Thread by @andhen25

1/ Why tuberculosis has a preferential apical localization?
Is it the host? Is it the pathogen?
To answer this question, let’s review some cool stuff
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2/ Arnold Rich stated: “There is no more puzzling circumstance in the pathogenesis of pulmonary tuberculosis […] than the peculiar fact that in the adult the disease begins almost invariably in the upper portion of the upper lobe“
ncbi.nlm.nih.gov/pubmed/6839825

3/ M. tuberculosis is an obligate aerobe (they have an absolute requirement of O2 to grow).
*Tubercle bacilli respire maximally in vitro at O2 concentrations of 20-40%
*Experimental infections in animals are inhibited by very low O2 concentrations
Am Rev Tuberc 1939; 40:157-68

4/ Let’s review the oxygen tension levels in different parts of the lung
The partial pressure of alveolar oxygen (Pa02) at sea level is approximately 132 mm Hg at the level of the first anterior interspace, decreasing to 89 mm Hg at the level of the fifth anterior interspace

5/Higher apical alveolar oxygen pressures correlate with higher ventilation to perfusion (V/Q) ratios; favoring M. tuberculosis growth — at least initially.

6/ Tuberculosis is relatively rare in individuals with mitral stenosis, and this has been attributed to higher pulmonary artery pressure and increased apical pulmonary artery blood flow
ncbi.nlm.nih.gov/pubmed/21023380
sciencedirect.com/science/articl…

7/ Conversely, in pulmonary stenosis— in which pulmonary artery pressure is abnormally low —the incidence of pulmonary tuberculosis is strikingly high.

*Auerbach et al. The development of pulmonary tuberculosis in congenital heart disease. Am J Med Sci 1944; 207:219-30

8/ William Dock postulated in 1946 that “pulmonary arterial pressures at the lung apices in the erect position are very low. This would result in diminished regional formation of tissue fluid or lymph, leading to accumulation of bacterial products and toxic substances”

9/ Summing up, the apical portions of the lungs offer an ideal higher oxygen tension levels for an optimal TB growth
*Disclosure: TB can infect any lung part or body part! But may preferentially start on the lung apices.