26
1011
Students
, particularly nursing students,
are often given blood values and asked to
determine (1) whether the
patient
is in
acidosis
or
alkalosis
, (2) the
cause
of the condition (respiratory or metabolic),
and (3) whether the condition is being
compensated
. If you approach these
questions systematically, they are not
nearly as difficult as they may appear.
To analyze a person’s acid-base
balance, scrutinize the blood values in the
following order:
1.
Note the pH.
This tells you whether the
person is in acidosis (pH below 7.35) or
alkalosis (pH above 7.45), but it does
not tell you the cause.
2.
Check the P
CO
2
to see if this is causing
the acid-base imbalance. Because
the respiratory system is a fast-acting
system, an excessively high or low
P
CO
2
may indicate that the respiratory
system is either causing the condition
or compensating for it. For example, if
the pH indicates acidosis and
a.
The P
CO
2
is over 45 mm Hg, then
the respiratory system is the cause
of the problem and the condition is
respiratory acidosis
b.
The P
CO
2
is below normal limits
(below 35 mm Hg), then the
respiratory system is
not the cause
but is compensating
c.
The P
CO
2
is within normal limits,
then the condition is
neither caused
nor compensated
by the respiratory
system
3.
Check the bicarbonate level.
If step 2
proves that the respiratory system is not
responsible for the imbalance, then the
condition is metabolic and should be
reflected in increased or decreased bicar-
bonate levels. HCO
3
2
values below
22 mEq/L indicate metabolic acidosis,
and values over 26 mEq/L indicate meta-
bolic alkalosis. Notice that whereas P
CO
2
levels vary inversely with blood pH (P
CO
2
rises as blood pH falls), HCO
3
2
levels vary
directly with blood pH (increased HCO
3
2
results in increased pH).
Beyond this bare-bones approach there
is something else to consider when you
are assessing acid-base problems. If an
imbalance is fully compensated, the pH
may be normal even while the patient is
in trouble. Hence, when the pH is normal,
carefully scrutinize the P
CO
2
or HCO
3
2
values for clues to what imbalance may be
occurring.
Consider the following two examples
of the three-step approach.
Problem 1
Blood values
: pH 7.6; P
CO
2
24 mm Hg;
HCO
3
2
23 mEq/L
Sleuthing: Using Blood Values to Determine
the Cause of Acidosis or Alkalosis
Analysis
:
1.
The pH is elevated: alkalosis.
2.
The P
CO
2
is very low: the cause of the
alkalosis.
3.
The HCO
3
2
value is within normal
limits.
Conclusion:
This is respiratory alkalosis
not compensated by renal mechanisms,
as might occur during short-term
hyperventilation.
Problem 2
Blood values:
pH 7.48; P
CO
2
46 mm Hg;
HCO
3
2
33 mEq/L
Analysis
:
1.
The pH is elevated: alkalosis.
2.
The P
CO
2
is elevated: the cause of
acidosis, not alkalosis. Thus, the
respiratory system is compensating and
is not the cause.
3.
The HCO
3
2
is elevated: the cause of
the alkalosis.
Conclusion
: This is metabolic alkalosis
being compensated by respiratory acidosis
(retention of CO
2
to restore blood pH to
the normal range).
Use the accompanying simple chart to
help you in your future sleuthing.
A
C L O S E R
LOOK
 
Normal Range in Plasma
Acid-Base Disturbance
pH 7.35–7.45
P
CO
2
35–45 mm Hg
HCO
3
2
22–26 mEq/L
Respiratory acidosis
if compensating
Respiratory alkalosis
if compensating
Metabolic acidosis
if compensating
Metabolic alkalosis
if compensating
respiratory disease. We can recognize these
respiratory
and
re-
nal compensations
by the resulting changes in plasma P
CO
2
and
bicarbonate ion concentrations (see
A Closer Look
). Because the
compensations act to restore normal blood pH, a patient may
have a normal pH despite a significant medical problem.
Respiratory Compensations
As a rule, changes in respiratory rate
and depth are evident when the respiratory system is attempting
to compensate for metabolic acid-base imbalances. In metabolic
acidosis, respiratory rate and depth are usually elevated—an indi-
cation that high H
1
levels are stimulating the respiratory centers.
Blood pH is low (below 7.35) and the HCO
3
2
level is below 22
mEq/L. As the respiratory system “blows off” CO
2
to rid the blood
of excess acid, the P
CO
2
falls below 35 mm Hg. By contrast, in res-
piratory acidosis, the respiratory rate is oFen depressed and
is the
immediate cause of the acidosis
(with some exceptions such as
pneumonia or emphysema where gas exchange is impaired).
Respiratory compensation for metabolic alkalosis involves
slow, shallow breathing, which allows CO
2
to accumulate in
the blood. Evidence of metabolic alkalosis being compensated
by respiratory mechanisms includes a pH over 7.45 (at least
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