Understanding Arterial Blood Gas (ABG) Interpretation
Arterial Blood Gas (ABG) analysis is a critical diagnostic tool used to determine the acid-base balance of the blood. It measures the acidity (pH), carbon dioxide pressure (PaCO2), and bicarbonate (HCO3) levels, providing vital information about a patient’s respiratory and metabolic function.
- pH: Normal 7.35–7.45. Below 7.35 is Acidosis; above 7.45 is Alkalosis.
- PaCO2: Respiratory component. Normal 35–45 mmHg. High = Acidosis (Respiratory), Low = Alkalosis (Respiratory).
- HCO3: Metabolic component. Normal 22–28 mEq/L. High = Alkalosis (Metabolic), Low = Acidosis (Metabolic).
| Component | Normal Range |
|---|---|
| pH | 7.35 – 7.45 |
| PaCO2 | 35 – 45 mmHg |
| HCO3 | 22 – 28 mEq/L |
| PaO2 | 80 – 100 mmHg |
Frequently Asked Questions
What is the difference between compensation and primary disturbance?
A primary disturbance is the original cause of the pH change. Compensation occurs when the body tries to counteract that change—for example, if the respiratory system (PaCO2) compensates for a metabolic (HCO3) imbalance. Compensation rarely fully restores the pH to normal, but it helps limit the shift.
Why is arterial blood used instead of venous blood?
Arterial blood is taken because it represents oxygenated blood freshly leaving the lungs, providing the most accurate snapshot of pulmonary gas exchange. Venous blood varies significantly depending on which tissues it has just passed through, making it less reliable for diagnosing acid-base disturbances.
What is the significance of the "Anion Gap"?
The Anion Gap is calculated using Sodium, Chloride, and Bicarbonate levels (Sodium - (Chloride + Bicarbonate)). It helps clinicians determine the underlying cause of metabolic acidosis by detecting the presence of unmeasured acidic ions in the blood, such as ketones in diabetic ketoacidosis.