Beta antagonists, which pretty much everyone just calls beta blockers (and which one of my favorite teachers calls “The Wall”), reduce cardiac output by affecting the heart. Some of the newest ones, like carvedilol, actually cause vasodilation (can you guess how?), but our prototypes, propanolol and metoprolol, do not. They are more correctly secondary vasorelaxers (which is a phrase I just made up). Remember, beta-1 receptors increase cardiac output by making the heart work harder. This also forces more blood through the blood vessels and causes them to resist. When they are blocked, the heart doesn’t pump blood quite as forcefully. So, a pure beta blocker, among other things, prevents the smooth muscle from feeling the need to squeeze the heck out of your arteries. This allows blood to flow more freely!
You may also remember that beta blockers are negative dromotropic, that is, they block some of the electrical conductivity of the heart at the sinoatrial (SA) and atrioventricular (AV) nodes.
Don’t forget that beta-2 is really important for the lungs! So beta blockers that are non-selective, that is, block both beta-1 and beta-2 receptors, can bronchoconstrict. We have to be very careful that no one with breathing problems gets these by mistake!
In this section is your overview of three of the various sympatholytics: the three generations of beta blockers.