EFFECT OF EXTREME METABOLIC-ACIDOSIS ON OXYGEN DELIVERY CAPACITY OF THE BLOOD - AN IN-VITRO INVESTIGATION OF CHANGES IN THE OXYHEMOGLOBIN DISSOCIATION CURVE IN BLOOD WITH PH VALUES OF APPROXIMATELY 6.30

Objectives: To determine the oxyhemoglobin dissociation curve in blood with pH of similar to 6.3 due to metabolic and superimposed respirato ry acidosis, and to evaluate the oxygen delivery capacity of the blood under these circumstances. Design: In vitro study. Setting: A blood g as laboratory in a university institute for respiratory physiology. Su bjects: Heparinized normal human blood. Interventions: The oxyhemoglob in dissociation curve was determined by measuring Po-2, pH, Pco(2), an d hemoglobin oxygen saturation at 37 degrees C in mixtures of blood fr om two reservoirs, both prepared by titration with lactic acid to a pH of 6.3 during tonometry with gases containing 4.2% CO2 and high and l ow oxygen percentages, respectively. For determination of the effect o f additional increases in Pco(2), the reservoir blood thus produced wa s prepared by further tonometry with gases containing 12.8% CO2 and th e same oxygen percentages.Measurements and Main Results: With the same degree of lactic acidosis (blood lactate concentration of 52 mmol/L), the position of the oxyhemoglobin dissociation curve was the same for blood with Pco(2) of 30 torr (4 kPa) and pH of 6.295 and for blood wi th Pco(2) of 90 torr (12 kPa) and pH of 6.165, During tonometry with a gas with Pco(2) of 30 torr (4 kPa) and Po-2 of 20 torr (2.7 kPa) and addition of increasing amounts of lactic acid, leading to a stepwise c hange in pH from 6.7 to 6.0, hemoglobin oxygen saturation decreased wi th decreasing pH from 6.7 to 6.4, but remained the same at a pH of bet ween 6.4 and 6.0, The measured rightward shift of the oxyhemoglobin di ssociation curve at such a low pH was clearly less pronounced than tha t calculated using commonly applied equations, in particular, at the l owest pH. The beneficial effects of the rightward shift of the oxyhemo globin dissociation curve on the estimates of extractable oxygen at a given venous Po-2 decrease with decreasing pH, and disappear rapidly w hen the Pao(2) is reduced below normal. Conclusions: The acidemia-indu ced rightward shift of the oxyhemoglobin dissociation curve does not i ncrease further at a pH <6.4, and is, at such extreme acidemia, less p ronounced than calculated by the commonly used equations, To obtain op timal tissue oxygenation in patients with severe circulatory failure a nd extreme metabolic acidosis, Pao(2) should be >250 torr (>33.3 kPa).