Review of Cardiac Conduction System

00:01 Welcome back to the ECG lecture series.

00:03 Today, we're going to talk about bundle branch blocks.

00:06 So, bundle branch blocks are when the normal QRS width is greater than 0.10 seconds.

00:14 And the wide QRS means that there's low electrical conduction through the ventricles.

00:21 So, let's review the conduction system again. Remember this from the very first lecture.

00:26 Let's look at it again in its normal state and then we can talk about the abnormal state.

00:32 So, here's the little diagram that we saw before.

00:36 Remember that the impulse starts in the sinoatrial node that depolarizes automatically and sets off the electrical wave that passes down through the atria to the atrioventricular node where there's a pause, right? And the pause is to allow the atrium to empty before ventricular contraction starts.

00:56 Then, the impulse travels down through a common bundle called the His-Purkinje and then splits into a right bundle and a left bundle.

01:06 The left bundle has two branches; an anterior fascicle or branch and a posterior fascicle or branch.

01:14 So, this is the electrical conduction system in the heart.

01:18 So, remember that each myocardial cell has an electrical activity which leads to the ventricular or the atrial contraction.

01:28 So, it's electrical impulse followed by mechanical impulse.

01:32 There's - in the action potential an initial upstroke.

01:35 There's early-fast depolarization, followed by a plateau.

01:40 And then the cell resets itself, ready for the next beat.

01:44 And then rest for a moment in diastole and then starts again with the rapid upstroke and so forth.

01:49 And as the wave passes through the myocardium, each cell goes through this action potential.

01:55 There are special cells in the right bundle and the two branches of the left bundle that enable the electrical activity to pass more rapidly into the ventricular muscle.

02:07 And of course, once the electrical activity gets into the muscle, it triggers the contraction.

02:12 So, you have the electrical wave coming to the heart muscle and that's followed by a myocardial cell contraction.

02:19 In a sense, the electrical wave is the trigger that sets off the mechanical myocardial cell contraction.

02:26 Remember again, we've just gone over this.

02:29 You start with the sinus node, the pacemaker that has this funny channel that leaks sodium ions causing the cell to spontaneously depolarizes and initiate the wave of electrical activity in the heart that starts the electrical wave of activity.

02:45 It passes through small pathways in the atrium then gets to the AV node where there's the delay, goes into the bundle of His, also called the His-Purkinje bundle.

02:57 Then goes down into the right and left bundle branches and gets out into the small fibers of the Purkinje system throughout the ventricles and that coordinates the contraction of atria and ventricles.

03:10 And you'll remember this. We've gone over this before.

03:12 And so here you see the wave passing into the ventricle down through the septum, then it’s - hits the apex and then starts to go back up towards the atria and then concludes.

03:26 And so, you see that the wave is moving in a sense when seen by the electrodes in two different directions and that causes sometimes when it moves towards the electrode, there's an upward deflection of the EKG.

03:39 When it's moving away from the electrode, there's a downward deflection as we've talked about in the first lecture.

03:46 The ventricular depolarization in the intact heart generates a changing electrical field and a changing QRS depending upon the electrode, you're seeing the impulse coming towards it or seeing the impulse going away from it.