How do I interpret an arterial blood gas in detail?

Arterial Blood Gas Interpretation: A Systematic Approach

Use a three-step systematic method to interpret ABG results: 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 base excess/bicarbonate to identify the metabolic component. 1, 2

Step 1: Evaluate pH Status

• pH < 7.35 indicates acidemia 1, 2
• pH > 7.45 indicates alkalemia 1, 2
• pH 7.35-7.45 is normal but examine other parameters for compensated disorders 3, 4

Step 2: Identify the Respiratory Component

• Examine PaCO2 to determine respiratory contribution 1, 2
• Normal PaCO2 is 35-45 mmHg 5
• Respiratory acidosis: PaCO2 elevated (>45 mmHg) with decreased pH 6
• Respiratory alkalosis: PaCO2 decreased (<35 mmHg) with increased pH 6
• Use the "Respiratory opposite" principle: if pH and PaCO2 move in opposite directions, the primary disorder is respiratory 4

Step 3: Identify the Metabolic Component

• Evaluate bicarbonate (HCO3) and base excess to identify metabolic contribution 1, 2
• Normal HCO3 is 22-26 mmol/L 5
• Use the "Metabolic equal" principle: if pH and HCO3 move in the same direction, the primary disorder is metabolic 4
• Metabolic acidosis: HCO3 < 22 mmol/L with decreased pH 3
• Metabolic alkalosis: HCO3 > 26 mmol/L with increased pH 3

Step 4: Calculate Delta Ratio for Mixed Disorders (When Anion Gap Elevated)

• Calculate delta ratio as (Anion Gap - 12) / (24 - HCO3) 1
• Delta ratio < 1 suggests concurrent normal anion gap metabolic acidosis 1
• Delta ratio > 2 suggests concurrent metabolic alkalosis 1
• This step is essential in critically ill patients where multiple pathophysiologic processes may coexist 1

Assessment of Oxygenation Status

• Check PaO2, with normal values >90 mmHg on room air at sea level 1
• Severe hypoxemia is PaO2 <60 mmHg, requiring immediate intervention 1
• Normal arterial oxygen saturation is >94% 1
• Calculate P(A-a)O2 gradient, with normal values <15 mmHg (or <20 mmHg if age ≥65 years) 1

Critical Indications for ABG Testing

• All critically ill patients require ABG testing to assess oxygenation, ventilation, and acid-base status 1, 2, 6
• Patients with shock or hypotension must have initial blood gas from an arterial sample 1, 2, 6
• SpO2 fall below 94% on room air or supplemental oxygen 6
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 6
• Suspected metabolic conditions such as diabetic ketoacidosis or metabolic acidosis due to renal failure 6

Management Based on ABG Results

Respiratory Acidosis Management

• Initiate non-invasive ventilation for pH < 7.35 and PaCO2 > 6.5 kPa (49 mmHg) despite optimal medical therapy 1, 2
• Start oxygen at 1 L/min and titrate up in 1 L/min increments until SpO2 >90% 1, 6
• Use controlled oxygen therapy targeting SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 1, 2
• For severe respiratory acidosis unresponsive to NIV, consider mechanical ventilation 6

Oxygen Therapy Monitoring

• Repeat ABG within 60 minutes after starting or changing oxygen therapy in patients at risk for CO2 retention 1, 6
• After each titration of oxygen flow rate in patients with baseline hypercapnia, perform ABG analysis 1, 2, 6
• A rise in PaCO2 > 1 kPa (7.5 mmHg) indicates clinically unstable disease requiring further medical optimization 1, 6

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 1, 2, 6
• Use local anesthesia for all ABG specimens except in emergencies 2, 6
• Obtain informed consent with discussion of possible risks 1
• Either arterial or venous blood may be used for carboxyhemoglobin measurement, as levels are similar 7

Alternative Sampling Methods

• Capillary blood gases can replace ABG for re-measuring PaCO2 and pH during oxygen titration 1
• Cutaneous capnography can replace ABG for re-measuring PaCO2 alone but not pH 1
• For non-critical patients, arterialized earlobe blood gases may be used to measure acid-base status and ventilation 1, 6

Common Pitfalls to Avoid

• A normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia 1, 2, 6
• Failing to repeat ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention, is a common management error 1, 2, 6
• Standard pulse oximeters using two wavelengths cannot differentiate carboxyhemoglobin from oxyhemoglobin 7
• Older blood gas machines without CO oximeters may calculate oxygen saturation from PaO2 and pH, missing elevated carboxyhemoglobin levels 7
• Blood pH has a potent effect on pulmonary vascular tone; awareness of arterial blood gas measurements during procedures is critical for accurate interpretation 7

Compensation Patterns

• Uncompensated: pH abnormal, only one system (respiratory or metabolic) abnormal 4
• Partially compensated: pH abnormal, both PaCO2 and HCO3 abnormal 4
• Fully compensated: pH normal (7.35-7.45), both PaCO2 and HCO3 abnormal 4