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Current time:0:00Total duration:8:53

Video transcript

let's look at a single heartbeat and EKG so here on the right side we have a single beat on EKG and we're looking at lead to lead to has a negative electrode on the right hand and a positive electrode on the left foot and a healthy heart the wave of depolarization overall runs from the top right to the bottom left so you can see that lead two measures voltage created in this direction therefore it lead to is commonly singled out for more simple information such as heart rate and rhythm because it gives you a pretty good view of the waves particularly the P wave often people say that lead to is the best lead to see the P wave an EKG gives us information on voltage over time or the change in voltage over time so voltages on the y-axis time is on the x axis every small box running in the x axis or time Direction represents 0.04 seconds and there are 1 2 3 4 5 small boxes per every big box so you can do the math and you can calculate that each big box represents 0.2 seconds conduction starts at the hearts dominant pacemaker the SA node the SA node initiates waves of depolarization that run through the atria and go to the AV node and on EKG this depolarization through the atria is seen as this purple wave here called the P wave the P wave is usually about two small boxes or 0.08 seconds and again the P wave represents atrial depolarization and what happens after the atria depolarize well you get subsequent contraction after the signal reaches the AV node conduction slows down and this is for a couple reasons one is that AV nodal cells are smaller in diameter than the other cells and as a rule the small as I am or the cell the slower the conduction in fact AV nodal cells are among the smallest diameter cells in the body also conduction runs primarily through calcium channels in the AV node calcium channels have inherently slow kinetics and this is in contrast to fast sodium channels that we see in the reticular conduction system this delay in conduction is really important because it allows time for the atria to contract and relax which optimizes a time for the ventricles to fill so again we have the signal leaving the SA node going to the AV node where conduction slows down this all happens before ventricular depolarization on the EKG the time it takes from the beginning of atrial depolarization all the way up until ventricular depolarization is seen as the P R interval and the PR interval is typically 0.12 seconds or 3 boxes it shouldn't ever be longer than 0.2 seconds or something's wrong the signal travels from the AV node to the HISP undal and once we hit the hissed bundle this is where conduction starts moving rather quickly the his bundle bifurcates into the left and right bundle branches and these branches branch into terminal Purkinje fibers the depolarization from the HISP undal down the bundle branches and to the Purkinje fibers leads to mass depolarization of ventricular cells and is seen as the q r s complex on EKG again the q r s complex represents ventricular depolarization and like we said before this is very rapid because we're using fast-acting sodium channels now so typically the QRS complex is about 0.12 seconds it shouldn't be any longer than that early on depolarization the septum C polarizing and the septum depolarize is from the left septum to the right septum so you have the wave of depolarization going from the left septum to the right septum you'll notice that this wave of depolarization moves in a different direction than lead to which is why you get this first initial negative selecting Q wave on EKG however quickly after the wave depolarization spreads throughout the ventricles which is why you have the positive high amplitude R wave shortly after something else note is that during ventricular depolarization you also have atrial repolarization so at the same time as the ventricles are depolarizing the atria are repolarizing however the EKG signal that you would see from atrial repolarization is actually lost because the QRS complex dominates since there are far more ventricular cells and there are atrial cells so you don't see the atrial repolarization on EKG because it's buried in the QRS complex so just after ventricular depolarization and in the initial phase of ventricular repolarization there's a flat segment on the EKG known as the ST segment and the ST segment represents a period where there is no net current that's to say there's no large electrical vectors in any direction however this doesn't mean there's not a lot going on during the ST segment is when the ventricles are contracting and are pumping blood to the aorta and to the pulmonary artery finally the ventricular cells repolarize and this is seen as the T wave on EKG the T wave is flatter and longer than the QRS complex because repolarization is a slower process and depolarization and again this represents ventricular repolarization the T wave typically takes up about four boxes or 0.16 seconds now something you might be asking yourself is why is the PR interval called the PR interval when it goes from the P to the Q wave well remember we said that the PR interval is from the beginning of atrial depolarization up until the start of ventricular depolarization sometimes there's no Q wave on the Keiji it's just not found so ventricular depolarization or the start of it is actually the start of the R wave which is why it's called the P R interval if there is a Q it can call the P Q interval however to generalize we call it the PR interval for the times that the Q wave is lost something else might be wondering is why is the T wave positive as well as the R wave why are the ways representing ventricular depolarization and ventricular repolarization in the same direction and to answer this you need to remember two things one is that the EKG gives you information on the difference in charge so it gives you information on the changing imbalance of potential the other thing to know is that the epicardial cells repolarize before the endocardial cells what do we mean by that well remember how we said that the wave of depolarization goes down the bundle of hysts the right and left bundle branches into the Purkinje fibers so you can imagine that the cells on the inside or the endocardium depolarize first and the wave of depolarization spreads to the outside because we had the Purkinje fibers spreading the depolarization to the outer cells or the cells of the epicardium let's cut out this part of the heart so I can show you in more detail remember the EKG see cells at rest as being positive on the outside the wave of depolarization comes down and depolarizes the cells in the endocardium first so those was seen as negative while they're still positive on the outside because the depolarization hasn't gone through yet eventually the endocardium and the epicardium will be depolarized and while typically cells that depolarize first repolarize first it's different in the heart ventricles epicardial cells actually repolarize before and no cardio cells so the EKG sees them as positive before endocardial cells are positive you'll notice that during depolarization the vector created and during repolarization created are both in the same direction which is why the peak of the QRS complex and the T wave are in the same direction