Abg analysis
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
Finding Sources
Reading File
Finding Sources
Reading File
Finding Sources
Reading File
Finding Sources
Reading File
I now have comprehensive textbook content to write a thorough ABG analysis guide. Let me compile everything.
Arterial Blood Gas (ABG) Analysis
What Is an ABG?
An ABG measures the partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2) in arterial blood, along with direct serum pH and calculated bicarbonate (HCO3-). It is the reference standard for evaluating oxygenation, ventilation, and acid-base status. Arterial blood is used because it reflects gas composition after pulmonary circulation, before peripheral tissue exchange.
Normal Values
| Parameter | Normal Range |
|---|---|
| pH | 7.35 - 7.45 |
| PaCO2 | 35 - 45 mmHg |
| PaO2 | 80 - 100 mmHg |
| HCO3- | 22 - 26 mEq/L |
| SaO2 | 95 - 100% |
Stepwise Approach to ABG Interpretation
This 6-step framework is used universally:
Step 1 - Determine pH (Acidemia vs. Alkalemia)
- pH < 7.35 = Acidemia
- pH > 7.45 = Alkalemia
- pH 7.35-7.45 = normal range (but a disorder with compensation may still be present)
Step 2 - Identify the Primary Process
Look at PaCO2 and HCO3- together with the pH direction:
| pH | PaCO2 | Disorder |
|---|---|---|
| ↓ | ↑ | Respiratory Acidosis |
| ↓ | ↓ | Metabolic Acidosis |
| ↑ | ↓ | Respiratory Alkalosis |
| ↑ | ↑ | Metabolic Alkalosis |
Rule of thumb: If pH and PaCO2 move in opposite directions - primary respiratory disorder. If pH and HCO3- move in the same direction - primary metabolic disorder.
Step 3 - Assess Compensation
Compensation always moves pH back toward normal but never fully corrects it (except for respiratory alkalosis in pregnancy).
| Primary Disorder | Compensatory Response | Formula |
|---|---|---|
| Metabolic acidosis | Hyperventilation (↓ PaCO2) | Winter's: PaCO2 = (1.5 × HCO3-) + 8 ± 2 |
| Metabolic alkalosis | Hypoventilation (↑ PaCO2) | PaCO2 = 40 + 0.7 × (HCO3- - 24) ± 5 |
| Acute resp. acidosis | Minimal renal (↑ HCO3-) | ΔHCO3- = ΔPaCO2 × 0.1 |
| Chronic resp. acidosis | Renal HCO3- retention | ΔHCO3- = ΔPaCO2 × 0.35 |
| Acute resp. alkalosis | Minimal renal (↓ HCO3-) | ΔHCO3- = ΔPaCO2 × 0.2 |
| Chronic resp. alkalosis | Renal HCO3- excretion | ΔHCO3- = ΔPaCO2 × 0.5 |
If the measured compensation does not match the predicted value, a mixed disorder exists.
Step 4 - Calculate the Anion Gap (AG)
AG = Na+ - (Cl- + HCO3-) ; Normal = 8-12 mEq/L (some labs use < 13)
Always calculate the AG, even when a metabolic acidosis isn't the primary diagnosis - it may unmask a hidden mixed disorder.
Albumin correction: Because albumin is a major unmeasured anion, correct for hypoalbuminemia:
Corrected AG = Measured AG + 2.5 × (4 - measured albumin [g/dL])
High AG Metabolic Acidosis causes (MUDPILES or GOLDMARK):
- Methanol
- Uremia (CKD)
- Diabetic ketoacidosis (DKA) / Alcoholic / Starvation ketoacidosis
- Propylene glycol / Paraldehyde
- Isoniazid / Iron
- Lactic acidosis
- Ethylene glycol
- Salicylates
Normal AG (Hyperchloremic) Metabolic Acidosis causes:
- GI bicarbonate loss (diarrhea, fistulas)
- Renal tubular acidosis (RTA)
- Saline infusion
- Carbonic anhydrase inhibitors
Step 5 - Urine Anion Gap (if non-AG acidosis present)
Urine AG = Urine (Na+ + K+) - Cl-
- Negative urine AG: appropriate renal NH4+ excretion - GI HCO3- loss (diarrhea)
- Positive urine AG: impaired NH4+ excretion - renal cause (RTA)
Step 6 - Delta-Delta Ratio (ΔΔ) when AG is elevated
ΔΔ = ΔAG / ΔHCO3- = (Measured AG - 12) / (24 - Measured HCO3-)
| ΔΔ Value | Interpretation |
|---|---|
| < 1.0 | Mixed AG + non-AG metabolic acidosis |
| 1 - 2 | Pure AG metabolic acidosis |
| > 2.0 | AG metabolic acidosis + concurrent metabolic alkalosis (or compensated chronic resp. acidosis) |
Oxygenation Assessment
PaO2 and Hypoxemia
| PaO2 (mmHg) | Classification |
|---|---|
| 80 - 100 | Normal |
| 60 - 79 | Mild hypoxemia |
| 40 - 59 | Moderate hypoxemia |
| < 40 | Severe hypoxemia |
Alveolar-Arterial (A-a) Gradient
PAO2 = FiO2 × (Patm - PH2O) - (PaCO2 / RQ)
On room air: PAO2 ≈ 150 - (PaCO2 / 0.8)
A-a gradient = PAO2 - PaO2
Normal A-a gradient = 2.5 + (0.21 × age) approximately, or roughly 5-15 mmHg in a young adult.
