Evidence of shunting can be obtained by injecting a harmless dye into the blood and observing the pattern downstream (Figure 4.02). Normally, dye infused into a major vein shows an increase in concentration, followed by a fall when the blood is sampled in a major artery.
If a right to left shunt is present in the heart (e.g. Tetralogy of Fallot), the time the dye bolus takes to reach the sampling point will be decreased because all of the dye has not had to pass through the lungs. It is said that the "appearance time" has been shortened.
If a left to right shunt is present, the time it takes for dye to reappear after the initial peak will be shortened because some of the blood which reaches the left heart passes through the shunt (ASD, VSD) and reaches the aorta by quickly traversing the pulmonary circulation rather than going through the entire systemic circulation. In this case it is said that the "reappearance time" has been shortened.
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