A New Approach
to Acid-Base Metabolism
PouI Ash-up
A technic of high accuracy for determining the acid-base values of blood is based
on the equilibration principle. Exact measurement of blood pH allows calculation
of all acid-base data. Capillary blood may be used for the determination.
E
the distinction between the two kinds of disturbances in the acid-base metabolism:
the respiratory,
and the nonrespiratory-.
Moreover,
everyone agrees that the respiratory
disturbances ought to be characterized
by the CO2 tension of the arterial
blood, as this tension is supposed to correspond
with that of the
alveolar air.
When trying
to characterize
the nonrespiratory
disturbances
however, we are faced with difficulties.
The purpose must be to
cheose a value that reflects the nonrespiratory
disturbances
only,
and which is completely
uninfluenced
by the changes in the acidbase content of the blood caused by hyper- and hypoventilation.
Various kinds of bicarbonate
values have been, and still are being
used for this. Most commonly used are total CO2 of plasma, the
COs-combining
power of plasma, and what we call the standard
bicarbonate
of plasma. The latter value is defined as the bicarbonate
of true plasma, separated
from the blood with the hemoglobin completely oxygenated,
at a pCO2 of 40 mm. Hg and at 38#{176}.
These three values vary in a blood sample when the pCOs and
the oxygen saturation
are varied.
An experiment
illustrating
this
is conducted as follows.
A sample of venous blood is drawn into
an Ehrlenmeyer
flask, well mixed, and distributed
among six tubes.
In the three tubes (No. 1-3, Fig. 1) the hemoglobin is oxygenated,
and in three (No. 4-6), reduced.
The blood is equilibrated
with
C02, so that the pCO2 will be 20, 40, and 80 mm., respectively,
in
one tube each of the oxygenated
and of the reduced blood (Fig. 1).
VEBYO}TB
KNOWS
Front the Central Laboratory, Rigahospitalet, Copenhagen, Denmark.
The W. S. Bauld Memorial Lecture delivered before the Toint Meeting of the American
Association of Clinical Chemists and the Canadian Society for Clinical Chemistry, Aug. 29,
1960, Montreal, P.Q., Canada.
ASTRUP
2
Clinical Ch.mistry
If we then determine the total C02, C02-combining
power, and the
standard bicarbonate
in each of the six tubes, values are found as
shown in Table 1.
Only the standnrd bicarbonate
is influenced by oxygen saturation
and variations
in pCO2-that
is, in variations
caused by respiratory
changes.
It seems evident then that standard
bicarbonate
is the
best of the bicarbonate
values to measure when one wishes to eliminate the influence of respiration,
and to determine
the nonrespiratory component only.
The standard
bicarbonate
value, like all bicarbonate
values, has
the drawback that it does not give directly, in milliequivalents
per
liter of blood, the amount of fixed acid or base causing a change
in the base content of a blood sample.
The exact correction can be
carried out if one knows the buffer capacity of the blood, which to
all practical
purposes means the hemoglobin
concentration.
The concept of “base excess” and “buffer base” is helpful in
determining
these accumulated
amounts of acid or base.
FIg. 1. Sample of venous
blood
distributed
in six
tubes for equilibration with
gas mixtures.
1. TOTAL 00,, COrCOsINmG
Powxa, AND S nmnn BIcsBBoNAra
TBE SAMX BlooD Ssarpan AT VAEflNG p00, AND Viarmo
OXYGEN
Table
20 mm. Hg
Fully ozygenated
blood
Total CO,
COg-combining
power
Standard bicarbonate
Fully reduced
CO,-combining
power
bIcarbonate
#{149}Iii
milliequivalents
per liter.
FaOM
80 ,n,n. Hg
16.8
22.2
18.0
21.0
22.2
21.0
30.0
27.5
21.0
19.6
25.7
34.8
21.0
25.7
31.9
21.0
21.0
21.0
blood
Total CO1
Standard
p00,
40 mni. Hg
xx PLASMA
SATUn.aIIox
Vol. 7. No. I, 1961
ACID-BASE METABOLISM
3
The term base excess (BE) was recently introduced
by us in
Copenhagen.
Normally
the blood contains a certain amount of
base. If the normal value is defined as zero, the positive values of
base excess signify a real excess of base in blood, while negative
values reflect a deficit of base. A deficit of base is, of course, the
same as an excess of acid.
Typical values in a case of diabetic
coma, for example, would be pH, 7.08; pCO2, 20 mm. Hg; and BE,
-22 mEq./L.
