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ABG Analysis: From Basic to Clinical

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PART 1 - What Is an ABG?

"By using the combination of an arterial blood gas sample and serum electrolyte levels, a patient's acid-base status can be evaluated." - Roberts and Hedges' Clinical Procedures in Emergency Medicine

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PART 2 - Normal Values (Memorize These)

• pH < 7.35 = Acidosis
• pH > 7.45 = Alkalosis
• PaCO2 up = More acid (respiratory)
• HCO3- down = More acid (metabolic)

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PART 3 - The Henderson-Hasselbalch Equation (The Foundation)

pH = 6.1 + log [HCO3-] / (0.03 × PaCO2)

• CO2 = respiratory acid (controlled by lungs, minute-to-minute)
• HCO3- = metabolic base (controlled by kidneys, over hours to days)

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PART 4 - The 4 Primary Disorders

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PART 5 - Compensation Formulas (The Numbers)

Metabolic Disorders (Respiratory Compensation - Fast, minutes-hours)

Respiratory Disorders (Metabolic Compensation - Slow, 3-5 days)

Important: "Respiratory compensation for metabolic disorders is more rapid and occurs through a change in the respiratory rate... Metabolic compensation for a respiratory disturbance requires renal adjustment of HCO3- and can take 3 to 5 days." - Schwartz's Principles of Surgery

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PART 6 - Step-by-Step ABG Interpretation (The 5-Step Method)

Step 1 - Is the patient acidemic or alkalemic?

• pH < 7.35 → Acidosis
• pH > 7.45 → Alkalosis
• pH 7.35-7.45 → Could still have a disorder (look at CO2 and HCO3-)

Step 2 - What is the primary disorder?

• Acidosis + PaCO2 ↑ → Respiratory Acidosis
• Acidosis + HCO3- ↓ → Metabolic Acidosis
• Alkalosis + PaCO2 ↓ → Respiratory Alkalosis
• Alkalosis + HCO3- ↑ → Metabolic Alkalosis

If the pH is normal but CO2 and HCO3- are both abnormal, there is likely a mixed disorder.

Step 3 - Is there appropriate compensation?

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

Anion Gap (AG) = Na+ - (Cl- + HCO3-)
Normal = 8-12 mEq/L (or up to 16 if not albumin-corrected)

• AG > 12 = High AG metabolic acidosis (MUDPILES mnemonic below)
• AG normal (8-12) = Non-AG (hyperchloremic) metabolic acidosis (HARDUP mnemonic below)

Step 5 - Assess oxygenation

• PaO2 < 80 mmHg = Hypoxemia
• Check the P/F ratio = PaO2 / FiO2
  • Normal: > 400
  • Mild ARDS: 200-300
  • Moderate ARDS: 100-200
  • Severe ARDS: < 100

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PART 7 - Anion Gap Acidosis: The MUDPILES Causes

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PART 8 - Non-Anion Gap (Normal AG) Acidosis: HARDUP Causes

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PART 9 - The Delta-Delta (ΔΔ) Ratio (Advanced: Detecting Mixed Disorders)

ΔAG = Calculated AG - 12  (rise in AG)
ΔHCO3- = 24 - Measured HCO3-  (fall in HCO3-)

Delta-Delta ratio = ΔAG / ΔHCO3-

"In pure AG metabolic acidosis, the decrease in serum HCO3- should be roughly the same as the ΔAG (i.e., ratio of ΔAG to ΔHCO3- is roughly 1:1)." - Frameworks for Internal Medicine

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PART 10 - The Acid-Base Map

• Zone 1 (pink/red, high pH + high PaCO2): Mixed respiratory and metabolic acidosis
• Zone 2 (purple, low pH + low PaCO2): Mixed respiratory and metabolic alkalosis
• Zone 3 (tan, high pH + high PaCO2): Metabolic alkalosis + respiratory acidosis
• Zone 4 (gold, low pH + low PaCO2): Metabolic acidosis + respiratory alkalosis
• N = Normal point (pH 7.40, PaCO2 40)

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PART 11 - Common Causes of Each Disorder (Clinical Scenarios)

Respiratory Acidosis (PaCO2 ↑)

• COPD exacerbation, severe asthma
• Opioid/sedative overdose (respiratory depression)
• Neuromuscular disease (Guillain-Barre, myasthenia crisis)
• Obesity hypoventilation syndrome

Respiratory Alkalosis (PaCO2 ↓)

