Beta blockers, especially non-selective ones, block adrenaline's action on both beta-1 receptors in the heart, slowing heart rate and force, and beta-2 receptors in the lungs, which can cause dangerous bronchoconstriction in asthma. Cardioselectivity matters immensely because a beta-1 selective agent minimizes this lung risk. In my cath lab, I've seen an asthmatic patient's wheezing spike after a resident mistakenly prescribed propranolol for angina. My own baba has a weak heart and reactive airways back in the village, so when his reports come, I triple-check that any beta blocker is cardioselective—his safety is worth my slower, careful checking.
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