Blood gas temperature correction

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

Print this page

This article have been viewed 2416 times

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

Parameter	Effect of Hypothermia	Effect/Change in Temperature
PCO₂	↓	4.5%/° C
PO₂	↓	4.5%/° C
pH	↑	0.015 unit/° C

pH-stat versus α-stat method

Two major approaches have been proposed to handle the management of blood gases during cardiopulmonary bypass–induced hypothermia.

The pH-stat method

In the pH-stat approach, which was used almost exclusively for the first several decades of cardiopulmonary bypass, CO₂ is added to the inspired gases in an attempt to keep the temperature-corrected PaCO₂ normal at 40 mm Hg and the pH normal at 7.4 or greater. The goal of this method is to maintain a constant pH despite varying patient temperatures. Because, during cardiopulmonary bypass using hypothermia, the patient’s temperature is much lower than 37° C, a temperature-corrected system would produce a decreased PaCO₂ and an elevated pH in vivo.

The α-stat method

In the early 1980s, many physicians began using the α-stat system to manage patient care because of its potential theoretic benefits, despite a lack of randomized trials or clinical outcome studies. The goal of this approach is to keep a constant ionic charge on amino acids in proteins, principally the α-imidazole ring of histidine, which functions as an important pH buffer in hemoglobin and other body proteins. The ratio (α) of dissociated to undissociated imidazole groups stays constant (α-stat) with cooling because of changes of blood pH when CO₂ content is held constant. Constant imidazole ionization makes optimal enzyme function possible. As the patient cools, pH must rise because less H⁺ is dissociated. However, equally less OH^– is available, and, therefore, electrochemical neutrality is maintained. Proponents argue that the α-stat method keeps a patient’s uncorrected PaCO₂ and pH at normal levels, as this method preserves more physiologic values by maintaining electrochemical neutrality despite varying body temperature. Temperature correction of blood gases is therefore unnecessary.

Comparison of the two systems

Patients managed by the pH-stat approach would be considered hypercarbic and have a lower pH (i.e., a respiratory acidosis) by the α-stat approach, and α-stat management would be considered a relative respiratory alkalosis by the pH-stat system (Table 24-2). The blood of cold-blooded animals (ectotherms) undergoes pH changes during cooling, parallel to the changes that water undergoes (α-stat). On the other hand, homeothermic mammals, in effect, have “corrected” blood gases during hibernation, and their decreased metabolic function produces anesthetic effects.

Table 24-2

Effects of Temperature Correction

Parameter	pH-Stat as Viewed by α-Stat	α-Stat as Viewed by pH-Stat
CO₂	↑	↓
pH	↓	↑
Condition	Respiratory acidosis	Respiratory alkalosis

Some controversy exists as to whether the α-stat or the pH-stat method yields better patient outcomes. Concerns exist with either system as to how the fluctuating pH and PCO₂ levels affect cerebral and cardiac function. Very few outcome studies exist in the literature, however, and those that do exist have failed to demonstrate any important differences in outcome of these regimens.

Related

Fausts Anesthesiology Review Expert Consult 4e