If the coarctation is prior to the branching point of the L subclavian artery, then it will cause the arterial hypertension that is standard from coarctation, but only in the right arm. The left arm will join the leg arteries in having significantly decreased BP. GENERALLY, the coarctation is distal to the branch off point of the L subclavian artery. This usually causes arterial HTN in both arms, with lower blood pressures in the legs (and a low ABI)
For subclavian artery stenosis (which is more common in the left than the right) the decreased lumen will cause an increase in blood pressure due to the same amount of blood going through a smaller tube faster (= more pressure). Therefore the left arm will have higher pressure than the right arm.
