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Arterial Blood Gas (ABG) - Comprehensive Overview

Normal ABG Values

• pH < 7.35 = acidemia; pH > 7.45 = alkalemia
• An arterial sample is the gold standard - it reflects the blood after pulmonary circulation, before any peripheral gas exchange.

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Henderson-Hasselbalch Equation

pH = pK + log ([HCO3-] / [0.03 × PaCO2])

• Lungs regulate CO2 (fast, minutes)
• Kidneys regulate HCO3- (slow, hours to days)

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Step-by-Step ABG Interpretation

Step 1 - Assess the pH

Step 2 - Identify the Primary Disorder

If pH is normal but PaCO2 and HCO3- are both abnormal in opposite directions, a combined/mixed disorder is likely. (Washington Manual, p. 467)

Step 3 - Check Compensation

Winter's Formula (for metabolic acidosis): Expected PaCO2 = (1.5 × HCO3-) + 8 ± 2 (Roberts and Hedges' Clinical Procedures in Emergency Medicine)

Step 4 - Calculate the Anion Gap (if metabolic acidosis is present)

AG = Na+ - (Cl- + HCO3-) Normal AG = 8-12 mEq/L (some labs use 10 ± 2)

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The 4 Primary Acid-Base Disorders

1. Metabolic Acidosis

• Definition: Primary decrease in HCO3-, pH falls
• Compensation: Hyperventilation (lowers PaCO2)
• Two subtypes by anion gap:

High Anion Gap (AG > 12-15) - Mnemonic: MUDPILES

Normal Anion Gap (Hyperchloremic) - Mnemonic: HARDUP

Osmolar gap tip: If high AG acidosis + osmolar gap → suspect methanol or ethylene glycol poisoning. High molecular weight acids (lactate, ketoacids, salicylate) typically do NOT produce osmolar gaps.

Delta-Delta Ratio (for high AG metabolic acidosis)

• Checks for a hidden concurrent metabolic alkalosis or normal AG acidosis
• Delta-Delta = (AG - 12) / (24 - HCO3-)
• < 0.4: pure non-AG acidosis
• 0.4-0.8: mixed AG + non-AG acidosis
• 1-2: pure AG metabolic acidosis

• 2: AG metabolic acidosis + metabolic alkalosis

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2. Metabolic Alkalosis

• Definition: Primary rise in HCO3-, pH rises
• Compensation: Hypoventilation (raises PaCO2)
• Causes divided by urine chloride response:

• Vomiting, NG suction
• Chloride-wasting diarrhea
• Villous adenoma
• Diuretic use

• Primary hyperaldosteronism
• Cushing syndrome
• Bartter / Gitelman syndrome
• Severe hypokalemia

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3. Respiratory Acidosis

• Definition: Primary rise in PaCO2 (hypoventilation), pH falls
• Compensation: Renal HCO3- retention (takes 3-5 days for chronic)
• Causes: COPD exacerbation, neuromuscular disease, opioid overdose, chest wall disorders, severe pneumonia, upper airway obstruction

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4. Respiratory Alkalosis

• Definition: Primary fall in PaCO2 (hyperventilation), pH rises
• Compensation: Renal HCO3- excretion
• Causes: Anxiety, pain, fever, PE, salicylate toxicity (early), pregnancy, liver failure, iatrogenic hyperventilation

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ABG vs Venous Blood Gas (VBG)

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Important Clinical Pearls

1. Pulse oximetry cannot detect hypoventilation in patients on supplemental oxygen - the oxyhemoglobin dissociation curve's flat upper portion means PaO2 can fall significantly with rising PaCO2 while SpO2 remains normal. An ABG or capnography is required in that setting.
2. pH normal with abnormal PaCO2 and HCO3- = always suspect a mixed disorder.
3. Lactic acidosis is the most common cause of high AG metabolic acidosis (~50% of cases).
4. Bicarbonate therapy in metabolic acidosis: commonly used to target pH > 7.10 in severe cases, or > 7.20 with concurrent AKI. Optimal use remains debated.
5. Acute vs. Chronic respiratory disorders: renal compensation takes days, so acute respiratory acidosis will show a more abnormal pH (less HCO3- compensation) than the same degree of chronic hypercapnia.

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