True shunt

Shunt flow is blood that enters the arterial circulation without passing through ventilated lung. Most anatomical shunts are extrapulmonary, although arteriovenous malformations in the lung also cause shunt. Blood flowing through consolidated or atelectatic lung behaves as if it were shunt flow, although it can also be considered as one extreme of ventilation-perfusion mismatch. This is the most frequent cause of shunt encountered in clinical practice.

In venous blood, oxygen saturation is highest in the inferior vena cava, lower in the superior vena cava (which is most equivalent to mixed venous saturation), and lowest in the coronary sinus. Therefore the saturation of shunted blood depends on the level of the shunt.

Because of normal pressure gradients, blood ordinarily flows through congenital or acquired defects from the left side of the circulation to the right until pulmonary hypertension develops in response to chronic right-sided volume overload. At that point, pressures on the right may equal or exceed those on the left (in any given chamber) and venous blood enters the arterial circulation directly through the defect. A left-to-right shunt can also be acutely converted to a right-to-left shunt when pulmonary artery pressures rise abruptly. Pulmonary emboli due to air, fat, or clot, for example, cause acute increases in pulmonary artery pressure and can cause right-to-left shunting in susceptible patients such as those with a 'probe-patent' foramen ovale. Moreover, in patients with complex congenital heart disease, the pulmonary circulation is dynamically responsive to changes in the partial pressure of oxygen and CO 2, and a decrease in the former or an increase in the latter can result in life-threatening diversion of blood flow to the systemic circulation.

The shunt equation

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Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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