What are the considerations and implications for performing an arterial blood gas (ABG) in a critically ill patient, particularly those with severe respiratory distress or known respiratory conditions like chronic obstructive pulmonary disease (COPD)?

Arterial Blood Gas in Critically Ill Patients

When ABG is Mandatory

For critically ill patients, particularly those with severe respiratory distress or COPD, arterial blood gas (ABG) analysis is mandatory and should be obtained from an arterial sample—not venous—especially in patients with shock, hypotension (systolic BP <90 mmHg), or severe respiratory compromise. 1, 2, 3

Critical Indications Requiring ABG

• All critically ill patients require ABG analysis at initial presentation, including those with major trauma, sepsis, shock, or anaphylaxis 1, 3
• Patients with shock or hypotension (systolic BP <90 mmHg) must have arterial sampling, as venous blood gas cannot reliably assess oxygenation status 1, 2, 3
• Unexpected fall in SpO2 below 94% in patients breathing air or oxygen requires urgent ABG measurement 1
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia (e.g., severe COPD) mandates ABG analysis 1

Special Considerations for COPD Patients

Pre-Procedure Assessment

• Patients with suspected severe COPD (FEV1 <40% predicted and/or SpO2 <93%) should have arterial blood gas tensions measured before any procedure like bronchoscopy 1
• Spirometric parameters should be checked before procedures in patients with suspected COPD 1

Oxygen Therapy Targets

For COPD patients at risk of hypercapnic respiratory failure, target SpO2 is 88-92% (not 94-98%) while awaiting blood gas results. 1, 4

• Start with 24% Venturi mask at 2-3 L/min or 28% Venturi mask at 4 L/min, or nasal cannulae at 1-2 L/min 1, 3
• Increase Venturi mask flow by up to 50% if respiratory rate exceeds 30 breaths/min 1, 3
• Adjust target range to 94-98% only if PCO2 is normal and there is no history of previous non-invasive or invasive mechanical ventilation 1

Critical Timing for Repeat ABG

• Recheck blood gases after 30-60 minutes of starting oxygen therapy in at-risk patients 1, 4, 3
• Immediate repeat ABG is required if SpO2 falls ≥3% or patient develops drowsiness suggesting CO2 retention 3
• Any increase in FiO2 must be followed by repeat blood gases within 1 hour (or sooner if conscious level deteriorates) 1

Identifying Respiratory Acidosis and Need for Ventilatory Support

Critical Thresholds

If pH <7.35 (or [H+] >45 nmol/L) AND PCO2 >6.0 kPa, this indicates respiratory acidosis requiring immediate senior review and consideration of non-invasive ventilation (NIV) or invasive ventilation. 1, 4

• NIV should be initiated when pH <7.35 and PaCO2 >6.5 kPa persist despite optimal medical therapy 4
• Monitor for improvement in clinical state and arterial blood gas tensions after 1-2 hours of NIV, then again after 4-6 hours 1
• If no improvement in PaCO2 and pH after 4-6 hours despite optimal ventilator settings, discontinue NIV and consider invasive ventilation 1

Common Pitfalls to Avoid

Oximetry Limitations

• A normal SpO2 does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen 1, 2
• Pulse oximetry will be normal in a patient with normal PO2 but abnormal blood pH or PCO2, or with low blood oxygen content due to anaemia 1, 2
• For this reason, blood gases and full blood count are required as early as possible in all situations where these measurements may affect patient outcomes 1

Sedation Risks

• Sedation should be avoided where pre-procedure arterial CO2 is raised, as it may lead to further CO2 retention 1
• Oxygen supplementation given only with extreme caution in patients with raised arterial CO2 1

Technical Considerations

Sampling Technique

• Local anesthesia should be used for all ABG specimens except in emergencies 1, 3
• Arterial samples are preferred over capillary samples in critically ill patients 1, 2
• For most patients requiring blood gas sampling, either ABG or arterialized earlobe blood gases may be used for pH and PCO2, but PO2 is less accurate in earlobe samples (underestimates by 0.5-1 kPa) 1

Alternative Sampling Sites

• In ECMO patients, ABG samples should come from a right radial arterial line as this best represents cerebral perfusion 2, 4
• Venous blood gas is insufficient for assessing oxygenation in critically ill patients and should not be used as a substitute for ABG in shock or severe respiratory compromise 3, 5