THE BLOOD GAS LABORATORY
ABG TEMPERATURE CORRECTION: TO CORRECT OR NOT TO CORRECT; THAT IS THE QUESTION
by Wesley Granger PhD, RRT A
colleague recently asked me what the latest recommendations were for ABG temperature correction. I had to admit that I was
not sure what the current thinking was, so I went on a journey through the literature to see if I could determine the current recommendations. As I discuss temperature correction there are two terms I will be using that I should define accurately here at the beginning. When I use the term "Corrected" I will be referring to the ABG values as corrected to the patient’s body temperature by the blood gas analyzer and "non- corrected" will refer to the ABG analyzer readings at 37°C. One of the interesting facts that I came across was discussed in
detail back in the 1960s. When the temperature of a blood sample changes in a closed system the pH, PCO2, and PO2 will change, but the SO2, O2 Content, CO2 Content, and HCO3- will not change. A closed system occurs within the blood vessels of the pa- tient and within the sampling chamber of the blood gas analyzer. You must also keep in mind that the oxyhemoglobin dissociation curve will shift with changes in body temperature. When the pa- tient’s temperature decreases the oxyhemoglobin dissociation curve shifts to the left resulting in an increased affinity of hemoglobin for oxygen and this means that at the tissue level the hemoglobin will not release oxygen as easily which could result in tissue hypoxia. THE ALPHA-STAT & pH-STAT MECHANISMS
Hypothermia has been studied in more detail than hyperther-
mia because of the clinical interest in induced hypothermia during surgery, especially during cardiopulmonary bypass. Also, tempera- ture increases are limited to about 4°C while decreases may be in- duced down to 25°C. Some the early studies looked at how different animals respond to hypothermia. In endothermic animals (including humans) as the body temperature is lowered the corrected pH will increase and the corrected PCO2 will decrease. This mechanism is thought to attempt to keep the ratio of [H+]/ [HCO3-] at a constant value which assures continued functioning of proteins (enzymes) at normal levels. If you look at the non-corrected pH, PCO2, and PO2 values they will be normal (i.e. 7.40, 40 and 95). This mechanism has been called alpha-stat regulation. The research also found that animals that have a hibernation
cycle will change their ventilation as their metabolism changes so that as their body temperature drops the temperature corrected pH and PCO2 will remain close to normal (e.g. 7.40 and 40) this means that the non-corrected values will show an increased PCO2 and de- creased pH (e.g. 7.24 and 72 measured at 37°C when body tem- perature 25°C) indicating a respiratory acidosis. This mechanism appears to maintain cerebral blood flow and better cerebral oxy- genation during hypothermia and has been given the name pH-stat regulation. The alpha-stat and pH-stat mechanisms have both been
6 Focus Journal Fall 2011
studied with induced hypothermia during surgery. Studies have found better cerebral perfusion and oxygenation with the pH-stat method but more cardiac arrhythmias. The pH-stat method re- quires changes in minute ventilation or during bypass requires the addition of CO2 into the oxygenator of the bypass pump. The debate and controversy about which method is best to use even continues in the current literature. The current general consensus seems to be that with moderate hypothermia (down to 30°C - 32°C) the alpha-stat method is preferred while with deep hy- pothermia (below 30°C) the pH-stat method may be preferred to better maintain cerebral oxygenation.
PROS and CONS OF TEMPERATURE CORRECTION The biggest problem with using temperature corrected blood
gas values is the lack of knowledge about what is "normal" at tem- peratures other than 37°C. Based on the fact that the SO2, O2 Content and HCO3- do not change with changes in temperature, it is argued that acid-base status does not change when temper- ature changes under the alpha-stat regulation mechanism. How- ever, with pH-stat regulation, the change in ventilation or the addition of CO2 results in a respiratory acidosis. To use some ex- amples, if a person is cooled from 37°C to 25°C, starting with normal acid-base balance, the pH will change from 7.40 to 7.57, the PCO2 will change from 40 to 23, the PO2 will change from 95 to 23 but remember that HCO3-, SO2 , CO2 content and O2 Content will all remain close to normal (< 2% change). Therefore, it can be argued that at 25°C a pH of 7.57, PCO2 of 23, and PO2 of 23 (assuming room air) represents the "normal" for that tem- perature. We cannot use the traditional "normal" ranges except at 37°C. Therefore, several articles I reviewed recommend that assessment of acid-base and oxygenation status be carried out on non-corrected (37°C) ABG values regardless of the patient’s actual temperature. The 37°C ABG results will show if and what kind of acid-base disorder is present and the assessment is conducted in the same manner using the well recognized "normal" values. If a patient is under going induced hypothermic surgery with the sur- gical team using the pH-stat protocol, then they will want to see the temperature corrected values since the goal is to keep the cor- rected pH close to 7.40 and the corrected PCO2 close to 40. A review of this topic in 1995 by Shapiro (Respir Care Clinics
N Am, 1995; 1(1):69-76) pointed out that oxygenation is a little harder to assess during hypothermia. There is no data describing or quantifying the balance between tissue oxygen demand and oxygen delivery at temperatures other than 37°C. Advocates of the pH-stat method argue that since hemoglobin’s affinity for oxy- gen increases as temperature decreases the addition of CO2 or
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