Blood Gas Temperature Correction
The creation of the first blood gas apparatus can be attributed to John Severinghaus, Leland Clark, and multiple other inventors with their respective contributions. This adjunct to standard care practices revolutionized the assessment of critical patients. Contemporary devices have evolved from the apparatus originally used by Severinghaus, but the techniques underlying blood gas detection are largely unchanged. Notably, modern devices reach temperatures of approximately 37 degrees centigrade. In many cases, this temperature is within the margin of error for an approximation of the patient’s temperature. However, in cases of hyperthermic or hypothermic patients, there must be consideration of the influence of temperature upon the detected partial pressure of oxygen (PO2), the partial pressure of carbon dioxide (PCO2), and pH. In this article, arterial blood gas detection will be described with attention to methods of management as they relate to the temperature correction.
Anatomy and Physiology
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The primary influences on temperature changes include physical properties of the gas, human metabolism as it relates to temperature, and the acid-base buffering system. The Henry law demonstrates that as temperature increases, gas becomes less soluble in solution. The Clark and Severinghaus electrodes both determine gaseous concentrations of oxygen and carbon dioxide, respectively. Therefore, as temperature increases, the gases detected by the electrodes become less soluble with higher partial pressures.
- Henry Law: Xgas = S * Pgas
Xgas is the gas concentration within the liquid (mM/L), S is the gas's liquid coefficient of solubility, and Pgas is the partial pressure of the gas (torr)
As the liquid coefficient of solubility decreases with increased temperature, a decreasing gas concentration within the solution and increased partial pressure of the gas will result.
In regards to metabolism, carbon dioxide (CO2) is found mostly as bicarbonate, dissolved in the blood, and bonded to nitrogenous bases. During the heating of blood, imidazole loses protons that bind to bicarbonate and result in an increase of CO2 through buffering. As demonstrated by the Henderson-Hasselbach equation, the pH will decrease as the CO2 increases. The inverse relationship remains true as the temperature of the blood decreases.
- Henderson-Hasselback Equation: pH = pKa + log10 (A-/HA-)
Oxygen blood gas readings also increase following the Henry law. Additionally, the patient's hematocrit influences the readings. The partial pressure of oxygen may vary upon the temperature dependency of hemoglobin, which is the influence that temperature has on altering the P50 of hemoglobin. In humans, as blood temperature increases, the P50 increases, and more oxygen is released to exit the solution and be detected by the electrode. Considering the effect of hematocrit, a severely anemic patient may not have as robust of an increase in PO2 upon heating, as expected, or predicted by certain algorithms. This should be acknowledged when interpreting corrected blood gas values.
The Bohr effect of increasing P50 as environmental pH decreases, and the Haldane effect of increased oxygen affinity with decreasing hemoglobin-bound CO2 are interrelated to temperature effects on blood gases but have a modest influence on values.
The benefits of temperature correction in practice are equivocal. However, it remains important that the practitioner considers the physiology of hyperthermic and hypothermic states to determine the appropriate approach to a patient. That is, practitioners should recognize that a patient displays hypocarbia, hypoxia, and has higher alkalemia when their body temperature is lower compared to uncorrected blood gas values. Conversely, a hyperthermic patient displays hypercarbia, hyperoxia, with higher acidemia compared to uncorrected blood gas lab values.
Temperature correction of blood gases is most often noted in cardiopulmonary bypass and post-cardiac arrest states. In these instances, tissue perfusion is diminished, resulting in an enduring anaerobic metabolism and resultant acidemia. The tissues will consume less oxygen and produce less CO2 during low perfusion states. Depending on the perfusion to the brain and concerns for increased intracranial pressure due to hypercarbia-related increased cerebral blood flow or the opposite effect of cerebral ischemia in states of hypocarbia, a practitioner may want to use one of two common methods for patient management.
The methods of alpha-stat and pH-stat differ because, in alpha-stat management, homeostasis (e.g., PCO2 of 40 mmHg, pH of 7.4, and PO2 of greater than 60 mmHg) is targetted at the 37°C laboratory values without being corrected for patient's temperature. It is thought that alpha-stat is more appropriate for patients who are prone to decreased cerebral perfusion as the practitioner is using an overestimate of CO2 partial pressure to guide management. Alpha-stat follows the hemoglobin disassociation curve, which is thought to preserve intracellular pH and cerebral autoregulation. Electrochemical neutrality and intracellular pH remain intact with this method.
During pH-stat management, homeostasis is targetted at the laboratory values corrected for the patient's temperature. Ph-stat, or temperature corrected management, aims to maintain normocarbia, normoxia, and an approximately normal pH at the patient's temperature. Ph-stat is thought by some to be more appropriate for patients who are prone to increased intracranial pressure because management is determined by the corrected blood gas values influenced by a hypothermic state with lower CO2 values. Regardless of the method used, there is no evidence of superior neurological outcomes with one particular approach. Some resources state that the general consensus of The Royal College of Anesthetists in the United Kingdom (UK) is to use the alpha-stat method.
