Conduction System – Cardiac Dysrhythmia

00:00 I’m going to show a number of diagrams of the electrocardiogram and take you through showing you what each part consists of. You'll see the first small wave there is called the P wave and that is the atrial depolarization, it’s followed by a large upward deflection that’s the QRS, that’s ventricular depolarization and then followed by a final rounded wave that’s called the T wave, which is repolarization of the ventricle. You may say, “Wait a minute, why call them P, QRS and T? Why not call them A, B, C, D, E and F?” In fact, in the early history of electrocardiography, back at the beginning of the twentieth century, a number of waves were described A, B and C and so forth, that all turned out to be artefacts and the real ones actually ended up being finally called P, QRS and T. They went through a lot of artefacts in the alphabet before they got the P, QRS and T and those are terms we’ve used now for over a hundred years. What the electrocardiogram is reflecting is the electrical wave that is being conducted through the heart muscle and you'll remember from our earlier lectures, the electrical wave triggers the mechanical activity. So, each electrical complex is actually telling us something about the mechanical activity of the heart. So, where does the impulse begin? It begins high in the right atrium, in the sinus node. The sinus node is the pacemaker of the heart. It’s the one that we hope is always controlling the rhythm in the heart, it’s the normal pacemaker of the heart.

01:36 The impulse, the electrical impulse, passes down through some special channels in both atria and enters the atrioventricular node which is right where the atria and the ventricles meet with the valves, the mitral and tricuspid valve on either side and the beginning of the septum that separates the two ventricles. There's a delay there. I'll tell you about the...

02:00 why there's a delay there. The impulse then enters the septum of the ventricles between the right and left ventricles and passes into some special electrical fibers - the bundle of His and the Purkinje fibers and passes out into the ventricle resulting in activation of the ventricle with contraction. So, why should there be a delay? The reason you have to have a delay is you want the electrical activity in the atria to cause an atrial contraction and then you want the valves to be able to be open and empty all of the atrial blood into the ventricle and then you want the ventricle to contract. If there were no delay, the atria and ventricles would contract at the same time and the blood wouldn’t move forward.

02:45 So, the delay in the AV node is important, but you don’t want excessive delay and we'll talk about conditions where there's excessive delay later and where often patients need pacemakers, but that’s in the final phase of this lecture because that’s very advanced. So, let’s talk about each component in place.

03:04 Here's the sinoatrial node, it has a spontaneous depolarization, so it slowly slowly depolarizes and then fires, resets and slowly slowly depolarizes and then fires. It is affected by the autonomic that is the automatic nervous system. So, when you exercise, impulses reach the sinus node increasing the heart rate of the patient. When you rest and you're quiet, there are impulses from the vagus nerve that slow the sinus node and this is, of course, in response to your activity which is exactly what you'd like. If you're running up a flight of stairs, you want the heart to increase an activity to pump more blood. If you're lying down, sleeping, you want the heart to rest and relax and have a nice slow heartbeat. From the sinus node, which is high in the right atrium, the electrical impulse passes through both atria, both the right and the left atrium, and into the AV node. And as I have already told you, there is a delay here which is required so that the mechanical activity, the systole - the squeeze of the atria can empty all of their blood down into the ventricles before the ventricles start to contract. In the AV node, there are a number of specialized fibers which in long life they can become disrupted and even injured and fibrotic and the patients may develop a very slow heart rhythm because the impulse cannot get through from the atria.

04:36 You can see from the electrocardiogram, the P wave is… begins with the sinus node and the rest of the P is the atrial depolarization and then you can see a little pause before the big deflection, which is the ventricular deflection. That little pause shown in green on this is the delay in the AV node. And then we get to the ventricle, the QRS is ventricular depolarization. When the electrical impulses passing out through the His-Purkinje system - the specialized fibers in the ventricular muscle transmitting the electrical wave to the ventricles, which then contract and you can see again in green where the QRS is and the impulse is in the ventricles.