Physiologic Shunt (also called venous admixture or wasted blood flow) refers to the blood entering the arterial system without passing through ventilated areas of lung causing the PO2 of arterial blood to be less than that of alveolar PO2.
In this indice of ventilation-perfusion inequlaity, one pretends that all of the leftward movement of the arterial point (a) away from the ideal point (i) (the hypoxemia) is caused by the addition of mixed venous blood (v).
In a normal lung, some bronchial artery blood perfuses the bronchi and becomes partially O2 depleted. Another source of shunt is the coronary venous blood that drains directly into the cavity of the left ventrical throught the Thebesian veins. This poorly oxygenated blood lowers the arterial PO2. It is possible to calculate the amount of shunt flow if the shunt is caused by the addition of mixed venous blood to blood from the capillaries. The total amount of O2 leaving the system is (Total Blood Flow)*(O2 concentration in the arterial blood). This must equal the amounts of O2 in the shunted blood (Shunt Flow)*(Concentration of O2 in the venous blood) and end-capillary blood (Total Blood Flow - Shunt Flow)*(Concentration of O2 in end-capillary blood). The O2 concentration of end-capillary blood is calculated from the alveolar PO2 and oxygen dissociation curve.
If shunt occurs, hypoxemia cannot be abolished by giving the subject 100% O2. The arterial PO2 is elevated somewhat by an increase of dissolved O2. A shunt, however, does not usually raise PCO2 because chemoreceptors battle high PCO2 with increased ventilation.
(Qs/Qt)=(CiO2 - CaO2)/(CiO2 - CvO2)
Sample problems:
A man with normal lungs and right-to-left shunt is found at catheterization to O2 concentrations in his arterial and mixed venous blood of 20 and 16 ml/100ml, respectively. If the O2 concentration of the blood leaving the pulmonary capillaries is calculated to be 22ml/100ml, how large is his shunt? Will his arterial PO2 rise if he is given 100% O2 to breathe?
Answers:
(Qs/Qt) = (22 - 20)/(22 - 16) = 2/6 = 1/3 Therefore, one third of the total blood flow is shunted. YES, the arterial PO2 will rise upon breathing 100% O2.
For more information on this topic, please refer to West , page 159.
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