Carbon dioxide transportation and haemoglobin oxygen affinity during haemodilution

The increase in the PCO2 of tonometered, haemodiluted (Hct 15%) blood samples was on an average 8 mm Hg higher than in the undiluted samples of the same blood when a fixed amount of Co2 was added to both samples. Concurrently the actual P50 increased on an average 2 mm Hg more in the diluted than in the undiluted samples. In a series of patients undergoing coronary artery bypass grafting the arterial-mixed venous PCO2 difference was on an average 5 mm Hg higher during haemodilution (Hct 16%) than in the control situation. In another series of similar patients the arterial-coronary sinus PCO2 difference increased from 9 +/- 1 to 14 +/- 2 mm Hg when the Hct fell to the level of 22%. The haemodilution-induced decrease in the total CO2-transporting capacity of the blood leads to an increased PCO2 and a decreased pH as compared to the situation without haemodilution, if the same amount of CO2 is added. In the clinical situation the gained decrease of the haemoglobin oxygen affinity probably plays a part in the compensation for the decreased oxygen capacity of the blood caused by the haemodilution.