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ABG Analysis: Basic to Pro

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PART 1 - NORMAL VALUES (Reference Anchor)

Age-adjusted PaO2: PaO2 sitting = 104.2 - (0.27 × age in years)

• Goldman-Cecil Medicine

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PART 2 - THE HENDERSON-HASSELBALCH EQUATION

pH = 6.1 + log₁₀ [HCO3-] / (0.003 × PaCO2)

• Goldman-Cecil Medicine, p. 1039

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PART 3 - THE 6-STEP SYSTEMATIC APPROACH

Step 1: Determine pH Status

• pH < 7.35 = Acidemia
• pH > 7.45 = Alkalemia
• pH 7.35-7.45 = Normal (but a compensated or mixed disorder may still be present)

Step 2: Identify the Primary Disorder

Step 3: Assess Compensation

Roberts and Hedges' Clinical Procedures in Emergency Medicine

Expected PaCO2 = (1.5 × HCO3-) + 8 ± 2

• Measured PaCO2 above expected = concomitant respiratory acidosis (impending failure?)
• Measured PaCO2 below expected = concomitant respiratory alkalosis (e.g., salicylate toxicity)
• A quick trick: expected PaCO2 ≈ last two digits of predicted pH

Roberts and Hedges', Goldman-Cecil Medicine

Step 4: Calculate the Anion Gap

AG = Na+ - (Cl- + HCO3-)
Normal = 8-12 mEq/L (12 used as cutoff in most formulas)

Corrected AG = Observed AG + 2.5 × (4 - measured albumin in g/dL)

• Barash Clinical Anesthesia, 9e

• Methanol
• Uremia (renal failure)
• Diabetic ketoacidosis
• Propylene glycol / Phenformin
• Isoniazid / Inborn errors of metabolism
• Lactic acidosis
• Ethylene glycol / Ethanol
• Salicylates / Starvation ketosis

• Diarrhea (most common GI cause - diarrheal fluid has 20-50 mEq/L of HCO3-)
• Ureteral diversion / Ureterosigmoidostomy
• Renal tubular acidosis (types 1, 2, 4)
• Hyperalimentation (amino acid infusions)
• Acetazolamide / Ammonium chloride
• Massive normal saline infusion (dilutional hyperchloremic acidosis)

Morgan & Mikhail's Clinical Anesthesiology, 7e

Step 5: Urine Anion Gap (for Normal AG acidosis)

Urine AG = Urine (Na+ + K+) - Urine Cl-

Morgan & Mikhail's Clinical Anesthesiology, 7e

Step 6: Delta-Delta Ratio (for High AG acidosis - detect mixed metabolic disorders)

Δ/Δ = ΔAG / ΔHCO3-
     = (Measured AG - 12) / (24 - Measured HCO3-)

Miller's Anesthesia 10e, Barash Clinical Anesthesia 9e

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PART 4 - THE FOUR PRIMARY DISORDERS

Respiratory Acidosis (pH↓, PaCO2↑)

• CNS depression (opioids, sedatives, brainstem lesion)
• Neuromuscular weakness (GBS, myasthenia, ALS)
• Obstructive lung disease (COPD, severe asthma)
• Chest wall deformity, obesity hypoventilation

Respiratory Alkalosis (pH↑, PaCO2↓)

Metabolic Acidosis (pH↓, HCO3-↓)

• High AG: Accumulation of organic or inorganic acids
• Normal AG: Loss of HCO3- from GI or kidneys, or gain of chloride
• Compensation: Immediate hyperventilation (Kussmaul breathing in DKA)

Metabolic Alkalosis (pH↑, HCO3-↑)

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PART 5 - OXYGENATION ASSESSMENT

Alveolar-Arterial (A-a) Gradient

P(A-a)O2 = FiO2 × (Patm - PH2O) - (PaO2 + PaCO2/0.8)
         = [FiO2 × 713] - [PaO2 + (PaCO2 / 0.8)]   (at sea level, FiO2=0.21)

Simplified estimate: P(A-a)O2 = Age/4 + 4   (in mmHg, room air)

PaO2/FiO2 Ratio (P/F ratio)

P/F = PaO2 / FiO2

Six Mechanisms of Hypoxemia

Goldman-Cecil Medicine, Table 89-1

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PART 6 - VENOUS BLOOD GAS (VBG) CORRELATION

• Central VBGs are more reliable than peripheral VBGs
• VBGs are unreliable in hypotensive patients and severe hypercapnia

Tintinalli's Emergency Medicine, Goldman-Cecil Medicine

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PART 7 - BASE EXCESS (Copenhagen Approach)

Base Excess (BE) = amount of acid (HCl) or base (NaOH) needed to return
                   1L of blood to pH 7.4 at PaCO2 40 mmHg at 37°C
Normal: -2 to +2 mEq/L

• Negative BE (base deficit) = metabolic acidosis - quantifies severity
• Positive BE = metabolic alkalosis
• BE isolates the metabolic component independent of the respiratory CO2 effect
• Whole-blood BE is influenced by hemoglobin concentration; standard BE uses a standardized Hb of 5 g/dL

Miller's Anesthesia 10e

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PART 8 - WORKED EXAMPLES

Example 1 - Acute Respiratory Alkalosis

1. pH 7.49 → Alkalemia
2. PaCO2 ↓ → Primary respiratory alkalosis
3. Check compensation: ↓PaCO2 = 40-25 = 15 mmHg. Expected ↓HCO3- = 15/10 × 2 = 3 mEq/L. Actual ↓HCO3- = 25-22 = 3 ✓
4. Conclusion: Pure acute respiratory alkalosis with appropriate compensation

Example 2 - Metabolic Acidosis with Check for Mixed Disorder

1. pH 7.22 → Acidemia
2. HCO3- ↓ → Primary metabolic acidosis
3. Winter's formula: Expected PaCO2 = (1.5×10) + 8 = 23 ± 2. Measured = 28 → Above expected = concurrent respiratory acidosis
4. AG = 140 - (100+10) = 30 (high AG)
5. Δ/Δ = (30-12)/(24-10) = 18/14 = 1.3 → Pure AG metabolic acidosis (no hidden metabolic alkalosis)
6. Conclusion: High-AG metabolic acidosis (MUDPILES list) WITH concurrent respiratory acidosis - watch for respiratory failure

Example 3 - The Triple Disorder (Pro Level)

1. pH and PaCO2 both normal - superficially "normal ABG"
2. AG = 140 - (92+24) = 24 - looks elevated
3. Albumin-corrected AG = 24 + 2.5×(4-2) = 24 + 5 = 29 - markedly elevated
4. Conclusion: Hidden high-AG metabolic acidosis (e.g., sepsis/lactic acidosis) + concurrent metabolic alkalosis (HCO3- should be lower but isn't) - a triple disorder masquerading as a normal ABG. Only albumin correction reveals it.

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PART 9 - QUICK REFERENCE MNEMONIC

• Respiratory: pH and PaCO2 move in Opposite directions
• Metabolic: pH and HCO3- move in Equal (same) directions

• HCO3- = 10 → expected pH ≈ 7.23 → expected PaCO2 ≈ 23 mmHg

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• Goldman-Cecil Medicine, International Edition
• Morgan & Mikhail's Clinical Anesthesiology, 7e
• Roberts and Hedges' Clinical Procedures in Emergency Medicine
• Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e
• Miller's Anesthesia, 10e
• Tintinalli's Emergency Medicine
• Ganong's Review of Medical Physiology, 26e