ABG Interpretation

An arterial blood gas (ABG) describes acid–base balance, ventilation and arterial oxygenation at the time of sampling. Interpret it alongside the patient’s work of breathing, consciousness, perfusion, oxygen delivery and relevant chemistry results. An apparently reassuring pH can hide more than one acid–base disorder; the goal is to explain the pattern and its cause, not simply label the pH. Start by confirming the sample type, sampling time and oxygen or ventilator settings. Typical adult arterial reference intervals are pH 7.35–7.45, PaCO2 35–45 mmHg and bicarbonate approximately 22–26 mmol/L; use the reporting laboratory’s intervals. Acidemia describes a low measured pH, whereas acidosis describes a process tending to lower pH. A low bicarbonate supports a metabolic acidosis; a high PaCO2 supports a respiratory acidosis. More than one process can coexist. Assess whether compensation fits the primary disorder. In metabolic acidosis, Winter’s formula estimates expected PaCO2 in mmHg: 1.5 × bicarbonate in mmol/L + 8, with an approximate range of ±2. A measured PaCO2 above that range suggests an additional respiratory acidosis; a value below it suggests an additional respiratory alkalosis. Do not...