Why is arterial blood checked?

Arterial Blood Gas Analysis: Indications and Clinical Utility

Arterial blood gas (ABG) analysis is primarily performed to assess oxygenation, ventilation, and acid-base status in critically ill patients, with specific indications for respiratory compromise, metabolic disturbances, and monitoring treatment responses.

Critical Care Indications

• ABG measurement is recommended for all critically ill patients to provide essential information about oxygenation, ventilation, and acid-base status 1
• After return of spontaneous circulation following cardiopulmonary resuscitation, ABG measurement guides ongoing oxygen therapy 1
• In major trauma, sepsis, shock, and anaphylaxis, ABG measurement is essential for patient assessment 1, 2
• During CPR, arterial blood gas monitoring is not a reliable indicator of tissue hypoxemia, hypercarbia, or tissue acidosis, and has uncertain value (Class IIb, LOE C) 3

Respiratory Indications

• ABG sampling is indicated when there is an unexpected or inappropriate fall in oxygen saturation below 94% in patients breathing air or oxygen 1, 2
• Patients with deteriorating oxygen saturation or increasing breathlessness with previously stable chronic hypoxemia require ABG analysis 1
• Any patient with risk factors for hypercapnic respiratory failure who develops acute breathlessness, deteriorating oxygen saturation, or drowsiness should have ABG measurement 1, 2
• In patients with known pulmonary gas exchange abnormalities (interstitial lung disease, pulmonary vascular disease, COPD with low DLCO), arterial blood sampling should be considered during cardiopulmonary exercise testing 3

Metabolic Assessment

• ABG analysis is indicated in patients with suspected metabolic conditions such as diabetic ketoacidosis or metabolic acidosis due to renal failure 1, 2
• ABG measurement assesses the major functional consequences of respiratory muscle weakness, with hypercapnia being a predictor of shorter survival in conditions like Duchenne muscular dystrophy 3
• ABG analysis provides comprehensive information about acid-base status that cannot be obtained through pulse oximetry alone 4, 5

Monitoring and Treatment Guidance

• After oxygen titration, ABG analysis confirms whether adequate oxygenation has been achieved without precipitating respiratory acidosis 1, 2
• Within 60 minutes of starting oxygen therapy in patients with COPD or other risk factors for hypercapnic respiratory failure, ABG measurement should be performed 3, 2
• When there is an unexpected change in "track and trigger" systems such as a sudden rise in early warning scores or an unexpected fall in oxygen saturation of 3% or more, ABG measurement is indicated 1
• Arterial blood sampling may be needed when pulse oximetry is considered less reliable (e.g., in dark-skinned individuals) 3

Special Considerations and Technical Aspects

• Local anesthesia should be used for all ABG specimens except in emergencies 1, 2
• Patients undergoing radial ABG should have an Allen's test performed first to ensure dual blood supply to the hand 1, 2
• For most non-critical patients requiring blood gas sampling, arterialised earlobe blood gases may be used as an alternative to obtain accurate pH and PCO₂ measurements, though PO₂ is less accurate in these samples 1
• In carbon monoxide poisoning, pulse oximetry readings may be falsely normal, necessitating ABG regardless of oximeter readings 1, 6

Common Pitfalls

• A normal SpO₂ does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen therapy 1, 2
• Pulse oximetry will appear normal in patients with normal PO₂ but abnormal pH or PCO₂, or with low oxygen content due to anemia 1, 2, 6
• Central venous blood gas parameters may be used as surrogates for arterial values in stable patients, but the difference between central venous and arterial pH/PCO₂ is significantly greater in patients with circulatory failure 7
• When using earlobe blood gas samples, oximetry should be monitored carefully and a repeat arterial specimen taken if there are concerns about accuracy 1

Clinical Applications Beyond Critical Care

• ABG analysis serves as an exceptional diagnostic tool for acid-base diseases including severe sepsis, diabetic ketoacidosis, renal tubular acidosis, chronic respiratory failure, and various metabolic diseases 5
• In patients with respiratory muscle weakness, daytime hypercapnia is unlikely unless respiratory muscle strength is reduced to 40% of predicted and vital capacity is reduced to 50% of predicted 3
• ABG analysis is essential for home oxygen assessment, with patients requiring two measurements at least 3 weeks apart during a period of clinical stability 1