Acid Base Disorders - Blood Gas Interpretation
Steps for Blood Gas Interpretation:
1. Acidemia or Alkalemia?
2. Respiratory or Metabolic?
3. Compensated? Acute or chronic?
4. Anion gap? Delta-delta?
5. Differentials?
ABG And BMP Normal Values
• pH: 7.35-7.45
• PaCO2: 35-45 mmHg
• PaO2: 80-100 mmHg
• HCO3 (on BMP): 22-26 mmol/L
Step 1 - Acidemia or Alkalemia:
• pH <7.35 = Acidemia
• pH >7.45 = Alkalemia
Step 2 - Respiratory or Metabolic:
pH pCO2
↓ ↓ Metabolic Acidosis
↑ ↑ Metabolic alkalosis
↓ ↑ Resp Acidosis
↑ ↓ Resp Alkalosis
Step 3 - Compensation, Acute vs Chronic:
• Metabolic compensation
• Respiratory compensation
Step 4 - Anion gap, Delta-delta:
Anion Gap (AG) = {Na - (Cl + HCO3)} Normal = 12 +/- 2
Corrected Anion Gap = AG + 2.5(4-albumin)
Delta: Delta = (AG-12) / (24-HCO3)
Delta: Delta Interpretation For Metabolic Acidosis
<0.4 Pure Normal AG metabolic acidosis
0.4-0.8 Normal + High AG metabolic acidosis
0.8-2.0 Pure High AG metabolic acidosis
>2.0 Metabolic acidosis with superimposed Metabolic alkalosis/Resp acidosis
Step 5 - Differential Diagnosis:
Causes of High Anion Gap Metabolic acidosis:
G Glycols - ethylene glycol “antifreeze” and propylene glycol (present in IV benzodiazepines)
O Oxoprolin (associated with acetaminophen dosing)
L L-lactate (common form of lactate)
D D-lactate (short bowel syndrome, intestinal bacterial overgrowth, propylene glycol)
M Methanol
A Aspirin (salicylates)
R Renal failure (uremia)
K Ketoacidosis (starvation, diabetic)
Causes of Normal Anion Gap Metabolic acidosis:
- Diarrhea
- Renal tubular acidosis/Chronic renal failure
- Adrenal insufficiency
- Rapid saline infusion
- Acetazolamide
Causes of Metabolic Alkalosis:
- Vomiting, NG suction
- Volume depletion (diuresis)
- Mineralocorticoid excess
Causes of Respiratory alkalosis:
- Hyperventilation (Anxiety, pain, fever, hypoxia)
- “Classically” noted with pulmonary embolism (with associated hypoxia)
- Salicylates
Causes of Respiratory acidosis:
- CNS depression (sedation, narcotics, CVA)
- Neuromuscular weakness (GBS, Myasthenia gravis)
- Obstructive or restrictive lung disease (COPD, OSA, Asthma, Obesity hypoventilation)
- Airway obstruction (foreign body, aspiration)
M. Daniyal Hashmi, MD @MDaniyalHashmi1
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