the various serum electrolytes in the pregnant with those in the non-pregnant condition.

The reduction of serum proteins in pregnancy first observed by Zangemeister (69) in 1903 has been abundantly confirmed by others (14, 34, 47, 48) and by the authors. Plass and Bogert (47) and Plass and Matthew (48) have recently found that there is a progressive decrease of serum protein from the 3rd month of pregnancy to the 6th, after which there is a gradual rise, which

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H. C. Oard and J. P. Peters 19

does not, however, restore the normal level until several weeks post partum.

A consistent decrease of about 4 mM (10 volumes per cent) BHC03 in pregnancy has been found by the authors, a finding

NON-PREGNANT PREGNANT

DTAL BASE

155.7

~2 2.4 Ca 5. K 5.

I

1

-HCO;

25.6

PROT.

16.3

-Cl

103.5

3 ACID

‘OTAL

BASE

146.1

.HC03

22.1

‘ROT.

IS.2

-Cl

03.7

~-pq2.3 UND. 2.6

FIQ. 1.

which substantiates Hasselbalch and Gammeltoft (27), Denis and King (13), and Marrack and Boone (39).

No appreciable changes of chlorides or inorganic phosphate have been found in the authors’ series. This confirms the findings

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20 Acid and Base of Serum in Pregnancy

of Krebs and Briggs (33), Marrack and Boone (39), and Denis and King (13).

Total base in pregnancy is lowered about 5 per cent (8 mM) from the normal, the average value in pregnancy being about 146 mM and that for the normal 154 mM (Fig. 1 and Table III). Furthermore, in pregnancy the maximum value, 149 mM, found by the authors, exceeds by only 2 mM the minimum value, 147 mM, observed by Peters, Bulger, Eisenman, and Lee (43) in normals. The findings of Denis and King (13) do not agree with the above. Their values for pregnancy would be high even for normals. Without more definite knowledge of their technique in handling blood, it can only be pointed out that they are at variance with three other groups of workers, Krebs and Briggs (33), Marrack and Boone (39), and the authors.

Pathogenesis of Acid-Base Changes.

Fetal Requirements.-The work here presented has thrown no light on the cause of the reduction of fixed base in the serum in pregnancy. Shohl (55) has calculated that the fetus requires, during the last 100 days of gestation, a daily excess of 8.5 milli- equivalents of basic radicles over acid. This he believes accounts for the “acidosis of pregnancy.” Harding (25) does not concur in this. He points out that the reduction of reserve alkali has repeatedly been demonstrated in the 3rd month (11, 12, 27, 28). In Case 31392 (Table I) the reduction of fixed base was fully developed by the 18th week.

Acid Production and Excretion.-Marrack and Boone (39) found in the serum of pregnancy an excess of 18 mM of anion unaccounted for by inorganic phosphorus, chloride, and bicarbonate. They assumed that this excess consisted chiefly of protein as anion. From the work of Plass and Bogert (47), Plass and Matthew (48), and the author, we may assign a value of about 15 mM to the anion present as serum protein. This leaves an excess, on the basis of Marrack and Boone’s findings, of about 3 mM of unde- termined acid. The average value found in the authors’ work for undetermined acid is also 3 mM. The undetermined acid in pregnancy is less than half the amount found by Peters, Bulger, Eisenman, and Lee (43) for normals (Table III and Fig. 1).

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H. C. Oard and J. P. Peters 21

