Interpretation of Acid – Base Status
1. Using a pH of 7.40 as absolute normal, classify the pH as being either Acidic (acidosis)
or Basic (alkalosis)
2. Using a PaCO2 of 40 mm Hg as absolute normal, classify the PaCO2 as being either
Acidic (PaCO2 > 40 mm Hg) or Basic (PaCO2 < 40 mm Hg)
3. Using a HCO3- of 24 mEq/L as absolute normal, classify the HCO3- as being either
Acidic (HCO3- < 24 mEq/L) or Basic (HCO3- > 24 mEq/L)
pH
PaCO2 mm Hg
HCO3- mEq/L
Normal Ranges
7.35 – 7.45
35 - 45
22 – 26
Absolute Normal
7.40
40
24
Acidic
< 7.40
> 40
< 24
Basic
> 7.40
< 40
> 24
4. Identify the parameter (PaCO2 or HCO3-) which correlates with the pH.
a.
If the pH and PaCO2 correlate, this indicates a respiratory impairment
or
b.
If the pH and HCO3- correlate, this indicates a metabolic impairment
5. Identify if there is respiratory or renal compensation present
a.
If the non-correlating parameter is within normal limits, this would be
classified as an uncompensated or acute impairment
or
b.
If the non-correlating parameter is outside of normal range, this would be
classified as either a compensated or partially compensated impairment
i.
If the pH is in normal range, the ABG is interpreted as compensated
ii.
If the pH is not in normal range, the ABG is interpreted as partially
compensated
c.
Respiratory compensation is usually complete in 12 – 24 hours.
d.
Renal compensation takes 3 – 5 days to complete. The kidneys will retain or
excrete HCO3- to provide compensation.
Case Examples for Arterial Blood Gas Interpretation
Respiratory Alkalosis
Example 1
Acid or Base Comments
pH
7.47
B
Outside normal range
PaCO2 mm Hg 30
B
Correlates with pH (respiratory problem)
HCO3- mEq/L 23
A
Within normal range (uncompensated)
Interpretation: Uncompensated Respiratory Alkalosis (Acute Alveolar Hyperventilation)
Acid or Base Comments
Example 2
pH
7.46
B
Outside normal range (partially compensated)
PaCO2 mm Hg 30
B
Correlates with pH (respiratory problem)
HCO3 mEq/L 21
A
Outside normal range (compensated)
Interpretation: Partially Compensated Respiratory Alkalosis
Acid or Base Comments
Example 3
pH
7.44
B
In normal range (compensated)
PaCO2 mm Hg 30
B
Correlates with pH (respiratory problem)
HCO3- mEq/L 19
A
Outside normal range (compensated)
Interpretation: Compensated Respiratory Alkalosis (Chronic Alveolar Hyperventilation)
Respiratory Acidosis
Acid or Base Comments
Example 4
pH
7.30
A
Outside normal range
PaCO2 mm Hg 65
A
Correlates with pH (respiratory problem)
HCO3 mEq/L 25
B
Within normal range (uncompensated)
Interpretation: Uncompensated Respiratory Acidosis (Acute Ventilatory Failure)
Acid or Base Comments
Example 5
pH
7.33
A
Outside normal range (partially compensated)
PaCO2 mm Hg 65
A
Correlates with pH (respiratory problem)
HCO3 mEq/L 27
B
Outside normal range (compensated)
Interpretation: Partially Compensated Respiratory Acidosis
Acid or Base Comments
Example 6
pH
7.36
A
In normal range (compensated)
PaCO2 mm Hg 65
A
Correlates with pH (respiratory problem)
HCO3 mEq/L 30
B
Outside normal range (compensated)
Interpretation: Compensated Respiratory Acidosis (Chronic Ventilatory Failure)
Metabolic Alkalosis
Example 7
Acid or Base Comments
pH
7.49
B
Outside normal range
PaCO2 mm Hg 42
A
Within normal range (uncompensated)
HCO3 mEq/L 29
B
Correlates with pH (metabolic problem)
Interpretation: Uncompensated Metabolic Alkalosis
Example 8
pH
PaCO2 mm Hg
HCO3- mEq/L
7.47
47
29
Acid or Base
B
A
B
Comments
Outside normal range (partially compensated)
Outside normal range (compensated)
Correlates with pH (metabolic problem)
Interpretation: Partially Compensated Metabolic Alkalosis
Acid or Base Comments
Example 9
pH
7.45
B
In normal range (compensated)
PaCO2 mm Hg 49
A
Outside normal range (compensated)
HCO3 mEq/L 29
B
Correlates with pH (metabolic problem)
Interpretation: Compensated Metabolic Alkalosis
Metabolic Acidosis
Example 10
Acid or Base Comments
pH
7.31
A
Outside normal range
PaCO2 mm Hg 38
B
Within normal range (uncompensated)
HCO3- mEq/L 20
A
Correlates with pH (metabolic problem)
Interpretation: Uncompensated Metabolic Acidosis
Acid or Base Comments
Example 11
pH
7.33
A
Outside normal range (partially compensated)
PaCO2 mm Hg 33
B
Outside normal range (compensated)
HCO3 mEq/L 20
A
Correlates with pH (metabolic problem)
Interpretation: Partially Compensated Metabolic Acidosis
Acid or Base Comments
Example 12
pH
7.35
A
In normal range (compensated)
PaCO2 mm Hg 31
B
Outside normal range (compensated)
HCO3 mEq/L 20
A
Correlates with pH (metabolic problem)
Interpretation: Compensated Metabolic Acidosis
Mixed Alkalosis
Acid or Base Comments
Example 13
pH
7.58
B
Outside normal range
PaCO2 mm Hg 30
B
Outside normal range
HCO3 mEq/L 28
B
Outside normal range
Interpretation: Mixed Respiratory and Metabolic Alkalosis
Mixed Acidosis
Acid or Base Comments
Example 14
pH
7.29
A
Outside normal range
PaCO2 mm Hg 55
A
Outside normal range
HCO3- mEq/L 20
A
Outside normal range
Interpretation: Mixed Respiratory and Metabolic Acidosis
Exceptions to the Rule
Acid or Base Comments
Example 15
pH
7.55
B
Outside normal range (partially compensated)
PaCO2 mm Hg 53
A
Outside normal range (compensated)
HCO3 mEq/L 32
B
Correlates with pH (metabolic problem)
Interpretation: Acute Alveolar Hyperventilation Superimposed on Chronic Ventilatory
Failure
Comments: Looks like partially compensated metabolic alkalosis. This would be seen in
a patient with a history of COPD who presents with hypoxia, increased work of
breathing, dyspnea, tachycardia and tachypnea due to an acute episode. The patient
appears to have a metabolic alkalosis because their PaCO2 is normally higher and renal
compensation has occurred. The hypoxemia causes an increased respiratory rate which
decreases the PaCO2 from baseline. This patient will develop Acute on Chronic
Ventilatory Failure (see example 16) and may require ventilatory support if treatment is
not effective.
Acid or Base Comments
Example 16
pH
7.29
A
Outside normal range (partially compensated)
PaCO2 mm Hg 75
A
Correlates with pH (respiratory problem)
HCO3- mEq/L 30
B
Outside normal range (compensated)
Interpretation: Acute on Chronic Ventilatory Failure
Comments: Looks like partially compensated respiratory acidosis. This would be seen in
a patient with a history of COPD who develops respiratory failure as the result of an
acute episode such as pneumonia in addition to their COPD. The patient’s PaCO2 is
higher than baseline causing the pH to fall below normal range. This patient will require
aggressive treatment and may require ventilatory support.