There are no contraindications to blood gas temperature correction as blood gas correction poses no risk to the patient. The data is inconclusive regarding the definitive long-term benefits of a particular approach between alpha-stat and pH-stat. The approach should be tailored to the individual.
pH-stat management requires an oxygenator to alter carbon dioxide receipt as well as an in-line blood gas analyzer.
- The provider ordering the blood gas
- The proceduralist providing venous or arterial access
- The technician drawing the blood for the sample
- The personnel transporting the sample for processing
- The personnel who process and report the findings
Preparation for arterial access and venipuncture is beyond the scope of this review. After access to blood is secured, preparation for blood gas correction requires adequate training for the individual drawing the sample, a known, reliable temperature of the patient at the time of the draw, rapid transport to processing, and a known temperature of the sample while being processed.
Technique or Treatment
The practitioner may use a rudimentary estimate of the effect of temperature on blood gas values with the following simple equations:
For each 1°C decrease in body temperature below 37°C, the PO2 will decrease by 5 mmHg, PCO2 will decrease by 2 mmHg, and pH will increase by 0.012. Arterial CO2 increases by 4.6% for each 1°C over 37°C.
- logcPao2 = logPao2 + [T-37]/2.303 * f, where f = 0.058/(0.243[Pao2/100] * 3.88 + 1) + 0.013
The method of Nunn is one algorithm that can be used to determine CO2 with temperature changes: (PatcCO2); PatcCO2 PaCO2/10^0.0185 (change in T), where the change in T = 37°C - body temperature(C).
Most temperature correction guidelines are based on empirical data and may not take specific patient qualities into account.
It was common practice to cool blood gas tubes in order to slow leukocyte metabolism, which would result in inflated PCO2 values and an underestimate of PO2 and pH. Cooling is no longer recommended as PVC syringes are widely used, and as the tubes cool, they become more permeable, releasing PO2 and resulting in an underestimate. Instead, practitioners are encouraged to process blood gas samples within 15 minutes for more accurate PO2 results with minor metabolic interference and within 30 minutes for all other desired values.
Other influences on blood gas readings that can result in inaccurate findings include capturing an air bubble in the sample, shaking the sample vigorously, drawing from a line containing fluid other than blood, and improper phlebotomy technique.
There is conflicting evidence concerning temperature corrected blood gas values and their usefulness in patient management. Correcting temperature when interpreting blood gases should be used only when specifically requested. However, it is very important to understand the effects of temperature on blood gases, and the lack of knowledge of these effects can lead to potential abnormal interpretations and erroneous clinical decisions.
Enhancing Healthcare Team Outcomes
A laboratory's acquisition of blood samples includes provider order, identification, collection, transportation, and separation/preparation. This is only the preanalytical phase of a blood gas order, and numerous steps are liable to go awry. Using point-of-care testing (POCT) minimizes the number of teams involved. Depending on the situation, the person using POCT may also be the ordering practitioner, thereby closing the feedback loop. Depending on the technology, data may be immediately uploaded to the electronic medical record (EMR), allowing for efficient transmission. Although the process is simplified by eliminating the variable of multiple team members, there are common and potentially devastating errors that can still occur. One of the most impactful mistakes, regardless of the method of testing used, is the misidentification of the patient. This error may be avoided by standard practices akin to time out in the operating room, wherein the patient and correct test are identified prior to drawing the sample. An additional error pertinent to blood gas temperature correction is temperature recording. In order to accurately correct the readings for a patient's temperature, the correct temperature must be taken. The correct temperature method must be recorded at the time of the draw, and this information relayed to the laboratory. The sample should be transported and processed within 15 minutes to 30 minutes, depending on the desired data. The laboratory must have the equipment and availability to process the information. The medical center should have the infrastructure to be able to communicate the findings to the ordering practitioner in a timely manner. Each step of this cascade could be examined for effectiveness. Should any shortcomings be identified, the medical center could implement a plan to address those shortcomings.
Nursing, Allied Health, and Interprofessional Team Interventions
Each party involved in the hand-off and readback of a blood gas sample provides an opportunity for error. Jacobs et al. intervened by creating a committee to oversee all POCT conducted through their medical institution. Furthermore, after its establishment, the committee continued to intervene once monthly to monitor teams, revealing that particular teams were delinquent in upholding quality standards in regards to POCT. The intervention included a questionnaire to identify deficiencies and a plan for re-training. These same interventions could be effective in blood gas submissions.
Nursing, Allied Health, and Interprofessional Team Monitoring
Monitoring for blood gas POCT is also covered in Jacobs et al.'s proposal. Monitoring is conducted via monthly investigation of quality indicators involving proficiency testing, maintenance performance, patient identification, alert value confirmations, and quality control performance. In instances of blood gas readings aside from POCT, teams can implement the same protocol, this time involving the multiple teams included in laboratory-processed samples. These measures would ensure higher confidence that testing is being performed correctly. For example, monitoring blood sample labeling or the laboratory's processing time for blood gases could result in upholding a higher standard of patient care and improved patient data.
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