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Arterial Blood Gas (ABG) Analysis and Interpretation

1. Introduction

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2. Normal ABG Values

• Murray & Nadel's Textbook of Respiratory Medicine

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3. Physiological Basis: Henderson-Hasselbalch Equation

• A fall in HCO3- OR a rise in PaCO2 → acidosis
• A rise in HCO3- OR a fall in PaCO2 → alkalosis

• Costanzo Physiology, 7th Ed.

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4. Buffer Systems

• Current Surgical Therapy, 14e

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5. Regulatory Systems

Respiratory Regulation (minutes to hours)

• pH falls → hyperventilation → CO2 blown off → pH rises
• pH rises → hypoventilation → CO2 retained → pH falls

Renal Regulation (hours to days)

• Controlling HCO3- reabsorption and excretion
• Regulating H+ secretion
• Modulating ammonia production

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6. The Four Primary Acid-Base Disorders

• Costanzo Physiology, 7th Ed., Table 7.2

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7. Compensation Formulae (Expected Compensation)

Metabolic Acidosis

• Winters' Formula: Expected PaCO2 = (1.5 × HCO3-) + 8 ± 2
• Rule of 15: HCO3- + 15 = expected PaCO2 (and last 2 digits of pH, ±2)
• If HCO3- < 10 mmol/L: expected PaCO2 ≈ 15 mmHg (corollary to Rule of 15)

Metabolic Alkalosis

• Expected PaCO2 = 40 + 0.7 × (HCO3- measured - 24) [± 5]

Acute Respiratory Acidosis

• For every ↑ 10 mmHg PaCO2: HCO3- ↑ 1 mEq/L; pH ↓ 0.08

Chronic Respiratory Acidosis (3-5 days)

• For every ↑ 10 mmHg PaCO2: HCO3- ↑ 3.5-5 mEq/L; pH approaches normal

Acute Respiratory Alkalosis

• For every ↓ 10 mmHg PaCO2: HCO3- ↓ 2 mEq/L

Chronic Respiratory Alkalosis

• For every ↓ 10 mmHg PaCO2: HCO3- ↓ 4-5 mEq/L
• Murray & Nadel's; Barash Clinical Anesthesia, 9e

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8. Stepwise Approach to ABG Interpretation (Systematic Method)

Step 1: Assess the pH

• pH < 7.35 → Acidemia
• pH > 7.45 → Alkalemia
• pH 7.35-7.45 → Normal (may still have a mixed disorder)

Step 2: Identify the Primary Disorder

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

Step 3: Assess Compensation

• Apply the appropriate compensation formula (see above)
• If PaCO2 equals predicted: simple/compensated disorder
• If PaCO2 lower than predicted: superimposed respiratory alkalosis
• If PaCO2 higher than predicted: superimposed respiratory acidosis
• Check if compensation is acute or chronic using renal response tables

Step 4: Calculate the Anion Gap (AG)

Step 5: If High Anion Gap - Calculate Delta-Delta (ΔΔ)

• ΔΔ < 1 → mixed HAGMA + NAGMA
• ΔΔ = 1-2 → pure HAGMA
• ΔΔ > 2 → HAGMA + concurrent metabolic alkalosis
• Barash Clinical Anesthesia, 9e, Table 16-8; Rosen's Emergency Medicine

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9. Metabolic Acidosis: Classification by Anion Gap

High Anion Gap Metabolic Acidosis (HAGMA)

Normal/Non-Anion Gap Metabolic Acidosis (NAGMA)

• H - Hyperalimentation/Hospital-acquired saline (hyperchloremic)
• A - Acid infusion, Addison's disease, Carbonic Anhydrase Inhibitors (acetazolamide)
• R - Renal Tubular Acidosis (RTA)
• D - Diarrhea (HCO3- loss via GI tract)
• U - Ureterosigmoidostomy
• P - Pancreatic fistula/drainage

Osmol Gap

• Osmol gap = Measured osmolality - Calculated osmolality
• Calculated osmolality = 2[Na+] + BUN/2.8 + Glucose/18 + Ethanol/4.6
• Elevated osmol gap (>10-15) suggests unmeasured solute (methanol, ethylene glycol)
• Current Surgical Therapy, 14e; Rosen's Emergency Medicine

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10. Metabolic Alkalosis

• Associated with decreased circulating volume
• GI chloride losses corrected with NaCl IV fluids (saline-responsive)
• Saline-resistant: due to impaired renal NaCl excretion (hyperaldosteronism, Cushing's)

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11. Respiratory Acidosis

• CNS depression (opioids, sedatives, brain injury)
• Chest wall disorders, neuromuscular disease
• Obstructive lung disease (COPD, severe asthma)
• Rib fractures, pneumothorax, pleural effusion

