Arterial Blood Gas

Blood gas analysis is a commonly used diagnostic tool to evaluate the partial pressures of gas in blood and acid-base content. Understanding and use of blood gas analysis enable providers to interpret respiratory, circulatory, and metabolic disorders.

A "blood gas analysis" can be performed on blood obtained from anywhere in the circulatory system (artery, vein, or capillary). An arterial blood gas (ABG) tests explicitly blood taken from an artery. ABG analysis assesses a patient's partial pressure of oxygen (PaO2) and carbon dioxide (PaCO2). PaO2 provides information on the oxygenation status, and PaCO2 offers information on the ventilation status (chronic or acute respiratory failure). PaCO2 is affected by hyperventilation (rapid or deep breathing), hypoventilation (slow or shallow breathing), and acid-base status. Although oxygenation and ventilation can be assessed non-invasively via pulse oximetry and end-tidal carbon dioxide monitoring, respectively, ABG analysis is the standard.

When assessing the acid-base balance, most ABG analyzers measure the pH and PaCO2 directly. A derivative of the Hasselbach equation calculates the serum bicarbonate (HCO3) and base deficit or excess. This calculation frequently results in a discrepancy from the measured due to the blood CO2 unaccounted for by the equation. The measured HCO3 uses a strong alkali that liberates all CO2 in serum, including dissolved CO2, carbamino compounds, and carbonic acid. The calculation only accounts for dissolved CO2; this measurement using a standard chemistry analysis will likely be called a "total CO2". For that reason, the difference will amount to around 1.2 mmol/L. However, a larger difference may be seen on the ABG, compared to the measured value, especially in critically ill patients.

The calculation has been disputed as both accurate and inaccurate based on the study, machine, or calibration used and must be interpreted appropriately based on your institutional standards.

Arterial blood gases are frequently ordered by emergency medicine, intensivist, anesthesiology, and pulmonology clinicians but may also be needed in other clinical settings. Many diseases are evaluated using an ABG, including acute respiratory distress syndrome (ARDS), severe sepsis, septic shock, hypovolemic shock, diabetic ketoacidosis, renal tubular acidosis, acute respiratory failure, heart failure, cardiac arrest, asthma, and inborn errors of metabolism.