🔬 Arterial Blood Gas

The Arterial Blood Gas (ABG) is a point-of-care test used to evaluate a patient's acid-base balance, oxygenation, and ventilation. It measures the partial pressures of oxygen and carbon dioxide, pH, bicarbonate, electrolytes, and lactate. It is a cornerstone in managing acute respiratory failure and metabolic disturbances in the emergency and intensive care settings.

Assessment of acute respiratory distress or suspected respiratory failure. Monitoring of acid-base balance in critically ill patients, including those with sepsis, multi-organ failure, or diabetic ketoacidosis. Evaluation of carboxyhaemoglobin in suspected carbon monoxide poisoning. Monitoring response to supplemental oxygen therapy or mechanical ventilation. Measurement of lactate in suspected tissue ischaemia or shock states.

A sample of arterial blood is typically obtained from the radial artery at the wrist, though the brachial or femoral arteries may be used in emergencies. The Allen's test may be performed prior to ensure collateral circulation via the ulnar artery. The sample is collected into a heparinised syringe and must be analysed immediately or placed on ice to prevent heparin dilution and continued cellular metabolism from altering the results.

pH: 7.35–7.45; pO2: 10.6–13.3 kPa (on room air); pCO2: 4.7–6.0 kPa; HCO3-: 22–26 mmol/L; Base Excess: -2 to +2 mmol/L; Lactate: <2 mmol/L. Normal findings indicate adequate alveolar ventilation, effective gas exchange across the alveolar-capillary membrane, and balanced metabolic acid production and excretion.

Interpretation follows a systematic approach: first, check the pH to determine acidity or alkalinity. Second, evaluate the pCO2 to see if there is a respiratory cause for the pH change. Third, look at the bicarbonate (HCO3-) or base excess to identify metabolic compensation or primary metabolic derangement. Finally, assess the pO2 relative to the inspired oxygen fraction (FiO2) to evaluate shunt or V/Q mismatch. Compensation is never 'complete' enough to return pH to normal.

Metabolic acidosis (low pH and bicarbonate) may indicate sepsis, DKA, or renal failure. Respiratory acidosis (low pH, high pCO2) suggests type 2 respiratory failure, commonly seen in COPD or neuromuscular weakness. Type 1 respiratory failure is characterised by hypoxia with normal or low pCO2. High lactate (>2 mmol/L) is a critical marker of tissue hypoperfusion or anaerobic metabolism. Electrolyte derangements such as hyperkalaemia or hypoglycaemia are also frequently identified.

The ABG is a vital tool in the assessment of the 'A, B, and C' of resuscitation. It provides real-time data on gas exchange and acid-base status, which is more accurate than venous sampling for pO2 and pCO2. It is essential for determining the need for escalation of respiratory support, such as Non-Invasive Ventilation (NIV) or intubation, and for monitoring the response to treatment in metabolic emergencies.

Delay in analysis which leads to 'pseudohypoxaemia' and 'pseudohypercapnia' due to ongoing erythrocyte respiration. Failure to record the patient's inspired oxygen concentration (FiO2) at the time of sampling, making interpretation of pO2 impossible. Accidental venous sampling, which will show a significantly lower pO2 and slightly higher pCO2/lower pH. Over-reliance on the ABG while ignoring the clinical trend and physical examination of the patient.

Performing an ABG is an invasive and painful procedure for the patient. It provides only a 'snapshot' in time and may not reflect rapidly changing clinical states. Result accuracy depends heavily on correct sampling technique; for example, air bubbles in the syringe can falsely elevate pO2 and lower pCO2. It does not provide information on the underlying anatomical cause of the respiratory or metabolic distress.

Students must distinguish between Type 1 (hypoxic) and Type 2 (hypercapnic) respiratory failure. Understanding the Henderson-Hasselbalch principle and the role of the kidneys versus lungs in acid-base homeostasis is essential. Note that venous blood gases (VBG) are often sufficient for pH and lactate screening, but ABG remains the gold standard for oxygenation.