Which one of the following statements is true regarding the pathophysiology of pulmonary shunts?
a. Alveolar ventilation and the alveolar–arterial (A-a) gradient are both increased
b. Alveolar ventilation and gas exchange are both reduced
c. Alveolar ventilation is decreased and dead space is increased
d. Alveolar ventilation is normal while perfusion is decreased
e. Alveolar ventilation and the alveolar–arterial (A-a) gradient are both unaffected.
The answer is b. Alveolar ventilation and gas exchange are both reduced in a pulmonary shunt, while the alveolar-arterial (A-a) gradient is increased.
A pulmonary shunt is a volume of lung with adequate perfusion but poor or absent ventilation. This creates regions of little or no gas exchange so that blood leaving the shunt remains de-oxygenated. When the deoxygenated blood from the shunt mixes with the oxygenated blood from rest of the lung, it lowers the overall arterial oxygen concentration (PaO2) and if the shunt is large enough, cause systemic arterial hypoxia. Shunts may be as small as a few alveoli in a tiny patch of atelectasis or large as an entire lung. A common cause of pulmonary shunting is pneumonia where the alveoli fill with inflammatory fluid (consolidation).
The alveolar-arterial (A-a) gradient is a measure of the difference between the alveolar concentration of oxygen (PAO2) and the arterial concentration of oxygen (PaO2): A-a gradient = PAO2 - PaO2. Now, the ‘ideal’ alveolar oxygen concentration (PAO2) calculated by the alveolar gas equation is largely unaffected by pulmonary shunts while the arterial oxygen concentration (PaO2) measured by blood gas analysis is markedly reduced, resulting in an increase in the A-a gradient.
Dead space refers to areas of lung that are ventilated but not perfused (the opposite of a shunt) and therefore shunts do not affect dead space.