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Arterial Blood Gas (ABG) Analysis: Basic to Clinical

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1. What Is an ABG?

The HCO3- on the ABG printout is calculated (not directly measured) from pH and PaCO2 via the Henderson-Hasselbalch equation: pH = 6.1 + log([HCO3-] / 0.003 × PaCO2). The "total CO2" on a basic metabolic panel is a direct measurement and usually runs ~1-2 mEq/L higher.

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2. The Underlying Chemistry

• ↑ PaCO2 → reaction shifts right → more H+ → pH falls (respiratory acidosis)
• ↓ PaCO2 → reaction shifts left → less H+ → pH rises (respiratory alkalosis)
• ↑ HCO3- → reaction shifts left → less H+ → pH rises (metabolic alkalosis)
• ↓ HCO3- → reaction shifts right → more H+ → pH falls (metabolic acidosis)

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3. The 7-Step Systematic Approach

Step 1 - Determine pH (Acidosis vs. Alkalosis)

• pH < 7.35: Acidemia
• pH 7.35 - 7.45: Normal
• pH > 7.45: Alkalemia

pH < 7.20 or > 7.60 is life-threatening and needs urgent action.

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Step 2 - Identify the Primary Disorder (Respiratory vs. Metabolic)

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Step 3 - Check for Appropriate Compensation

Compensation Formulas:

Clinical pearl: Metabolic compensation is respiratory (fast, minutes to hours). Respiratory compensation is metabolic/renal (slow, 3-5 days to complete).

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

• Normal: 12 ± 4 mEq/L (some labs use 7-9 if they omit K+)
• If albumin is low, correct: Corrected AG = measured AG + 2.5 × (4 - measured albumin)

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Step 5 - Calculate the Delta-Delta Ratio (for elevated AG acidosis)

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Step 6 - Assess Oxygenation

• Young healthy adult: ~5-10 mm Hg
• Age-adjusted: (Age / 4) + 4 mm Hg
• On supplemental O2: up to 100 mm Hg can be normal

• 300: Normal

• 200-300: Mild ARDS
• 100-200: Moderate ARDS
• < 100: Severe ARDS

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Step 7 - Identify Mixed Disorders

1. Compensation is outside the predicted range
2. pH is normal but both PaCO2 and HCO3- are abnormal
3. The delta-delta ratio is outside 1-2

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4. The Four Primary Disorders - Causes and Clinical Features

Metabolic Acidosis

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Metabolic Alkalosis

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Respiratory Acidosis

• Acute: pH low, PaCO2 high, HCO3- only mildly elevated (~normal)
• Chronic: pH near-normal (compensated), PaCO2 high, HCO3- clearly elevated (> 28-30)

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Respiratory Alkalosis

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5. Base Excess (BE)

• Normal: -2 to +2 mEq/L
• Negative BE (base deficit): metabolic acidosis
• Positive BE: metabolic alkalosis
• Base deficit > -10 in trauma = marker of severe hemorrhagic shock

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6. Venous Blood Gas vs. ABG

VBGs are increasingly used in emergency medicine. A normal venous PaCO2 reliably excludes hypercapnia. However, VBGs are unreliable in shock or severe hypercapnia.

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7. Worked Clinical Examples

Case 1 - Diabetic Ketoacidosis

1. pH 7.15 → Acidemia
2. HCO3- 6 → Primary metabolic acidosis
3. Winter's formula: Expected PaCO2 = (1.5 × 6) + 8 = 17 ± 2 → actual 20 is within range → appropriate compensation, no secondary respiratory disorder
4. AG = 138 - (100 + 6) = 32 (markedly elevated)
5. Delta-delta = (32-12)/(24-6) = 20/18 = 1.1 → pure elevated AG metabolic acidosis
6. Diagnosis: DKA with appropriate respiratory compensation (Kussmaul breathing)

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Case 2 - Septic Patient (UTI/Pyelonephritis)

1. pH 7.32 → Acidemia
2. HCO3- 14 → Primary metabolic acidosis
3. AG = 138 - (102+14) = 22 (elevated) → anion gap metabolic acidosis
4. Delta-delta = (22-12)/(24-14) = 10/10 = 1.0 → pure AG acidosis
5. Clinical context: fever, hypotension, leukocytosis → Lactic acidosis from sepsis

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Case 3 - COPD Exacerbation

1. pH 7.30 → Acidemia
2. PaCO2 65 → Primary respiratory acidosis
3. Compensation check (acute): expected HCO3- = 24 + (65-40)/10 × 1 = 24 + 2.5 = 26.5 - but actual HCO3- is 30, suggesting chronic respiratory acidosis with renal compensation: expected HCO3- = 24 + (25/10) × 3.5 = ~33 → HCO3- is 30, slightly below → mostly compensated chronic resp. acidosis, possibly during an acute-on-chronic exacerbation