The value for BE indicates that 22 mEq. of acid are
accumulated
per liter of blood. The pCO2 is about half the normal
value, indicating
a doubling of the alveolar ventilation
to keep the
pH as normal as possible.
In another example, a case of pyloric stenosis values might be
pH, 7.54; pCO2, 52 mm. Hg; and BE, +18 mEq./L.
Here we have a positive value for BE, indicating
a surplus of
base of 18 mEq./L. of blood-in
this case, due to a loss of acid from
the stomach.
The pCO2 is a little too high, reflecting the respiratory
compensating
mechanism for keeping the pH near the normal value.
In my opinion, the term base excess may be easily understood
by
everyone-medical
students,
as well as doctors.
It signifies both
qualitatively
and quantitatively
the nonrespiratory
disturbances
of the acid-base metabolism, whether they are primary or compensatory.
The term “buffer
base”
(BB) was introduced
by Singer and
Hastings in 1948. It can be defined as the sum of all buffer anions
(not cations) in 1 L. of blood, including the hemoglobin ions.
The data in Table 2 show that changes in BB correspond
to the
amounts of acid or base added to the blood. The same happens to
BE, but not to standard bicarbonate.
A drawback in the use of BB is the fact that the value of a blood
sample depends on its hemoglobin
concentration.
Table 3 shows
‘The use of the modern acid-base concept developed first by Br#{216}nsted
should be accepted
in medicine, as it has been for a long time in chemistry and biochemistry. In Scandinavia
-and
perhaps in all other regious-children
are taught acid-base chemistry according to
these concepts.
Medical students too are taught Br#{216}nsted
‘a concepts In biochemistry, but
In the clinical disciplines, they are (at least in Denmark) taught according to the old
theory that cations are bases. This creates great confusion. According to Br#{216}nsted,
an
acid is defined as a substance that is able to give of H+, while a base is defined as a
substance able to accept H+. Cations, such as those of Na and K, are neither acids nor
bases, and have nothing to do with the acid-base metabolism; hence, their turn over can
be treated as separate from acid-base metabolism.
In this way, for instance, a so-called
hyperchloremic acidosis is explained by the absorption
of NCI,
where NH4+ is the acid
that will deliver the H+
Many problems in biology and medicine may be better understood
and explained if one
keeps
to this
acid-base
concept.
ASTRUP
4
Clinical Chemistry
values from the same blood as used for the values given in Table 2,
but with erythrocytes
withdrawn
until a hemoglobin
concentration
of 7.5 gm./100 ml. was achieved.
Here the values for BB are
lower, but the same for BE.
Hence, the use of BE is recommended
where one wants exact
values for the accumulation
of fixed acids or base in a blood sample.
If an ion value for the base content is wanted, the standard
bicarbonate
should be used, also for practical
reasons, as it is very
easy to determine, especially in serial analyses.
Table
2.
V&tEs
ron Brma
BASE,
SAMPLE
BASE
EXCESS,
AND
or NORMAL
BlooD’
STANDARD
Addition,
blood
base, mEq./L.
Base excess, mEq./L.
Standard
bicarbonate,
Buffer
‘Hemoglobin
Table
3.
blood
mEq./L.
content,
15 gm./100
won Burnz
vALuss
BlooD
plasma
SAMPLE
par Star
Non.
lOmEq.
.trong acid
46.2
0
36.2
-10
22.9
15.9
BASS
2 WITH
EXCESS,
HEMOGLOBIN
AND
Non.
base,
mEq./L.
Base excess, mEq./L.
Standard
bicarbonate,
‘To
7.5 gm./100
blood
blood
mEq./L.
A
blood
1O,nEq.
strong baa.
56.2
+10
STANDARD
CONTENT
Add Wont
Buffer
OF
30.4
ml.
EASE,
OP TABLE
BICARBONATE
43.5
0
plasma
22.9
p.r
Stir
10mPg.
strong acid
33.5
-10
15.4
BIosasoNars
Ton
Rsrncxi’
Mood
lOrnEq.
strong baa.
53.5
+10
31.0
ml.
Analytical
Principle
Our technic for determining
the acid-base values of blood is based
on the equilibration
principle.
The curve of the relationship
between log pCOR and pH for a blood sample is, for all practical purposes, a straight line (Fig. 2); i.e., the line may be determined
if
one knows two points on it. The slope of the curve depends on the
buffer capacity of the blood (Fig. 3.). When fixed acid is added to
the blood samples (used for Fig. 3), the curves are displaced to the
left (Fig. 4); when the base is added, the curves are displaced to
the right.
vol. 7, No. I, 1961
5
ACID-BASE METABOLISM
Different
amounts of acid or base added to such blood samples
produce corresponding
log pCO2-pH curves the points of intersection of which form the BE graph (Fig. 4). This means that the
graph expresses the displacement
of a log pCOa-pH line caused by
pCO1
mm Hg
100
90
80
70
60
50
40
30
20
15
pH
10
6.8
6.9
Fig. 2.
