Measurement of Blood Gases

Measurement and monitoring of blood gases can be invasive or noninvasive. Transcutaneous pulse oximetry is the most widely used noninvasive test. It provides a fairly accurate measure of oxygen saturation (So2) of hemoglobin at values ranging from 70% to 100% by measuring the difference between oxyhemoglobin and reduced hemoglobin in the absorption of light of specific wavelengths. So2 of 98% corresponds to an arterial oxygen partial pressure (Pao2) of 100 mm Hg, and 95% to a Pao2 of 80 mm Hg, demonstrating the challenge of interpreting a test with a 95% confidence interval of ±5%. An oxygen saturation of less than 89% corresponds to a Pao2 of less than 60 mm Hg. Decreased tissue perfusion or color changes caused by jaundice or intravascu-lar dyes can degrade accuracy. Arterial oxygen levels can also be measured transcutaneously (tcPo2) with a skin surface oxygen electrode, but its accuracy is also affected by tissue perfusion, skin temperature, and other factors.

Exhaled carbon dioxide can also be measured noninva-sively, most often in the ICU for patients on mechanical ventilation or in operating rooms during general anesthesia. Capnography, colorimetric techniques, and CO2 sensors can detect failure of mechanical ventilation or improper endo-tracheal tube placement, which generate hypercapnia secondary to hypoventilation.

The invasive technique most often used for measuring oxygen, CO2, and acid-base blood chemistries is the arterial blood gas (ABG) measurement. Although it requires an arterial needle puncture and several milliliters of blood, it is highly accurate and reproducible. ABG measurement is indicated in any patient with acute respiratory distress or in managing the patient with respiratory failure. In addition to Pao2 and arterial carbon dioxide partial pressure (Paco2) measurements, ABG testing also provides a measure of pH, bicarbonate (HCO3-), and the anion gap, which can be used to detect respiratory (rather than metabolic) causes of acidosis and alkalosis. Patients with moderate to severe COPD can have chronic hypoxia plus chronic hypercapnia (decreased Pao2 and increased Paco2). They can also show signs of primary respiratory acidosis (reduced pH with elevated CO2), and a compensatory metabolic alkalosis (partial normalization of the pH despite elevated Paco2) mediated by renal HCO3-retention. Nomograms or software used in personal digital assistants (PDAs) allow simultaneous plotting of pH, CO2, and HCO3- to facilitate interpretation of mixed respiratory and metabolic acid-base disturbances.

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