PH And Co2 Monitoring On Cpb

There are several methods of assessing blood pH and carbon dioxide levels. One must first understand the normal physiologic change of carbon dioxide concentrations and blood pH changes with the temperature. At 37°C, the pH is 7.4 whereas in the periphery where the temperature is 25°C, the pH is 7.6. A formula to describe this is that the change in pH for every degree Celsius is -0.017, i.e. for every degree Celsius the patient drops, the pH goes up by 0.017. This tempera-ture-pH slope is similar to that of water's neutral pH and results from a constant OH- concentration with regard to to H+ concentration over a great spectrum of temperatures. This is done to preserve the charge state of the dominant protein buffer imidazole. Thus, when progressing from high temperature to low temperature, to maintain pH the body compensates by decreasing the CO2. Thus at lower temperatures organisms have a higher pH and lower CO2. One must keep this in mind when measuring pHs while on cardiopulmonary bypass. The alpha stat method of blood pH maintenance on cardiopulmonary bypass is such that the carbon dioxide and pH are not corrected for temperature; i.e. the increased pH and decreased CO2 at hypothermic situations is not corrected. With the pH stat method of blood pH maintenance on cardiopulmonary bypass, the CO2 and pH are corrected, i.e. the increased pH and decreased CO2 at hypothermic conditions are corrected for by infusing more CO2 into the cardiopulmonary bypass circuit, thus raising the patient's CO2 and lowering the pH. Most authorities now agree that the pH stat method is less physiologic than the alpha stat method. Some say, however, that the pH stat method of infusing CO2 will result in better cerebral blood perfusion because of cerebral vasodilation, but this has not been shown to be clinically important.

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