Arterial Blood Gas

Excerpt

Blood gas analysis is a commonly used diagnostic tool to evaluate the partial pressures of gas in blood and acid-base content. Understanding and using blood gas analysis enables 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) explicitly tests blood taken from an artery. ABG analysis assesses the patient's partial pressures 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 value 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 uses a standard chemistry analysis and will likely be called a "total CO₂". Therefore, the difference will amount to around 1.2 mmol/L. However, a more considerable difference may be seen in 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 institutional standards.

Emergency medicine, intensivist, anesthesiology, and pulmonology clinicians frequently order arterial blood gases, which may also be used in other clinical settings. Healthcare professionals evaluate many diseases 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.