Relationship
7.0
7.1
between
7.2
7.3
7.4
7.5
7.6
7.?
7.8
log pCO. and pH for a blood sample.
any amount of acid or base, independent
of the hemoglobin
concentration.
By use of the same arrangement
as in Fig. 4 a graph
can be constructed
expressing
the content of BB (Fig. 5).
It is thus seen that exact measurement
of blood pH at the actual
pOOs and at two known values for pCO2 will allow the calculation
of all blood data concerning
the acid-base status.
Point A and
Point B (Fig. 5) correspond
to the measured pH values at a high
and a low pCOa, respectively.
Point F corresponds
to the actual
pH of the anaerobically
drawn blood and allows the calculation
of
the actual pCO2. Point C indicates the BE, and Point P the BB.
6
ASTRUP
ClInkal Chemistr,
Point E indicates
the standard
bicarbonate.
Further,
the total
CO2 and the C02-combining
power of the plasma can be calculated.
Analytical Procedure
The methods we have developed
for determining
the three pH
values are simple in principle.
They involve an equilibration
of
the blood sample with two O2-CO2 mixtures
at a high and a low
pCOa, respectively,
followed by the measuring
of the corresponding
pH values.
The equilibration
can be done on a macro scale (about
3 ml. of blood is necessary)
or on a micro scale for about 100
of
blood. Both determinations
can be carried out in less than 10 min.
The micro method makes it possible to use capillary blood for
the determination,
which is very convenient when frequent analyses
10
6.8
6.9
7.3
7.4
7.5
7.6
7.
FIg. 3. Log pCOrpH curves for a blood sample. The hemoglobin content
(Curve A), 10 gm./100 ml. (Curve B), and 20 gm./100 ml. (Curve (3).
varies
for 0
Vol. 7, No. I, 1961
7
ACID-BASE METABOLISM
are needed-for
instance, at intervals of minutes.
The use of capillary blood is preferable
in pediatric cases also.
For sampling,
we use small glass capifiary
tubes containing
heparin.
After the tube is ifiled with blood, one end is closed with
ordinary wax. A piece of iron wire is then put into the tube and
the other end is closed with wax. The blood in the tube is then
stirred magnetically.
The tubes can be kept in ice water for some
hours without any significant change in the acid-base values. Before
determining
these values, the blood is stirred again, the ends of the
tube are cut off, and the blood is sucked up into the glass electrode.
Figure 6 shows the glass capillary electrode that has been developed according to the principles described by Sanz. The electrode,
with a polyethylene
tube attached,
can be put into the capillary
pcog mmHg
Fig. 4. Displacement
of log pCO1-pH curves
respectively,
per liter of blood.
by addition
of 15 mEq.
of acid and base,
a
ASTRUP
Clinical
Chemsfry
sampling tube and blood can be sucked up into it; the pH can then
be measured.
Temperature
of the calomel electrode and the glass
electrode are kept at 38#{176}.
Figure 7 shows the equilibration
unit, with two chambers:
one
for a high pCO and one for a low. About 50-75 jLl. of blood is
placed in each chamber.
The whole unit is then shaken mechanically
(2500 reciprocations
per minute), while conditioned 02-C02 mixtures
are allowed through.
After 3 mlii., the pH values of the two blood
samples are measured
and, by using the described nomogram,
all
pCO2 mm Hg
Standard
Fig.
5.
Calculation
of the relevant
acid-base
values
7.7 pH
rr
45
55
Blcarbonats
moq/ I
of a blood
sample
(see text).
Vol. 7, No. I. 1961
9
ACID-BASE METABOLISM
the relevant
acid-base
data can then be found by drawing
the
straight line.
The accurae
is high. Within normal limits for BE and pCO2,
the error is about ±2 per cent. The normal values are, for pH,
7.35-7.42; for pCO2, 34-45 mm. Hg; and for BE, -2.3 to +2.3
mEq./L.
for thermostatmg water
Shielded cable to
glass electrode”
suction pump
Inlet for
Outlet for
thermostating
water
Fig. 6. Capillary glass electrode.
Capillary glass electrode-calomel
electrode system, in
po8ition for measuring,
is temperature
controlled.
