Measurement of Blood Gas Tensions & pH

MEASUREMENT OF BLOOD GAS TENSIONS & pH

Values obtained by routine blood gas measurement include oxygen and carbon dioxide tensions (Po₂ and Pco₂), pH, [HCO3−], base excess, hemoglobin, and the percentage oxygen saturation of hemoglo-bin. As a rule, only Po2, Pco2, and pH are measured. Hemoglobin and percentage oxygen saturation are measured with a cooximeter. [HCO3−] is derived using the Henderson–Hasselbalch equation and base excess from the Siggaard–Andersen nomogram.

Sample Source & Collection

Arterial blood samples are most commonly utilized clinically, though capillary or venous blood can be used if the limitations of such samples are recognized. Oxygen tension in venous blood (normally 40 mm Hg) reflects tissue extraction, not pulmonary func-tion. Venous Pco₂ is usually 4–6 mm Hg higher than Paco₂. Consequently, venous blood pH is usually 0.05 U lower than arterial blood pH. Despite these limitations, venous blood is often useful in determin-ing acid–base status. Capillary blood represents a mixture of arterial and venous blood, and the values obtained reflect this fact. Samples are usually collected in heparin-coated syringes and should be analyzed as soon as possible. Air bubbles should be eliminated, and the sample should be capped and placed on ice to prevent significant uptake of gas from blood cells or loss of gases to the atmosphere. Although heparin is highly acidic, excessive amounts of heparin in the sample syringe usually lower pH only minimally, but decrease Pco₂ in direct proportion to percentage dilu-tion and have a variable effect on Po₂.

Temperature Correction

Changes in temperature affect Pco₂, Po₂, and pH. Decreases in temperature lower the partialpressure of a gas in solution—even though the total gas content does not change—because gas solubil-ity is inversely proportionate to temperature. Both Pco₂ and Po₂ therefore decrease during hypother-mia, but pH increases because temperature does not appreciably alter [HCO₃⁻]: Paco₂ decreases, but [HCO₃⁻] is unchanged. Because blood gas tensions and pH are always measured at 37°C, controversy exists over whether to correct the measured values to the patient’s actual temperature. “Normal” values at temperatures other than 37°C are not known. Many clinicians use the measurements at 37°C directly (“α-stat”), regardless of the patient’s actual tempera-ture .