An arterial blood gas measures the acid-base balance, oxygenation, and ventilation status of arterial blood. Unlike most blood tests drawn from veins, ABGs are drawn from an artery (typically the radial artery in the wrist), which provides direct measurement of oxygen and carbon dioxide levels as they’re being delivered to tissues. ABGs are common in critical care, emergency medicine, and respiratory disease management. This guide covers what ABGs measure, normal ranges, and how to interpret common abnormalities.
What ABGs measure
- pH: Acid-base status
- PaCO2: Partial pressure of carbon dioxide; reflects ventilation
- PaO2: Partial pressure of oxygen; reflects oxygenation
- HCO3- (bicarbonate): Major buffering system; reflects metabolic component
- SaO2: Oxygen saturation
- Base excess: Calculated metabolic component
Normal ranges
- pH: 7.35-7.45
- PaCO2: 35-45 mmHg
- PaO2: 80-100 mmHg (room air, sea level)
- HCO3-: 22-26 mEq/L
- SaO2: 95-100%
Acid-base interpretation
Step 1: Identify the primary disturbance
- Acidosis: pH below 7.35
- Alkalosis: pH above 7.45
Step 2: Identify whether respiratory or metabolic
- If pH and PCO2 move in opposite directions: respiratory disturbance
- If pH and HCO3 move in same direction: metabolic disturbance
Step 3: Assess compensation
- Acute disturbances: minimal compensation
- Chronic disturbances: full compensation, sometimes near-normal pH
Common acid-base patterns
Respiratory acidosis: Low pH, high PCO2. Causes: COPD exacerbation, respiratory depression, neuromuscular disease.
Respiratory alkalosis: High pH, low PCO2. Causes: hyperventilation from anxiety, pulmonary embolism, sepsis, pain, salicylate toxicity.
Metabolic acidosis: Low pH, low HCO3. Causes: diabetic ketoacidosis, lactic acidosis, kidney failure, severe diarrhea.
Metabolic alkalosis: High pH, high HCO3. Causes: vomiting, diuretic therapy, hypokalemia.
The anion gap
For metabolic acidosis, calculating the anion gap (Na – Cl – HCO3) helps identify the cause. Normal: 8-12 mEq/L. Elevated anion gap acidosis (mnemonics MUDPILES, GOLD MARK) includes diabetic ketoacidosis, lactic acidosis, methanol/ethylene glycol poisoning, and others.
Oxygenation assessment
PaO2 below 60 mmHg or SaO2 below 90% indicates significant hypoxemia warranting intervention. The A-a gradient (calculated from PaO2, atmospheric pressure, and FiO2) helps distinguish causes — elevated A-a gradient suggests pulmonary embolism, pneumonia, or other gas exchange problems; normal A-a gradient with low PaO2 suggests hypoventilation.
When ABGs are typically ordered
- Critical care assessment
- Respiratory failure evaluation
- Sepsis and lactic acidosis assessment
- Diabetic ketoacidosis monitoring
- Drug overdose evaluation
- Mechanical ventilation management
- Severe metabolic disturbances
Frequently Asked Questions
Are ABGs painful?
Yes, more so than venous draws. Local anesthesia is often used for elective ABGs.
Why arterial rather than venous?
Arterial blood reflects oxygen and CO2 levels being delivered to tissues. Venous blood gives different values.
Can a venous blood gas substitute?
Sometimes. Venous blood gases provide reasonable estimates of pH and PCO2 but not oxygenation. They’re often used when ABG is impractical.
What’s a normal pH for blood?
7.35-7.45. Tighter than the body tolerates well — even small deviations have clinical consequences.
How quickly do ABG results return?
Usually within 5-15 minutes since they’re typically run in critical care settings.
The bottom line on arterial blood gas
ABGs provide essential acid-base, oxygenation, and ventilation information for critically ill patients. Interpretation follows a systematic approach identifying the primary disturbance, classifying it as respiratory or metabolic, and assessing compensation. ABGs are typically performed in hospital settings rather than outpatient routine care.