Read the ABG in the same order every time
Start with pH, then compare PaCO2 and bicarbonate with the reference ranges supplied in the question or laboratory report. Common adult teaching ranges are pH 7.35–7.45, PaCO2 35–45 mm Hg and bicarbonate 22–26 mEq/L, but local ranges can vary.
An acidic pH points toward acidosis and an alkaline pH toward alkalosis. Next decide which component explains that direction. Carbon dioxide behaves as an acid; bicarbonate behaves as a base.
Respiratory opposite, metabolic equal
In a primary respiratory disorder, PaCO2 and pH move in opposite directions: high PaCO2 with low pH suggests respiratory acidosis, while low PaCO2 with high pH suggests respiratory alkalosis.
In a primary metabolic disorder, bicarbonate and pH move in the same direction: low bicarbonate with low pH suggests metabolic acidosis, while high bicarbonate with high pH suggests metabolic alkalosis. Use this pattern only after reading the actual values and client presentation.
- Low pH + high PaCO2: respiratory acidosis pattern.
- High pH + low PaCO2: respiratory alkalosis pattern.
- Low pH + low bicarbonate: metabolic acidosis pattern.
- High pH + high bicarbonate: metabolic alkalosis pattern.
Determine compensation
If the non-primary system remains within range, the disorder is commonly described as uncompensated. If it has moved in the compensatory direction while pH remains outside range, compensation is partial. If pH has returned within range while both PaCO2 and bicarbonate are abnormal, compensation may be complete; use the direction of pH and the client context to identify the primary process.
Compensation is a physiological response, not proof of recovery. A client can have a near-normal pH while remaining seriously ill.
Connect the pattern to nursing priority
Respiratory acidosis can result from inadequate ventilation, so assess airway, respiratory effort, sedation and gas exchange. Respiratory alkalosis can accompany hyperventilation, pain or early deterioration. Metabolic acidosis may occur with shock, severe diarrhoea, renal failure or ketoacidosis, while metabolic alkalosis may follow gastric losses or certain diuretics.
Prioritise airway, breathing, perfusion and neurological status. Review oxygenation, electrolytes, glucose, lactate and treatment history as relevant. Escalate acute deterioration and reassess whether interventions improve both the client and the trend.
- Identify the acid-base pattern.
- Find the likely cause in the scenario.
- Assess the immediate safety threat.
- Choose an intervention that addresses that cause.
- Evaluate the client’s response and repeat data as ordered.
Sources and further reading
This guide summarises public educational and regulatory information. It does not contain live or recalled NCLEX items and does not replace your nursing program, local policy or professional advice.