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Emphysema
14%
29/210
Pulmonary fibrosis
10%
21/210
Pulmonary embolism
49%
103/210
Foreign body obstruction distal to the trachea
22/210
Exercise
17%
35/210
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Pulmonary embolism (PE) effectively results in a blockage of flow in a vessel to its corresponding alveoli. As a result, there is no extraction of oxygen or ventilation in this region (i.e. dead space physiology) and the alveoli have a gas composition similar to the trachea (PAO2=150, PACO2=0). There are two main V/Q alterations: dead space (V/Q = infinity) and shunts (V/Q = 0). Dead space is caused by an obstruction in blood flow (e.g. PE) resulting in ventilation of lung regions that are not perfused, and therefore participate in no gas exchange. A shunt is caused by an airway obstruction or atelectasis from a variety of causes and results in perfusion of lung regions that are not ventilated. In a pure shunt, 100% O2 does not improve PaO2, while it would improve PaO2 when dead space physiology is present. Wilbur and Shian describe PE in the context of deep vein thrombosis (DVT). A PE is typically a consequence of DVT. CT angiography is the diagnostic test of choice when available and not contraindicated. D-dimer has a high negative predictive value, but in a post-surgical patient, is almost universally elevated. Burrowes et al. describe the pathophysiology of acute PE. They report that this condition causes redistribution of blood in the lung, which impairs ventilation/perfusion matching and gas exchange. As a result, pulmonary vascular resistance increases, elevating pulmonary arterial pressure. Illustration A is a simple schematic which displays changes in the V/Q relationship where panel A represents the normal relationship, panel B depicts shunt physiology, and panel C depicts dead space physiology. Incorrect Answer: Answer 1: Emphysema is a diffuse process and does not change the oxygen content of an affected alveoli, but instead reduces the number of functioning alveoli. Answer 2: Pulmonary fibrosis is a diffuse process that does not change the oxygen content of an affected alveoli, but instead reduces the number of functioning alveoli. Answer 4: An airway foreign body would reduce the oxygen content of the affected alveoli (not increase as in PE), and would result in shunt physiology of the alveoli distal to the obstruction. Answer 5: Exercise does not significantly change the oxygen content of the alveoli; PAO2 may increase slightly with exercise in normal subjects, as PaCO2 decreases with increased ventilation: PAO2 = PiO2 - PaCO2 x [FiO2 + (1 - FiO2)/R]
3.6
(19)
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