• Anxiety/hyperventilation
• Sepsis (early), pregnancy
• High altitude
• Salicylate poisoning (early phase - stimulates the respiratory center)
• Hepatic encephalopathy

Metabolic Acidosis (HCO3- ↓)

• Lactic acidosis (sepsis, shock, ischemia)
• DKA
• Renal failure (uremia)
• Diarrhea
• Toxic ingestions (methanol, ethylene glycol, salicylates)

Metabolic Alkalosis (HCO3- ↑)

• Vomiting (loss of HCl)
• NG suction
• Diuretics (loop and thiazides - volume contraction alkalosis)
• Hyperaldosteronism
• Exogenous bicarbonate or antacid overuse

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PART 12 - Worked Clinical Examples

Example 1 - Diarrhea (from Roberts & Hedges)

58-year-old woman, 1 week of profuse diarrhea. Na+ 133, K+ 2.8, pH 7.26, Cl- 118, PaCO2 13, HCO3- 5.

1. pH 7.26 → Acidosis
2. HCO3- 5 (low), PaCO2 13 (low, not elevated) → Metabolic Acidosis
3. AG = 133 - (118 + 5) = 10 → Normal AG → Non-AG metabolic acidosis (diarrhea causing HCO3- loss)
4. Winter's formula: Expected PaCO2 = 1.5(5) + 8 = 15.5 ± 2. Actual PaCO2 = 13. Within range → Appropriate respiratory compensation only

Example 2 - Sepsis/Respiratory Alkalosis (from Roberts & Hedges)

74-year-old, septic shock, E. coli bacteremia. pH 7.49, PaCO2 25, HCO3- 22.

1. pH 7.49 → Alkalosis
2. PaCO2 25 (low) → Respiratory Alkalosis
3. Expected HCO3- decrease: 2 mEq/L per 10 mmHg drop in PaCO2. Drop = 15 mmHg → expected ↓HCO3- = 3 mEq/L → expected HCO3- = 24 - 3 = 21 mEq/L. Actual = 22 → Appropriate compensation
4. Cause: Sepsis drives hyperventilation via inflammatory cytokines stimulating the respiratory center

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PART 13 - Quick Reference: ABG Interpretation Cheat Sheet

Step 1: pH → Acidosis (<7.35) or Alkalosis (>7.45)?
Step 2: CO2 or HCO3-? → Respiratory or Metabolic?
Step 3: Does compensation match the formula? → Simple or Mixed?
Step 4: If Met Acidosis → Calculate AG = Na - (Cl + HCO3-)
         AG > 12 → MUDPILES
         AG normal → HARDUP
Step 5: If High AG → Calculate Delta-Delta ratio
Step 6: PaO2 and P/F ratio → Assess oxygenation

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Sources

• Roberts and Hedges' Clinical Procedures in Emergency Medicine, Chapter on Acid-Base Balance
• Schwartz's Principles of Surgery, 11th Edition, Chapter 3
• Frameworks for Internal Medicine, Chapter 32
• Tintinalli's Emergency Medicine, ABG Analysis section
• Fishman's Pulmonary Diseases and Disorders, 2-Volume Set

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🫁 ABG Analysis: Simplified from Basic to Clinical

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PART 1 - What Is an ABG?

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PART 2 - Normal ABG Values (Memorize These First)

Simple rule to remember:

• pH low = Acidosis
• pH high = Alkalosis
• CO2 up = more acid (respiratory)
• HCO3- down = more acid (metabolic)

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PART 3 - The Core Concept: CO2 vs HCO3-

        pH (balance point)
       /                  \
  CO2 (acid)          HCO3- (base)
  Lungs control        Kidneys control
  (fast - minutes)     (slow - days)

• When CO2 rises → blood becomes more acidic (respiratory acidosis)
• When HCO3- falls → blood becomes more acidic (metabolic acidosis)
• The body always tries to compensate by adjusting the other side

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PART 4 - The 4 Primary Acid-Base Disorders

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PART 5 - Compensation Formulas

Metabolic Disorders (Lungs compensate - Fast)

Respiratory Disorders (Kidneys compensate - Slow, 3-5 days)

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PART 6 - The 5-Step ABG Interpretation Method

✅ Step 1 - Look at the pH

• pH < 7.35 → Acidosis
• pH > 7.45 → Alkalosis
• pH normal but CO2 and HCO3- both abnormal → likely mixed disorder

✅ Step 2 - Identify the Primary Disorder

• Acidosis + CO2 ↑ → Respiratory Acidosis
• Acidosis + HCO3- ↓ → Metabolic Acidosis
• Alkalosis + CO2 ↓ → Respiratory Alkalosis
• Alkalosis + HCO3- ↑ → Metabolic Alkalosis

