Arterial Blood Gas

Blood gas analysis is a commonly used diagnostic tool to evaluate the partial pressures of gas in blood as well as acid-base content. Understanding and use of blood gas analysis enables providers to interpret respiratory, circulatory and metabolic disorders. [1] 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) specifically tests blood taken from an artery. Arterial blood gas analysis assesses a patient's partial pressure of oxygen (PaO2), providing information on the oxygenation status; the partial pressure of carbon dioxide (PaCO2), providing information on the ventilation status (chronic or acute respiratory failure, and is changed by hyperventilation (rapid or deep breathing) and 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, blood gas analysis is the standard. When assessing the acid-base balance, most ABG analyzers measure the pH and PaCo2 directly and through a derivative of the Hasselbach equation calculate the serum bicarbonate (HCO3) and base deficit or excess. The calculation frequently results in a discrepancy from the measured; this may be from CO2 in blood that is not accounted for by the Henderson-Hasselbach 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 measured value, especially in critically ill patients. [2] 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. [3] Arterial blood gases are frequently ordered by emergency medicine, intensivist, anesthesiology and pulmonology physicians, but may also be needed in other clinical settings. There are many diseases that are evaluated using an ABG which include 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.