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ABG Analysis - Complete Guide for NEET PG / INICET

References: Harrison's Principles of Internal Medicine 22e (Ch. 58, DuBose) | Ganong's Review of Medical Physiology 26e | Symptom to Diagnosis: Evidence-Based Guide 4e | Roberts & Hedges' Clinical Procedures in Emergency Medicine | Barash Clinical Anesthesia 9e

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PART 1: NORMAL VALUES - THE "PHONE NUMBER" to Memorize

Quick recall trick: pH 7.4 | PaCO2 40 | HCO3 24 - just like 7-40-24

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PART 2: THE HENDERSON-HASSELBALCH EQUATION (Why ABG Works)

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

pH depends on HCO3/PaCO2 ratio

• HCO3 = "Kidney controls this" (slow, days)
• PaCO2 = "Lungs control this" (fast, minutes)

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PART 3: THE 7-STEP ABG APPROACH (Most High-Yield for NEET PG)

STEP 1: Is the patient Acidotic or Alkalotic?

• pH < 7.35 = Acidosis
• pH > 7.45 = Alkalosis
• pH 7.35-7.45 = Normal (but may still have mixed disorder!)

Exam tip: If pH is on the acidic side (< 7.4) even within normal range - underlying disorder is acidosis. If pH is on alkalotic side (> 7.4) - underlying disorder is alkalosis.

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STEP 2: Is it Metabolic or Respiratory?

• PaCO2 HIGH (> 45) → Respiratory Acidosis (lungs retaining CO2)
• HCO3 LOW (< 22) → Metabolic Acidosis (kidneys losing buffer)

• PaCO2 LOW (< 35) → Respiratory Alkalosis (lungs blowing off CO2)
• HCO3 HIGH (> 26) → Metabolic Alkalosis (too much buffer)

Golden Rule: The primary disorder and its cause always move in the SAME direction. CO2 and pH move OPPOSITE. HCO3 and pH move SAME direction.

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STEP 3: Calculate Anion Gap (for Metabolic Acidosis)

AG = Na+ - (Cl- + HCO3-)
Normal = 8-12 mEq/L (most commonly cited as 12 ± 4)

High Anion Gap Metabolic Acidosis - Mnemonic: "A MUD PILE CAT" (from Roberts & Hedges)

Normal Anion Gap (Hyperchloremic) Metabolic Acidosis - Mnemonic: "USED CARP" or think Diarrhea/RTA

Key concept: High AG = hidden acid accumulating (the body is making acids). Normal AG = bicarbonate is being lost (through gut or kidney) and Cl- rises to compensate - that's why it's called "hyperchloremic."

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STEP 4: Check Compensation (Most Important for MCQs!)

Compensation Formulas (HIGH YIELD - NEET PG)

Expected PaCO2 = (1.5 × HCO3) + 8 ± 2
OR: PaCO2 falls 1.25 mmHg per 1 mEq/L fall in HCO3

Memory trick: "1.5 × bicarb + 8" - like 1.5 liters water + 8 cups coffee = the Winters recipe!

• If actual PaCO2 < expected → additional respiratory alkalosis
• If actual PaCO2 > expected → additional respiratory acidosis

Expected PaCO2 = 0.75 × HCO3 + 0 (or rises 0.75 mmHg per 1 mEq/L rise in HCO3)

(Lungs hypoventilate to retain CO2 and bring pH down)

Acute:   HCO3 rises 1 mEq/L per 10 mmHg rise in PaCO2
Chronic: HCO3 rises 4 mEq/L per 10 mmHg rise in PaCO2

Daily life analogy: Acute = you just held your breath for 10 seconds. Chronic = COPD patient who has been retaining CO2 for months. Kidneys have had time to compensate fully in chronic.

Acute:   HCO3 falls 2 mEq/L per 10 mmHg fall in PaCO2
Chronic: HCO3 falls 5 mEq/L per 10 mmHg fall in PaCO2

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STEP 5: Delta-Delta Ratio (Δ/Δ) - Detecting Mixed Disorders

Δ/Δ = (Observed AG - Normal AG) / (Normal HCO3 - Observed HCO3)
     = (AG - 12) / (24 - HCO3)

Daily life analogy: Think of the Delta-Delta like checking if there is a hidden problem behind a visible one. Example: A diabetic with DKA (visible problem = high AG acidosis) but who has also been vomiting for days (hidden problem = metabolic alkalosis from vomit loss). The Δ/Δ > 2 exposes this hidden alkalosis.

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PART 4: THE 4 SIMPLE DISORDERS - Causes & Clinical Features

1. Respiratory Acidosis

Daily life analogy: Respiratory acidosis = someone sitting on your chest. You can't breathe out CO2. CO2 builds up. Blood gets acidic.

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2. Respiratory Alkalosis

Daily life analogy: Respiratory alkalosis = anxious student breathing rapidly before an exam. You breathe so fast, you blow out too much CO2. Blood pH rises. You feel tingling in fingertips (from Ca2+ binding more tightly to albumin in alkalosis → hypocalcemia symptoms).

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3. Metabolic Acidosis

Daily life analogy: Kussmaul breathing = a car engine overheating. The cooling fan (lungs) goes on overdrive to cool the engine (remove CO2 = remove acid).

