The Arterial Blood Gas (ABG) test measures the acidity (pH) and the levels of oxygen and carbon dioxide in arterial blood. This test provides a snapshot of how effectively the lungs move oxygen into the blood and remove carbon dioxide. It also assesses the balance between acids and bases, regulated by both the respiratory and renal (kidney) systems. ABG results are frequently used to evaluate patients in acute settings, such as those with severe breathing problems, uncontrolled diabetes, or critical infections. Understanding ABG results is paramount for diagnosing and monitoring conditions affecting the respiratory system, circulatory system, and metabolic processes.
The Essential ABG Parameters
An ABG report contains five primary measurements. The blood pH indicates the concentration of hydrogen ions and determines the blood’s acidity or alkalinity. The normal range is 7.35 to 7.45; below 7.35 is acidosis, and above 7.45 is alkalosis.
The next two parameters represent the body’s primary acid-base regulatory systems. The partial pressure of carbon dioxide (PaCO2) reflects the respiratory component, regulated by the lungs. PaCO2 acts as an acid; its normal range is 35 to 45 millimeters of mercury (mmHg). A high PaCO2 indicates acid retention.
Bicarbonate (HCO3) reflects the metabolic or renal component, acting as the body’s primary base (alkali). The normal range for HCO3 is 22 to 26 milliequivalents per liter (mEq/L); an elevated HCO3 indicates base retention.
The final two parameters relate to oxygenation. The partial pressure of oxygen (PaO2) measures dissolved oxygen pressure, indicating oxygen movement from the lungs into the blood (normal range: 75 to 100 mmHg). Oxygen saturation (SaO2) is the percentage of hemoglobin carrying oxygen, normally between 95% and 100%.
A Systematic Approach to Interpretation
Interpreting an ABG requires a structured approach to identify the primary disorder. The first step is assessing the blood pH to determine the overall acid-base status (acidemia if pH < 7.35, alkalemia if pH > 7.45).
The next step is to examine the PaCO2 (respiratory component) and HCO3 (metabolic component) to identify the driving system. PaCO2 is an acid, and HCO3 is a base.
The third step is matching the pH change with the corresponding component. For example, if the pH is low (acidemia) and the PaCO2 is high (acidic), the primary problem is respiratory. Conversely, if the pH is low and the HCO3 is low (deficit of base), the primary problem is metabolic. This comparison identifies the originating system responsible for the disturbance.
Identifying the Four Primary Imbalances
The systematic approach identifies four primary, uncompensated acid-base imbalances.
Respiratory Acidosis
This occurs when the lungs fail to remove carbon dioxide, causing PaCO2 to rise. The ABG shows a low pH and an elevated PaCO2. A common cause is hypoventilation, such as from chronic obstructive pulmonary disease (COPD) or respiratory muscle weakness.
Respiratory Alkalosis
This results from excessive breathing (hyperventilation), causing too much carbon dioxide to be exhaled. This state is characterized by a high pH and a decreased PaCO2, often triggered by anxiety or fever.
Metabolic Acidosis
This is characterized by a low pH and a decreased HCO3, signifying a loss of base or an increase in non-carbonic acids. Common causes include uncontrolled diabetes (ketoacidosis) or severe diarrhea (loss of bicarbonate).
Metabolic Alkalosis
This occurs due to an excess of bicarbonate or a loss of acid, leading to a high pH and an elevated HCO3. Protracted vomiting, which causes a significant loss of stomach acid, is a frequent cause.
Understanding Compensation
The body counteracts a primary acid-base disturbance through compensation. This is the attempt by the unaffected system to restore the pH toward the normal range (7.35 to 7.45). For a respiratory problem, the kidneys (metabolic system) compensate by adjusting bicarbonate levels. For a metabolic problem, the lungs (respiratory system) compensate by altering carbon dioxide levels through changes in breathing rate.
Partially Compensated
If the compensating value (PaCO2 or HCO3) is outside its normal range, but the blood pH remains abnormal, the state is partially compensated. This means the body is working to correct the imbalance. For example, in respiratory acidosis, the HCO3 would be high, but the pH would still be below 7.35.
Fully Compensated
If the pH has returned to the normal range (7.35 to 7.45), but both the PaCO2 and HCO3 remain abnormal, the disorder is fully compensated. To determine the original problem, the pH value must be examined: a pH closer to 7.35 suggests the original problem was an acidosis, while a pH closer to 7.45 suggests an alkalosis. Compensation does not cause the pH to overshoot the normal range.