Hello. Welcome to Byte Size Med. This video is on the cardiac cycle and we're going to be looking at the events that take place, the volume and pressure changes that happen along with the electrocardiogram and the heart sounds.
The diagram that successfully summarizes all of that into one neat little picture is called Wiggers diagram and you would have seen it in almost every textbook. If you're trying to understand what that picture is about, hopefully I can help you out. Now I have broken down each part into separate videos and I'll leave links in the description box in case you do want to use them.
This is really just putting all of it together and looking at it from a different angle. We'll start off with the diagram itself. We've got six parts.
That's the three pressure changes: the aorta, the left ventricle, and the left atrium. We usually use the left side of the heart on this diagram. The ventricular volume change, the electrocardiogram and the heart sounds.
I'm gonna break this down into six steps. Step one: we'll be learning the layout. Each cardiac cycle is around 0.
8 seconds and that's assuming that the heart rate is around 72 to 75 beats per minute. Obviously if the heart rate changes, the cardiac cycle length would also change. Along the horizontal axis is the time, and along the vertical axis is both pressure and volume changes.
Depending on which curve we're looking at, the volume changes from around 50 to 120 to 130 ml in the ventricles during each cycle. The pressure, we'll plot from 0 to 120 mm of mercury. Of course all numbers have individual variation.
We just use these to understand it. I'll be focusing on the shapes of the curves in this video. I talk about the numbers more in my videos on volume and pressure.
So you can check them out if you want to. At the top of the picture are the three pressure changes. The aorta, the left ventricle, and the left atrium.
Then we have the curve for the ventricular volume change. The electrocardiogram and the heart sounds are usually at the bottom. Now this is one cardiac cycle, and it can be divided into phases.
There's systole, which is the phase of contraction and diastole, which is the phase of relaxation and they happen both in the atria and in the ventricles. To make it easier for me to explain, I'm going to split this picture, such that the pressure changes are all on the left and the rest of the curves are on the right. If we start with atrial systole, that lasts a short duration.
So the rest of the cycle is in atrial diastole. Ventricular systole starts after atrial systole and it's longer. It overlaps with atrial diastole.
Ventricular systole is followed by ventricular diastole, which completes the cycle. So the phases overlap between the chambers. The ventricular events can be divided.
After atrial systole, there's isovolumetric contraction in the ventricles, then the ejection phase, which has two further parts. That's rapid ejection first and then slow ejection after. Those are the three parts of ventricular systole.
Ventricular diastole has isovolumetric relaxation, then the filling phase, first rapid filling and then slow filling. Now usually we end the picture with another atrial systole, because the cycle is going to repeat again. So this would be the start of the next cycle and would overlap with ventricular diastole.
Next we are going to look at what happens during each of these phases. Step two: we'll pick up atrial systole. Atrial systole is when the atria contract.
Now most of the blood actually fills in the ventricles during that filling phase, but a little bit at the end gets pushed into the ventricles by contraction of the atria. So obviously if blood is entering the ventricles, that means the ventricular volume will increase. The atria are contracting, so the atrial pressure rises.
That's the 'a' wave on the atrial pressure curve. It's the first peak in that curve. The push into the ventricles causes a slight rise in the ventricular pressure as well.
This atrial systole is overlapping with the end of ventricular diastole. So during this time, the aortic pressure is actually just coming down and this will make sense once we reach the end of the picture. Now what about the electrocardiogram?
What made the atria contract? Atrial depolarization. That's the 'P' wave of the ECG.
That would have to happen before the atria contract. So we put the 'P' wave just before atrial systole. At the end of atrial systole, the atrioventricular valves, which would be the mitral valve in the left heart, it closes.
That closure gives us the first heart sound, S1. So S1 would happen just after atrial systole, which would be during isovolumetric contraction. That brings us to step three: isovolumetric contraction.
Both the atrioventricular and the semilunar valves are now closed. The ventricles start systole. So they start contracting, but the valves are closed.
Blood doesn't leave the ventricles during this stage. So the ventricular volume would not change. That's why it's called isovolumetric, volume doesn't change, but the ventricular pressure rises.
Aortic pressure is still up here. But what about atrial pressure? The atria were now relaxing, so their pressure was coming down.
