Let's talk about sympathetic blockers in the eye. The
sympathetic blockers are used to block that fight or flight
reflex. So remember sympathetic system wants to open up the
iris. So blocking that ability to see in the dark blocks the
dilation of the iris. And blocking the creation of intraocular
pressure because it blocks the secretion of the aqueous
humor or the liquid inside the eye. So timolol and levobunolol
are eye drops that are used for glaucoma. It decreases the
secretion of aqueous humor and the side effect is pinpoint
pupil or miosis. Partial agonist activity is kind of a unique
concept where the beta 1 blockers act as a blocker but those
same drugs have a little bit of agonist activity on the beta 2
receptor. So this is good for the asthmatic patient. An example
of that is acebutolol and labetolol. So remember that you have
one heart and two lungs, right. So beta 1 is for the heart,
beta 2 is for the lung because you have two lungs.
So if you block the effects of adrenaline on the heart, you
slow down heart rate. If you block the effect of adrenaline
on the lungs which are designed to normally breathe when you
are under stimulation. If you block the beta receptors in the
lung, you actually stimulate asthma. But if you are a agonist
for beta 2, you will treat asthma, you will help the asthma
get better so you can breathe better. That's why acebutolol
and labetolol are theoretically better in asthmatic patients.
I just want to mention very quickly another beta blocker called
nebivolol. This newest beta blocker has also activity on
nitric oxide. So this drug is now available in the United
States, recently available in Canada. It's been in Europe
for about 8 years. It's an excellant drug for heart failure and
it's probably the best beta blocker for the treatment of
high blood pressure as well. Now finally let's talk about
toxicity of these medications. One of the greatest problems
that you are going to get in overdosage of beta blockers or
excessive prescription doses of beta blockers is excessive
bradycardia. So watch your patients for low heart rate. The
heart rate drop can be so severe as to actually cause
AV blockade. So you can get first, second or even third degree
heart blocks. Heart failure is also a potential problem.
Those patients who have a low EF may actually have exacerbation
of their heart failures symptoms when you use too much
beta blockade. Another potential problem is bronchoconstriction.
Not so much through the beta 1 receptor but through cross
blockage of the beta 2 receptor. So just be aware that you
may exarcebate asthma in patients that you have given too
much medications to or if they have overdosed on them. A more
complicated issue is masking the symptoms of hypoglycemia.
Diabetic patients who have drops in their sugar under 4 can
have symptoms of tachycardia, tremelousness and diaphoresis.
Sometimes beta blockade will mask those symtoms so patients
are not aware that their blood sugars have dropped.
And finally and probably most commonly is sexual dysfunction.
I see this even at regular doses of beta blockade in a lot
of my patients who are in their 50 to 60 age range. This is
one of the reasons why we try not to use beta blockade
as a primary treatment of high blood pressure in patients who
are over the age of 50, but in patients younger than the age of 50
it's usually okay because it does'nt cause a lot of problems.
Okay, so there you have it. This is the overview of the
sympathetic system and a little bit of information on
beta blockade. We're going to be covering other areas
of autonomic function in the next lectures. Keep in mind
that we are going to have questions at the end of
the last of the lectures. Thanks very much.