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Arterial Blood Gas Interpretation

Acid-Base Disorder Analysis

Normal range: 7.35–7.45

Normal range: 35–45 mmHg

Normal range: 22–26 mEq/L

Normal range: 80–100 mmHg (breathing room air)

Reference Values

Parameter Normal Range Unit
pH 7.35–7.45 —
PaCO₂ 35–45 mmHg
HCO₃⁻ 22–26 mEq/L
PaO₂ 80–100 mmHg
SaO₂ > 95 %
Base Excess −2 to +2 mEq/L

Acid-Base Disorders

Respiratory Acidosis:

pH ↓, PaCO₂ ↑ (hypoventilation, COPD, respiratory depression)

Respiratory Alkalosis:

pH ↑, PaCO₂ ↓ (hyperventilation, anxiety, pain, hypoxaemia)

Metabolic Acidosis:

pH ↓, HCO₃⁻ ↓ (ketoacidosis, renal failure, diarrhoea, lactic acidosis)

Metabolic Alkalosis:

pH ↑, HCO₃⁻ ↑ (vomiting, diuretic use, hypovolaemia)

Compensation Rules

Acute Respiratory Acidosis: HCO₃⁻ increases by 1 mEq/L for every 10 mmHg rise in PaCO₂

Chronic Respiratory Acidosis: HCO₃⁻ increases by 3.5 mEq/L for every 10 mmHg rise in PaCO₂

Acute Respiratory Alkalosis: HCO₃⁻ decreases by 2 mEq/L for every 10 mmHg fall in PaCO₂

Chronic Respiratory Alkalosis: HCO₃⁻ decreases by 4–5 mEq/L for every 10 mmHg fall in PaCO₂ (calculation uses 4 mEq/L as reference)

Metabolic Acidosis (Winter's Formula): Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 (±2)

Metabolic Alkalosis: PaCO₂ increases by 0.7 mmHg for every 1 mEq/L rise in HCO₃⁻

Note: Compensation never fully restores pH — it only approximates it towards normal. The acute/chronic distinction in respiratory disorders requires clinical context beyond laboratory values.

Classification of Hypoxaemia

Mild: PaO₂ 60–79 mmHg
Moderate: PaO₂ 40–59 mmHg
Severe: PaO₂ < 40 mmHg

Note: PaO₂ values should be adjusted for age and altitude.

IMPORTANT

This calculator is a decision-support tool and does not replace clinical judgement. Interpretation must always be correlated with a complete clinical history, detailed physical examination, serum electrolytes, and other investigations. Mixed disorders are common and require careful analysis.

References

  • 1. Adrogué HJ, Madias NE. "Management of life-threatening acid-base disorders. First of two parts." N Engl J Med. 1998;338(1):26–34. PubMed ↗
  • 2. Berend K, de Vries AP, Gans RO. "Physiological approach to assessment of acid-base disturbances." N Engl J Med. 2014;371(15):1434–1445. PubMed ↗
  • 3. Albert MS, Dell RB, Winters RW. "Quantitative displacement of acid-base equilibrium in metabolic acidosis." Ann Intern Med. 1967;66(2):312–322. PubMed ↗
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