ABG Analysis: Diagnosing Acid-Base Disorders
The study of acid-base disorders would not be complete without a thorough understanding of arterial blood gases (ABGs). Since the lungs play a vital role in regulating acid-base balance, it is logical that assessment of the respiratory status of a patient with an acid-base disorder should be performed, and one of the most important examinations to achieve this is the ABG analysis. Armed with this important information, the clinician can then make important diagnostic and therapeutic decisions to optimize patient outcomes. It also makes more complex phenomena such as contraction alkalosis easier to understand.
Arterial Blood Gas Analysis
The ABG analysis is a test used to measure the levels of oxygen and carbon dioxide in arterial blood. Aside from these, pH and bicarbonate levels can be calculated. The ABG is a test used to evaluate lung function and acid-base status.
The components of the ABG are:
- pH – the negative logarithm of hydrogen ion concentration. Normal value: 7.35 – 7.45
- PO2 or PaO2 – this is the arterial oxygen pressure and represents the amount of oxygen dissolved in the blood. Normal value: 80-100 mmHg
- PCO2 or PaCO2 - this is the arterial carbon dioxide pressure and represents the amount of carbon dioxide dissolved in the blood. Normal value: 35-45 mmHg
- HCO3 – the amount of bicarbonate ion (calculated, not directly measured). Normal value: 22-28mEq/L
- SaO2 – this is the amount of oxygen bound by hemoglobin (calculated, not directly measured). Normal value; >90%
Interpretation of Arterial Blood Gas Results
These are very basic steps to correctly interpret blood gas results and identify the acid-base disorder: (Note: the steps below deal only with identification of uncomplicated acid-base disorders and do not include steps to assess lung function. Interpretation of complex acid-base disorders involve steps not enumerated below.)
Check the pH
- Is the pH normal, acidic, or basic?
Check the pH
- High PCO2 indicates respiratory acidosis in the presence of a low pH
- Low PCO2 indicates respiratory alkalosis in the presence of a high pH
Check the pH
- High HCO3 indicates metabolic alkalosis in the presence of a high pH
- Low HCO3 indicates metabolic acidosis in the presence of a low pH
Understanding the concepts, principles, and interpretation of ABGs is essential for every healthcare professional. ExpertCollege offers interactive distance learning courses on respiratory and metabolic alkalosis and other acid-base disorders designed to help students of all levels gain a fuller understanding of acid-base physiology.