Abg analysis
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Now I have enough content for a thorough, well-structured ABG analysis guide. Let me compile everything.
Arterial Blood Gas (ABG) Analysis
Normal Values
| Parameter | Normal Range |
|---|---|
| pH | 7.35 - 7.45 |
| PaCO2 | 35 - 45 mm Hg |
| PaO2 | 70 - 95 mm Hg (age-dependent) |
| HCO3- | 22 - 26 mmol/L |
| Base Excess (BE) | -2 to +2 mEq/L |
| SaO2 | 95 - 100% |
- pH < 7.35 = acidemia; pH > 7.45 = alkalemia
- PaCO2 < 35 = hyperventilation; PaCO2 > 45 = hypoventilation/hypercapnia
Step-by-Step Interpretation
Step 1 - Determine the pH
- < 7.35 = Acidosis
- > 7.45 = Alkalosis
- 7.35-7.45 = Normal (but a disorder may still exist - check other values)
Step 2 - Identify the Primary Disorder
| Disorder | pH | PaCO2 | HCO3- |
|---|---|---|---|
| Respiratory acidosis | ↓ | ↑ | Normal/↑ |
| Respiratory alkalosis | ↑ | ↓ | Normal/↓ |
| Metabolic acidosis | ↓ | Normal/↓ | ↓ |
| Metabolic alkalosis | ↑ | Normal/↑ | ↑ |
Rule: The parameter that matches the pH change is the primary driver.
Step 3 - Assess Compensation
Compensation is never complete - it only partially corrects the pH.
| Primary Disorder | Compensatory Response | Formula |
|---|---|---|
| Metabolic acidosis | ↓ PCO2 (hyperventilation) | Expected PCO2 = 1.3 × ΔHCO3- below normal; or Winter's formula: PCO2 = (1.5 × HCO3-) + 8 ± 2 |
| Metabolic alkalosis | ↑ PCO2 (hypoventilation) | Expected ΔPCO2 = 0.6 × ΔHCO3- |
| Respiratory acidosis (acute) | ↑ HCO3- (buffering) | HCO3- ↑ 1 mmol/L per 10 mm Hg ↑ PCO2 |
| Respiratory acidosis (chronic) | ↑ HCO3- (renal) | HCO3- ↑ 4 mmol/L per 10 mm Hg ↑ PCO2 |
| Respiratory alkalosis (acute) | ↓ HCO3- | HCO3- ↓ 2 mmol/L per 10 mm Hg ↓ PCO2 |
| Respiratory alkalosis (chronic) | ↓ HCO3- (renal) | HCO3- ↓ 5 mmol/L per 10 mm Hg ↓ PCO2 |
Key: If measured compensation does NOT match predicted, a mixed disorder is present.
- Respiratory compensation for metabolic disorders: rapid (minutes-hours)
- Metabolic compensation for respiratory disorders: slow (3-5 days for full renal compensation)
Step 4 - Calculate the Anion Gap (if metabolic acidosis)
Anion Gap = Na+ - (Cl- + HCO3-)
- Normal AG: 8-12 mEq/L (some use 12 as upper limit)
- Correct for albumin: for every 1 g/dL drop in albumin below 4, add 2.5 mEq/L to measured AG
| High AG Metabolic Acidosis (MUDPILES) | Normal AG Metabolic Acidosis (HARDUP) |
|---|---|
| Methanol | Hyperalimentation / Hospital saline |
| Uremia | Acid infusion / Addison's disease / Carbonic anhydrase inhibitors |
| DKA (diabetic ketoacidosis) | Renal tubular acidosis |
| Paraldehyde / Propylene glycol | Diarrhea |
| Iron / Isoniazid | Ureteral diversion |
| Lactic acidosis | Pancreatic fistula |
| Ethylene glycol | |
| Salicylates |
Step 5 - Assess Oxygenation
A-a Gradient = FiO2 × (Patm - PH2O) - (PaO2 + PaCO2/0.8)
Simplified estimate of normal A-a gradient: (Age/4) + 4
| Mechanism of Hypoxemia | A-a Gradient |
|---|---|
| V/Q mismatch (e.g., pneumonia, PE) | Increased |
| Diffusion impairment (e.g., ILD) | Increased |
| R-to-L shunt | Increased |
| Hypoventilation | Normal |
| Altitude / low FiO2 | Normal |
| Hepatopulmonary syndrome | Increased |