The freely movable glass electrode
part consists of a p11-sensitive glass capillary,
4, surrounded
by the constant-pH
liquid in
glass tube, B. The electrode, C, isiside B, is connected to the pH meter by a cable fixed
in the ebonite handle, L. The glass jacket, D, for circulating
thermostat
water is fused
with Tubes
A and B in the spouts, G and H. The thermostat
water is led in and out
through the handle, L. The capillary, A, is filled via the polyethylene
capillary, B, held by
the polyvinyl-chloride
adapter, F, which is pressed
on the spout, G. A suction
device for
the filling of A consists of a small glass housing, I, glued on D, with a sinafl hole, J, (to
be closed by a fingertip to make vacuum effective), and with a side tube, K, through L to
be connected to a auction pump. The metal pin, M, is adapted to a holder (not shown)
for the whole setup. The stationary
reference electrode has its calomel filling, 0, pIeced in
a U-tube
system with saturated
potassium
chloride solution, N, Q. A porous plate, F, is
inserted to protect the filling, 0, against
contamination.
Thus a small compartment,
Q, is
formed.
A liquid junction in the electrode system is established
when the capillary,
F,
with the sample to be measured is dipped into Q. Temperature
of the ealomel cell is controlled by circulating
water
in the water jacket, R.
10
ASTRUP
Figure 8 ifiustrates
the use
analyses.
It shows values for
normal person during a short
apnoea, and intensive muscular
as 13 mEq. of acid accumulate
a few minutes.
of the micro method for frequent
pH, pCO2, and BE of blood from a
period of maximal hyperventilation,
work. It is interesting
that as much
per liter of blood, in the course of
Clinical Chemistry
Fig. 7. Apparatus
for equilibration
of micro samples of blood at two carbon dioxide
tensions.
Equilibration
unit, made of glass, is composed of two pairs of equilibration
tubes (height, 35 mm.; width, 5 mm.) sealed into the water jacket (length, 120 mm.;
diameter, 35 mm.). The gases are conditioned
in two temperature-control1ed
humidifiers
(not shown) of some current type by bubbling through water at 38g. Thus conditioned,
each gas mixture
is passed to the corresponding
pair of equilibration
tubes through
plastic
tubing
enclosed
in a water
jacket
for temperature
control.
Figure 9 illustrates
the use of this method with infants.
At
delivery most newborn infants have an accumulation
of fixed acids,
due to periods of anoxia.
The first values were determined
in
blood obtained from skin puncture of the head, before the child was
born.
A slight metabolic
acidosis and a high pCOa were found.
After some hours the values were normal.
Clinical Acid-Base Problems
In many hospitals diabetic coma is treated with both insulin and
bicarbonate.
It seems quite unnecessary
in most cases to compen-
Vol. 7, No. I, 1961
11
ACID-BASE METABOLISM
sate in this manner, as insulin alone will eliminate
the acidosis,
causing combustion
of the organic acid to carbonic acid, which is
excreted by the lungs.
If bicarbonate
is given in addition,
the
result will be an over-compensation
of the acidosis, as the bicarbonate is excreted slowly by the kidneys (Fig. 10).
I
E
E
OU)
4
0
5
10
15
20
25
3D
Values for pH, pCO and BE of blood from a normal person
of hyperventilation,
apnoea,
and intensive
muscular
work.
FIg. 8.
period
3$
during
a short
When acid or base is lost extrarenally
and replacement
therapy
is required, it is most important
to determine
this extrarenal
loss
of acid (commonly hydrochloric
acid in gastric fluid) or base. This
loss should be determined
by titrating
the fluid to pH 7.40. In the
more severe cases, this loss must be replaced by parenteral
administration
of acid. We use ammonium
chloride intravenously.
In
less severe cases, where the renal function is normal, an acid deficit
12
ASTRUP
Clinicsl Ch.mistry
can be corrected by giving
sodium chloride.
To a chemist it seems
peculiar to neutralize
with sodium chloride.
The explanation
of
its acidifying
effect is that these ions enable the kidneys to excrete
a urine less acid than usual, thus retaining
acid in the body.
Aiso of clinical importance
is the need for a quantitative
determination of acid or base accumulated
in the body. The problems involved are rather complex and, for proper treatment,
knowledge of
the distribution
of acid and base in the different body spaces and the
rate of exchange between them is required.
When sodium bicarbonate
is infused intravenously,
it is distrib-
xl,
EW
Q-Q.
Acid-bas,
of an infantvalues
before
7,40
and after birth.
