An Arterial Blood Gas (ABG) test is a common diagnostic procedure that measures the acidity, or pH, of the blood, along with the levels of oxygen and carbon dioxide. This information is obtained from a small sample of blood drawn from an artery, typically in the wrist. The ABG analysis provides a rapid, real-time snapshot of how effectively the lungs and kidneys are working to maintain the body’s internal balance.
The Clinical Purpose of Blood Gas Analysis
The primary goal of the ABG test is to evaluate two major physiological systems: ventilation and metabolic function.
Ventilation
Ventilation refers to how well the lungs are moving air in and out, which directly controls the level of carbon dioxide in the blood. Inadequate ventilation, for example, can lead to a buildup of carbon dioxide, signaling a potential problem with the patient’s breathing mechanics or lung tissue.
Metabolic Function
The test also assesses the metabolic status, which is largely regulated by the kidneys. The kidneys manage the body’s store of bicarbonate, a chemical buffer that neutralizes acids. By measuring the bicarbonate level, clinicians can gauge how well the body is handling the acids produced by normal metabolism.
Defining the Normal Ranges
These reference values represent the narrow boundaries within which the body functions optimally.
The measure of acidity or alkalinity in the blood is the pH, which should fall within the range of 7.35 to 7.45. A value below 7.35 indicates acidosis, while a value above 7.45 signifies alkalosis. The partial pressure of carbon dioxide (PaCO2) is the respiratory component, reflecting how effectively the lungs are removing this gaseous acid. The normal range for PaCO2 is 35 to 45 millimeters of mercury (mmHg).
The bicarbonate ion (HCO3-) represents the metabolic component and is the main chemical buffer controlled by the kidneys. Normal HCO3- levels are between 22 and 26 milliequivalents per liter (mEq/L). The partial pressure of oxygen (PaO2) measures the amount of oxygen dissolved in the arterial blood, which assesses oxygenation. A typical PaO2 should be between 75 and 100 mmHg.
Interpreting Acid-Base Balance
The body tightly regulates the pH of the blood because enzymes and other proteins function best within that narrow 7.35 to 7.45 window. This stability is maintained through the interplay of the respiratory system, which controls PaCO2, and the renal system, which controls HCO3-.
An increase in PaCO2 above 45 mmHg means more carbon dioxide is dissolved in the blood, leading to a drop in pH and a state called respiratory acidosis. Conversely, if the lungs expel too much CO2, the PaCO2 drops below 35 mmHg, causing the pH to rise, which is known as respiratory alkalosis.
The metabolic arm of the balance involves the HCO3- buffer. A decrease in HCO3- below 22 mEq/L indicates that the body has consumed its buffer supply to neutralize excess non-respiratory acids, resulting in metabolic acidosis. An increase in HCO3- above 26 mEq/L means there is an excess of base, which causes the pH to rise, leading to metabolic alkalosis.
When one system is the primary cause of an imbalance, the other system attempts to restore the pH back toward the normal range through a process called compensation. For instance, in a metabolic acidosis, the respiratory system compensates by increasing the breathing rate to “blow off” more CO2, thereby lowering the PaCO2 and raising the pH. This compensatory action is reflected in the ABG by having both the PaCO2 and HCO3- values out of their normal ranges, yet the pH is closer to the 7.40 midpoint or even completely normalized.