Anomogram for venous shunt (Qs-Qt) calculation.

Venous shunt in cardiopulmonary physiology means that a fraction of the total cardiac output from the right side of the heart does not take part in gas exchange in the lungs and so passes unchanged into the arterial circulation (Berggren, 1942; Riley and Cournand, 1951; and Mcllroy, 1965). Since the shunted blood is mixed venous blood, the terms venous admixture, right-to-left shunt, and venous-to-arterial shunt are commonly used. When a patient has such a shunt, his arterial blood contains excessive mixed venous blood which has bypassed the lungs and mixed with welloxygenated blood. Normally, there is some mixed venous blood bypassing the lungs into the arterial stream through (1) normal true shunts of bronchial veins and Thebesian veins, and (2) normal virtual shunts due to slight deviations of the ventilation-perfusion ratio from the ideal value. But the total amount of such shunts is less than 7% of the cardiac output from the right ventricle (Comroe, Forster, DuBois, Briscoe, and Carlsen, 1962) and this has been called physiological shunt. In abnormal conditions, mixed venous blood can enter the arterial stream through (1) an abnormal anatomical rightto-left shunt due to a congenital cardiac anomaly, and (2) an abnormal virtual shunt from the alveoli which has a low ventilation-perfusion ratio. The magnitude of such a shunt is dependent on the size of the communication or ratio of regional uneven ventilation to perfusion. The venous-to-arterial shunt originating from a congenital cardiac anomaly is easily demonstrated by the injection of dye or contrast medium during cardiac catheterization. Venous-to-arterial shunt due to pulmonary disease can be determined by the oxygen method, provided alveolar ventilation is adequate to produce an alveolar oxygen tension of 100 mm. Hg or more and there is no significant diffusion barrier across the alveolar-capillary membrane. The relationship of shunted blood to the total cardiac output can be analogous to dead space ventilation to total pulmonary ventilation. The basis of the calculation, stated in words, is similar to Bohr's equation for the calculation of respiratory dead space. The total blood flow (QT) is equal to the amount of blood that has travelled the pulmonary capillaries (Qc) plus the amount of blood shunted (Qs) (Comroe et al., 1962). Similarly, the amount of oxygen in arterial blood is the sum of the amount of oxygen in blood that flows through the pulmonary capillaries and the amount of oxygen in shunted blood. Because the amount of oxygen is the production of the oxygen content (Co2) and blood flow (Q), the above statement, written as an equation, will be: