How is an arterial blood gas (ABG) clinically useful in assessing and managing critically ill patients, particularly those with respiratory or cardiac conditions such as chronic obstructive pulmonary disease (COPD) or heart failure?

Clinical Utility of Arterial Blood Gas (ABG) Analysis

ABG analysis is essential for assessing oxygenation, ventilation, and acid-base status in critically ill patients, and should be obtained in all patients with shock, respiratory distress, or unexplained deterioration, as pulse oximetry alone cannot detect hypercapnia, acidosis, or metabolic derangements. 1, 2

Primary Clinical Applications

Assessment of Oxygenation and Ventilation

• ABG directly measures arterial oxygen tension (PaO2) and carbon dioxide tension (PaCO2), providing critical information that pulse oximetry cannot detect 1, 3
• A normal SpO2 does not exclude significant respiratory or metabolic problems—patients can have normal oxygen saturation while having dangerous hypercapnia, acidosis, or low oxygen content from anemia 1, 2
• ABG is superior to pulse oximetry in carbon monoxide poisoning, where oximetry readings are falsely reassuring 2

Acid-Base Status Determination

• ABG measures pH directly and calculates bicarbonate (HCO3-), enabling diagnosis of metabolic acidosis (diabetic ketoacidosis, renal failure, sepsis) and respiratory acidosis/alkalosis 3, 4, 5
• This is particularly critical in patients with breathlessness who may have metabolic conditions such as diabetic ketoacidosis or metabolic acidosis from renal failure 1, 6

Mandatory Indications for ABG Testing

Critical Illness

• All critically ill patients require ABG measurement from an arterial sample 1, 2, 6
• Patients with shock or hypotension (systolic BP <90 mmHg) must have arterial sampling, not venous or capillary 1
• Critical illness including major trauma, sepsis, shock, and anaphylaxis mandates ABG analysis 1, 2
• After return of spontaneous circulation following cardiopulmonary resuscitation, ABG guides ongoing oxygen therapy 1, 7, 2

Respiratory Deterioration

• Unexpected fall in SpO2 below 94% in patients on air or oxygen requires ABG 1, 2, 6
• Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia (e.g., severe COPD) 1, 2, 6
• Any patient requiring increased FiO2 to maintain constant oxygen saturation needs ABG to assess for hypercapnia 1, 2

Monitoring Oxygen Therapy in At-Risk Patients

• Within 60 minutes of starting oxygen therapy in COPD patients or those at risk for hypercapnic respiratory failure, ABG must be obtained 1, 7, 2
• After each titration of oxygen flow rate in patients with baseline hypercapnia, ABG should be performed to detect respiratory acidosis 2, 6
• This is critical because supplemental oxygen can worsen hypercapnia and cause respiratory acidosis in susceptible patients 1

Guiding Oxygen Therapy and Ventilation Decisions

Target Saturation Determination

• ABG results determine whether target saturation should be 88-92% (for patients with hypercapnic respiratory failure) or 94-98% (for patients without CO2 retention) 1, 7
• If ABG shows pH <7.35 and PCO2 >6.0 kPa, this indicates respiratory acidosis requiring immediate senior review and consideration of non-invasive ventilation 1, 7

Ventilatory Support Decisions

• ABG guides decisions about initiating non-invasive ventilation (NIV) or mechanical ventilation in acute respiratory failure 1
• In acute heart failure with respiratory distress, ABG helps determine need for NIV or intubation 1

Technical Considerations for Optimal Results

Sampling Technique

• Local anesthesia should be used for all ABG specimens except in emergencies 1, 2, 6
• For patients in shock, on vasopressors, or with severe peripheral edema, use arterial or venous whole blood sampling instead of finger-stick capillary testing 1
• Allen's test should be performed before radial artery puncture to ensure dual blood supply to the hand 2, 6

Alternative Sampling Methods

• For most non-critical patients, arterialized earlobe blood gases may be used to obtain accurate pH and PCO2, though PO2 is less accurate (underestimates by 0.5-1 kPa) 1, 2
• Central venous blood gas can be used as a screening tool in stable patients, with arterial pH approximately equal to venous pH + 0.05 units and arterial PCO2 approximately equal to venous PCO2 - 5 mmHg 8
• However, in patients with circulatory failure, the difference between central venous and arterial values is 4-fold greater, making venous sampling unreliable 8

Common Pitfalls and How to Avoid Them

Overreliance on Pulse Oximetry

• The most dangerous error is assuming a normal SpO2 means the patient is stable—pulse oximetry cannot detect hypercapnia, acidosis, or anemia 1, 2, 6
• Blood gases and full blood count are required in all situations where these measurements may affect patient outcomes 1

Delayed ABG in Oxygen-Dependent Patients

• Failing to obtain ABG within 60 minutes of starting or changing oxygen therapy in at-risk patients can miss dangerous hypercapnia 1, 2
• Never discontinue oxygen therapy to obtain an oximetry measurement on room air in patients who clearly require oxygen 1

Inappropriate Sampling Site

• Using finger-stick capillary samples in patients with shock or on vasopressors produces unreliable results 1
• In critically ill patients, arterial sampling is mandatory, not optional 1, 2

Misinterpreting Results in Special Populations

• In COPD patients aged 50 years or more, check ABG before giving FiO2 >28% via Venturi mask or >2 L/min via nasal cannulae 2
• Patients with morbid obesity, neuromuscular disease, or chest wall deformity may require ventilatory support even with seemingly acceptable ABG values 1