Normal PaO2 by age (sitting): PaO2 = 104.2 - (0.27 × age in years)
Acid-Base Map
This diagram plots pH vs PCO2 with labeled zones for each primary and mixed disorder:
Roberts and Hedges' Clinical Procedures in Emergency Medicine
- Zone 1 (upper right, red): Mixed respiratory + metabolic acidosis
- Zone 2 (lower left, pink): Mixed respiratory + metabolic alkalosis
- Zone 3 (lower right, yellow): Metabolic alkalosis + respiratory acidosis
- Zone 4 (upper left, orange): Metabolic acidosis + respiratory alkalosis
- N (center): Normal
Worked Examples
Example 1 - Metabolic acidosis with compensation:
A 58-year-old with 1 week of diarrhea: Na+ 133, K+ 2.8, pH 7.26, Cl- 118, PCO2 13, HCO3- 5
- pH < 7.40 → acidosis
- HCO3- is low, PCO2 is not elevated → primary metabolic acidosis
- Expected PCO2 = 40 - (1.3 × [25-5]) = 40 - 26 = 14 mm Hg; actual = 13 → appropriate compensation, no mixed disorder
Example 2 - Respiratory alkalosis (sepsis):
A 74-year-old with sepsis: pH 7.49, PCO2 25, HCO3- 22
- pH > 7.45 → alkalosis
- PCO2 low, HCO3- normal → primary respiratory alkalosis
- Cause: hyperventilation from sepsis/fever
Quick Reference: Henderson-Hasselbalch Equation
pH = 6.1 + log10([HCO3-] / 0.003 × PaCO2)
This is the backbone of ABG interpretation. Any change in HCO3- (metabolic) or PaCO2 (respiratory) drives a change in pH.
Venous Blood Gas (VBG) vs. ABG
- Venous pH averages ~0.03 lower than arterial
- PvCO2 averages 3-8 mm Hg higher than PaCO2
- VBG is unreliable in hypotensive patients and severe hypercapnia
- A normal PvCO2 can be used to exclude hypercapnia
Sources: Goldman-Cecil Medicine, Roberts & Hedges' Clinical Procedures in Emergency Medicine, Barash's Clinical Anesthesia, ROSEN's Emergency Medicine
Abg analysis from.basics.to.clinicals
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This is a complete ABG guide covering everything from physiology to clinical application. Here's what's included:
Part 1 - Physiology: Why the body needs tight pH control, Henderson-Hasselbalch equation, the three buffer systems (chemical, respiratory, renal) and their speeds.
Part 2 - Normal Values: All parameters with clinical significance.
Part 3 - The 6-Step Approach:
- pH status
- Primary disorder identification
- Compensation assessment (with all formulas)
- Anion gap + albumin correction (MUDPILES / HARDUP)
- Urine anion gap for normal-AG acidosis
- Delta-delta ratio to unmask hidden mixed disorders
Part 4 - Four Primary Disorders in Depth: Each with pathophysiology, causes, clinical features, and management - including lactic acidosis Type A/B, metabolic alkalosis chloride-responsive vs. resistant, permissive hypercapnia in ARDS, and respiratory alkalosis as an early sepsis sign.
Part 5 - Osmolar Gap: Toxic alcohol ingestion, estimation formulas.
Part 6 - Oxygenation: A-a gradient and all six mechanisms of hypoxemia.
Part 7 - Mixed Disorders: Using the acid-base map and Δ/Δ ratio with classic clinical scenarios.
Part 8 - Four Worked Clinical Cases: DKA with hidden metabolic alkalosis, ethylene glycol ingestion, early sepsis, and acute-on-chronic COPD.
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