Formula
1.Broadly Define Venous Admixture. What are the forms of venous admixture?
2.Define Anatomic Shunt
3.Define Physiologic Shunt
4.Show me a shunt equation
1.请叙述混合静脉血的定义。都包括哪些成分？
2.何谓解剖分流？
3.何谓生理短路？
4.分流方程。
参考答案：
1.请叙述静脉血掺杂的定义。其形式是什么？
Nunn定义静脉血掺杂为：肺毛细血管混合静脉血的参杂程度，通过其可计算出动脉血和肺毛细血管PO2间的差别（常用于替代理想的肺泡PO2）。Nunn进一步解释分流方程与波尔方程相似，其建立在公式性关系之上，即1min的动脉血流所含的氧量等于1min内通过肺毛细血管的氧含量与分流氧含量之和。需要注意的是，该模型假设，所有动脉血来自于正常的通气血流比或分流。然而，必须记住，实际静脉血掺杂不同于计算所得，因为thesbian和支气管静脉分流的存在使其与静脉血PO2不相等。
此外，动脉血的形成更多的来自于通气血流比大于0的肺泡，但可能低于正常。
混合静脉血的来源包括：
•Thesbian静脉
•支气管静脉
•先天性心脏病
•肺部感染
•肺水肿
•肺部新生物
•肺动静脉瘘
2.何谓解剖分流？
解剖分流指循环中动脉血直接与静脉血混合的情况。这些血来自于胸膜、细支气管、Thesbian静脉，因此其分流肺的血液而直接进入左心。正常的解剖分流占心输出量的2-5%。
3.何谓生理短路？
生理短路指肺有血流灌注但通气不足，致气体交换和氧合的不足。生理短路因此被认为是回到左心和体循环的心输出量的一部分，但未经过肺的氧合。
4.分流方程。
Qs/Qt = Cc O2- CaO2/CcO2-CvO2
Qs：生理短路
Qt：全心输出量
Cc O2 ：终末毛细血管静脉血氧含量
Cv O2 ：混合静脉氧含量
Ca O2 ：动脉血氧浓度
编者注：有兴趣的读者可以参考Cruz与Metting的介绍。
Cruz与Metting指出，危重病患者中常计算肺分流指数(Qs/Qt以监测肺部氧合的效果。纯氧呼吸常导致计算的Qs/Qt增高，因为存在肺不张、通气灌注失衡，或二者兼而有之。当吸入氧浓度变化时，为恰当解释计算的Qs/Qt的变化，必须了解可能影响Qs/Qt的所有因素。作者对影响Qs/Qt计算的四个因素进行了详细的分析：VO[sub]2[/sub] (氧摄取), Qt (心输出量), Cc'O2(肺终末毛细血管氧含量)，和CvO2 (混合静脉血氧含量)。这些变量与下列方程有关，该方程通过合并Fick与经典分流方程获得：
Qs/Qt = 1 - [(VO2 /Qt)/(Cc'O2 - CvO2 )]
有趣的是，作者还通过三维立体图表现了这些变量，以帮助读者理解这些变量的作用。
英文参考答案：
Editors' note: The following definitions are derived from Nunn's Applied Respiratory Physiology, 4th edition (1).
Broadly Define Venous Admixture. What are the forms of venous admixture?
According to Nunn (1) venous admixture refers refers to the degree of admixture of mixed venous blood with pulmonary end-capillary blood which would be required to produce the observed difference between arterial and pulmonary end-capillary P O2 (usually taken to equal ideal alveolar P O2). Nunn further explains that the shunt equation is similar to the Bohr equation and is based on the axiomatic relationship that the total amount of oxygen in 1 minute's flow of arterial blood equals the sum of the amount of oxygen in 1 minute's flow through the pulmonary capillaries and the amount of oxygen in 1 minute's flow through the shunt. Remember, this model makes the assumption that all of the arterial blood is derived from the alveoli with normal V/Q ratio or from a shunt. However, it must be remembered that the actual venous admixture will differ from that calculated since the thesbian and bronchial venous drainage may very well not have the same PO2 as the venous blood. Furthermore, the contribution to the arterial blood is derived from alveoli having V/Q ratios greater than zero, but possibly less than normal.
Sources of venous admixture include:
•Thesbian veins
•Bronchial veins
•Congenital heart disease
•Pulmonary infection
•Pulmonary edema
•Pulmonary collapse
•Pulmonary neoplasm
•Pulmonary arteriovenous shunts
Define Anatomic Shunt
Anatomic shunt refers to the amount of venous blood which mixes with pulmonary end-capillary blood on the arterial side if the circulation. This blood, derived from the pleural, bronchiolar, and thesbian veins, thus "bypasses" the lungs and enters the left side of the heart. Normal anatomic shunt accounts for 2-5 % of cardiac output.
Define Physiologic Shunt
Physiologic shunt originates from lung that is perfused but poorly ventilated, resulting in impairment in gas exchange and thus oxygenation. Physiologic shunt can thus be thought of as the portion of cardiac output that returns to the left heart and systemic circulation without receiving oxygen in the lung.
Show me a shunt equation!
Qs/Qt = Cc O2- CaO2/CcO2-CvO2
where
Qs = Physiologic shunt
Qt = total cardiac output
Cc O2 = end-capillary venous oxygen content
Cv O2 = mixed venous oxygen content
Ca O2 = concentration of oxygen in arterial blood
Editors' note: The interested reader is referred to a tutorial by Cruz and Metting (reference #2).
Cruz and Metting note that the pulmonary shunt fraction (Qs/Qt) is frequently calculated in critically ill patients to monitor the effectiveness of pulmonary oxygenation. The breathing of pure oxygen often results in higher calculated Qs/Qt values that have been attributed to the development of atelectasis, ventilation-perfusion imbalance, or both. To interpret properly the changes in calculated Qs/Qt that occur when the inspired oxygen fraction is altered, the changes produced in all the variables affecting Qs/Qt must be known. The authors have developed a tutorial which presents an in-depth analysis of the four variables affecting the calculation of Qs/Qt: VO2 (oxygen uptake), Qt (cardiac output), Cc'O2 (oxygen content in pulmonary end capillaries), and CvO2 (oxygen content in mixed venous blood). These variables are related according to the following equation, which is derived by combining the Fick and the classic shunt equations: Qs/Qt = 1 - [(VO2/Qt)/(Cc'O2 - CvO2)]. Interestingly, the authors also include three-dimensional surface representations relating these variables to help the reader understand the effects of these variables on the calculated Qs/Qt.
References:
1.Nunn JF. Distribution of pulmonary ventilation and perfusion. In: Nunn's applied respiratory physiology. 4th ed. Boston: Butterworth Heinemann; 1993, 178-81.
2.Cruz JC, Metting PJ. Understanding the meaning of the shunt fraction calculation. J Clin Monit 1987;3:124-34
编辑：西门吹血
