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.
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.
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 .
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