How do you interpret an Arterial Blood Gas (ABG) result?

How to Interpret an Arterial Blood Gas (ABG) Result

Use a systematic three-step approach: first evaluate pH to determine acidemia (pH < 7.35) or alkalemia (pH > 7.45), then examine PaCO2 to identify the respiratory component, and finally evaluate bicarbonate/base excess to identify the metabolic component. 1

Systematic Interpretation Method

Step 1: Evaluate the pH

• pH < 7.35 = Acidemia 1
• pH > 7.45 = Alkalemia 1
• pH 7.35-7.45 = Normal (but may still have compensated disorders) 2

Step 2: Identify the Respiratory Component

• Examine PaCO2 to determine if respiratory processes are contributing 1
• PaCO2 > 45 mmHg = Respiratory acidosis 2
• PaCO2 < 35 mmHg = Respiratory alkalosis 2
• Use the "RoMe" technique: Respiratory moves opposite to pH (high CO2 = low pH), while Metabolic moves equal to pH (low HCO3 = low pH) 3

Step 3: Identify the Metabolic Component

• Examine bicarbonate (HCO3) and base excess/deficit 1
• HCO3 < 22 mEq/L or base deficit > -4 mEq/L = Metabolic acidosis 4
• HCO3 > 26 mEq/L = Metabolic alkalosis 2

Step 4: Assess Oxygenation Status

• PaO2 < 60 mmHg = Hypoxemia requiring intervention 5
• Target PaO2 ≥ 60 mmHg (8 kPa) on supplemental oxygen 5
• Remember: Normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia 1, 6

Critical Clinical Applications

When to Obtain ABG

• All critically ill patients including those with shock, sepsis, major trauma, or anaphylaxis 6
• Unexpected SpO2 fall below 94% in patients breathing room air or oxygen 6
• Any patient with risk factors for hypercapnic respiratory failure who develops acute breathlessness, drowsiness, or deteriorating oxygen saturation 6
• Within 60 minutes of starting oxygen therapy in COPD patients or those at risk for CO2 retention 6
• After return of spontaneous circulation following cardiopulmonary resuscitation 7

Management Based on ABG Results

For Hypercapnic Respiratory Failure:

• Initiate non-invasive ventilation (NIV) when pH < 7.35 and PaCO2 > 49 mmHg (6.5 kPa) despite optimal medical therapy 1
• Use controlled oxygen therapy targeting SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 1

For Patients with Baseline Hypercapnia:

• Monitor for respiratory acidosis and worsening hypercapnia with ABG after each oxygen titration 5
• If PaCO2 rises > 7.5 mmHg (1 kPa) during oxygen titration, the patient may have clinically unstable disease requiring further medical optimization 5

Technical Considerations

Proper Sampling Technique

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries 5, 1
• Use local anesthesia for all ABG specimens except in emergencies 1, 6
• Obtain consent with discussion of possible risks 5
• Prefer arterial samples over venous sources in patients with shock, hypotension, or on vasopressor therapy 7

Timing of Repeat ABG

• Recheck ABG after oxygen titration is complete to confirm adequate oxygenation without precipitating respiratory acidosis 5
• For home oxygen assessment, obtain two ABG measurements at least 3 weeks apart during clinical stability 5

Common Pitfalls to Avoid

Critical Interpretation Errors

• Normal pulse oximetry does not exclude significant acid-base disturbances - SpO2 will appear normal in patients with abnormal pH or PCO2 1, 7, 6
• In carbon monoxide poisoning, pulse oximetry readings may be falsely normal - always obtain ABG regardless of oximeter readings 6
• Failing to repeat ABG after changes in oxygen therapy is a common management error, especially in patients at risk for CO2 retention 1

Clinical Context Matters

• Metabolic acidosis (pH < 7.35) at presentation predicts higher mortality in acute pancreatitis and other critical illnesses 4
• Bicarbonate < 22 mEq/L and base deficit > -4 mEq/L are useful early markers for predicting adverse outcomes 4
• ABG provides critical information about PaO2, PaCO2, and pH that cannot be obtained through pulse oximetry alone 7