Wednesday, December 15, 2010

Cause Of Apical Cavitating Lung Lesions

A patient comes in to the doctor because of a chronic cough. He notes occasional streaks of blood in his sputum. Chest x-ray reveals multinodular, cavitating lesions in the apical posterior segments of both lungs with evident satellite lesions. The condition described is likely to occur in the apices of the lungs because they  

         A. are better perfused than the base 
         B. are more acidic than the base 
         C. contain more alveolar macrophages than the base 
         D. have a higher PO2 than the base 
         E. ventilate better than the base  


         The correct answer is D. The presentation is typical for reactivation pulmonary tuberculosis. The patient may also note fever, malaise, and weight loss. The high PO2 found in the upper portion of the lungs provides a favorable environment for growth of Mycobacterium tuberculosis, leading to reactivation tuberculosis. (In contrast, primary tuberculosis tends to occur in the lower and middle lobes, where small infectious particles are most likely to lodge after being inhaled.)  

         Ventilation increases from the top to the bottom of the lung, so choice E is wrong. Perfusion increases even more rapidly than ventilation, so choice A is also wrong. As a result, the ventilation-perfusion ratio decreases from the top to the bottom of the lung. The higher ratio at the apex of the lung results in a relatively elevated PO2 at that location.  

         The apex of the lung has a higher pH than the base, so choice B is wrong. Because the ventilation-perfusion ratio is higher at the apex, PCO2 would be lower, thus increasing the pH.  

         Regional differences in the density of alveolar macrophages (choice C) are not known to cause the described predisposition.