Why is an Arterial Blood Gas (ABG) test recommended when a patient desaturates while sitting?

Why Arterial Blood Gas (ABG) is Recommended When a Patient Desaturates While Sitting

Arterial Blood Gas (ABG) testing is essential when a patient desaturates while sitting because it provides critical information about oxygenation, ventilation, and acid-base status that pulse oximetry alone cannot detect, allowing for accurate diagnosis and appropriate management of the underlying cause. 1

Clinical Significance of Desaturation While Sitting

Desaturation while sitting (or during minimal exertion) is a concerning clinical sign that warrants immediate investigation for several reasons:

• It indicates potential respiratory compromise that may not be apparent at rest
• It may signal early deterioration in patients with underlying cardiopulmonary conditions
• It can identify patients at risk for hypercapnic respiratory failure

When to Obtain an ABG

An ABG should be performed in the following situations:

• 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 1
• Patients requiring increased FiO2 to maintain a constant oxygen saturation 1
• Any patient with risk factors for hypercapnic respiratory failure who develops acute breathlessness or deteriorating oxygen saturation 1
• Significant arterial oxygen desaturation (SpO2 <90% or fall of 4% or more) that is prolonged (>1 min) 1

What ABG Provides That Pulse Oximetry Cannot

Pulse oximetry has several important limitations that ABG analysis overcomes:

1. Detection of Hypercarbia: Pulse oximetry cannot detect early decreases in ventilation adequacy or the onset of hypercarbia that may occur before the development of apnea 1

2. Acid-Base Status: ABG provides pH and bicarbonate levels, essential for identifying metabolic or respiratory causes of desaturation 1

3. Accurate PaO2 Measurement: Direct measurement of arterial oxygen tension rather than an estimate through peripheral saturation 1

4. Masked Hypoxemia: Administration of supplemental oxygen may delay the onset of hypoxemia detection by pulse oximetry, masking hypoventilation 1

Clinical Scenarios Requiring ABG When Desaturation Occurs

Chronic Respiratory Conditions

• COPD and Cystic Fibrosis: Patients with these conditions are at risk for hypercapnic respiratory failure and require ABG to assess if target saturation of 88-92% is appropriate 1
• Neuromuscular Disorders: Patients with respiratory failure due to neurological disorders or muscle disease require ABG to determine if non-invasive ventilation will be needed 1

Acute Clinical Situations

• Procedural Sedation: When desaturation occurs during procedural sedation, ABG helps assess ventilation adequacy that may not be apparent with oximetry alone 1
• Post-COVID-19 Patients: Exertional desaturation (≥4 points decrease in SpO2) during sit-to-stand testing may warrant ABG analysis to assess gas exchange abnormalities 2
• Complicated GI Endoscopy: Prolonged desaturation during endoscopy, especially in patients with cardiorespiratory comorbidity, requires ABG to assess for hypercapnia 1

Interpreting ABG Results in Context of Desaturation

When interpreting ABG results in a patient who desaturates while sitting, assess:

1. Oxygenation: PaO2 < 60 mmHg indicates significant hypoxemia 3
2. Ventilation: PaCO2 > 45 mmHg suggests hypoventilation/respiratory acidosis 3
3. Acid-Base Status: pH < 7.35 with elevated PaCO2 indicates respiratory acidosis; pH < 7.35 with normal PaCO2 but low HCO3- suggests metabolic acidosis 3

Important Caveats and Considerations

• The presence of normal SpO2 does not negate the need for ABG, especially if the patient is on supplemental oxygen 1
• Local anesthesia should be used for all ABG specimens except in emergencies 1
• For most patients requiring blood gas sampling, either arterial blood gases or arterialised earlobe blood gases may be used to obtain accurate pH and PCO2, but PaO2 is less accurate in earlobe samples 1
• Sudden cessation of supplementary oxygen therapy can cause life-threatening rebound hypoxemia 1

Monitoring After Initial ABG

• For patients with interstitial lung disease associated with systemic autoimmune rheumatic diseases, monitoring for ambulatory desaturation every 3-12 months is recommended 1
• Patients with time to desaturation less than 1 minute during exertion tests have a 74% probability of desaturating during daily living activities and may require more frequent ABG monitoring 4

By obtaining an ABG when a patient desaturates while sitting, clinicians can accurately diagnose the underlying cause and implement appropriate management strategies to prevent further deterioration and improve patient outcomes.