Temperature correction of blood gas values

A blood sample with fixed O2 and CO2 contents will manifest different gas tensions when analyzed at various temperatures. Adjustment of the temperature of the measuring electrode to the patient's core temperature would add at least 30 min to the time required for each set of measurements and would severely complicate quality assurance in the laboratory. To avoid these impractical and undesirable factors, the pH, PCO2, and PO2 electrodes are encased in a constant 37 °C environment to which the blood sample chamber is also exposed.

The term 'temperature correction' refers to the application of mathematical adjustments to the values measured at 37 °C in order to produce a more accurate reflection of the gas tensions as they existed in the artery from which they were obtained. The availability of microprocessor technology makes the practice of temperature correction convenient and practical. Justification for temperature correction is based on the belief that 'knowing' the precise in vivo values is an advantage in patient care. This implies that either the normal ranges for pH, PCO2, and PO2 remain constant regardless of body temperature variation, or that the normal ranges are well established for all body temperatures. All available physiological and clinical data suggest that neither of these alternatives is valid.

The following factors (S.h,apiroJ995) strongly support the contention that there is no scientific basis for the routine practice of applying temperature corrections to blood gas measurements when alterations in body temperature are present.

1. The 37 °C values for pH and PCO2 reliably reflect the in vivo acid-base status at the patient's actual temperature.

2. The pH and PCO2 changes attributable to temperature variation do not affect the calculated bicarbonate value.

3. Since CO2 production and PaCO2 undergo parallel changes with temperature variation, clinical assessment of alveolar ventilation is most reliably reflected by applying the well-established homeostatic reference points at 37 °C.

4. When hypocapnia is desired to minimize the intracranial blood volume, the PaCO2 measurement at 37 °C is as clinically reliable as the temperature-corrected values.

5. Data reliably quantifying the balance between oxygen delivery and oxygen demand at temperatures other than 37 °C are unavailable; therefore temperature-corrected PO2 values do not improve our ability to make clinically relevant interpretations.

The popularity of the routine temperature correction of pH, PCO2, and PO2 values is based on the observation that large differences in blood gas values are present when the patient's temperature is profoundly hypo- or hyperthermic. This observation leads some clinicians to the unsubstantiated conclusion that the uncorrected 37 °C values are 'wrong'. The danger in this superficial thought process is that one might reach the unfounded conclusion that temperature-corrected values are 'right'.

Chapter References

Cerveri, I., et al. (1995). Reference values of arterial oxygen tension in the middle-aged and elderly. American Journal of Respiratory and Critical Care Medicine, 152, 934-41.

Shapiro, B.A. (1995). Temperature correction of blood gas values. Respiratory Care Clinics of North America, 1, 69-76.

Shapiro, B.A., Peruzzi, W.T., and Kozelowski-Templin, R (1994). Clinical application of blood gases (5th edn). Mosby Year Book, Chicago, IL.

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