When should blood gases be checked?

When to Check Blood Gases

Blood gases should be checked in all critically ill patients, patients with unexpected or inappropriate SpO2 below 94%, deteriorating oxygen saturation, increasing breathlessness in patients with chronic hypoxemia, patients requiring increased oxygen therapy, and those at risk of hypercapnic respiratory failure who develop acute symptoms. 1

Essential Indications for Blood Gas Analysis

Critical Clinical Scenarios

• All critically ill patients 1, 2
• Unexpected or inappropriate fall in SpO2 below 94% in patients breathing air or oxygen 1
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia (e.g., severe COPD) 1
• Previously stable patients who deteriorate clinically and require increased FiO2 to maintain constant oxygen saturation 1
• Patients with risk factors for hypercapnic respiratory failure who develop acute breathlessness, deteriorating oxygen saturation, drowsiness or other features of carbon dioxide retention 1

Monitoring Oxygen Therapy

• Patients at risk of hypercapnic respiratory failure (target saturation 88-92%) should have blood gases measured within 30-60 minutes of starting oxygen therapy 1
• This applies even to patients with normal PCO2 on initial measurement 1, 2
• After any increase in oxygen therapy for patients at risk of hypercapnic respiratory failure 1

Other Important Indications

• Patients with breathlessness who may have metabolic conditions (e.g., diabetic ketoacidosis or renal failure) 1
• Unexpected changes in vital signs or tracking systems (e.g., sudden rise in NEWS score) 1
• Any clinical situation where blood gas results would guide management 1

Blood Gas Sampling Methods

Arterial vs. Arterialised Samples

• For critically ill patients or those with shock/hypotension (systolic BP <90 mmHg), obtain arterial samples 1, 2
• For most other patients, either arterial blood gases (ABGs) or arterialised earlobe blood gases can be used 1
• Note that PO2 is less accurate in earlobe samples (underestimates by 0.5-1 kPa) 1, 2
• Use local anesthesia for all ABG specimens except in emergencies 1

Repeat Blood Gas Measurements

When to Recheck

• For patients with target saturation 88-92% (at risk of hypercapnic respiratory failure), recheck within 30-60 minutes 1
• After any increase in oxygen therapy for patients at risk of hypercapnic respiratory failure 1
• If oxygen saturation fails to rise after 5-10 minutes of increased oxygen therapy 1
• If there is clinical concern following medical review 1

When Repeat Gases May Not Be Needed

• Stable patients whose oxygen saturation is within target range of 94-98% with no risk of hypercapnic respiratory failure 1
• Patients requiring reduced concentration of oxygen or cessation of oxygen therapy to maintain target saturation 1
• Patients with no risk of hypercapnic respiratory failure who require increased oxygen (though clinical review is still needed) 1

Common Pitfalls and Caveats

• Normal SpO2 does not exclude abnormalities: Pulse oximetry will be normal in a patient with normal PO2 but abnormal pH, PCO2, or with anemia 1, 2
• Delayed hypercapnia: Patients with COPD or risk factors for hypercapnic respiratory failure may develop respiratory acidosis even with initially normal PCO2 1, 2
• Excessive oxygen: Avoid excessive oxygen use in patients at risk of hypercapnic respiratory failure; target 88-92% saturation 1, 2
• Monitoring limitations: All clinical staff using oximeters must understand their limitations and potential for artifact 1
• Sampling errors: Ensure proper technique for blood gas collection to avoid pre-analytical errors 2

Special Considerations for COPD Patients

• For COPD patients aged 50+ years, initially use no more than 28% oxygen via Venturi mask or 2 L/min via nasal cannulae until blood gases are known 1
• Check blood gases within 60 minutes of starting oxygen and within 60 minutes of any change in inspired oxygen concentration 1
• If pH falls (due to rising PCO2), consider alternative strategies including NIV 1, 2
• If the patient is initially acidotic or hypercapnic, repeat blood gas measurement within 60 minutes 1

By following these evidence-based guidelines for blood gas analysis, clinicians can effectively monitor respiratory and metabolic status, guide oxygen therapy, and improve patient outcomes in critical care settings.