How is an Arterial Blood Gas (ABG) result computed and interpreted?

ABG Computation and Interpretation

Systematic Three-Step Approach to ABG Analysis

Use a systematic three-step 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
• pH > 7.45 indicates alkalemia 1
• This first step determines the primary direction of the acid-base disturbance 1

Step 2: Assess the Respiratory Component (PaCO2)

• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 1
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 1
• Use the "RoMe" technique: Respiratory disorders move pH in the opposite direction of PaCO2 3

Step 3: Assess the Metabolic Component (HCO3/Base Excess)

• Base excess < -2 or HCO3 < 22 mEq/L indicates metabolic acidosis 1
• Base excess > +2 or HCO3 > 26 mEq/L indicates metabolic alkalosis 1
• Metabolic disorders move pH in the equal direction as HCO3 3

Primary Indications for ABG Testing

All critically ill patients require ABG testing to assess oxygenation, ventilation, and acid-base status 1, 2, 4.

Specific Clinical Scenarios Requiring ABG:

• Shock or hypotension - initial blood gas must be from arterial sample 1, 4
• SpO2 fall below 94% on room air or supplemental oxygen 1, 4
• Deteriorating oxygen saturation (≥3% fall) or increasing breathlessness in patients with previously stable chronic hypoxemia 4
• Suspected diabetic ketoacidosis or metabolic acidosis from renal failure, trauma, shock, or sepsis 1, 4
• COPD patients starting oxygen therapy, especially with known CO2 retention 1, 2, 4

Management Based on ABG Results

Acute Hypercapnic Respiratory Failure

Initiate non-invasive ventilation (NIV) for pH < 7.35 and PaCO2 > 6.5 kPa (49 mmHg) despite optimal medical therapy 1, 2.

• Use controlled oxygen therapy targeting SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 1, 2
• Start oxygen at 1 L/min and titrate up in 1 L/min increments until SpO2 >90% 5, 4
• Perform ABG after each titration to monitor for worsening hypercapnia 1, 2, 4

Oxygen Titration Protocol

• Initiate at 1 L/min flow rate 5, 4
• Titrate up in 1 L/min increments until SpO2 >90% 5, 4
• Confirm target PaO2 ≥ 60 mmHg (8 kPa) with repeat ABG 5, 2
• Perform ABG within 60 minutes of starting oxygen or changing FiO2 in COPD patients 4

Monitoring Hypercapnia During Oxygen Therapy

Patients with baseline hypercapnia must have ABG monitoring after each flow rate titration 5, 2, 4.

• Rise in PaCO2 > 1 kPa (7.5 mmHg) indicates clinically unstable disease requiring further medical optimization and reassessment after 4 weeks 5, 4
• Patients developing respiratory acidosis on two repeated occasions while clinically stable should only receive domiciliary oxygen with nocturnal ventilatory support 5, 4

Technical Considerations for ABG Sampling

Pre-Procedure Assessment

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries 5, 2, 4
• Obtain informed consent with discussion of possible risks 5, 4
• Use local anesthesia for all ABG specimens except in emergencies 2, 4

Timing of Repeat Measurements

• For long-term oxygen therapy (LTOT) assessment, obtain two ABG measurements at least 3 weeks apart during clinical stability 5, 2
• Perform repeat ABG after oxygen titration is complete to confirm adequate oxygenation without precipitating respiratory acidosis 5, 2, 4

Critical Pitfalls to Avoid

A normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia 2, 4.

• Pulse oximetry will be normal in patients with abnormal acid-base status or ventilation if oxygen levels are adequate 4
• Failing to repeat ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention, is a common management error 2, 4
• Never rely solely on pulse oximetry when acid-base status and ventilation need assessment 4
• Patients on supplemental oxygen can have normal SpO2 despite severe metabolic or respiratory acidosis 4

Alternative Sampling Methods

• Capillary blood gases (CBG) can replace ABG for re-measuring PaCO2 and pH during oxygen titration 5
• Cutaneous capnography can replace ABG for re-measuring PaCO2 alone but not pH 5
• For non-critical patients, arterialized earlobe blood gases may be used to measure acid-base status and ventilation 4