Harding and Allin (26), as a result of careful metabolism studies, conclude that the normal mechanism of ketogenesis is not dis- turbed in pregnancy. They point out that those observers who claim to have demonstrated a greater tendency to ketone pro- duction have failed to take into account the raised metabolism of pregnancy, especially during the later stages, and the amount of antiketogenic material removed from the metabolic mixture by the fetus for the formation of new tissue. These two factors may cause the appearance of ketosis on diets which would not have the same effect in a normal individual. Even if we were to grant such a mild ketosis as is reputed to exist in pregnancy, one would not expect a reduction of base to ensue. In studies of acid-base equilibrium in diabetes, Peters, Bulger, Eisenman, and Lee (45) found that in moderate or severe, slowly or rapidly developing ketoses, base was released first by bicarbonate and next by chloride to neutralize ketone acids. Except in an overwhelming acidosis, the normal concentration of base in the serum remained unaffected. The same phenomena have been demonstrated by Gamble, Ross, and Tisdall (23) in fasting. These two groups have, however, conclusively shown that the depletion of body stores of base by acid excretion results in diuresis sufficient to maintain the concen- tration of serum base approximately at the normal level. This close relationship between total base concentration and water metabolism is also substantiated by many others. Cohnheim (lo), for example, found that weight lost by excessive sweating during exercise could not be restored by drinking distilled water. Only when salt so lost was replaced, could weight be recovered. The diuresis produced by acidifying salts was found by Gamble, Blackfan, and Hamilton (20) to result because of loss of base. The well known diuretic effect of salt-poor diets rests on this same basis. It may be stated empirically that loss or acquisition of body base is accompanied, except under the most extreme conditions, by loss or acquisition of an equivalent amount of water (21, 22). Pregnancy, on the other hand, in spite of the reduced concentration of serum electrolytes, is characterized by a positive water balance (56).

Furthermore, increased ammonia excretion may no longer be considered prima facie evidence of increased acid production and excretion. The urinary ammonia, as Gamble, Ross, and Tisdall

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22 Acid and Base of Serum in Pregnancy

(23) have shown, is merely a device to spare body stores of base, and it does not reveal how the necessity for alkali conservation has arisen. The increased ratio of ammonia nitrogen to total nitrogen excretion observed by Hasselbalch and Gammeltoft (27) and Slemons (56) in pregnancy may be merely an expression of the alkaline depletion which the authors have found. An actual quantitative increase of ammonia excretion was not observed by those workers. The increased ammonia nitrogen to total nitrogen ratio was due to a diminution of the denominator; ammonia excretion remained unchanged from the normal. Moreover, the actual acidity of the urine in Hasselbalch and Gammeltoft’s cases was less (pH 5.80) before delivery than after (pH 5.46).

In view of the facts set forth in the introduction and in the above discussion, the theory that the “acidosis of pregnancy” is the result of an unusual production of acid is untenable.

Hyperventilation.-On the basis of an increased serum alkalinity (a shift of pH from 7.30 to 7.45 in normals to 7.35 to 7.55 in pregnancy) found by Marrack and Boone (39)) Austin and Cullen (1) attribute the reduction of reserve alkali to hyperventilation. These pH values were determined calorimetrically, a method which is open to criticism (Eisenman (16)) Austin, Stadie, and Robinson (3))) especially when applied to pathological blood (unpublished studies from this laboratory). At the alveolar COz tensions exist- ing before and after delivery, Hasselbalch and Gammeltoft (27) found no change of pH, the average value at both periods being 7.44, and furthermore neither Menten (40) nor Williamson (68) found a change of pH in pregnancy. pH determinations on sera in pregnancy by methods of sufficient accuracy are as yet not available in the literature.

Were the reduction of reserve alkali, however, the result of hyperventilation, one would expect an electrolyte picture entirely different from that which is seen in pregnancy. Peters, Bulger, Eisenman, and Lee (44) found that in hyperventilation, chronic (postencephalitic) or acute, reduced bicarbonate was compensated for by increased chlorides. The concentration of total base always remained within normal limits, 152 to 160 mM. This contrasts sharply with the unchanged chloride and lowered total base in pregnancy.

Hydremia in Pregnancy.-The idea that there is in pregnancy a

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H. C. Oard and J. P. Peters 23

hydremia is supported by considerable evidence; namely, lowered specific gravity of the blood (42, 61, 69), decreased serum or plasma proteins (14, 17, 18, 34, 38, 47, 48, 69, and the authors), diminished freezing point depression (69), and increased water content and decreased total solids of the blood (60, 66, 69). But true dilution of the blood in GVO, except under extremely pathological conditions, is probably impossible. The findings of the authors do not lend support to a theory of blood dilution as an explanation for the diminution of serum proteins, bicarbonate, and total base. By plotting these constituents one against the other, no quantitative correlation can be detected. The per- centage diminutions from the normal are widely different, those for serum protein and bicarbonate being about 16 per cent, for total base about 5 per cent. Chlorides, we have seen, show no change. Moreover, according ,to Plass and Matthew (48) the reduction of serum protein is chiefly at the expense of the albumin fraction, the globulin remaining essentially normal.

