Sympathetic Blocking Agents

Alpha blocking agents

If you stimulate the alpha receptor, you get vasoconstriction (and sweat/salivary secretions and decreased intra-ocular pressure).  But now we switch gears to alpha blockers.  If you block the alpha receptor, the blood vessels dilate and blood pressure goes down.

Pheochromocytoma is a tumor in the adrenal gland that causes the secretion of norepinephrine which causes vasoconstriction and the blood pressure goes sky high.  We need to block those alpha receptors.  Most commonly phenoxybenzamine (dibenzyline) is used.

Remember how lervarterenol has extremely strong alpha properties and some beta-1 properties?  A nurse may titrate that together with a drug called phentolamine (regitine) to block its very powerful alpha effects so we get the beta-1 effects to speed up the heart rate and increase blood pressure without the extreme vasoconstriction.

Beta blockers (non selective beta 1-2 blocker)

So, what happens when you block beta 1?  You will decrease the heart rate.  Why do you want to decrease heart rate?  Because it’s too fast!

What happens when you block beta 2? Bronchoconstriction, so it becomes harder to breath.

Why would you ever broncho-constrict somebody?  Truthfully, only one answer comes up, and that’s if you want to kill them.  So asthmatics are going to have a problem with beta blockers, BUT… let’s look at this drug list.  Notice the beta blocker group is broken into beta-1/2 non selective and primarily beta-1. If you had to use a beta blocker on an asthmatic you HAVE to use a primarily beta-1 active drug.

Beta-1/Beta-2 (non-selective)

• Propranolol(Inderal)
• Nadolol (Corgard)
• Timolol (Blocadren)
• Pindolol (Visken) – also ISA activity
• Labetalol (Normodyne, Trandate) – also alpha blocking activity

Primarily Beta-1 activity

• Atenolol (Tenormin)
• Metoprolol (Lopresor)
• Acebutolol (Sectral) – also ISA activity

Ophthalmic (Topical)

• Timolol (Timoptic)
• Betaxolol (Betoptic)
• Levobunolol (Betagan) – Primarily beta-1 
  

Therapeutic Use of Beta Blockers

Why do we utilize beta blockers?

Cardiac arrhythmia or tachycardia.

Hypertension (if you lower the heart rate it brings down blood pressure but we rarely use it only for that purpose).

High renin hypertension. Usually we don’t know why someone has hypertension, but usually in younger men we can see that their renin is high and that’s telling us their blood pressure is high.  Renin is secreted from the kidney and angiotensin-1 goes to angiotensin-2 which then goes to the angiotensin-2 receptor and causes constriction.  Angiotensin-2 is our bodies most potent vasoconstrictor.  This is good if you’re dehydrated.  Then aldosterone is released which causes retention of sodium and water (and excretion of K+) which is also good if you’re dehydrated and your blood pressure needs to go up.  But if you’re not dehydrated and you’re a young man and your kidneys decide to secrete more renin than usual, your BP goes up and you become hypertensive.  Beta blockers decrease renin release.  If any of this is confusing, review the RAA system.

Angina:  Angina is like saying a person is having a pre-heart attack.  O2 demand exceeds o2 delivery.  In an MIA there’s permanent damage.  In angina, the balance goes back in balance and there is no damage.  But next time, there might be damage.  What will a beta blocker do to their heart?  It will decrease the heart rate which means it will decrease cardiac workload which decreases oxygen demand and causes us to go back into balance.

Historically, we found that after a heart attack a person is vulnerable for about 18 months for having a fatal cardiac arrhythmia so we put them on beta-blockers to prevent them from having that.  This month a study came out that says we’re not sure whether beta blockers are doing anything for this situation.  Time will tell whether this will change the protocol.

Migraine we talked about earlier, beta blockers are used prophylactically but we don’t know why, they just work.

Beta blocker cautions

1. If a person has asthma, and we have to use a beta blocker, we have to use a primarily beta-1, but no guarantees that that’s safe.
2. Insulin dependent diabetic:  Our big concern is that a diabetic is hypoglycemic, their symptoms would normally include shaking, sweating and their heart is racing.  If they are on a beta blocker, they can’t shake, they cant sweat as much and their heart won’t race, effectively masking the symptoms of hypoglycemia.
3. Peripheral vascular disease