It’s all about hypoxemia and hypercapnia. A big thick thorax can’t expand well and ventilation is affected. This is specially true during sleep, worsen by co-existing sleep apnea. High pCO2 will condition pulmonary capillary vasoconstriction as a way to try to improve VQ mismatch (lung “thinks” there is little perfusion so compensation occurs by increasing capillary pressure). Resulting hypertension is a burden to the heart. The right ventricle needs to work harder to pump blood into the lungs, creating a strain and eventually as decompensation occurs, upward congestion and ultimately right heart failure with generalized venous congestion. Check out this next illustration as a summary:
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-The Plague Doctor