Probable Effect of Observed Changes.-No adequate explanation of the observed changes appears to be available. Yet the reduc- tion of t6tal electrolytes (of which total base may be considered the measure since acid cannot exist as such in the blood) and of serum protein must be accompanied by considerable reduction of osmotic pressure in the pregnant organism. Even though it is impossible to interpret the electrolyte or freezing point changes quantitatively in terms of osmotic pressure because of our ignorance of activity coefficients and other factors in media as complex as serum, the electrolyte changes here observed are, nevertheless, of the same order of magnitude as the freezing point depressions previously observed by Zangemeister (69).

Plass and Bogert (47), in considering this question are of the opinion that the reported changes in pH and the increase of fibrin which have been observed in pregnancy, probably prevent osmotic changes. The lack of proof of pH changes has been discussed. The decrease of albumin found by Plass and Matthew (48) is about 4 times as great as the increase of fibrin, whereas the minimal atomic weights of albumin and fibrin are 45,000 and 42,000 re- spectively (9), so that there is no basis for the assumption that the fibrin increase may compensate for the diminution of serum protein.

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24 Acid and Base of Serum in Pregnancy

In view of the remarkable constancy of the concentration of serum electrolytes in normal individuals and the resistance, even in pathological conditions, offered by the body to fluctuations of that concentration (21-23, 4345), the pregnant organism is unique in its ability to tolerate such changes without obvious symptomatology. This point of view may be emphasized by the following examples.

Moss (41) reports that when stokers and miners, who work at high temperatures and lose considerable body salts by excessive sweating, drink large quantities of water, they develop severe cramps. These can be prevented or relieved by ingestion of salt. In dogs a 15 per cent dilution of the blood produced by forced water intake was found by Greene and Rowntree (24) and Rown- tree (52, 53) to result in convulsions, stupor, and eventually death. Intravenous injection of hypertonic saline would rapidly restore a moribund animal to normal, apparently none the worse for the experience. The peculiar diminution of serum electrolytes together with the reduction of alkaline reserve in pregnancy may be of significance as regards the delicate physiological balance existing in that condition. The violence and suddenness of the reactions of pregnant individuals to minor accidents, which are comparatively well sustained by the non-pregnant, is a matter of universal ciinical experience.

Postpartum Changes .-It remains to point out the rapidity with which, following delivery, the normal blood picture is re- stored. Serum proteins were found by Plass and Matthew (48) to return to the normal level about the 3rd week after delivery; the alveolar CO* tension by Hasselbalch and Gammeltoft (27) by about the 15th day. In Case 66517 of the authors’ series, by the 9th day serum bicarbonate had reached practically the normal level, serum proteins had increased 1 per cent, and total base had increased 3 mM. Cases 66436 and 67359, especially the former, show the same phenomena.

SUMMARY AND CONCLUSION.

1. Total electrolyte studies on the sera of twelve normal pregnant women and one non-pregnant woman, and serum total base deter- minations on ten normal non-pregnant individuals have been pre-

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H. C. Oard and J. P. Peters 25

sented. The values for the individual ions have been shown to be in close agreement with those of many other observers.

2. Inorganic phosphorus and chlorides in pregnancy show no changes from the normal.

3. There is a reduction of about 5 per cent (8 mM) of total base of the serum in pregnancy. From the work of others this reduc- tion would appear to be almost entirely at the expense of sodium.

4. There is a concomitant and equal reduction of the anion content of the serum in pregnancy. This reduction is seen in serum protein, serum bicarbonate, and organic acid, chiefly in the latter two.

5. The term, “acidosis of pregnancy,” as connoting an actual collection of abnormal acids in the blood is misleading. On the contrary, the reduction of reserve alkali is associated with’ a diminution of base.

6. No satisfactory explanation for the observed changes is available.

7. The pregnant organism appears to have a unique ability to tolerate a reduced concentration of serum electrolytes.

The authors wish to express their appreciation to Dr. Arthur H. Morse, Professor of Obstetrics and Gynecology, and his staff for their interest and cooperation, which made it possible to secure the material for this investigation.

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Harry C. Oard and John P. PetersPREGNANCY

BASE IN THE SERUM IN NORMAL THE CONCENTRATION OF ACID AND

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