50
7,30
-.5
p CO1
BE
60
#{149}10
7,20-
pH
60
45
bIrth
10
7,10-
0
Fig. 9. Values
Blood
obtained
for pH, pCO,
by skin puncture
30
60-.’minutes
and BE of blood from an infant
of the head.
before
and after
birth.
Vol. 7, No. I. 1961
ACID.BASE METABOLISM
13
uted rapidly into what we call the bicarbonate
distribution
space.
This is not quite comparable
to the extracellular
body space. The
deficit or excess of base can be estimated in mEq. by multiplying
the
negative or positive BE by 0.3 and by the body weight in kilograms,
where the factor 0.3 is found experimentally.
The result corresponds
directly to the amount of sodium bicarbonate
or of ammonium
i:,,
=
L)
7.
4
0-
30
BASEOSIS
-
-
-
-Ci pH
-A
pCO2mmHg
-0
BASEXCESSmeq/1
4
ACIDOSIS
/
DAYS
4
0
1
3
Fig. 10. Values in case of severe diabetic coma. The treatment
with a small dose of
bicarbonate
besides insulin (and hydration)
causes the development of a metabolic aikalosis
with BE values of about +8.
14
Clinical Chemistry
ASTRUP
chloride required to neutralize
a nonrespiratory
disturbance
in the
bicarbonate
distribution
space, so that normal blood values for BE
are obtained.
Figure 11 shows the values from a patient with uremia.
The BE
I
7,50
I!
pCO2
7
norDH
7,30
area
-5
BE
-1
7,20
7,10:
1
46 years old
weight: 70 kg.
diagnosis: uremia
220 meq
#{244}
NaHcO3
10 1’
FIg. 11. Values in case of uremie acidosis
for obtaining normal BE values of blood.
1’6
‘
given the amount
2o
24 hours
of bicarbonate
calculated
Vol. 7, No. I, 1961
ACID.BASE METABOLISM
15
was found to be -10.6 mEq./L.,
and the estimated
amount
of
NaHCO8
to be given, according to the above-mentioned
formula,
was 220 mEq.
After intravenous
administration
of the NaHCO8,
the BE value was found to be -0.5.
In the next 24 hr. the value fell
to -7.3.
The pCO2of the patient did not change very much in the course
of those 24 hr. The pH values rose above normal limits. The course
follows that of the BE curve very closely.
As the rate of exchange of bicarbonate
between the extracellular
and the intracellular
space varies from patient to patient,
such
patients
should be treated-if
treatment
is clinically indicatedwith just the amount of bicarbonate
calculated
to give a normal
blood value; then, after some hours, the blood value should be determined again.
In this way overtreatment
is avoided.
If one considers the acid-base problem as a whole, there is no
justification
in its clinical importance
(relative
to that of other
fields of medicine) for the extensive work carried out through the
years to clarify its many intricate theoretical
aspects.
This, however, does not lessen the significance
in biology and medicine of
the acid-base problem for the understanding
of fundamental
processes an intimate knowledge of which is absolutely essential to any
doctor or biologist.
References
Astrup,
P., Siggaard
Andersen,
0., J#{216}rgensen,
K., and Engel, K., The acid-base
metabolism.
A new approach.
Lancet 1, 1035 (1960).
Singer,
H. B., and Hastings,
A. B., An improved
clinical
method
for the estimation
of the
acid-base balance of human blood. Medicine 27, 223 (1948).
Mellemgaard,
K., and Astrup, P., Quantitative
determination
of surplus amounts
of acid or
base in the human body. Scand. J. din.. 4. Lab. Invest.
12, 187 (1960).
Mellemgaard,
K., Lassen, U., and Astrup,
P. Unpublished
results.
Siggaard
Andersen, 0., Engel, K., J#{216}rgensen,
K., and Astrup, P., A micro method for
determination
of pH, carbon dioxide tension, base excess and standard
bicarbonate
in capillary blood. Scand. .T. dUn. 4. Lab. Inve8t. 12, 172 (1960).
Astrup,
P., A simple
electrometric
technique
for the determination
of carbon
dioxide
tension
in blood
and plasma,
total
content
of carbon
dioxide
in plasma,
and
bicarbonate
content
in “separated”
plasma
at a fixed carbon
dioxide
tension
(40
mm. Hg).
Scand. J. dUn. 4. Lab. Invest. 8, 33 (1956).
Siggaard Andersen, 0., and Engel,
Invest. 12, 177 (1960).
K., A new acid-base
nomogram.
Scand.
J. GUn. 4. Lab.