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ABG (Arterial Blood Gas) - Detailed Clinical Analysis
What is an ABG?
An arterial blood gas is the gold standard for assessing a patient's respiratory and metabolic status. It directly measures:
| Parameter | Normal Value | Normal Range |
|---|---|---|
| pH | 7.40 | 7.35 - 7.45 |
| PaCO2 | 40 mmHg | 35 - 45 mmHg |
| PaO2 | 100 mmHg | 75 - 100 mmHg |
| HCO3- (calculated) | 24 mEq/L | 22 - 26 mEq/L |
| Base Excess/Deficit | 0 | -4 to +4 |
HCO3- is not directly measured - it is calculated from the Henderson-Hasselbalch equation. Base excess is also a calculated value.
- Murray & Nadel's Textbook of Respiratory Medicine, p. 974
- Current Surgical Therapy 14e, p. 1608
Physiologic Basis: The Henderson-Hasselbalch Equation
pH = pKa + log [HCO3-] / (0.03 × PaCO2)
Blood pH is determined by the ratio of HCO3- to CO2. Any change in either drives a change in pH. The buffer equation:
H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-
- Acidemia: pH < 7.35 (increased H+)
- Alkalemia: pH > 7.45 (decreased H+)
Two systems tightly regulate pH:
- Respiratory system - responds within minutes to hours (alveolar ventilation)
- Renal system - responds within hours to days (HCO3- reabsorption / H+ secretion)
Step-by-Step ABG Interpretation
Step 1: Evaluate pH
- pH < 7.35 = Acidemia
- pH > 7.45 = Alkalemia
- pH 7.35 - 7.45 = Normal (but a disorder may still be present if compensation is occurring)
Step 2: Evaluate PaCO2
- In respiratory disorders: PaCO2 changes opposite to pH
- PaCO2 ↑ + pH ↓ = Respiratory acidosis
- PaCO2 ↓ + pH ↑ = Respiratory alkalosis
- In metabolic disorders: PaCO2 changes same direction as pH (compensatory)
Step 3: Evaluate HCO3-
- HCO3- < 22 mEq/L = Metabolic acidosis
- HCO3- > 26 mEq/L = Metabolic alkalosis
Step 4: Evaluate Base Excess/Deficit
- Normal: -4 to +4
- Positive (base excess) = Alkalosis
- Negative (base deficit) = Acidosis
The base deficit is calculated as shown below:
For every ΔPaCO2 of 10, pH changes by 0.08 in the opposite direction. The difference between measured and expected pH, multiplied by 2/3, gives the base deficit.
Bicarbonate deficit formula:
Bicarbonate deficit (mEq HCO3-) = 1/3 × body weight (kg) × base deficit
Step 5: Evaluate PaO2 and Oxygenation
- Hypoxemia (PaO2 < 75 mmHg) may be the cause or trigger of an acid-base disorder
- Calculate the A-a gradient: Alveolar PO2 - Arterial PO2
- Normal A-a gradient ≈ 10 mmHg (varies with age)
- A-a gradient > 20 = suggests underlying pulmonary pathology
Step 6: Check for Compensation and Mixed Disorders
The Four Primary Acid-Base Disorders
Summary Table
| Disorder | Primary Change | pH | Compensation | Renal/Respiratory Response |
|---|---|---|---|---|
| Metabolic Acidosis | ↓ HCO3- | ↓ | Hyperventilation | ↑ HCO3- reabsorption |
| Metabolic Alkalosis | ↑ HCO3- | ↑ | Hypoventilation | ↑ HCO3- excretion |
| Respiratory Acidosis | ↑ PaCO2 | ↓ | None (cause is respiratory) | ↑ HCO3- reabsorption |
| Respiratory Alkalosis | ↓ PaCO2 | ↑ | None | ↓ HCO3- reabsorption |
Key rule: If the disturbance is metabolic, the compensation is respiratory (and vice versa). The compensatory response always moves in the same direction as the original disturbance.
- Costanzo Physiology 7th Edition, p. 330
Compensation Formulas (Critical for Identifying Mixed Disorders)
| Disorder | Formula |
|---|---|
| Metabolic Acidosis | Expected PaCO2 = 1.5 × [HCO3-] + 8 ± 2 (Winter's Formula) |
| Metabolic Alkalosis | Expected PaCO2 = 0.7 × [HCO3-] + 20 ± 5 |
| Acute Respiratory Acidosis | ↑ HCO3- = ΔPaCO2 / 10 |
| Chronic Respiratory Acidosis | ↑ HCO3- = 4 × (ΔPaCO2 / 10) |
| Acute Respiratory Alkalosis | ↓ HCO3- = 2 × (ΔPaCO2 / 10) |
| Chronic Respiratory Alkalosis | ↓ HCO3- = 4 × (ΔPaCO2 / 10) |
- Current Surgical Therapy 14e, p. 1608
The Acid-Base Map
If the patient's values fall within a shaded band = simple disorder. If outside all bands = mixed disorder.
