Arterial blood gas interpretation in five steps, with four worked examples

Reference ranges to carry

These adult ranges are commonly taught. Laboratories differ, so use the range printed on the report. In exams, the values given are usually clear enough that small differences do not matter.

This is exam-preparation revision, not clinical guidance. In practice, follow your own organisation’s policies and the current guideline, and check doses and thresholds against them.

• pH: 7.35 to 7.45.
• PaCO2 (carbon dioxide): 4.7 to 6.0 kPa, or 35 to 45 mmHg.
• HCO3 (bicarbonate): 22 to 26 mmol/L.
• PaO2 (oxygen): about 10 to 13 kPa on room air, or 75 to 100 mmHg.

The five steps

Use the same order every time so you do not get pulled to the most striking number.

1. Oxygenation: is the PaO2 low? Compare it with the oxygen the patient is receiving. A normal PaO2 on high-flow oxygen is not reassuring.
2. pH: below 7.35 is acidaemia, above 7.45 is alkalaemia.
3. Carbon dioxide: high PaCO2 acidifies (respiratory acidosis), low PaCO2 alkalinises (respiratory alkalosis).
4. Bicarbonate: low HCO3 acidifies (metabolic acidosis), high HCO3 alkalinises (metabolic alkalosis).
5. Compensation: does the other system move to pull the pH back toward normal? If pH is normal with both abnormal, it is fully compensated. If pH is still abnormal, it is partly compensated or uncompensated.

Finding the primary problem

Match the direction of the pH with the direction of the CO2 or bicarbonate. If the pH is low and the CO2 is high, the CO2 is the cause, so it is a respiratory acidosis. If the pH is low and the bicarbonate is low, the cause is metabolic. The other value, moving the opposite way to pull the pH back, is compensation.

Four invented examples

Every value below is made up for teaching.

• pH 7.28, PaCO2 3.2 kPa, HCO3 14: low pH with low bicarbonate is a metabolic acidosis. The low CO2 is the lungs compensating by breathing fast. Think diabetic ketoacidosis or sepsis. See diabetic emergencies.
• pH 7.30, PaCO2 8.5 kPa, HCO3 28: low pH with high CO2 is a respiratory acidosis. The raised bicarbonate suggests the kidneys have been compensating for some time, as in long-standing COPD with an acute flare. See oxygen targets.
• pH 7.50, PaCO2 3.5 kPa, HCO3 24: high pH with low CO2 is a respiratory alkalosis. Think pain, anxiety or overbreathing. Bicarbonate has not had time to compensate.
• pH 7.50, PaCO2 5.5 kPa, HCO3 34: high pH with high bicarbonate is a metabolic alkalosis. Think prolonged vomiting or diuretic use.

The anion gap

In a metabolic acidosis, the anion gap helps sort the cause. It is sodium minus the sum of chloride and bicarbonate. A raised gap points to added acid, such as ketones, lactate or toxins. A normal gap points to bicarbonate loss, as in diarrhoea. Normal ranges vary by laboratory, so compare with the range on the report.

How exams use it

Questions usually give a short history and a gas, then ask for the interpretation or the first nursing action. If the question asks what to do, treat the patient and the cause, not the number. For a drowsy patient with a high CO2, think airway and ventilation, and escalate. Link the gas to the setting: see fluids and electrolytes and sepsis. To rehearse, add one card per pattern to your flashcards.

Sample and technique points

Exams sometimes ask about collecting the sample. Check the patient’s identity, record the oxygen the patient is receiving, and follow your policy for arterial puncture, including checking circulation to the hand where relevant, applying firm pressure afterwards and removing air bubbles from the syringe. Send the sample promptly, since delay can change the result. If the patient is anticoagulated, tell the person taking the sample and apply pressure for longer, as your policy says. Venous gases are used in some pathways, and the pH and bicarbonate are close to arterial values in many cases, but oxygenation needs an arterial sample or a saturation reading.

Frequently asked questions