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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