| A-a Gradient | Cause of Hypoxemia |
|---|---|
| Normal (< 15 mmHg) | Pure hypoventilation (normal lungs) |
| Elevated | V/Q mismatch, shunt, diffusion defect |
Four Primary Acid-Base Disorders
1. Respiratory Acidosis
- pH ↓, PaCO2 ↑
- Cause: Hypoventilation (CNS depression, neuromuscular disease, severe COPD, obesity hypoventilation)
- Acute: HCO3- rises ~1 mEq/L per 10 mmHg ↑ PaCO2
- Chronic: HCO3- rises ~3.5 mEq/L per 10 mmHg ↑ PaCO2
2. Respiratory Alkalosis
- pH ↑, PaCO2 ↓
- Cause: Hyperventilation (anxiety, pain, sepsis, PE, pregnancy, altitude, mechanical overventilation)
- Acute: HCO3- falls ~2 mEq/L per 10 mmHg ↓ PaCO2
- Chronic: HCO3- falls ~5 mEq/L per 10 mmHg ↓ PaCO2
3. Metabolic Acidosis
- pH ↓, HCO3- ↓ (PaCO2 ↓ as compensation)
- Divided into high AG vs. normal AG (see Step 4 above)
- Respiratory compensation: Winter's formula
4. Metabolic Alkalosis
- pH ↑, HCO3- ↑ (PaCO2 ↑ as compensation)
- Causes: Vomiting, NG suction, diuretics, hyperaldosteronism, exogenous NaHCO3
- Divided by urinary chloride: Cl-responsive (urine Cl < 20) vs. Cl-resistant (urine Cl > 20)
Venous Blood Gas (VBG) vs. ABG
| Parameter | ABG vs. VBG Difference |
|---|---|
| pH | VBG ~0.03-0.05 units lower |
| PCO2 | VBG ~5-6 mmHg higher (can vary up to 20 mmHg) |
| PO2 | VBG cannot substitute for arterial oxygenation |
| Lactate | Normal VBG lactate reliably rules out elevated arterial lactate |
VBG pH correlates closely enough for most clinical decisions; abnormal values should be confirmed with arterial sampling.
Quick Clinical Example
ABG: pH 7.14 | PaCO2 73 | HCO3- 21 | Na 138 | Cl 105
- Step 1: pH 7.14 - Acidemia
- Step 2: PaCO2 ↑ with pH ↓ - Primary respiratory acidosis
- Step 3: Expected HCO3- for chronic resp. acidosis = 24 + 0.35 × (73-40) = 24 + 11.5 ≈ 35.5 mEq/L; measured is only 21 - suggests a concurrent metabolic acidosis
- Step 4: AG = 138 - (105 + 21) = 12 - normal AG
- Interpretation: Acute-on-chronic respiratory acidosis with concurrent non-AG metabolic acidosis
Rule of 15 (Quick Bedside Tool for Metabolic Acidosis)
For a simple metabolic acidosis with appropriate respiratory compensation:
- Expected PaCO2 ≈ HCO3- + 15
- Expected pH last two digits ≈ HCO3- + 15
Example: HCO3- = 20 → expected PaCO2 = 35, expected pH ≈ 7.35
Sources:
- Barash, Cullen & Stoelting's Clinical Anesthesia, 9e (Table 16-8/16-9 ABG interpretation)
- Sabiston Textbook of Surgery (Acid-Base Regulation, compensation formulas)
- Rosen's Emergency Medicine (Anion gap, Rule of 15, clinical cases)
- Tintinalli's Emergency Medicine (ABG vs. VBG, clinical utility)
- Murray & Nadel's Textbook of Respiratory Medicine (Oxygenation, A-a gradient)
This is a shared conversation. Sign in to Orris to start your own chat.