1. See Marieb & Hoehn 9th ed., Chapter 26
HESC310
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Acid-Base Analysis
W. Rose
See Marieb & Hoehn 9th ed., Chapter 26
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2. Normal Arterial Blood Gas
HESC310
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Normal Arterial Blood Gas
pH =
PaCO2 = mmHg
HCO3- = mEq/L
PaO2 = mmHg
We will skip base excess and anion gap
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3. Arterial pH<7.35: Acidosis Arterial pH >7.45: Alkalosis
HESC310
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Abnormal Blood Gases
Arterial pH<7.35: Acidosis
Arterial pH >7.45: Alkalosis
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4. Arterial pH < 7.35: Acidosis
HESC310
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Arterial pH < 7.35: Acidosis
Is it respiratory, metabolic, or both?
Respiratory acidosis = acidosis with high PaCO2.
pH<7.35, PaCO2>45 mmHg
PaO2 likely to be low (<80 mmHg)
Metabolic acidosis = acidosis with low HCO3.
pH<7.35, HCO3<22 mEq/L
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5. Arterial pH > 7.45: Alkalosis
HESC310
12/4/2018
Arterial pH > 7.45: Alkalosis
Is it respiratory, metabolic, or both?
Respiratory alkalosis = alkalosis with low PaCO2.
pH>7.45, PaCO2<35 mmHg
Look for hyperventilation
PaO2 often high normal ( mmHg)
Metabolic alkalosis = alkalosis with high HCO3.
pH>7.45, HCO3>26 mEq/L
Gain of acid or loss of base
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6. Respiratory Alkalosis
HESC310
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pH < 7.35: Acidosis
pH > 7.45: Alkalosis
Respiratory Acidosis
PaCO2 > 45 mmHg
Respiratory Alkalosis
PaCO2 < 35 mmHg
Metabolic Acidosis
HCO3<22 mEq/L
Metabolic Alkalosis
HCO3>26 mEq/L
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7. Arterial blood gas diagram

8. HESC310
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Compensation
There is immediate but limited compensation from the bicarbonate buffer system of blood.
Lungs can compensate (in minutes) for a metabolic disturbance. Increased breathing to compensate for metabolic acidosis is a more robust and reliable response than decreased breathing in response to metabolic alkalosis.
Kidneys can compensate (in hours to days) for a respiratory disturbance. Renal compensation for respiratory acidosis is slow but may be nearly complete after 3-4 days; renal compensation for respiratory alkalosis is slow and incomplete.
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9. Compensation Abnormally low PaCO2 (<35 mmHg) in
HESC310
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Compensation
Abnormally low PaCO2 (<35 mmHg) in
metabolic acidosis indicates respiratory compensation (hyperventilation).
Pure (uncompensated) metabolic acidosis seldom seen
since respiratory system compensates quickly.
PaO2 is often high normal ( mmHg)
when there is respiratory compensation.
Compensated (or chronic) metabolic acidosis
pH=7.25, PaCO2=25 mmHg, HCO3=10 mEq/L
There’s respiratory compensation
It’s acidosis
The acidosis is metabolic
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10. Compensation Abnormally high PaCO2 (>45 mmHg) in
HESC310
12/4/2018
Compensation
Abnormally high PaCO2 (>45 mmHg) in
metabolic alkalosis indicates respiratory compensation (hypoventilation).
PaO2 may be normal or slightly below normal (<=80 mmHg)
Uncompensated metabolic alkalosis
pH=7.58, PaCO2=44 mmHg, HCO3=40 mEq/L
Compensated metabolic alkalosis
pH=7.50, PaCO2=55 mmHg, HCO3=40 mEq/L
It’s alkalosis
There’s respiratory compensation
The alkalosis is metabolic
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11. Compensation Abnormally high HCO3 (>26 mEq/L) in
HESC310
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Compensation
Abnormally high HCO3 (>26 mEq/L) in
respiratory acidosis indicates renal compensation.
Uncompensated (or acute) respiratory acidosis
pH=7.25, PaCO2=60 mmHg, HCO3=25 mEq/L
Compensated (or chronic) respiratory acidosis
pH=7.35, PaCO2=60 mmHg, HCO3=32 mEq/L
Near normal
Abnormally high – this person isn’t getting rid of CO2
Higher than normal – kidneys must be compensating for the high CO2
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12. Compensation Abnormally low HCO3 (<22 mEq/L) in
HESC310
12/4/2018
Compensation
Abnormally low HCO3 (<22 mEq/L) in
respiratory alkalosis indicates renal compensation.
Compensated (or chronic) respiratory alkalosis
pH=7.44, PaCO2=26 mmHg, HCO3=17 mEq/L, PaO2=53 mmHg, pneumonia
It’s alkalosis
The alkalosis is not metabolic – quite the opposite
The alkalosis is respiratory
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13. HESC310
12/4/2018
Sources
Rose, W.C., in Fluids and Electrolytes with Clinical Applications, 8th ed., 2009.
Chapter 11. Regulatory Mechanisms for pH Control.
Chapter 12. Determination of Acid-Base Imbalances.
Chapter 13. Metabolic Acidosis and Alkalosis.
Chapter 14. Respiratory Acidosis and Alkalosis.
Acid-base pHysiology'. K. Brandis,
GlobalRPh: Arterial blood gases.
Köse et al. (2014). Acid-Base Disorders in the Emergency Department: Incidence, Etiologies and Outcomes. J. Acad. Emerg. Med. 13: 4-9.
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