- Costanzo Physiology 7th Edition, p. 332
1. Metabolic Acidosis
Definition: HCO3- < 22 mEq/L, pH < 7.35
Key step: Calculate the Anion Gap (AG)
AG = Na+ - (Cl- + HCO3-) Normal AG = 8-12 mEq/L
High Anion Gap Metabolic Acidosis - MUDPILES
| Letter | Cause |
|---|---|
| M | Methanol, Muscle injury (rhabdomyolysis), Metformin |
| U | Uremia (renal failure) |
| D | Diabetic ketoacidosis, alcoholic/starvation ketoacidosis |
| P | Propylene glycol, Paraldehyde |
| I | Isoniazid, Iron |
| L | Lactic acidosis (most common cause, ~50% of high-AG cases) |
| E | Ethanol, Ethylene glycol (antifreeze) |
| S | Salicylates, Short gut |
If AG is elevated but MUDPILES doesn't explain it, check the Osmol Gap:
Osmol gap = Measured osmolality - Calculated osmolality Calculated Osm = 2[Na+] + BUN/2.8 + glucose/18 + ethanol/4.6 Elevated osmol gap suggests toxic ingestion (methanol, ethylene glycol)
Normal Anion Gap Metabolic Acidosis (Hyperchloremic)
Causes: Diarrhea, Renal tubular acidosis (RTA), Excessive normal saline administration, GI fistulas, nasogastric suctioning
Remember: In normal saline (154 mEq/L each of Na and Cl), excess Cl displaces HCO3-, causing hyperchloremic metabolic acidosis.
Treatment: Address the underlying cause. For severe acidosis (pH < 7.15), exogenous bicarbonate may be used temporarily.
2. Metabolic Alkalosis
Definition: HCO3- > 26 mEq/L, pH > 7.45
Causes:
- Loss of H+ (vomiting, nasogastric drainage, loop/thiazide diuretics)
- Gain of HCO3- (exogenous bicarbonate, massive blood transfusion with citrate)
- Volume contraction (contraction alkalosis)
- Hyperaldosteronism
Compensation: Hypoventilation (CO2 retention), PaCO2 rises
- Expected PaCO2 = 0.7 × [HCO3-] + 20 ± 5
Risks: Associated with electrolyte disturbances (hypokalemia, hypocalcemia) and arrhythmias.
3. Respiratory Acidosis
Definition: PaCO2 > 45 mmHg, pH < 7.35 (due to hypoventilation/CO2 retention)
Acute vs. Chronic:
- Acute: For every ↑10 mmHg PaCO2, pH falls by 0.08 and HCO3- rises by ~1 mEq/L
- Chronic (3-5 days): Renal compensation - for every ↑10 mmHg PaCO2, HCO3- rises by 3.5-5 mEq/L; pH normalizes toward normal
Causes:
- CNS depression (opioids, sedatives, stroke, TBI)
- Neuromuscular disease (myasthenia gravis, GBS, ALS)
- Airway obstruction (COPD exacerbation, severe asthma)
- Chest wall disease (flail chest, kyphoscoliosis)
- Mucus plugging, pneumothorax
Treatment:
- Increase minute ventilation (↑ tidal volume or respiratory rate on ventilator)
- Non-invasive ventilation (BiPAP) - especially for COPD, heart failure
- If no improvement in 2 hours: consider intubation
- Salvage: VV-ECMO for severe refractory cases
A-a gradient guide:
- Normal A-a gradient + hypercapnia = reduced ventilatory drive (CNS, neuromuscular)
- Elevated A-a gradient (>20) + hypercapnia = pulmonary parenchymal disease
4. Respiratory Alkalosis
Definition: PaCO2 < 40 mmHg, pH > 7.45 (due to hyperventilation/CO2 loss)
Causes:
- Pain, anxiety, agitation
- Fever, sepsis (gram-negative)
- Brain tumors, CNS disorders
- Pulmonary disorders: PE, pneumothorax, pneumonia
- Pregnancy (chronic), heart failure, liver failure
- High altitude
- Classic: Salicylate toxicity initially causes respiratory alkalosis, then anion-gap metabolic acidosis
Compensation: Renal HCO3- excretion
- Acute: HCO3- falls by 2 mEq/L per 10 mmHg fall in PaCO2
- Chronic: HCO3- falls by 4 mEq/L per 10 mmHg fall in PaCO2
Treatment: Treat the underlying cause.
Mixed Acid-Base Disorders
A mixed disorder is present when the measured compensation differs from the predicted compensation (using formulas above). Examples:
- Respiratory acidosis + Metabolic acidosis: Inadequate ventilation (CO2 retention) + poor perfusion (lactic acidosis) - e.g., septic shock with respiratory failure
- Respiratory alkalosis + Metabolic alkalosis: Liver failure (hyperventilation) + vomiting
- Metabolic acidosis + Metabolic alkalosis: DKA with prolonged vomiting
Rule: If PaCO2 or HCO3- falls outside the predicted compensation range, a second disorder is superimposed.
Acute vs. Chronic Hypercapnia Quick Summary
| Feature | Acute | Chronic |
|---|---|---|
| PaCO2 | > 45 mmHg | > 45 mmHg |
| pH | < 7.35 (acidotic) | Near normal |
| HCO3- change per 10 mmHg ↑PaCO2 | +1 mEq/L | +3.5 - 5 mEq/L |
| Time course | Minutes to hours | 3-5 days |
- Murray & Nadel's Textbook of Respiratory Medicine, p. 974
Clinical Tips for Practice
- Always use a systematic approach (Steps 1-6 above) - never jump to conclusions
- HCO3- on ABG is calculated, not measured - use the simultaneous BMP for the true measured value when checking for mixed disorders
- PaO2 can be normal even with respiratory failure if the patient is on supplemental O2 - the PaCO2 is the better indicator of ventilation
- Pain or anxiety during arterial puncture can cause transient hyperventilation - use local lidocaine to minimize this artifact
- Base deficit is a useful trend marker for illness severity and correlates with mortality in surgical/trauma patients
- Check osmol gap whenever toxic alcohol ingestion is on the differential for a high AG metabolic acidosis
Sources:
- Current Surgical Therapy 14e (Enumah & Harris), pp. 1607-1610
- Costanzo Physiology 7th Edition, pp. 330-333
- Murray & Nadel's Textbook of Respiratory Medicine, pp. 974-975
- ROSEN's Emergency Medicine, pp. 1481-1649
- Fishman's Pulmonary Diseases and Disorders
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