hi every Ron Dr Mike here in this video we're taking a look at the cardiac cycle this is a great way to make sense of what's Happening both electrically and mechanically in the heart so looking at the conduction system and then the contractions that follow all right let's take a [Music] look so to begin you can see I've drawn up 1 2 3 four five hearts and it's going to show what's happen at the heart at different stages of what we call that cardiac cycle but to begin we need to understand what's happening electrically in the heart because the electrical or the conduction system of the heart precedes the contraction or the mechanical aspects of the heart so we need to understand how they Link in order to show where they link so first thing is I hope you've got an understanding of an ECG Trace if not don't stress we'll go through the basics of it first thing is if you look at an ECG Trace from what we call lead 2 you tend to have something that looks a little bit like this you see these Peaks and troughs like that now this is happening over time and this is from an ECG an electrocardiogram and it's measuring the conduction the electrical events in the heart if we were to take a heart for example so I'm going to draw a heart like this and I'm going to draw a very simplistic version of it perfect like that and I'll just draw that for the valves we know that the heart in order for it to contract an electrical signal needs to propagate through it and we call this a depolarization event if you want to know more watch my video on cardiac Action potentials it goes through all the details of that but for this video we need to know that there is a node here called the SA node and that SA node will spontaneously send an electrical signal like I said called depolarization and that wave of depolarization spreads through the heart muscle of the Atria in a manner like this now remember if you watch my ECG video I told you the ECG cheat sheet that if you have depolarization going in the direction of an ECG lead you get a bump up on the ECG okay if you get a depolarization event going away from the lead it's probably unsurprising that you get a dip down on the ECG so in this case we're going to look at the typical lead that people look at in an ECG Trace which we call lead two and lead two looks at the heart from this perspective so what you can see here the electrical signal or depolarization event that's happening from the SA node through the The myocardium the muscle of the Atria goes in the direction of the lead so we get a bump up on the ECG so when the Atria depolarizes we get this wave here that's our bump up we call that the p wve and the P wve like I said represents atrial depolarization brilliant then what you'll see is that there's a fibrous tissue here that the electrical signal cannot move past what it does is the electrical signal funnels through another node which we call the AV node and that takes a little bit of time so what we end up getting is this Flatline so this Flatline is telling us what's happening with the conduction going through the AV node then what we get is once it's moved through the AV node it moves through some more conduction branches of the heart and we've got branches that look like this which we call the bundle branches and the depolarization event moves through the bundle branches like this now if you have a look that is away from the heart and because this septum isn't very big it's sharp and it's it's it's it's short not sharp it's short and it's sharp which which means a depolarization event away from the lead you get a bump down there's our short sharp bump down we call that the Q wave and that Q wave is representative of sepal depolarization sepal depolarization great to know all right then this electrical signal this depolarization event spreads through what we call the pingi fibers in know fact like this now you might be thinking right these ones are going towards the lead these ones are sort of going away from the lead what do we get remember the left ventricular wall is three times thicker than the right so whatever is happening here is overwhelming what's happening anywhere else so we get a depolarization event in the direction of the lead thick myocardium of the ventricles so you get a very big bump up on the ECG and that's this bump up here and that's the r w that's representative of ventricular depolarization okay brilliant then we need to depolarize the rest of the heart which is the rest of the walls here that is short and sharpen it's away from the lead so we get that final wave there which we call the S Wave as you can see going from Q to R to S that's septum most of the muscle and then the end of the muscle that is all of this in actual fact from the QR and S so the QR and S together we call the QRS complex and the whole thing represents ventricular depolarization brilliant now what we get is a break everything's depolarized brilliant we need to reset the heart now so we've got a little break there where nothing's happening but now we need to reset it and when we reset it we need to repolarize the heart and the repolarization event for the heart starts where it finishes and has a big wave that moves backwards so repolarization moves like this and as you can see that's away from the lead repolarization to reset so if depolarization in the direction of the lead gives you a bump up repolarization in the direction of the lead will give you a bump down and repolarization away from the lead will give you a bump up in this instance we've got repolarization mostly away from the lead and we get the bump up right so this is what we call the t-wave and it's representative of ventricular repolarization resetting of the heart now what have we done we've set up our electrical scene but what is the importance of understand understanding what's happening electrically because what follows is the mechanical function of the heart contraction so now what I want to do is look at what's happening in the heart and correlate that to different phases of the ECG Trace so let's start here what can the heart do well it can either contract which we call cyol or syy or it can relax which we call diol or diast so we're going to start with diol relaxation so I'm going to write that down so we're going to start with diol the heart is relaxed and what's happening in this particular phase we've got ventricular filling so blood is moving into the ventricles passively as you can see it's coming up on the right hand side but it's passively moving into the ventricles and it's going to be happening on the left hand side as well now there's no contraction in this process but blood is starting to fill the ventricles so we call this again ventricular filling so we have ventricular filling a passive process it's diast all right what's the next step well once we've filled the ventricles we want the ventricles well actually we haven't finished filling the ventricles have we because we need the Atria to contract to try and push the remaining blood into the ventricles so the next step here is we need the Atria the muscle of the Atria to contract and this is obviously not a diastolic phase this is a systolic phase but it's systolic for the Atria and so what we call this is atrial contraction and what do you think is happening in atrial contraction the blood is going from the Atria into the ventricles in an active process to try and fill up the ventricles to their fullest capacity so a couple of things here there's valves in the heart oneway valves you've got the valves that separate the at let's look at it here the Atria