00:01
Now, we have pacemaker activity.
00:05
We talked about the most important things.
00:07
That was how the nerves control the heart rate.
00:10
Let's talk about a couple other
things that control heart rate
because it's not only about the nerves,
although it’s mainly about the nerves.
00:17
It’s not only about the nerves.
00:19
There are a few hormones
that can affect heart rate,
such as you have a high amount of thyroid hormone.
00:25
So, that's T3, T4 in a hyperthyroid condition.
00:30
You have an increase in heart rate.
00:32
If you have low thyroid hormones,
T3 and T4,
you have a low heart rate.
00:39
Catecholamines can increase heart rate.
00:42
Now, what do we mean by catecholamines.
00:44
Catecholamines is a broad term that we use
for epinephrine and norepinephrine.
00:50
Now, these are ones that are
circulating around in the body,
not necessarily what is released by the nerves
that will bind directly to the SA node.
01:01
So, these are circulating hormones
traveling around in the blood,
traveling around in the blood.
01:07
We are mainly talking about
epinephrine, norepinephrine
or adrenaline and noradrenaline.
01:13
Those increase heart rate.
01:17
We have a few ions that also can change heart rate.
01:21
Potassium is probably our biggest
one we want to think about.
01:24
If potassium levels are high,
heart rates are a little bit lower.
01:27
If potassium levels are lower,
heart rate is higher.
01:30
So, they kind of work in an inverse relationship
between potassium and heart rate.
01:35
Why this might occur?
Think of how that potassium changes
resting membrane potential.
01:42
So, if you have a lower resting membrane potential
versus a higher resting membrane potential,
that's going to affect the speed at which
or the distance that phase 4
will reach threshold.
01:56
A couple other things that affect heart rate.
01:58
Ischemia is a big one.
02:00
So, what is ischemia?
Ischemia is the decrease in blood flow to a certain spot.
02:06
So, this is making it hypoxic.
02:09
It doesn't have enough oxygen.
02:10
So, what –
when can this happen?
If you think about it,
how about coronary artery disease,
If you think about it,
how about coronary artery disease,
a disease in which there's a decreased level
of blood flow to a certain spot of the heart.
02:20
If it’s severe enough,
you might cause a myocardial
infarction or a heart attack.
02:25
But if it's just minor in nature,
it's ischemic.
02:29
Sometimes it can cause angina or chest pain,
but in this case,
we’re talking about just a small decrease in blood flow
and a small decrease in the
amount of oxygen that is delivered.
02:40
So, hypoxic.
02:42
In this condition,
there's a decrease in heart rate
and this works through a special potassium channel
called a KATP channel.
02:51
So, it's not the same channel that was
involved for a normal heart rate,
in which we had a transient potassium
or a potassium kind of occurring that
happened during the repolarization process.
03:05
This is opening up a whole new channel
that wasn't opened up during
normal action potential.
03:11
And, of course,
especially as a physician,
you have a lot of choices
in how else you might be
able to change heart rate.
03:19
Beside the nerves,
besides the hormones,
besides clinical conditions such as ischemia,
there are a lot of different drugs.
03:26
You have four different classes of antiarrhythmic drugs.
03:30
You have calcium channel blockers.
03:32
You have beta-adrenergic agonists and antagonists.
03:36
The one you hear about most often
is called a beta blocker.
03:40
What a beta blocker is,
is blocking that beta-1 adrenergic response,
so you have less of an increase in heart rate
when the sympathetic nervous system is engaged.
03:50
And then you also have drugs like digoxin,
which helps with contractility,
and even though it has the heart beat harder,
it also slows it down.