Variability of the temperature coefficients for pH, pCO₂, and pO₂ in blood

Abstract

Blood pH, pCO₂ and pO₂ were measured at 37 °C and 25 °C and the temperature coefficients were calculated as δpH/δT; δlg pCO₂/δT, and δln pO₂/δT. A total of 204 blood specimens were obtained from 10 patients undergoing coronary artery bypass grafting with extracorporeal circulation including hypothermia and hemodilution. Multivariate regression analysis gave the following equations:

The constants in the parentheses are the mean values of the variables. There is a significant negative correlation between the two temperature coefficients. Unfortunately pH(37) and pCO₂(37) were so closely correlated that it is impossible to separate the individual effects of the two, and the same applies to cHb and the plasma protein concentration. Therefore it may be fortuitous that the pH-temperature coefficient appears to vary with pCO₂ (not pH) while the pCO₂-temperature coefficient appears to vary more with pH than with pCO₂.

The pO₂-temperature coefficient was found to vary with pO₂ and cHb as previously predicted. At a very high pO₂ the temperature coefficient drops to an average value of+ 0.013; at low pO₂ it increases to about 0.069. Ahigh hemoglobin concentration results in a higher temperature coefficient especially at intermediate pO₂ values.

Accurate correction of pH and blood gas values measured at 37 °C to values referring to the temperature of the patient, or conversion of values measured at the actual temperature (using extracorporeal or catheter tip continuous monitoring) to values referring to 37 °C requires more accurate temperature coefficients than are currently employed in most commercial equipment.

Key words:

• Blood gas analyzers
• carbon dioxide
• hydrogen ion
• hypothermia
• oxygen