Can Arterial Blood Gases (ABGs) be used to help diagnose disorders?

Can Arterial Blood Gases (ABGs) Be Used to Help Diagnose Disorders?

Yes, ABG analysis is an essential diagnostic tool that provides critical information about respiratory function, metabolic status, and acid-base balance across a wide range of acute and chronic disorders. 1, 2

Primary Diagnostic Applications

ABG analysis serves as a fundamental diagnostic tool by directly measuring pH, PaCO2, and PaO2, while calculating bicarbonate and base excess through the Hasselbach equation. 3 This enables clear understanding of respiratory, circulatory, and metabolic disorders that cannot be obtained through pulse oximetry alone. 2

Critical Illness and Emergency Conditions

All critically ill patients require ABG measurement to assess oxygenation, ventilation, and acid-base status. 1, 2 Specific critical indications include:

• Peri-arrest situations and patients requiring immediate medical intervention 1
• Post-cardiac arrest after return of spontaneous circulation to guide ongoing oxygen therapy 1, 2
• Major trauma, sepsis, shock, and anaphylaxis 1
• Carbon monoxide poisoning, where pulse oximetry readings may be falsely normal 1

Respiratory Disorders

ABG is essential for diagnosing and managing respiratory conditions:

• Acute respiratory failure: PaO2 indicates oxygenation status while PaCO2 indicates ventilation status (chronic or acute respiratory failure) 3
• COPD exacerbations: During severe exacerbations, ABGs should be monitored for PaO2, PaCO2, and pH 4
• Neuromuscular disease: ABG criteria (PaCO2 >45 mm Hg) help determine need for noninvasive ventilation 4
• Obesity hypoventilation syndrome: ABG confirms daytime hypercapnia when serum bicarbonate >27 mmol/L 4
• Unexpected oxygen desaturation below 94% in patients breathing air or oxygen 1, 2

Metabolic Disorders

ABG analysis is crucial for diagnosing acid-base diseases including diabetic ketoacidosis, renal tubular acidosis, and metabolic acidosis due to renal failure. 1, 2, 3 The analysis encompasses severe sepsis, septic shock, hypovolemic shock, chronic heart failure, and diverse metabolic diseases. 3

Cardiovascular Conditions

In critically ill cardiovascular patients, ABG provides information that pulse oximetry cannot:

• Cardiogenic shock: Identifies metabolic acidosis associated with poor outcomes 5
• Acute heart failure: Differentiates between cardiac and pulmonary causes of respiratory distress 5
• Post-ECMO management: Assesses oxygenation and identifies differential oxygenation syndromes 5

Monitoring and Treatment Guidance

Oxygen Therapy Titration

After oxygen titration is complete, ABG should be taken to determine whether adequate oxygenation has been achieved without precipitating respiratory acidosis or worsening hypercapnia. 1, 2

• Within 60 minutes of starting oxygen therapy in patients with COPD or risk factors for hypercapnic respiratory failure 1, 2
• After each titration of oxygen flow rate in patients with baseline hypercapnia 1, 2
• Within 60 minutes of any change in inspired oxygen concentration in at-risk patients 1

Ventilatory Support Decisions

ABG results guide critical decisions about ventilatory support:

• NIV initiation in neuromuscular disease when PaCO2 >45 mm Hg 4
• Mechanical ventilation consideration in severe respiratory acidosis 2
• CPAP effectiveness assessment in acute heart failure 5

Critical Pitfalls to Avoid

A normal oxygen saturation does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen therapy. 1, 2, 5 Key caveats include:

• Pulse oximetry appears normal in patients with normal PO2 but abnormal pH or PCO2, or with low oxygen content due to anemia 1, 2
• Carbon monoxide poisoning produces falsely normal pulse oximetry readings, necessitating ABG regardless of oximeter readings 1
• Metabolic disturbances (abnormal pH, bicarbonate) are completely missed by pulse oximetry 2

When NOT to Obtain ABG

In lower-risk infants presenting with Brief Resolved Unexplained Events (BRUE), clinicians should not obtain measurement of venous or arterial blood gases. 4 The benefits of reducing unnecessary testing, pain, risk of thrombosis, and false-positive results outweigh the rare missed diagnostic opportunity for hypercapnia and acid-base imbalances in this specific population. 4

Technical Considerations

• Arterial samples are preferred over venous sources in critically ill patients, shock, or hypotension 1, 2
• Local anesthesia should be used for all ABG specimens except in emergencies 1, 2
• Allen's test should be performed before radial ABG to ensure dual blood supply to the hand 1, 2
• Arterialized earlobe blood gases may be used as an alternative for most non-critical patients to obtain accurate pH and PCO2, though PO2 is less accurate 1