- Right-Left Shunts occur when oxygen-poor blood from the right heart flows in the left heart without passing through functional, ventilated alveoli. In certain cases, the magnitude of the shunted blood can be large enough to result in a reduction in the partial pressure of arterial oxygen such that hypoxemia arises. In some sense, a right-left shunt is simply a particular instance of a ventilation-perfusion defect in which the ventilation-perfusion ratio is simply zero; however, we discuss this scenario separately in order to highlight some unique features.
- Right-left shunts result in a particular volume of de-oxygenated systemic venous blood mixing in with oxygenated blood leaving ventilated areas of the lung. The result is a dilution of oxygenation when these two sources of blood mix and thus an intermediate partial pressure of oxygen within the mixture. The final partial pressure of oxygen ultimately depends on the ratio of shunted blood to total arterial blood with larger shunt volumes reducing the oxygen tension closer to that of mixed systemic venous blood. When right-left shunts are large enough the partial pressure of oxygen may be reduced enough to yield bona fide hypoxemia. It is important to point out that right-left shunts typically do not result in hypercapnia likely because the compensatory hyperventilation in response to consequent hypoxemia is sufficient to eliminate the carbon dioxide from ventilated alveoli.
- A-a Gradient
- Because partial pressures of arterial carbon dioxide are typically normal in those with right-left shunts, the Alveolar Gas Equation used calculate the alveolar oxygen tension yields a partial pressure for alveolar oxygen that is largely normal. However, as mentioned above, right-left shunts can yield significant hypoxemia and thus substantialy lower-than-normal values for arterial oxygen. Consequently, the A-a Gradient in those with right-left Shunts is typically widened.
- Response to Oxygen Therapy
- Hypoxemia caused by right-left shunts prototypically cannot be corrected by oxygen therapy. This is an extremely important concept to understand as it is a key feature in the clinical diagnosis of hypoxemia. Oxygen therapy is of little use in contexts of right-left shunts because shunted blood, by definition, does not pass through ventilated alveoli. Thus, improving the partial pressure of alveolar oxygen cannot modify the the gas composition of blood traveling through the shunt. Although oxygen therapy will increase the partial pressure of oxygen in blood perfusing ventilated alveoli, this will yield little improvement in the total content of oxygen of the blood exiting ventilated alveoli as the hemoglobin oxygen saturation is already 97% at normal alveolar oxygen tensions (See: Oxygen Transport). Consequently, even though the oxygen partial pressure of this blood will be very high, the miniscule additional oxygen content will be far from sufficient to overcome it subsequent dilution with de-oxygenated shunted blood.
- A small right-left shunt is a natural consequence of the bronchial circulation which releases deoxygenated blood into blood entering the left atrium. Indeed, this small physiological right-left shunt is likely why the A-a Gradient in a healthy individual is not nearly zero and ranges between 4 - 8 mm Hg. Pathological right-left shunts typically arise due to causes of airway obstructions, consolidations from pneumonia, or atelectasis. Search for "ventilation-perfusion defect" to find other etiologies.