✅ Step 3 - Check Compensation

• If YES → Simple single disorder
• If NO (too much or too little) → Mixed disorder

✅ Step 4 - Calculate Anion Gap (if metabolic acidosis)

Anion Gap = Na+ - (Cl- + HCO3-)
Normal = 8 - 12 mEq/L

• AG > 12 → High AG metabolic acidosis → MUDPILES causes
• AG normal → Non-AG (hyperchloremic) acidosis → HARDUP causes

✅ Step 5 - Assess Oxygenation

• PaO2 < 80 mmHg = Hypoxemia
• P/F Ratio = PaO2 ÷ FiO2

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PART 7 - Anion Gap Causes: MUDPILES

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PART 8 - Non-Anion Gap Causes: HARDUP

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PART 9 - Delta-Delta Ratio (Detect Hidden Mixed Disorders)

Delta Ratio = ΔAG / ΔHCO3-

ΔAG = Calculated AG - 12
ΔHCO3- = 24 - Measured HCO3-

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PART 10 - The Acid-Base Map

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PART 11 - Clinical Causes at a Glance

Respiratory Acidosis (CO2 ↑)

• COPD exacerbation, severe asthma
• Opioid/sedative overdose
• Neuromuscular failure (Guillain-Barre, MG crisis)
• Obesity hypoventilation

Respiratory Alkalosis (CO2 ↓)

• Anxiety / panic attack
• Sepsis (early), pregnancy
• High altitude
• Salicylate poisoning (early)

Metabolic Acidosis (HCO3- ↓)

• Sepsis/shock → lactic acidosis
• DKA
• Renal failure
• Diarrhea, toxic ingestions

Metabolic Alkalosis (HCO3- ↑)

• Vomiting / NG suction
• Diuretics (loop, thiazides)
• Hyperaldosteronism
• Antacid overuse

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PART 12 - Worked Clinical Examples

Case 1 - COPD Patient in Respiratory Distress

pH 7.28, PaCO2 72, HCO3- 33, PaO2 52

1. pH 7.28 → Acidosis
2. PaCO2 72 (↑) → Respiratory Acidosis
3. HCO3- 33 - check chronic compensation: ↑ by 4 per 10 mmHg CO2 rise = 4 × 3.2 = ~13 → expected HCO3- = 24 + 13 = 37. Actual is 33 - slightly less → mild concurrent metabolic acidosis possible
4. PaO2 52 → Hypoxemia

• Diagnosis: Chronic respiratory acidosis with acute-on-chronic exacerbation + hypoxemia

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Case 2 - Diabetic in ER

pH 7.15, PaCO2 20, HCO3- 7, Na 138, Cl 100

1. pH 7.15 → Acidosis
2. HCO3- 7 (↓), PaCO2 not elevated → Metabolic Acidosis
3. Winter's: Expected PaCO2 = 1.5(7) + 8 = 18.5 ± 2. Actual = 20 → appropriate compensation
4. AG = 138 - (100 + 7) = 31 → High AG metabolic acidosis
5. Delta-Delta: ΔAG = 31-12 = 19; ΔHCO3- = 24-7 = 17; ratio = 19/17 = 1.1 → Pure high AG

• Diagnosis: DKA with pure high anion gap metabolic acidosis

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Case 3 - Post-Vomiting Patient

pH 7.52, PaCO2 48, HCO3- 38

1. pH 7.52 → Alkalosis
2. HCO3- 38 (↑) → Metabolic Alkalosis
3. Expected PaCO2 = 0.7(38) + 21 = 47.6 → Actual 48 → appropriate compensation

• Diagnosis: Metabolic alkalosis from vomiting (HCl loss) - fully compensated

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PART 13 - Master Cheat Sheet

STEP 1:  pH < 7.35 = Acidosis | pH > 7.45 = Alkalosis
STEP 2:  CO2 ↑ = Resp Acidosis | CO2 ↓ = Resp Alkalosis
         HCO3- ↓ = Met Acidosis | HCO3- ↑ = Met Alkalosis
STEP 3:  Apply compensation formula → Simple or Mixed?
STEP 4:  Met Acidosis → AG = Na - (Cl + HCO3-)
            AG > 12 → MUDPILES
            AG normal → HARDUP
STEP 5:  High AG → Delta-Delta ratio → hidden disorder?
STEP 6:  PaO2 and P/F ratio → oxygenation status

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