Pulmonary shunt

If every alveolus was perfectly ventilated and all blood from the right ventricle were to pass through fully functional pulmonary capillaries, and there was unimpeded diffusion across the alveolar and capillary membrane, there would be a theoretical maximum blood gas exchange, and the alveolar PO2 and arterial PO2 would be the same. The formula for shunt describes the deviation from this ideal. or deoxygenated blood flowing directly from pulmonary arterioles to nearby pulmonary veins through anastomoses, bypassing the alveolar capillaries. In addition, some of the smallest cardiac veins drain directly into the left ventricle of the human heart. This drainage of deoxygenated blood straight into the systemic circulation is why the arterial PO2 is normally slightly lower than the alveolar PO2, known as the alveolar–arterial gradient, a useful clinical sign in determining the cause of hypoxemia. The alveolar-arterial (A-a) gradient measures the difference between oxygen concentrations in the alveoli and the arterial system. This is an important clinical method of narrowing the differential diagnosis for hypoxemia. The gradient calculation is as follows: :A-a Gradient = PAO2 - PaO2 Where PAO2 represents the alveolar oxygen pressure and PaO2 represents the arterial oxygen pressure. The alveolar oxygen pressure is not easily measured directly and is therefor estimated using the alveolar gas equation. The A-a gradient should theoretically be zero in a healthy person, but almost never is. A normal person will have an A-a gradient estimated by: Normal Gradient = (Age in years/4) + 4. If the A-a gradient is significantly higher than the result of this equation, the person likely has a disorder of the alveoli, causing oxygen to be unable to diffuse into the blood. If the person has a normal A-a gradient but still has hypoxemia, then there is probably a cause unrelated to the alveoli, such as hypoventilation, obstructive lung disease, or shunting. ==Pathophysiology==

Efficient pulmonary gas exchange depends on matching alveolar ventilation (V) to pulmonary perfusion (Q), often expressed as the ventilation–perfusion (V/Q) ratio. A pulmonary shunt represents the extreme end of V/Q mismatch in which perfusion persists through lung units that receive little or no ventilation (V/Q≈0), so venous blood reaches the left heart without being oxygenated in the alveoli (venous admixture). When shunt is large or HPV is impaired, arterial hypoxemia worsens. Hypoxemia caused by V/Q mismatch generally improves with supplemental oxygen, but true shunt shows a comparatively poor response because part of the cardiac output bypasses ventilated alveoli. For this reason, breathing 100% oxygen (a hyperoxia test) can be used to help estimate the shunt fraction: after sufficient time on 100% oxygen, contributions from V/Q mismatch and diffusion limitation are minimized, and persistent arterial deoxygenation largely reflects shunted blood (with a small contribution from very low V/Q units). Hypercapnia is uncommon until the shunt fraction is large, because carbon dioxide elimination is often maintained by increased overall ventilation. In contrast, dead space represents the opposite extreme (very high V/Q), where ventilation is present but perfusion is reduced or absent, as can occur in pulmonary embolism. In such cases, hypoxemia commonly reflects redistribution of blood flow and the development of low V/Q units elsewhere in the lung rather than the dead-space regions themselves. ==See also==