• High AG = acid ADDED to blood (see A MUD PILE CAT above)
• Normal AG = HCO3 LOST from body (diarrhea, RTA)

Hyperkalemia in acidosis explained: Imagine K+ and H+ are tenants sharing an apartment (the cell). When too many H+ ions arrive (acidosis), some K+ tenants get kicked out into the blood. Hence hyperkalemia in acidosis, hypokalemia in alkalosis.

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4. Metabolic Alkalosis

Vomiting causes alkalosis - you lose HCl from stomach. Losing acid = blood becomes alkaline.

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PART 5: MIXED ACID-BASE DISORDERS (Harrison's 22e, Ch. 58)

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PART 6: CLASSIC NEET PG / INICET VIGNETTES

Vignette 1

A 30-year-old diabetic patient comes with vomiting and weakness. ABG: pH 7.15, PaCO2 20, HCO3 6. Na 138, Cl 100.

1. pH 7.15 - Acidosis
2. HCO3 6 (low) - Primary Metabolic Acidosis
3. AG = 138 - (100+6) = 32 → High AG Metabolic Acidosis → DKA likely
4. Expected PaCO2 (Winter's) = 1.5(6) + 8 = 17 ± 2 → Actual PaCO2 = 20 → slightly higher than expected → additional Respiratory Acidosis component
5. Final: DKA with mixed metabolic + mild respiratory acidosis (Case adapted from Symptom to Diagnosis 4e)

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Vignette 2

COPD patient on diuretics. ABG: pH 7.42, HCO3 42, PaCO2 67, Cl 88.

• Both PaCO2 and HCO3 are elevated (abnormal in opposite directions)
• pH is near normal - looks "normal" but is NOT
• This is Metabolic Alkalosis + Respiratory Acidosis (mixed)
• COPD causes chronic respiratory acidosis → compensatory high HCO3; diuretics cause additional metabolic alkalosis

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Vignette 3

Young woman hyperventilating due to anxiety. ABG: pH 7.56, PaCO2 24, HCO3 21.

1. pH 7.56 - Alkalosis
2. PaCO2 24 (low) - Primary Respiratory Alkalosis
3. Check compensation: Acute RAlk → HCO3 falls 2 mEq/L per 10 mmHg fall in PaCO2
   • PaCO2 fell from 40 to 24 = fall of 16 mmHg
   • Expected HCO3 fall = (16/10) × 2 = 3.2 → HCO3 should be ~21 ✓ (appropriate compensation)
4. This is Pure Respiratory Alkalosis (appropriate acute compensation)

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PART 7: QUICK MEMORY AIDS

pH Direction Rule

The primary disorder is the one whose change MATCHES the pH direction.

• pH down → acidosis → is PaCO2 up OR HCO3 down? That one is PRIMARY.

"ROME" mnemonic

• Respiratory = Opposite (pH and CO2 go opposite)
• Metabolic = Equal (pH and HCO3 go same direction)

Physiologic Compensation Logic

Urine Anion Gap (for Normal AG metabolic acidosis)

Urine AG = Urine Na+ + Urine K+ - Urine Cl-
Negative urine AG → Diarrhea (GI loss of HCO3)
Positive urine AG → RTA (kidney can't excrete NH4+/H+)

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PART 8: ONE-LINE HIGH-YIELD FACTS FOR MCQs

1. Aspirin overdose = early respiratory alkalosis (stimulates respiratory center), late metabolic acidosis
2. Methanol/Ethylene glycol = high AG metabolic acidosis + elevated osmolal gap
3. Vomiting = metabolic alkalosis (lose HCl) with hypokalemia and hypochloremia; saline-responsive
4. Diarrhea = normal AG metabolic acidosis; NEGATIVE urine anion gap
5. RTA Type 1 (distal) = normal AG acidosis, urine pH > 5.5, nephrocalcinosis, POSITIVE urine AG
6. RTA Type 2 (proximal) = normal AG acidosis, urine pH < 5.5, Fanconi syndrome
7. RTA Type 4 = hypoaldosteronism, hyperkalemia + normal AG acidosis (most common RTA)
8. Pregnancy = mild respiratory alkalosis (progesterone stimulates breathing; compensated chronically)
9. COPD = chronic respiratory acidosis (compensated); do NOT over-correct during mechanical ventilation
10. Kussmaul breathing = deep labored breathing in metabolic acidosis (e.g., DKA)
11. pH < 6.8 or > 7.8 = incompatible with life
12. Compensation NEVER overcorrects pH (except chronic respiratory alkalosis)
13. Loop/thiazide diuretics = metabolic alkalosis (lose H+ and Cl-)
14. Acetazolamide = normal AG metabolic acidosis (blocks carbonic anhydrase)
15. Hyperaldosteronism = metabolic alkalosis + hypokalemia; saline-RESISTANT

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• Harrison's Principles of Internal Medicine 22e, Ch. 58 (DuBose) - Acidosis and Alkalosis
• Ganong's Review of Medical Physiology 26e, Ch. 35 - Respiratory and Renal Compensation
• Symptom to Diagnosis: Evidence-Based Guide 4e, Ch. 4 - Stepwise ABG approach
• Roberts & Hedges' Clinical Procedures in Emergency Medicine - Acid-Base Balance, A MUD PILE CAT mnemonic
• Barash Clinical Anesthesia 9e, Ch. 16 - Delta-Delta ratio, ABG interpretation
• Tintinalli's Emergency Medicine - Arterial Blood Gas Analysis