Arterial Blood Gas (ABG) - Easy Guide

What is an ABG and Why Does it Matter?

• Heart: impaired pumping, dangerous arrhythmias, reduced response to adrenaline
• Lungs: exhausted respiratory muscles
• Metabolism: insulin resistance, high potassium, reduced energy production (ATP)
• Brain: confusion, coma

• Heart: arterial constriction, reduced coronary blood flow, arrhythmias
• Electrolytes: low potassium, low calcium and magnesium (causing muscle cramps, tingling, spasms)

The 4 Disorders - Simple Summary

pH < 7.35 = ACIDOSIS (too acid)
pH > 7.45 = ALKALOSIS (too alkaline)

1. Respiratory Acidosis

• What: Lungs can't blow off enough CO₂ → CO₂ builds up → acid rises
• ABG: pH ↓, PaCO₂ ↑
• Common causes: COPD, pneumonia, asthma, drug overdose (opioids/sedatives), Guillain-Barré, obesity hypoventilation
• The body compensates: Kidneys retain HCO₃⁻ (takes 48-96 hours)

2. Respiratory Alkalosis

• What: Lungs blow off too much CO₂ (hyperventilation) → acid drops
• ABG: pH ↑, PaCO₂ ↓
• Common causes: Anxiety/panic, pain, fever, sepsis, pregnancy, salicylate toxicity, pulmonary embolism
• Watch out: Causes low ionized calcium → tingling lips, hand spasms (carpal spasm), syncope
• The body compensates: Kidneys excrete HCO₃⁻ (takes 48-96 hours)

3. Metabolic Acidosis

• What: Too much acid produced OR too much bicarbonate lost → pH falls
• ABG: pH ↓, HCO₃⁻ ↓
• Common causes (use the mnemonic MUDPILES for high anion gap):

• The body compensates: Lungs hyperventilate to blow off CO₂ (within 12-24 hours). Use Winter's formula to check: PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2

4. Metabolic Alkalosis

• What: Too much bicarbonate gained OR too much acid lost → pH rises
• ABG: pH ↑, HCO₃⁻ ↑
• Common causes:
  • Chloride-responsive (urine Cl⁻ < 25): vomiting, nasogastric suction, diuretics, diarrhea (chloride-wasting)
  • Chloride-unresponsive (urine Cl⁻ > 40): hyperaldosteronism, Cushing's, severe hypomagnesemia, steroids
• The body compensates: Lungs hypoventilate to retain CO₂ (PaCO₂ = 0.7 × HCO₃⁻ + 20 ± 5)

How to Interpret an ABG - 4 Simple Steps

• pH < 7.35 = Acidosis
• pH > 7.45 = Alkalosis

• pH ↓ + PaCO₂ ↑ = Respiratory Acidosis
• pH ↓ + HCO₃⁻ ↓ = Metabolic Acidosis
• pH ↑ + PaCO₂ ↓ = Respiratory Alkalosis
• pH ↑ + HCO₃⁻ ↑ = Metabolic Alkalosis

Treatment

Metabolic Acidosis Treatment

• DKA → insulin + fluids
• Lactic acidosis → fix the cause of poor perfusion
• Salicylate toxicity → gastric lavage, activated charcoal, urinary alkalinization, hemodialysis in severe cases
• If pH < 7.10 and cause not quickly correctable → consider sodium bicarbonate (NaHCO₃)
• Renal failure → dialysis or ultrafiltration with alkali supplementation

Respiratory Acidosis Treatment

• Fix the lungs - treat the underlying cause (antibiotics for pneumonia, bronchodilators for COPD/asthma, corticosteroids)
• Oxygen via nasal cannula or Venturi mask
• If severe (hypercapnic encephalopathy or hemodynamic instability) → non-invasive ventilation (BiPAP/CPAP) first, then intubation and mechanical ventilation if that fails

Metabolic Alkalosis Treatment

• Chloride-responsive: Give isotonic saline (IV fluids) + withhold diuretics; replace potassium (KCl) if low
• Chloride-unresponsive: Treat the cause (e.g., remove aldosterone source); consider acetazolamide, HCl secretion inhibitors (omeprazole if due to gastric drainage), or ultrafiltration
• Congestive heart failure with alkalosis → acetazolamide or ultrafiltration with isotonic saline replacement

Respiratory Alkalosis Treatment

• Treat the underlying cause (reassure anxious patient, treat pain/fever/sepsis)
• Hyperventilation on a ventilator → reduce respiratory rate or add dead space
• The alkalosis itself resolves as cause is fixed; do NOT use CO₂ rebreathing (paper bag) routinely - it can be dangerous

Treatment Flowcharts (from Comprehensive Clinical Nephrology)

Quick Reference Cheat Sheet