Blood gas temperature correction

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 07/02/2015

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Blood gas temperature correction

John M. VanErdewyk, MD

The values of arterial CO2 tension (PaCO2), arterial O2 tension (PaO2), and pH are highly dependent on temperature. Most blood gas analysis machines are calibrated to 37° C and, thus, run samples at 37° C. If the patient’s actual temperature is near 37° C, then the machine values are approximately the same as those of the patient in vivo. However, the further the patient’s temperature is from 37° C, the greater the difference between the numbers reported by the machine and the patient’s actual values.

When the sample is analyzed, it is heated to 37° C, which causes a decrease in the solubility and falsely elevates partial-pressure values. Conversely, if the temperature of the blood decreases, the solubility of O2 and CO2 increases, which consequently lowers the partial pressures of these gases (i.e., decreases PO2 and PCO2). PCO2 decreases by approximately 4.5% for each 1° C temperature decrease (Table 24-1). Therefore, temperature correction—the process of correcting the machine values to the patient values in vivo at the patient’s actual temperature—is needed to determine the patient’s actual partial pressures of PO2 and PCO2. Changes in temperature also affect pH. As temperature decreases, less water is dissociated into OH and H+, and, thus, pH will rise, as will the pOH. The pH rises approximately 0.015 unit for each 1° C decrease in temperature. If the patient’s temperature is less than 37° C, but the blood sample is subsequently heated to 37° C in the analyzer, then elevated levels of H+ (and OH) will be recorded, leading to a falsely decreased pH value compared with the patient’s actual pH.

Table 24-1

Effects of Hypothermia on PCO2, PO2, and pH

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Parameter Effect of Hypothermia Effect/Change in Temperature
PCO2 ↓ 4.5%/° C
PO2 ↓