3 Arterial Blood Gas Interpretation

Reference values and pH

Typical adult reference values are pH $7.35$–$7.45$, PaCO₂ $35$–$45\ \text{mm Hg}$, and HCO₃⁻ $22$–$26\ \text{mEq/L}$. Reference intervals can vary by laboratory. PaO₂ also varies with age and other factors.

A pH below $7.35$ indicates Acidemia; a pH above $7.45$ indicates Alkalemia. Values within the usual interval do not alone establish that acid–base status is normal: an apparently near-normal pH can occur with compensation or with two opposing disorders.

A systematic interpretation method

Interpret the acid–base values in sequence:

1. Check the pH to determine whether it indicates Acidemia or Alkalemia.

2. Identify the primary change. PaCO₂ is the respiratory component, while HCO₃⁻ is the metabolic component. CO₂ moves pH in the opposite direction; bicarbonate moves pH in the same direction.

3. Assess Compensation by checking whether the other component changes in the direction that brings pH toward normal. Compensation does not by itself show that the primary disorder has resolved.

If the compensatory response is outside its expected range, consider a Mixed disorder. Also consider a mixed process when both PaCO₂ and HCO₃⁻ push pH in the same abnormal direction.

Estimating compensation

For Metabolic acidosis, estimate the expected PaCO₂ using Winter’s formula:

For respiratory disorders, expected bicarbonate changes depend on whether the disorder is acute or chronic. Per $10\ \text{mm Hg}$ change in PaCO₂:

• In acute Respiratory acidosis, HCO₃⁻ increases by approximately $1\ \text{mEq/L}$; in chronic Respiratory acidosis, it increases by approximately $3.5$–$4\ \text{mEq/L}$.

• In acute Respiratory alkalosis, HCO₃⁻ falls by approximately $2\ \text{mEq/L}$; in chronic Respiratory alkalosis, it falls by approximately $4$–$5\ \text{mEq/L}$.

These compensation estimates should be interpreted with the clinical picture.

Interpreting example blood gases

In each case, the values are given as pH / PaCO₂ / HCO₃⁻. The interpretations identify the primary disorder and assess the compensation described by the values.

• $7.28 / 55 / 25$: Acute Respiratory acidosis. The low pH and elevated PaCO₂ point to respiratory acidosis. HCO₃⁻ is near its usual range, as expected before substantial renal compensation.

• $7.25 / 27 / 12$: Metabolic acidosis with appropriate respiratory compensation. The low pH and low HCO₃⁻ identify metabolic acidosis. Winter’s formula gives an expected PaCO₂ of $26 \pm 2\ \text{mm Hg}$; the measured $27\ \text{mm Hg}$ is within that range.

• $7.52 / 28 / 22$: Acute Respiratory alkalosis. The high pH and low PaCO₂ identify the primary disorder. PaCO₂ is $12\ \text{mm Hg}$ below $40\ \text{mm Hg}$, and an acute bicarbonate decrease of about $2$–$3\ \text{mEq/L}$ is expected; HCO₃⁻ of $22\ \text{mEq/L}$ fits that pattern.

• $7.34 / 60 / 31$: Respiratory acidosis with renal compensation, consistent with a chronic pattern. The low pH and high PaCO₂ point to respiratory acidosis. For a PaCO₂ increase of $20\ \text{mm Hg}$, chronic compensation predicts an HCO₃⁻ increase of roughly $7$–$8\ \text{mEq/L}$ above $24\ \text{mEq/L}$; the measured $31\ \text{mEq/L}$ is consistent.

• $7.50 / 48 / 36$: Metabolic alkalosis with respiratory compensation. High pH and HCO₃⁻ identify the primary metabolic alkalosis. The elevated PaCO₂ is in the expected compensatory direction; assess compensation quantitatively if the clinical decision requires it.

• $7.18 / 55 / 20$: Mixed metabolic and respiratory acidosis. Low HCO₃⁻ and elevated PaCO₂ both push pH downward. This is not the expected compensation pattern for a single metabolic acidosis, because compensation would lower PaCO₂.

• $7.40 / 40 / 24$: No acid–base disturbance is apparent from these values; all three are within typical adult reference ranges.