What are Arterial Blood Gases (ABGs)?

Arterial Blood Gas (ABG) Analysis: A Comprehensive Guide

What is an ABG?

Arterial blood gas analysis is a diagnostic test that measures oxygen (PaO2), carbon dioxide (PaCO2), and pH levels in arterial blood to assess respiratory function, oxygenation status, and acid-base balance in the body. 1, 2, 3

ABG analysis directly measures three core parameters: pH (hydrogen ion content), PaO2 (blood oxygen tension), and PaCO2 (carbon dioxide tension), while calculating bicarbonate (HCO3) and base excess using the Hasselbach equation. 3, 4

Systematic Interpretation Approach

Use a 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: Assess pH Status

• pH < 7.35 indicates acidemia 2
• pH > 7.45 indicates alkalemia 2

Step 2: Identify Respiratory Component

• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 2
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 2

Step 3: Identify Metabolic Component

• Base excess < -2 or HCO3 < 22 indicates metabolic acidosis 2
• Base excess > +2 or HCO3 > 26 indicates metabolic alkalosis 2

Primary Indications for ABG Testing

All critically ill patients require ABG testing to assess oxygenation, ventilation, and acid-base status. 1, 2, 5

Mandatory Situations

• Patients with shock or hypotension must have initial blood gas measurement from an arterial sample 1, 5
• Oxygen saturation fall below 94% on room air or supplemental oxygen 5
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 5

Metabolic Indications

• Suspected diabetic ketoacidosis 2
• Metabolic acidosis from renal failure, trauma, shock, and sepsis 2
• Any breathlessness with risk of metabolic conditions 5

COPD and Hypercapnic Risk Patients

• Check ABG when starting oxygen in COPD patients, especially with known CO2 retention 1, 2, 5
• Perform ABG within 60 minutes of starting oxygen therapy and within 60 minutes of a change in inspired oxygen concentration in COPD patients 5
• After each titration of oxygen flow rate in patients with baseline hypercapnia, perform ABG analysis 1, 5

Management of Abnormal 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 with CPAP 4-8 cmH2O plus pressure support 10-15 cmH2O for NIV 2
• Monitor for worsening ABG/pH in 1-2 hours on NIV 2

Oxygen Titration Protocol

• For patients with COPD or risk factors for hypercapnic respiratory failure, start with low flow oxygen (1 L/min) and titrate up in 1 L/min increments until SpO2 >90%, then confirm with repeat ABG 5
• Repeat ABG after oxygen titration is complete to confirm adequate oxygenation without precipitating respiratory acidosis 1, 5

Intubation Criteria

• Lack of improvement after 4 hours of NIV 2
• Respiratory rate >35 breaths/min 2
• PCO2 rises >1 kPa (7.5 mmHg) despite NIV 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. 1, 5

• Use local anesthesia for all ABG specimens except in emergencies 1, 5
• Either arterial or venous blood may be used for COHb measurement, as the COHb levels are similar, provided the CO body stores are in near equilibrium 6
• For most non-critical patients, either arterial blood gases or arterialized earlobe blood gases may be used to measure acid-base status and ventilation 5

Critical Pitfalls to Avoid

A normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia. 1, 5

Common Errors

• Pulse oximetry will be normal in patients with normal oxygen levels but abnormal acid-base status or ventilation, or with low blood oxygen content due to anemia 5
• Standard pulse oximeters using two wavelengths (660 and 990 nm) cannot differentiate carboxyhemoglobin from oxyhemoglobin, resulting in falsely reassuring SpO2 readings in CO poisoning 6
• Older blood gas machines without CO oximeters calculate oxygen saturation from PaO2 and pH, potentially reporting 97-98% saturation even when 40% of hemoglobin is bound with CO 6
• Failing to repeat ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention 1, 5
• Overlooking the need for ABG in patients with metabolic conditions that may cause acid-base disturbances 5

Special Population Considerations

• For home oxygen assessment, obtain two ABG measurements at least 3 weeks apart during clinical stability 1
• For hepatopulmonary syndrome diagnosis in patients aged ≥ 65 years, use P(A-a)O2 ≥ 20 mmHg cutoff instead of ≥ 15 mmHg 2
• Patients who develop respiratory acidosis (rise in PaCO2 >1 kPa or 7.5 mm Hg) during oxygen therapy may have clinically unstable disease and should undergo further medical optimization 5