(a) Describe venous admixture and the sources contributing to it in an adult (60% of marks).
(b) Explain the effects of supplemental oxygen on arterial hypoxaemia (40% of marks).
(a) This part of the question required candidates to provide a definition of venous admixture and provide detail on the sources. This would include anatomical (including atelectasis/closing capacity) and V/Q Mismatch or Scatter. V/Q Scatter was commonly omitted from answers. The use of the verb “describe” implies that further details are required beyond a simple list of the sources (such as a description of their mechanism and relative importance).
(b) This part required the effect supplemental oxygen on the many causes of arterial hypoxaemia to be explained. This would require the effect that this has on areas of true shunt and different degrees of shunt fraction and also on V/Q scatter. Information for this can be found on Nunn's Respiratory Physiology.
a) Venous admixture is that amount of mixed venous blood which would have to be added to ideal pulmonary end-capillary blood to explain the observed difference between pulmonary end-capillary PO2 and arterial PO2.
Sources of it are:
b) Effect of supplemental oxygen on arterial hypoxemia:
| Cause of hypoxia | Effect of supplemental oxygen |
| Alveolar hypoventilation | As the cause is consumption of oxygen with insufficient supply in atmospheric gas alone, this is relieved by increasing the concentration of O2 in the inspired gas mixture. All other factors remaining equal, this should increase the arterial oxygen content in proportion to the product of the FiO2 and the minute volume. |
| Decreased atmospheric oxygen tension | |
| Decreased blood-gas barrier surface area | Supplemental oxygen increases the concentration gradient for diffusion and therefore increases the diffusion of oxygen across the membrane, proportionally to the increase in FiO2, and inversely proportionally to the decrease in surface area and increase in membrane thickness. Increased cardiac output decreases the efficiency of supplemental oxygen by decreasing the alveolar capillary transit time of the erythrocytes. |
| Decreased blood-gas barrier permeability | |
| V/Q mismatch/scatter | Increasing the FiO2 will reverse the hypoxia when it is due to a low V/Q ratio (V/Q scatter), but with less effect the lower the V/Q ratio. |
| Intrapulmonary shunt | Increasing the FiO2 will have no effect on hypoxia due to "true" shunt, where the V/Q ratio is 0. |
| Intracardiac shunt | As supplemental oxygen may reverse hypoxic pulmonary vasoconstriction, it will lower the afterload for the right ventricle, reducing right sided pressures and decreasing the fraction of a right-to-left shunt, thereby relieving some of the hypoxemia. |
| Decreased mixed venous oxygen content due to decreased cardiac output | Supplemental oxygen will reliably maintain the arterial oxygen content in these scenarios, as long as venous blood is delivered to the alveoli, irrespective of how anoxic the venous blood becomes. |
| Decreased mixed venous oxygen content due to increased metabolic rate |
Bigeleisen, Paul E. "Models of venous admixture." Advances in physiology education 25.3 (2001): 159-166.
Cruz, Julio C., and Patricia J. Metting. "Understanding the meaning of the shunt fraction calculation." Journal of clinical monitoring 3.2 (1987): 124-134.
Bigeleisen, Paul E. "Models of venous admixture." Advances in physiology education 25.3 (2001): 159-166.
Cruz, Julio C., and Patricia J. Metting. "Understanding the meaning of the shunt fraction calculation." Journal of clinical monitoring 3.2 (1987): 124-134.
Berggren SM. The oxygen deficit of arterial blood caused by nonventilating parts of the lung. Acta Physiol Scand 1942; 4:Suppl 11:1-92
Sarkar, Malay, N. Niranjan, and P. K. Banyal. "Mechanisms of hypoxemia." Lung India: official organ of Indian Chest Society 34.1 (2017): 47.