ABG temperature correction: to correct or not to correct; that is the question.
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 patient 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 patient'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 hyperthermia because of the clinical interest in induced hypothermia during surgery, especially during cardiopulmonary bypass. Also, temperature increases are limited to about 4[degrees]C while decreases may be induced down to 25[degrees]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 decreased pH (e.g. 7.24 and 72 measured at 37[degrees]C when body temperature 25[degrees]C) indicating a respiratory acidosis. This mechanism appears to maintain cerebral blood flow and better cerebral oxygenation during hypothermia and has been given the name pH-stat regulation. The alpha-stat and pH-stat mechanisms have both been 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 requires 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[degrees]C-32[degrees]C) the alpha-stat method is preferred while with deep hypothermia (below 30[degrees]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 temperatures other than 37[degrees]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 temperature changes under the alpha-stat regulation mechanism. However, with pH-stat regulation, the change in ventilation or the addition of CO2 results in a respiratory acidosis. To use some examples, if a person is cooled from 37[degrees]C to 25[degrees]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[degrees]C a pH of 7.57, PCO2 of 23, and PO2 of 23 (assuming room air) represents the "normal" for that temperature. We cannot use the traditional "normal" ranges except at 37[degrees]C. Therefore, several articles I reviewed recommend that assessment of acid-base and oxygenation status be carried out on non-corrected (37[degrees]C) ABG values regardless of the patient's actual temperature. The 37[degrees]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 surgical team using the pH-stat protocol, then they will want to see the temperature corrected values since the goal is to keep the corrected 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[degrees]C. Advocates of the pH-stat method argue that since hemoglobin's affinity for oxygen increases as temperature decreases the addition of CO2 or decrease in ventilation (resulting in a respiratory acidosis) during this method will counter the left shift and therefore oxygen release at the tissues will be improved. However, there is no quantitative data that shows O2 Delivery is significantly affected. Shapiro recommends that oxygen content and oxygen indices involving oxygen content (e.g. Shunt fraction) should be calculated using the non-corrected ABG values. When you want to calculate oxygen tension based indices such as the A-a Gradient, PAO2/PaO2 ratio or PaO2/FIO2 ratio the non-corrected results should be used. The comparison of end-tidal PO2 and PCO2 measurements to arterial values will require that the temperature corrected values be used since the end-tidal values are measured at the actual body temperature.
1. There is no evidence that we should routinely temperature correct ABG results.
2. When assessing acid-base status the non-corrected results at 37[degrees]C should be used.
3. Oxygen contents, Shunt fraction and oxygenation assessments using the A-a Gradient, PAO2/PaO2, or the PaO2/FIO2 should use the non-corrected results.
4. If assessment comparisons involve end-tidal measurements then use the temperature corrected ABG results.
5. Temperature correct when requested to do so by the physician (e.g. during surgery with pH-stat regulation), but also report the non-corrected values. The correct interpretation of the results is the responsibility of the requesting physician.
6. When reporting temperature corrected results mark them clearly as temperature corrected and also report the non-corrected results.
7. If you are comparing blood gas results during hypothermia or hyperthermia between intravascular sensors and a temperature controlled ABG analyzer, then the analyzer results must be corrected for temperature since the intravascular sensors are reading values at actual body temperature.
8. Get an accurate temperature reading if you need to report temperature corrected results. If the patient's correct temperature is not given the corrected results are useless.
by Wesley Granger PhD, RRT
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|Title Annotation:||THE BLOOD GAS LABORATORY|
|Publication:||FOCUS: Journal for Respiratory Care & Sleep Medicine|
|Date:||Jun 22, 2005|
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