abg
Reading File
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Reading File
Reading File
Reading File
Reading File
Reading File
Here is a thorough guide to ABG interpretation, drawn from multiple authoritative textbooks.
Arterial Blood Gas (ABG) Interpretation
What an ABG Directly Measures
| Parameter | Normal Range | What It Reflects |
|---|---|---|
| pH | 7.35 - 7.45 | Overall acid-base status |
| PaCO2 | 35 - 45 mmHg | Respiratory component (ventilation) |
| PaO2 | 80 - 100 mmHg | Oxygenation |
| HCO3- | 21 - 27 mEq/L | Metabolic component (calculated via Henderson-Hasselbalch) |
| SaO2 | 95 - 100% | Oxygen saturation |
The pH electrode measures the potential difference between a reference solution and the sample at 37°C. PaCO2 is measured by the CO2 electrode, and HCO3- is calculated using the Henderson-Hasselbalch equation:
pH = 6.1 + log10 ([HCO3-] / 0.0301 × PaCO2)
(where 6.1 = pKa of carbonic acid, 0.0301 = solubility coefficient of CO2 at 37°C)
Step-by-Step Interpretation
Step 1: Check the pH
- < 7.35 = Acidemia
- > 7.45 = Alkalemia
- 7.35-7.45 = Normal (may still have a compensated disorder)
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 moves in the same direction as pH change is the primary driver.
- pH ↓ + PaCO2 ↑ = respiratory acidosis
- pH ↑ + PaCO2 ↓ = respiratory alkalosis
Step 3: Assess Compensation
Compensation never fully corrects pH back to 7.40. If it does, suspect a mixed disorder.
| Primary Disorder | Expected Compensation | Formula |
|---|---|---|
| Metabolic Acidosis | Respiratory alkalosis (within 12-24h) | Winter's formula: PaCO2 = 1.5 × [HCO3-] + 8 ± 2 |
| Metabolic Alkalosis | Respiratory acidosis (within 24-48h) | PaCO2 = 0.7 × [HCO3-] + 20 ± 5 |
| Resp. Acidosis - Acute | Metabolic alkalosis | HCO3- rises +1 mEq/L per 10 mmHg ↑ PaCO2 |
| Resp. Acidosis - Chronic | Metabolic alkalosis (over 3-5 days) | HCO3- rises +3.5-5 mEq/L per 10 mmHg ↑ PaCO2 |
| Resp. Alkalosis - Acute | Metabolic acidosis | HCO3- falls -2 mEq/L per 10 mmHg ↓ PaCO2 |
| Resp. Alkalosis - Chronic | Metabolic acidosis | HCO3- falls -5 mEq/L per 10 mmHg ↓ PaCO2 |
Acute vs. Chronic Respiratory Acidosis (pH rule):
- Acute: pH falls 0.08 per 10 mmHg rise in PaCO2
- Chronic: pH falls 0.03 per 10 mmHg rise in PaCO2 (kidneys compensate over 3-5 days)
Step 4: Calculate the Anion Gap (if metabolic acidosis)
AG = Na+ - (Cl- + HCO3-) | Normal = 8-12 mEq/L (uncorrected) or <13 mEq/L
Albumin correction: Add 2.5 mEq/L to AG for every 1 g/dL drop in albumin below 4 g/dL (albumin is a major serum anion).
High AG metabolic acidosis - MUDPILES:
- Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Infection/Isoniazid/Ischemia, Lactic acidosis, Ethylene glycol, Salicylates
Normal AG (hyperchloremic) metabolic acidosis:
- Diarrhea, renal tubular acidosis, saline resuscitation, carbonic anhydrase inhibitors, early CKD, ureteral diversion
Step 5: Assess Oxygenation
A-a gradient = PAO2 - PaO2
- PAO2 = (FiO2 × [Patm - PH2O]) - (PaCO2 / R)
- On room air at sea level: PAO2 ≈ 150 - (PaCO2 / 0.8)
- Normal A-a gradient: ~10 mmHg (increases with age)
| A-a Gradient | Significance |
|---|---|
| Normal (<20 mmHg) with hypercapnia | Hypoventilation from central/neuromuscular cause |
| Elevated (>20 mmHg) with hypercapnia | Underlying lung disease (V/Q mismatch, shunt) |
The Four Primary Disorders at a Glance
Respiratory Acidosis
Cause: Hypoventilation (COPD, sedation, neuromuscular disease, obesity hypoventilation)
- pH ↓, PaCO2 ↑
- Acute: minimal HCO3- rise (+1 per 10 mmHg)
- Chronic: HCO3- rises significantly (+3.5-5 per 10 mmHg) - "chronic hypercapnia with near-normal pH"
Respiratory Alkalosis
Cause: Hyperventilation (anxiety, pain, PE, early sepsis, altitude, pregnancy, liver disease)
- pH ↑, PaCO2 ↓
- Most common acid-base disorder in hospitalized patients (~29-42%)
Metabolic Acidosis
Cause: Gain of acid or loss of HCO3-
- pH ↓, HCO3- ↓, compensatory hyperventilation (Kussmaul breathing in severe cases)
- Prevalence in hospitalized patients: ~10-12%
- Consequences of severe acidemia (pH <7.2): myocardial depression, reduced catecholamine response, arrhythmias, insulin resistance, hyperkalemia
Metabolic Alkalosis
Cause: Loss of acid (vomiting, NG suction) or gain of HCO3-
- pH ↑, HCO3- ↑, compensatory hypoventilation
- Most common metabolic disturbance in hospitalized patients (~16-28%)
- Categorized as chloride-responsive (urine Cl- <20, responds to saline) vs. chloride-resistant (urine Cl- >20, e.g., hyperaldosteronism)
- Consequences of severe alkalemia (pH >7.6): coronary vasoconstriction, arrhythmias, hypokalemia, hypocalcemia, decreased cerebral blood flow, tetany/seizures
Clinical Pearls
- Mixed disorders should be suspected when compensation is inappropriate (too much or too little), or when pH is normal but both PaCO2 and HCO3- are abnormal
- Venous blood gases can substitute for pH monitoring (VBG pH is ~0.05 units lower than ABG), but cannot substitute for oxygenation assessment
- Lactate is often co-measured on modern ABG analyzers; always check it in the critically ill
- Sodium bicarbonate for metabolic acidosis: no strong evidence to improve outcomes except possibly in patients with AKI (BICAR-ICU trial); pH <7.2 is associated with increased mortality in septic and hypovolemic shock
Sources: Murray & Nadel's Textbook of Respiratory Medicine; Barash's Clinical Anesthesia, 9e; Tintinalli's Emergency Medicine
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