1 Arterial Blood Gas Fundamentals and Interpretation

What an ABG measures

An arterial blood gas (ABG) helps assess acid–base status, ventilation, and oxygenation. Interpret the measurements together, but assess oxygenation separately from the primary acid–base pattern.

Measurements and typical ranges

Typical adult reference ranges are listed below. Laboratory reference intervals and clinical context may differ.

• Acid–base measurements: pH is typically $7.35$–$7.45$; $\mathrm{PaCO_2}$, which reflects the respiratory component, is $35$–$45$ mm Hg; and $\mathrm{HCO_3^-}$, which reflects the metabolic component, is $22$–$26$ mEq/L.

• Oxygenation measurements: $\mathrm{PaO_2}$, the oxygen dissolved in arterial blood, is about $80$–$100$ mm Hg. $\mathrm{SaO_2}$, the percentage of hemoglobin saturated with oxygen, is about $95$–$100\%$.

PaO₂ and SaO₂ indicate oxygenation; they do not determine whether the primary acid–base pattern is respiratory or metabolic. PaO₂ varies with factors such as age, altitude, and inspired oxygen.

A systematic interpretation sequence

Use the same sequence to organize interpretation:

1. Check the pH. A pH below $7.35$ indicates acidemia; a pH above $7.45$ indicates alkalemia. A pH within range does not, by itself, rule out an acid–base disturbance.

2. Compare the components with their ranges. Compare $\mathrm{PaCO_2}$ with $35$–$45$ mm Hg and $\mathrm{HCO_3^-}$ with $22$–$26$ mEq/L. PaCO₂ is the respiratory component, while HCO₃⁻ is the metabolic component.

3. Match a component’s direction to the pH change. With acidemia, high PaCO₂ fits a respiratory-acidosis pattern and low HCO₃⁻ fits a metabolic-acidosis pattern. With alkalemia, low PaCO₂ fits a respiratory-alkalosis pattern and high HCO₃⁻ fits a metabolic-alkalosis pattern.

4. Assess oxygenation separately. Review PaO₂ and SaO₂ together with inspired oxygen and the clinical context.

5. State the findings clearly. For example: “Acidemia with a metabolic-acidosis pattern; oxygenation is within the stated reference interval.” A full clinical interpretation may require additional information and assessment of compensation or mixed disorders.

Apply the patterns to ABG examples

Apply the typical ranges above to the following examples. In each case, first identify whether the pH shows acidemia, alkalemia, or a value within range; then identify the acid–base pattern indicated by PaCO₂ and HCO₃⁻. Consider oxygenation separately when PaO₂ is provided.

• Components within range: With pH $7.41$, PaCO₂ $39$ mm Hg, HCO₃⁻ $24$ mEq/L, and PaO₂ $92$ mm Hg, PaO₂ indicates oxygenation, while pH, PaCO₂, and HCO₃⁻ describe acid–base status. These results are within the typical ranges shown.

• Metabolic-acidosis pattern: With pH $7.28$, PaCO₂ $30$ mm Hg, and HCO₃⁻ $14$ mEq/L, the pH shows acidemia, and the low HCO₃⁻ fits a metabolic-acidosis pattern.

• Respiratory-alkalosis pattern: With pH $7.51$, PaCO₂ $29$ mm Hg, and HCO₃⁻ $23$ mEq/L, the pH shows alkalemia, and the low PaCO₂ fits a respiratory-alkalosis pattern.

• Respiratory-acidosis pattern: With pH $7.29$, PaCO₂ $55$ mm Hg, and HCO₃⁻ $25$ mEq/L, the pH shows acidemia, and the high PaCO₂ fits a respiratory-acidosis pattern.

• Metabolic-alkalosis pattern: With pH $7.48$, PaCO₂ $42$ mm Hg, and HCO₃⁻ $31$ mEq/L, the pH shows alkalemia, and the high HCO₃⁻ fits a metabolic-alkalosis pattern.

• Acid–base status and oxygenation considered separately: With pH $7.40$, PaCO₂ $40$ mm Hg, HCO₃⁻ $24$ mEq/L, and PaO₂ $58$ mm Hg on room air, the acid–base values are within the typical ranges shown. PaO₂ is below the typical adult reference interval, indicating reduced oxygenation that should be interpreted in clinical context.

Putting the interpretation together

Interpret an ABG in order: assess pH, compare PaCO₂ and HCO₃⁻ with their reference ranges, use their direction to identify the acid–base pattern, and assess oxygenation separately. Consider laboratory reference intervals and the patient’s clinical context.