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Update in Intensive Care and Emergency Medicine - part 3

Trong một trường hợp cực đoan, trong đó quá trình trao đổi chất hiếu khí là số không, sản xuất chuyển hóa CO2 (VCO2) cũng bằng không, và các nội dung tĩnh mạch bằng các nội dung động mạch. Tuy nhiên, mặc dù "tăng VCO2 'không thể xảy ra trong anaerobiosis | Adequate Hemodynamics A Question of Time 77 20 40 60 80 100 PCO2 mmHg Fig. 2. CO2 dissociation curve. CO2 content ml of whole blood vs. CO2 tension PCO2 . Each curve is described at constant base excess BE . As shown for the same CO2 content changing the base excess causes a great change in PCO2 see the broken line parallel to axes . and its clearance by the flow. In an extreme case in which aerobic metabolism is zero the metabolic production of CO2 VCO2 is also zero and the venous content equals the arterial content. However although increased VCO2 cannot occur in anaerobiosis there is no doubt that venous PCO2 or tissue PCO2 from gastric tonometry is increased during energy failure. The meaning of this phenomenon becomes clear if we consider the relationship between the CO2 content CvCO2 and the CO2 tension PvCO2 also called the CO2 dissociation curve. This is reasonably linear in the PCO2 range of 20 to 80 mmHg. However its position is strongly influenced by the acid base status of the medium Fig. 2 . During the passage into the tissue in normal conditions the decrease in oxygen saturation is associated with binding of H to hemoglobin. This effect Haldane buffers in part the acid-base changes induced by the addition ofVCO2 from the tissue. The overall picture is dramatically changed when a strong ion such as lactate is added from the tissue to venous blood. In this case part of the H increase due to the increase of the strong ion lactate is buffered by HCO3- which liberates dissolved CO2 PvCO2 according to the following reaction Added H HCO3V- CO2V H2O Indeed for a given venous CO2 content adding acid sharply increases the PvCO2. The phenomenon is quite clear if we consider the CO2 dissociation curve at different BE as shown in Figure 2. For the same CO2 content the change in BE 78 L. Gattinoni F. Valenza and E. Carlesso i.e. the addition of strong ions such as lactate results in a great change in PCO2. Indeed the large increase in venous PCO2 during critical .