• Acute (< 24-48 hr): HCO3- rises only 1 mEq/L per 10 mmHg ↑ PaCO2
• Chronic (3-5 days): HCO3- rises 3.5-5 mEq/L per 10 mmHg ↑ PaCO2 (full renal compensation)

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12. Respiratory Alkalosis

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13. Alveolar-Arterial (A-a) PO2 Gradient

• At sea level on room air (FiO2 = 0.21): PAO2 ≈ 150 - (PaCO2/0.8)

• Normal A-a gradient + hypercapnia → Pure hypoventilation (CNS/NMJ/chest wall disease) - lungs normal
• Elevated A-a gradient (>20 mmHg) + hypercapnia → Underlying lung disease contributing (V/Q mismatch, shunt, diffusion impairment)

• Murray & Nadel's Textbook of Respiratory Medicine

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14. Oxygenation Assessment from ABG

Hypoxemia Classification by PaO2

Mechanisms of Hypoxemia

1. V/Q mismatch - most common; corrects with supplemental O2
2. Shunt (intracardiac/intrapulmonary) - does NOT correct with O2
3. Diffusion impairment - rare, exercise-induced
4. Hypoventilation - normal A-a gradient
5. Low inspired FiO2 - altitude

P/F Ratio (PaO2/FiO2)

• Normal: > 400 mmHg
• Mild ARDS: 201-300 mmHg (on PEEP ≥ 5)
• Moderate ARDS: 101-200 mmHg
• Severe ARDS: ≤ 100 mmHg

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15. Base Excess / Deficit

• Normal: -2 to +2 mEq/L
• Negative BE (base deficit) → metabolic acidosis
• Positive BE → metabolic alkalosis

HCO3- deficit (mEq) = 1/3 × body weight (kg) × base deficit

• Current Surgical Therapy, 14e

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16. Mixed Acid-Base Disorders

• pH is normal but PaCO2 and HCO3- are both abnormal (opposite directions)
• Compensation is more or less than expected
• Delta-delta ratio outside 1-2 range

• Respiratory acidosis + Metabolic acidosis: Seen in cardiorespiratory arrest (CO2 retained + lactic acidosis). Severely low pH.
• Respiratory alkalosis + Metabolic alkalosis: Post-diuretic therapy in liver failure on mechanical ventilation
• HAGMA + NAGMA: ΔΔ < 1 (e.g., DKA + diarrhea)
• HAGMA + Metabolic alkalosis: ΔΔ > 2 (e.g., DKA + vomiting)

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17. Worked Clinical Examples

Example A: Acute Respiratory Acidosis

• Step 1: Acidemia
• Step 2: PaCO2 high → Respiratory acidosis
• Step 3: Expected HCO3- for acute = 24 + 1×[(73-40)/10] ≈ 27 - HCO3- is 21, no metabolic compensation → acute process
• Step 4: AG = Na(138) - (Cl 105 + HCO3 23) = 10 → Normal
• Diagnosis: Acute respiratory acidosis (e.g., opioid-induced hypoventilation)

Example B: HAGMA with Respiratory Compensation

• Step 1: Acidemia
• Step 2: HCO3- low → Metabolic acidosis
• Step 3: Rule of 15: 11 + 15 = 26 → expected PaCO2 ≈ 26; measured is 22 → lower than expected → superimposed respiratory alkalosis (or Winters': 1.5×11 + 8 = 24.5; measured 22 is within range → simple compensation)
• Step 4: AG = 132 - (98 + 11) = 23 → High AG
• Diagnosis: High anion gap metabolic acidosis (e.g., DKA with Kussmaul respiration)
• Barash Clinical Anesthesia 9e, Example Cases

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18. Practical ABG Interpretation Summary Table

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Key Exam Points to Remember

1. ABG is the gold standard for pH, PaCO2, PaO2; HCO3- is calculated (not measured directly)
2. Henderson-Hasselbalch: pH determined by HCO3-/CO2 ratio
3. Always use a systematic 5-step approach: pH → primary disorder → compensation → anion gap → delta-delta
4. Winters' formula for expected PaCO2 in metabolic acidosis; Rule of 15 is a quick bedside approximation
5. Always correct the anion gap for albumin in hypoalbuminemic patients
6. Delta-delta ratio unmasks hidden mixed disorders when HAGMA is present
7. A-a gradient separates pulmonary from non-pulmonary causes of hypercapnia
8. Acute vs chronic respiratory disorders are distinguished by the degree of HCO3- compensation
9. Do not normalize PaCO2 acutely in chronic hypercapnia - risk of sudden alkalosis
10. Base deficit correlates with severity of illness and guides bicarbonate replacement

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