2 ABG Interpretation

What an ABG can show

An arterial blood gas (ABG) helps assess acid–base status, ventilation, and oxygenation. Interpret the results alongside the patient’s condition, oxygen delivery, and other laboratory results—not in isolation. Assess the patient first if they are unstable.

A useful adult reference is $\mathrm{pH}\ 7.35\text{–}7.45$, $\mathrm{PaCO_2}\ 35\text{–}45\ \mathrm{mm\ Hg}$, and $\mathrm{HCO_3^-}\ 22\text{–}26\ \mathrm{mEq/L}$. Laboratory reference ranges can vary.

A systematic interpretation

Use the following sequence to organize interpretation:

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

2. Identify the primary change. A $\mathrm{PaCO_2}$ moving opposite the pH suggests a respiratory process. An $\mathrm{HCO_3^-}$ moving in the same direction as the pH suggests a metabolic process.

3. Check compensation. Compensation moves pH toward normal. If the measured response differs substantially from the expected response, consider a mixed disorder.

4. Connect the pattern to the clinical picture. Consider relevant symptoms, history, medications, and likely causes.

5. Assess oxygenation separately. Use $\mathrm{PaO_2}$ and oxygen saturation; pH, $\mathrm{PaCO_2}$, and $\mathrm{HCO_3^-}$ alone do not establish whether oxygenation is adequate.

Estimating respiratory compensation

In metabolic acidosis, estimate expected respiratory compensation with Winter’s formula:

Compare the measured $\mathrm{PaCO_2}$ with the estimated range. A value above the range suggests an additional respiratory acidosis; a value below the range suggests an additional respiratory alkalosis. Compensation rules are estimates and do not replace clinical judgment.

Deep, rapid breathing with elevated glucose

The low pH and low $\mathrm{HCO_3^-}$ identify metabolic acidosis. Winter’s formula predicts an expected $\mathrm{PaCO_2}$ of about $24.5 \pm 2\ \mathrm{mm\ Hg}$; the measured value of $25\ \mathrm{mm\ Hg}$ is within that range, indicating appropriate respiratory compensation. The rapid, deep breathing is consistent with that compensation.

In this clinical context, diabetic ketoacidosis is a concern, but further assessment and laboratory testing are needed to determine the cause.

Chronic obstructive lung disease

The elevated $\mathrm{PaCO_2}$ points to respiratory acidosis, while elevated $\mathrm{HCO_3^-}$ is consistent with renal compensation over time. The pattern is chronic respiratory acidosis with metabolic compensation and mild acidemia.

Chronic carbon dioxide retention can produce this pattern. However, new drowsiness warrants prompt clinical assessment for deterioration or another cause.

Repeated vomiting

The elevated pH and elevated $\mathrm{HCO_3^-}$ identify metabolic alkalosis. The elevated $\mathrm{PaCO_2}$ is in the compensatory direction, so the pattern is metabolic alkalosis with respiratory compensation.

Vomiting can contribute to metabolic alkalosis through loss of gastric acid. Assess volume status and relevant electrolyte results as well.

Sudden rapid breathing during distress

The high pH and low $\mathrm{PaCO_2}$ indicate acute respiratory alkalosis. The $\mathrm{HCO_3^-}$ is near the usual range, as expected early in a respiratory disturbance.

Anxiety may be relevant, but rapid breathing can also accompany serious conditions such as hypoxemia or pulmonary disease. Assess the patient and oxygenation rather than assuming anxiety is the cause.

Near-normal pH with abnormal values

The low $\mathrm{HCO_3^-}$ indicates metabolic acidosis, while the low $\mathrm{PaCO_2}$ is more than expected as compensation. Winter’s formula predicts about $26 \pm 2\ \mathrm{mm\ Hg}$, compared with a measured $\mathrm{PaCO_2}$ of $20\ \mathrm{mm\ Hg}$. This supports a mixed disorder: metabolic acidosis and respiratory alkalosis.

The two processes can bring the pH into the reference range, so a normal-looking pH does not exclude a serious mixed disorder. With suspected infection, urgent evaluation for causes such as lactic acidosis and other concurrent illness is important.

Putting the interpretation together

Start with pH, then identify whether $\mathrm{PaCO_2}$ or $\mathrm{HCO_3^-}$ best explains its direction and check whether compensation fits the expected pattern. Unexpected compensation, or a near-normal pH with markedly abnormal $\mathrm{PaCO_2}$ and $\mathrm{HCO_3^-}$, may indicate a mixed disorder. Relate the ABG to symptoms and clinical context, and evaluate oxygenation separately.