How do you interpret arterial blood gases (ABGs)?

How to Interpret Arterial Blood Gases (ABGs)

Use a systematic three-step approach: first assess pH to determine if acidemia or alkalemia is present, then evaluate PaCO2 to identify the respiratory component, and finally examine bicarbonate/base excess to identify the metabolic component. 1

Step 1: Assess Oxygenation Status

• Evaluate PaO2 (normal >80 mmHg) and oxygen saturation (normal >94% in most patients) to determine if hypoxemia is present 2
• Consider the PaO2/FiO2 ratio to assess severity of hypoxemia in critically ill patients 2
• A critical pitfall: normal pulse oximetry does not rule out significant acid-base disturbances, hypercapnia, or carbon monoxide poisoning 2, 3
• Pulse oximetry will appear falsely normal in patients with normal PO2 but abnormal pH or PCO2, or in carbon monoxide poisoning where carboxyhemoglobin is present 2, 3

Step 2: Determine Acid-Base Status Using pH

• pH < 7.35 indicates acidemia; pH > 7.45 indicates alkalemia (normal range: 7.35-7.45) 1, 2
• This is your starting point for determining the primary disturbance 1

Step 3: Identify the Respiratory Component

• Assess PaCO2 (normal range: 35-45 mmHg) 2
• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 1
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 1
• In respiratory acidosis, identify and treat the underlying cause while considering ventilatory support 2

Step 4: Identify the Metabolic Component

• Assess HCO3- (normal range: 22-26 mEq/L) and base excess (normal: -2 to +2) 1, 2
• Base excess < -2 or HCO3 < 22 indicates metabolic acidosis 1
• Base excess > +2 or HCO3 > 26 indicates metabolic alkalosis 1
• Base deficit is a sensitive marker for severity of shock and mortality risk, particularly in trauma patients 2

Step 5: Determine Compensation Status

• Apply the "RoMe" technique: Respiratory opposite, Metabolic equal 4
• In respiratory acidosis (high CO2, low pH), if HCO3 is elevated, metabolic compensation is occurring 4
• In metabolic acidosis (low HCO3, low pH), if PaCO2 is decreased, respiratory compensation is occurring 4
• Uncompensated: pH abnormal, only one system (respiratory or metabolic) is abnormal 4
• Partially compensated: pH still abnormal, but both systems show changes 4
• Fully compensated: pH normalized, both systems show compensatory changes 4

When to Obtain ABG Testing

Critical indications requiring ABG measurement: 5, 1, 2

• All critically ill patients 5, 2
• Shock or hypotension (systolic BP <90 mmHg) - use arterial samples, not venous 5, 2
• SpO2 fall below 94% on room air or supplemental oxygen 5
• Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 5
• Any patient with risk factors for hypercapnic respiratory failure who develops acute breathlessness, deteriorating saturation, or drowsiness 5
• Suspected diabetic ketoacidosis or metabolic acidosis from renal failure 1
• After return of spontaneous circulation following cardiopulmonary resuscitation 3

Management Based on ABG Results

Acute Hypercapnic Respiratory Failure

• Initiate non-invasive ventilation (NIV) when pH <7.35 and PaCO2 >6.5 kPa (49 mmHg) persist despite optimal medical therapy 5, 1
• Target SpO2 88-92% with controlled oxygen therapy in COPD and all causes of acute hypercapnic respiratory failure 5, 1
• Repeat ABG after each oxygen titration to monitor for worsening hypercapnia 1
• A rise in PaCO2 >1 kPa (7.5 mmHg) during oxygen titration indicates clinically unstable disease requiring reassessment 5

Severe Metabolic Acidosis

• Consider sodium bicarbonate therapy only for severe acidosis (arterial pH <7.1 and base deficit <10) 5, 2
• The best method of reversing acidosis is to restore spontaneous circulation and treat the underlying cause 5
• Monitor lactate levels as they provide information about tissue oxygenation and perfusion; elevated levels indicate shock 2

Technical Considerations for ABG Sampling

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries 5, 2
• Use local anesthesia for all ABG specimens except in emergencies 5, 2
• For critically ill patients or those with shock/hypotension, obtain arterial samples rather than capillary samples 5, 2
• Capillary blood gases can be used for measuring PaCO2 and pH during oxygen titration, but PO2 is less accurate (underestimates by 0.5-1 kPa) 5
• Consent patients for the procedure with discussion of possible risks 5

Common Pitfalls to Avoid

• Never assume normal pulse oximetry rules out acid-base disturbances - a patient can have normal SpO2 with severe metabolic acidosis or hypercapnia if on supplemental oxygen 5, 3
• Arterial blood gas analysis alone may be misleading - even simultaneous arterial and mixed venous samples may not accurately reflect myocardial and cerebral intracellular acid-base status 5
• In patients on ECMO, obtain ABG from right radial arterial line as this best represents cerebral perfusion 3
• Blood gases and full blood count are required in all situations where these measurements may affect patient outcomes, as pulse oximetry cannot detect anemia 5