What is Physiological Shunt?

[E.S.Prakash]

Question from Khoo Piggy (KP), Year 2 Student; Reply by E.S.Prakash (ESP) Inline

1. KP: I wish to ask what is physiological shunt. I have referred to many books and also lecture notes and there are many differences between them. 

2. KP: According to lecture notes, physiological shunt is due to anastomoses between pulmonary capillaries and capillaries in bronchioles and vein and also flow of blood from coronary veins into the left side of the heart.

3. ESP: Yes, correct, some of the bronchial capillaries drain into pulmonary veins thereby reducing the oxygenation of pulmonary venous blood. Since this happens normally, this is considered "physiological".

4. KP: According to another book, physiological shunt is due to some parts of the lungs being not ventilated, so the blood flow through there cannot be oxygenated and deoxygenated blood returns to the heart. 

5. ESP: Yes, correct. Even in health, while the average ratio of pulmonary ventilation and pulmonary perfusion ratio is 0.8 (i.e., about 4 liters of alveolar ventilation to 5 liters of cardiac output flowing through pulmonary circulation), there are considerable regional differences in the balance of alveolar ventilation and pulmonary perfusion. Especially, in lower lung zones, the V/Q ratio is lower (and the absolute perfusion higher) and to a 'small' extent this does contribute to incomplete oxygenation of pulmonary capillary blood. 

6. KP: According to another book that I have referred, physiological shunt is due to gravity. In the upright posture, the force of gravity will pull the blood to the bottom of the lung and the perfusion in the apex of the lung is less compared to the base, so this is the cause of the physiological shunt.

7. ESP: Yes, almost correct, but I wish to refine the statement to indicate that gravity is a contributor to regional differences in pulmonary perfusion (Zone 1, Zone 2, Zone 3); I am sure you are clear that the V/Q ratio is lower at the lung bases (Zone 3, upright posture) and the absolute perfusion is also higher (same as Point 5 then); this is one (not the only) cause of Physiologic Shunt.

8. KP: I am confused. I do not know which one is correct. 

9. ESP: all are!

10. KP: Is it due to physiological shunt that the partial pressure of oxygen in arterial blood is less than the partial pressure of oxygen in alveoli by about 5 mm Hg?

11. ESP: Yes; this suggests that you got the basic concept right. To sum it up, Physiologic Shunt refers to any mechanism that "normally" contributes to deoxygenation of arterial blood (there are physiological mechanisms that operate in the pulmonary circulation; and anatomical bases such as anastomoses between bronchial capillaries and pulmonary veins; and drainage of some coronary venous blood into the left ventricle)to now you have noted that the magnitude of the deoxygenation of arterial blood caused by these mechanisms is only about 3-5 mm Hg but does not normally exceed 10 mm Hg and an alveolar - arterial oxygen gradient (PAO2 - PaO2) exceeding 10 mm Hg is indicative of an "abnormal" defect in gas exchange - i.e, we would no longer consider it "physiological". 

12: KP: Thank you first for clearing my doubts.

13: ESP: Welcome.