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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.[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) explicitly tests blood taken from an artery. ABG analysis assesses the patient's partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2).[2] 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.[3] Although oxygenation and ventilation can be assessed non-invasively via pulse oximetry and end-tidal carbon dioxide monitoring, respectively, ABG analysis is the standard.[4] When assessing the acid-base balance, most ABG analyzers measure the pH and PaCO2 directly.[2] 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.[5] The measured HCO3 uses a strong alkali that liberates all CO2 in serum, including dissolved CO2, carbamino compounds, and carbonic acid.[6] 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.[7] 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.[6]

When assessing the acid-base balance, most ABG analyzers measure the pH and PaCO2 directly.[2] 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.[5] The measured HCO3 uses a strong alkali that liberates all CO2 in serum, including dissolved CO2, carbamino compounds, and carbonic acid.[6] 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.[7] 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.[6] 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.[3]

By obtaining an ABG and analyzing the pH, partial pressures, and comparing it to measured serum bicarbonate in a sick patient, multiple pathological conditions can be diagnosed.[1] The alveolar-arterial oxygen gradient is a useful measure of lung gas exchange, which can be abnormal in patients with a ventilation-perfusion mismatch.[8]

ABG should be used to assess a patient's ventilatory, acid-base, and oxygenation status. Additionally, blood gas analysis is recommended to assess a patient's response to therapeutic interventions and to monitor the severity and progression of documented cardiopulmonary disease processes.[44] Despite its clinical value, erroneous or discrepant values represent a potential drawback of blood gas analysis, so eliminating potential sources of error is paramount.[27] Therefore, attention to detail in the sampling technique and processing is essential. Rigorous quality control of the automated blood gas analyzers is also necessary for accurate results. However, machine performance and quality assurance advances have now made most errors in point-of-care analysis of ABGs attributable to clinical providers. Several pre-analytic steps must be followed to obtain a valid, interpretable ABG.[27] In most hospital settings, ABG analysis is a process that involves multiple healthcare providers (eg, physicians, respiratory therapists, and nurses). Hence, interprofessional coordination, cooperation, and communication are vitally important. The American Association for Respiratory Care has published Clinical Care Guidelines for Blood Gas Analysis and Hemoximetry that provide current best practices for sampling, handling, and analyzing ABGs.[44] Notable sources of erroneous values during blood draws include abnormal or misstated FiO2, barometric pressures, or temperatures. Temperature is a significant variable, leading to PaO2 and O2 saturation discrepancies, as do acid-base disturbances. Several physiological and clinical conditions, such as hyperleukocytosis and dyshemoglobinemias, can also lead to PaO2 and O2 saturation discrepancies. Sample dilution can be an additional error source, with liquid heparin and saline as potential culprits.[45] The mode of sample transportation is also of significance as discrepant values can result from air contamination after pneumatic tube system transport, compared with manual transport of the specimen, especially in the presence of inadvertent air bubbles.[45] Therefore, procuring samples using suitable syringes filled with adequate amounts of blood without air bubbles, maintained at the correct temperatures, and appropriately and promptly transporting them for rapid analysis can minimize erroneous values.