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Case 4 - Mixed Disorder (Sepsis + Vomiting)

1. pH 7.45 → Normal, but borderline
2. PaCO2 low + HCO3- low → could be respiratory alkalosis OR metabolic acidosis with overcompensation
3. AG = 140 - (92+20) = 28 (elevated)
4. Delta-delta = (28-12)/(24-20) = 16/4 = 4 → > 2, indicating underlying metabolic alkalosis (e.g., from vomiting) hidden behind the AG acidosis
5. Diagnosis: Triple disorder - elevated AG metabolic acidosis + metabolic alkalosis + respiratory alkalosis (from sepsis-driven hyperventilation)

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8. Quick Reference Summary

pH < 7.35 → Acidosis        pH > 7.45 → Alkalosis

↑ PaCO2 causes acidosis    ↓ PaCO2 causes alkalosis
↑ HCO3- causes alkalosis   ↓ HCO3- causes acidosis

Compensation rules:
  Met. acidosis  → lungs blow off CO2 (Winter's: PaCO2 = 1.5×HCO3 + 8 ± 2)
  Met. alkalosis → lungs retain CO2 (ΔPaCO2 = 0.6 × ΔHCO3)
  Resp. acidosis → kidneys retain HCO3 (acute: +1/10; chronic: +3.5/10)
  Resp. alkalosis → kidneys excrete HCO3 (acute: -2/10; chronic: -5/10)

Anion Gap = Na - (Cl + HCO3)   Normal: 8-12 (albumin-corrected)
Delta-delta < 1 → also normal AG acidosis
Delta-delta > 2 → also metabolic alkalosis

A-a gradient = PAO2 - PaO2    Normal ≈ age/4 + 4
Normal A-a gradient + hypoxia → hypoventilation or low FiO2
Elevated A-a gradient + hypoxia → V/Q mismatch, shunt, diffusion problem

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• Roberts & Hedges' Clinical Procedures in Emergency Medicine, 15th Ed., p. 1734-1735
• Symptom to Diagnosis: An Evidence-Based Guide, 4th Ed., pp. 69-72
• Goldman-Cecil Medicine, International Ed., p. 1040
• Barash Clinical Anesthesia, 9th Ed., pp. 4249-4250
• Miller's Anesthesia, 10th Ed., pp. 6495-6496
• Tintinalli's Emergency Medicine, p. 121

1. Prologue - Why pH is a matter of survival (the 0.1-unit window of life)
2. Act 1 - Where acid comes from (CO2 = volatile acid, fixed acids = the overflow)
3. Act 2 - The three buffer systems (bicarbonate as the "open" buffer; phosphate in urine; proteins/Hb intracellularly; the isohydric principle tying them all together)
4. Act 3 - The lungs (chemoreceptors → ventilation change in minutes; why compensation is never complete; Winter's Formula as the lung's accountability test)
5. Act 4 - The kidneys in detail (proximal tubule NHE3, carbonic anhydrase, distal H-ATPase, the ammonia trick - the kidney's most powerful but slowest tool)
6. Act 5 - Reading the ABG like an investigator (pH as verdict, PaCO2 as lungs' testimony, HCO3- as kidney's testimony, anion gap, delta-delta, compensation checks)
7. Act 6 - Oxygenation: the alveolar gas equation, A-a gradient, P/F ratio
8. Act 7 - All four disorders with full pathophysiology (including the COPD oxygen danger, paradoxical aciduria in metabolic alkalosis, salicylate triple disorder)

• Top-left (blue) - The Chemistry Engine: the core CO2/H2O reaction, Henderson-Hasselbalch equation, and the three buffer systems (bicarbonate, phosphate, protein/hemoglobin) plotted by pH range
• Top-right (green) - Lung as the Fast Responder: chemoreceptors → respiratory center → increased/decreased ventilation, all within seconds to minutes
• Right-middle (orange) - Kidney as the Slow Fixer: proximal tubule NHE3 for 80% HCO3 reabsorption, distal H+ secretion, and the ammonia trap - full effect takes 3-5 days
• Bottom-center (red) - The 4 Disorders: 2×2 grid with pH, HCO3, and key causes for each
• Bottom-left (purple) - Tools for the Internist: Anion Gap, Winter's Formula, Delta-Delta, A-a Gradient, and the compensation formulas table
• Bottom-right (teal) - Clinical Story Timeline: the arc from initial insult → buffer response → respiratory compensation → renal compensation → resolution or mixed disorder

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