But when the ventricle contracts with the valves closed, the atrioventricular valves bulge back into the atria. They were relaxing in atrial diastole, but that gets interrupted by this pressure rise. That's the 'c' wave of the atrial pressure curve.
It's the second peak. The ventricles contracted because they were depolarizing, that's the QRS complex on the electrocardiogram. That would have to start just before ventricular contraction begins, such that when the R wave reaches its peak, that's usually when ventricular systole starts.
At the end of this phase of isovolumetric contraction, the semilunar valves, that's the aortic valve on the left, it opens. That's because during this phase, the left ventricular pressure was rising and at this point, the left ventricular pressure overtook aortic pressure, opening the valve. That leads to step four: the ejection phase.
The ejection phase has a rapid and a slow part. But to make it slightly easier, we'll just look at it as one phase. The aortic valve open, because like I said ventricular pressure crossed aortic pressure.
The curves cross and the ventricular pressure reaches a peak. During this phase, the aortic pressure also peaks, because blood is being pumped into the aorta. Now though the curves cross, the aortic valve doesn't close just yet because of inertia of blood flow.
The ventricular volume would obviously drop, because blood is leaving the ventricle. The atrial pressure was dropping, as the atria relax. That's the x-descent of this curve.
But then the pressure slowly rises as the atria are filling, because this phase of the ventricular ejection overlaps with atrial diastole, and that's when they fill. They're filling against closed valves. At the end of the ejection phase, the aortic valve closes.
That results in another heart sound, the second heart sound. S2 is thus heard during the next phase, which is isovolumetric relaxation. And we've reached step 5: isovolumetric relaxation, where the ventricles are now relaxing with all the valves closed.
Now we're entering ventricular diastole, when the ventricles are going to relax. So on the ECG, before this phase, the ventricles will be repolarizing. That is the 'T' wave of the ECG.
So the 'T' wave happens just before isovolumetric relaxation. Valves closed, isovolumetric. Again the volume doesn't change in the ventricles.
No blood leaves or enters, but the pressure, it comes down. The aortic valve is closed and after that, there's a little notch. That's called a dicrotic notch or the incisura.
Now this is supposed to be from a slight backflow of blood towards the ventricle. That creates a little blip in the curve, as it's going down. The atrial pressure has been slowly rising as they fill, and it reaches a peak at the end of this phase.
That's the 'v' wave of the atrial pressure curve. The ventricular pressure has been going down and it goes lower than the atrial pressure. That opens the atrioventricular valve, which would be the mitral valve.
Here the atria empty into the ventricles, which are now in the filling phase. That's step six: filling of the ventricles. There's a rapid filling and a slow filling phase.
The volume will obviously increase in the ventricles as they fill, so that curve goes up. The atrial pressure reduces as they empty. That's the 'y' descent on the atrial pressure curve.
The ventricular pressure stays low even though they're filling, because they're relaxing and they're compliant. The aortic pressure drops as blood runs off into the arterial system. But even at the end of ventricular diastole, the aortic pressure stays higher than the ventricular diastolic pressure.
The elastic recoil of the aorta maintains it up there. Though usually we've got just two heart sounds, there is a third and a fourth heart sound, which can be heard in some pathological conditions, though the third heart sound can be physiological in some cases as well. If heard, the third heart sound is heard during the rapid filling of the ventricles.
So that would be during this phase. Most of the ventricular filling is done during this phase, but the last bit of blood is pushed into the ventricles by atrial contraction. That is during atrial systole, which is preceded by the 'P' wave on the electrocardiogram.
We're starting the next cycle, so all the changes that we saw at the beginning of the diagram are going to repeat again. The ventricular volume rises, the aortic pressure continues to drop, the atrial contraction creates the 'a' wave in its pressure curve, the ventricular pressure also rises slightly. The fourth heart sound, if heard, is during atrial systole, when the atria contract against a ventricle that's stiff and not very compliant.
An easy way to remember it is that the third heart sound is heard in early ventricular diastole, and the fourth heart sound is in late ventricular diastole. And the whole thing repeats over and over and over again. That is the cardiac cycle on Wiggers diagram.
I hope this video helped you out. If it did, you can give it a like and subscribe to my channel. You can check out my cardiac cycle playlist if you'd like help with learning it part by part.
Thanks for watching I'll see you on the next one!