from the ventricles we call them the AV valves you got the tricuspid on the right bicuspid on the left and you've got the semi Luna valves you've got the pulmonary semi Luna valve here at the trunk and the aoic semi Luna valve here at the aot aoic trunk it's important to know what's going to be open and closed at what time so let's start here with ventricular filling if we were to write up uh AV valves so Atri ventricular Valves and SL for semi lunar valves are they going to be open or closed so let's begin when we look at the AV valves well they're open right cuz it's filling so the AV valves are open what about the semi lunar valves well they're going to be closed because they only let blood up through this way and no blood's going out through that way so they are closed easy let's now have a look at atrial contraction it's going to be the same thing the blood is moving through the AV valves so they're open but the blood is not moving through the semi Luna valves so they are closed great let's keep that in mind now once we've got atrial contraction the ventricles are as filled as they possibly can be remember we call this preload they're as filled as they could possibly be the ventricles have a nice stretch to them and now the ventricles want to contract so now we are looking at a cystolic phase for the ventricles and so now we've got ventricular contraction but what's Happening Here is in a very short period of time there's a time in which the ventricles contract and as the blood pushes up just before it exits it completely fills the ventricles like this so you've got blood completely filling the ventricles but none has ejected which means that the AV valves now close cuz we don't want blood to regurgitate back into the Atria so the AV valves they shut all right they closed that's good to know because we can write here AV valves closed and what about the semi lunar valves well they're closed too because blood has not yet ejected from them so the semi lunar valves are still remained closed now keep this in mind cuz what we've just done open open close close close close so we've just shut the door keep that in mind we'll go come back to it in a sec we call this ISO volumetric contraction isov volumetric contraction simply ISO means same same volume at contraction so the the ventricles are basically filled with blood just before ejection right nothing's ejected all right next point is we now need to overcome that point and we have ejection remember that's just a fraction of a time period now we've got ventricular ejection ventricular ejection now remember this was systo and so is this both are systolic phases and again we've got the ventricles Contracting really really hard in this instance and what's going to happen with the blood the blood is going to move up and out through the semi lunar valves to go to either the lungs or to go to the rest of the body so that blood is now moving up what's happening with these AV valves they're shut AV valves are closed but the semi Luna valves now are open so AV valves closed semi Luna valves they're now open okay brilliant we're on the last phase we've had ventricular ejection the blood has ejected now what needs to happen now we have that passive process of filling again and you might think well we did filling but there's a there's another phase immediately before this particular phase of feeling so now we're in diol it's relaxation right it's relaxing and the blood is now entering again to fill but where is it filling it's going to be coming in and it's going to be filling the Atria the blood's going to be coming in and filling the Atria right and the blood here wants to fall back down right that blood that didn't get fully ejected wants to fall back down and closes the semi lunar valves right so what do we find here we find the AV valves blood hasn't entered them yet so they're closed still and the semi Luna valves now close as well cuz the blood's about remember they don't let blood fall back down so they're Clos now too what am I trying to highlight by showing you all this couple of important points when you open a door it doesn't make much sound but when you close it it does so when you close a valve that's when you hear a heart sound where do the valves close let's first look at the AV valves open open open closed this is where we hear the first heart sound great then let's look at the semil Luna close close close open we don't hear it because opening a door's quiet close this is where we hear the second heart sound when you close the door so under isovolumetric contraction you hear your first heart sound your first heart sound S1 first sound here at oh I didn't tell you what it was called I'm so sorry this is called isovolumetric relaxation isovolumetric relaxation same same volume right it's similar to this process here nothing's entering nothing's leaving right this nothing is entering in regards to the semul Luna vves and nothing's leaving in regards to going into the ventricles is volumetric contraction so this is where you hear the second heart sound S2 all right but how does this correlate with the ECG so when you have an electrical event it will immediately be followed by a mechanical event so if we start with ventricular filling there's no electrical event happening in this process no electrical event but there is for atrial contraction for the Atri to contract you needed the Atria to depolarize first for the Atria contract to contract that needs to happen first so it's not going to happen at the same time so what you can draw up here is a line if we want to connect this line here this event is going to start round about halfway through HL depolarization and when is it going to finish it's going to finish because the next is ventricular round about halfway through ventricular depolarization a CU remember when the ventricles depolarize then they contract which is what's happening here so this starts about halfway through the QRS complex and what happens in this process well you've got contraction here and contraction here but I told you isovolumetric contraction is a very short period of time and it is so what you're going to find is that this is going to be a very short period of time time for the ECG this is still contraction and this contraction continues all the way up until we start the repolarization the t-wave right and then what we get is that isov volumetric relaxation but I said these events are they happen in short periods of time so if I would to just wipe this off for a second so I can show you this one short period of time like that and then what's following that the rest of the ventricular filling so ventricular filling is going to be here moving across to here so what you can so I think the easiest way to remember this is anytime you have a contraction it's going to follow a wave and it generally starts halfway through the wave so h contraction starts halfway through the p-wave ventricular contraction starts halfway through the QRS complex and because that's just atrial contraction there it goes for this period these both two are systolic phases of ventricular contraction so effectively they both go from the middle of the QRS complex to the middle of the t-wave and then relaxation happens following that now let's look at the heart sounds when can you hear a heart sound regarding the ECG you should hear S1 at around about halfway through the QRS complex you should hear S2 at some point after the t-wave brilliant some point from halfway through the t-wave this is an overview of the cardiac cycle I'm Dr Mike and I really hoped it helped thank you