Influence on Pacemaker Activity – Heart Rate and Electricity

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.