Normal values
- pH: 7.35 – 7.45
- PaCO₂: 4.7 – 6.0 kPa || 35.2 – 45 mmHg
- PaO₂: 11 – 13 kPa || 82.5 – 97.5 mmHg
- HCO₃–: 22 – 26 mEq/L
- Base excess (BE): -2 to +2 mmol/L
Types of disturbance
Acid-base disturbance | pH | PaCO₂ | HCO₃ | Common causes |
Acute respiratory acidosis | < 7.35 | High | Normal | Acute asthma, suffocation |
Partly/fully compensated respiratory acidosis | < 7.35 or normal | High | High | COPD, MND |
Acute respiratory alkalosis | > 7.45 | Low | Normal | Pain, type 1 respiratory failure |
Partly/fully compensated respiratory alkalosis | > 7.45 or normal | Low | Low | CNS disturbance, pregnancy |
Acute metabolic acidosis | < 7.35 | Normal | Low | DKA, lactic acidosis; (anion gap present) |
Partly/fully compensated metabolic acidosis | < 7.35 or normal | Low | Low | Diarrhea, renal tubular acidosis; (no anion gap) |
Acute metabolic alkalosis | > 7.45 | Normal | High | Vomiting (hypochloremic), primary hyperaldosteronism |
Partly/fully compensated metabolic alkalosis | > 7.45 or normal | High | High | Diuresis (hypochloremic), Cushing’s syndrome |
- Mixed or complex acid-base disorders may also occur, and can be diagnosed due to a mismatch in expected compared to actual compensatory changes in ether HCO₃ (in respiratory acidosis or alkalosis) or pCO₂ (in metabolic acidosis or alkalosis)
How to interpret
- Identify the primary disorder: Look at the pH, PaCO₂, and HCO₃- values.
- If the pH is low (<7.35), it indicates acidemia.
- If the pH is high (>7.45), it indicates alkalemia
- Determine the cause of the disorder
- If the pH and PaCO₂ are moving in opposite directions, it’s a respiratory disorder.
- If the pH and HCO₃- are moving in the same direction, it’s a metabolic disorder
- Check for compensation
- Look at the value that is not part of the primary disorder (PaCO₂ for metabolic disorders, HCO₃- for respiratory disorders). If this value is also abnormal, it indicates compensation
- Evaluate the degree of compensation
- If the pH is within the normal range (7.35-7.45), it’s fully compensated.
- If the pH is outside the normal range, but the other values are changing in a direction to correct the pH, it’s partially compensated
- If the pH is abnormal and the other values are not changing in a direction to correct the pH, it’s uncompensated
- Remember, the body never overcompensates. Therefore, if the pH has crossed over to the other side of the normal range, it indicates a mixed disorder
- Example
- Mixed Respiratory + metabolic acidosis
- pH 7.18, pO₂ 67 mmHg (8.9 kPa), pCO₂ 74 mmHg (9.8 kPa), HCO₃ 11 mmol/l, Base Excess − 12 mmol/l
- The pH is not neutral so it is partially compensated
- HCO₃ is 11 which is low showing that the metabolic system is trying to compensate but have not been able to fully compensate yet
- Fully compensated respiratory acidosis
- pH 7.35, pO₂ 76 mmHg (10.1 kPa), pCO₂ 55 mmHg (7.33 kPa), HCO₃ 29 mmol/l, Base Excess − 1 mmol/l
- The pH is neutral so it is fully compensated
- HCO₃ is 29 which is high (the body has produced a lot of HCO₃ to take up the H+ from respi acdiosis)
- Mixed metabolic + respiratory acidosis
- pH 7.1, PaCO₂ 60 mmHg, HCO₃ 18 mEq/L
- The pH is lower than normal, indicating acidemia
- The PaCO₂ is higher than normal, indicating respiratory acidosis
- The bicarbonate (HCO₃) is lower than normal, indicating metabolic acidosis
- Partially compensated respiratory acidosis
- pH 7.33, PaCO₂ 55 mmHg, HCO₃ 29 mEq/L
- The pH is lower than normal, indicating acidemia
- The PaCO₂ is higher than normal, indicating primary respiratory acidosis
- The bicarbonate (HCO₃) is also higher than normal. This is the body’s metabolic response to compensate for the respiratory acidosis.
- Uncompensated respiratory acidosis
- pH 7.16, PaCO₂ 65 mmHg, HCO₃ 24 mEq/L
- The pH is lower than normal, indicating acidemia
- The PaCO₂ is higher than normal, indicating primary respiratory acidosis
- The bicarbonate (HCO₃) is within normal limits, indicating that the metabolic system has not started to compensate for the respiratory acidosis.
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