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Formula
1.What is oxygen content? How is it different from PO2? What are its units?
2.What is the formula used to calculate oxygen content? Calculate a normal patient's arterial and mixed venous oxygen content.
3.Which of the following will significantly increase arterial oxygen content?
A.Changing the inspired oxygen concentration from room air to 100% oxygen in a patient whose SaO2 on room air is 100%.
B.Transfusing packed red blood cells in a patient whose hemoglobin concentration is 10 g/dL.
C.Substituting fetal hemoglobin for adult hemoglobin when the PO2 = 30 mm Hg.
公式系列：
1.何谓氧含量？与PO₂有何不同？其单位是什么？
2.计算氧含量的方程是什么？计算一正常患者动脉和混合静脉氧含量。
3.下面哪个将明显增加动脉氧含量？
A.患者在吸空气下SaO₂为100%情况下，改变吸入氧浓度到100%。
B.患者Hb为10g/dL时，输入PRBC（浓缩红细胞）。
C.当PO₂ = 30 mm Hg时，用胎儿Hb替代成人Hb。

参考答案：
1.何谓氧含量？与PO2有何不同？其单位是什么？
氧含量可以定义为血标本所含的分子氧数量。常用单位为ml氧/dL血。其不同于PO2，PO2是血标本的氧分压。即使氧含量低，PO2也可以很高，因为全部氧含量Hb浓度的显著作用（见下）。一杯水与100%氧气平衡后PO2可达到760mmHg，但是氧含量却很低，低于1ml O2/dL。
2.计算氧含量的方程是什么？计算一正常患者动脉和混合静脉氧含量。
CaO2=1.34×[Hb] ×%sat+0.003×PaO2
第一步计算氧和Hb含量；其次，计算游离氧含量。如果我们考虑一Hb为15g/dL的正常患者，呼吸空气下PO2为100mmHg，SaO2为100%，动脉氧含量可以如下计算：
CaO2 = 1.34 (15) (100%) + 0.003 (100) = 20 + 0.3 = 20.3 ml/dL
对混合静脉血，Hb含量不变，但SvO2与PO2更低（大约75%和45mmHg），因此，氧含量更低：
CvO2 = 1.34 (15) (75%) + 0.003 (45) = 15 + 0.1= 15.1 ml/dL
3.下面哪个将明显增加动脉氧含量？
A.患者在吸空气下SaO2为100%情况下，改变吸入氧浓度到100%。
这影响很小，因为氧和Hb含量已经最大。只有游离部分增加了。重新计算上边患者的氧含量在PO2600时，表明氧含量仅轻度增加。
CaO2 = 1.34 (15) (100%) + 0.003 (600) = 20 + 1.8 = 21.8 ml/dL
B.患者Hb为10g/dL时，输入PRBC（浓缩红细胞）。
Manuever更有效果，是"blood doping" in athletes的基础。氧含量增加，因为第一步的数值明显增加：
输血前（Hb=10）：
CaO2 = 1.34 (10) (100%) + 0.003 (100) = 13.4 + 0.3 = 13.7 ml/dL
输血后（Hb=15）
CaO2 = 1.34 (15) (100%) + 0.003 (100) = 20 + 0.3 = 20.3 ml/dL
C.当PO2 = 30 mmHg时，用成人Hb替代胎儿Hb。
这也有很大区别，也是为什么胎儿能在脐静脉低PO2情况下存活的原因之一。胎儿Hb比成人Hb氧亲和力更高，因此低PO2实际上可合理的氧和血红蛋白。因此，任何增加Hb对氧亲和力的措施因为给与的PO2而将增加氧含量。

参考答案
What is oxygen content? How is it different from PO2? What are its units?
Oxygen content may be defined as the amount of molecular oxygen in a given sample of blood. The usual units are ml O2/ dL blood. It is not the same as PO2, which is the partial pressure of oxygen in a sample. PO2 can be very high even when total content is low, because of the overwhelming contribution of hemoglobin concentration in total oxygen content (see below). A glass of water equilibrated with 100% oxygen can have a PO2 approaching 760 mm Hg, but has a tiny oxygen content of much less that 1 ml O2/dL.

What is the formula used to calculate oxygen content? Calculate a normal patient's arterial and mixed venous oxygen content.

The first term calculates the portion of oxygen bound to hemoglobin, and the second calculates the contribution of dissolved oxygen. If we consider a normal patient with 15 g/dL of hemoglobin, breathing room air with a PO2 of 100 mm Hg, and with a SaO2 of 100%, then the arterial oxygen content can be calculated to be:
CaO2 = 1.34 (15) (100%) + 0.003 (100) = 20 + 0.3 = 20.3 ml/dL
For the mixed venous blood, the hemoglobin content and constants do not change, but the SvO2 and PO2 will be lower (about 75% and 45 mm Hg, respectively). Therefore the content will be much lower:
CvO2 = 1.34 (15) (75%) + 0.003 (45) = 15 + 0.1= 15.1 ml/dL

Which of the following will significantly increase arterial oxygen content?
Changing the inspired oxygen concentration from room air to 100% oxygen in a patient whose SaO2 on room air is 100%.
This will have little effect, since the contribution of oxygen bound to hemoglobin is already maximal. Only the dissolved component increases. Recalculating the above patient's content with a PaO2 of 600 shows a very modest increase in content:
CaO2 = 1.34 (15) (100%) + 0.003 (600) = 20 + 1.8 = 21.8 ml/dL

Transfusing packed red blood cells in a patient whose hemoglobin concentration is 10 g/dL.
This manuever is much more effective, and is the basis of the practice of "blood doping" in athletes. Content increases because the value of the first term markedly increases:
Before transfusion (Hb=10):
CaO2 = 1.34 (10) (100%) + 0.003 (100) = 13.4 + 0.3 = 13.7 ml/dL
After transfusion (to a Hb=15):
CaO2 = 1.34 (15) (100%) + 0.003 (100) = 20 + 0.3 = 20.3 ml/dL

Substituting fetal hemoglobin for adult hemoglobin when the PO2 = 30 mm Hg.
This also makes a big difference, and is part of the reason why the fetus can survive with such a low PO2 in the umbilical vein. Fetal hemoglobin has a higher affinity for oxygen than does adult hemoglobin, so that the low PO2 actually saturates the hemoglobin reasonably well. Indeed, anything that increases the affinity of Hb for oxygen will increase oxygen content for any given PO2.
References:
1.  West JB. Gas transport to the periphery. In: West JB. Respiratory Physiology, 3rd ed. Baltimore: Williams & Wilkins, 1985: 67-72.

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