Phases and Waveforms – Physiology of the Heart

by Joseph Alpert, MD

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    00:00 Here, we see a diagram of the pressures within the heart. First, we will look at diastole and then we will look at systole. So, what you see here is... this is at the top, you see the aortic pressure, superimposed on that is the left ventricular pressure and the hatched areas are diastole. During diastole, you can see that left ventricular pressure falls low, that it falls below right and left atrial pressure and blood flows into the left and right ventricles and you can see that on the right hand side. The little arrow is showing that during diastole, the mitral and tricuspid valves are open, blood is flowing in, pressure in the right and left ventricle is low, slightly lower than in the right atrium and the left atrium so that blood passes from the two atria into the ventricle. During ventricular systole, the ventricles both contract, the pressure rises that pushes the mitral and tricuspid valves closed and when the pressure rises to a high enough degree that is above aortic or pulmonary artery pressure, the pulmonary and the aortic valves open and blood is ejected into the pulmonary circuit on the right side and into the systemic circuit, that is the aorta on the left side. And the pressures are, of course, quite different as we will see.

    01:29 The average normal pressure in the lung, peak systolic pressure- that is during the maximum time of blood flow, from the right ventricle into the pulmonary artery, is generally somewhere around 20 to 25 mmHg. During peak systole on the left side, however, the pressure is often 120 mmHg or more, so you can see considerably higher on the left side.

    01:58 No surprise, the right ventricle just has to pump to the lungs, a nice low pressure system.

    02:03 The left ventricle has to pump to the entire body, a high pressure system.

    02:07 Here we see, in the hatch lines, ventricular systole. You can see aortic pressure.

    02:14 This is the left side, of course, but it would be similar on the right side, but at a much lower pressure, as we just talked about. And you can see the little diagram on the right hand side showing you the right ventricle and the left ventricle contracting.

    02:29 When the pressure in the right ventricle exceeds the pressure in the pulmonary artery during diastole, the pulmonic valve opens and blood is ejected. On the left side, when the pressure on the left ventricle exceeds the aortic pressure in diastole, the aortic valve opens and blood flows into the aorta. Following contraction, the pressure falls in the right ventricle and the left ventricle, the pulmonic and aortic valves close and then when the pressure falls low enough, the mitral and tricuspid valves open and blood flows into the ventricle in diastole, which we just looked at a moment ago. Let’s look at the pressures in each of the chambers and take our way through. In these little cartoons that you see, we are going to be using a little balloon catheter. Later, I’ll show you an actual picture of this catheter. It’s used very, very commonly to do right heart catheterizations, that is catheterizations that involve right atrium, right ventricle, the pulmonary artery.

    03:32 The left side, of course, has a separate catheter that goes in through the arterial system.

    03:35 So, we are going to follow this little balloon catheter through. Why do we have the balloon on it? It’s like the sail on a sailboat. It flows and is pulled along by the blood that’s flowing through the heart. So, you see here, the tip of the catheter is in the right atrium. You can see in the little tracings there, you can see the electrocardiogram above, the big deflection is the QRS, that’s the ventricular depolarization, that’s… that’s going to set off ventricular systole and you will notice a number of little waves in the right atrium. We are going to look at those little waves in greater detail.

    04:14 Here they are in greater detail. There is an A wave, a C wave and a V wave and two descents after the waves, the X descent and the Y descent. The A wave is right atrial and, by the way, you can sees the similar waves in the pulmonary capillary wedge or left atrial tracing.

    04:36 So, during atrial systole, you create the A wave, the pressure rises in the atrium as it contracts and then as the valve starts to close, there is a decrease in the pressure that’s the X descent. You then see a C wave, which is actually as the valve is shut and systole starts and the atrium fills, there is a little rise in pressure there. That’s followed then by relaxation of the atrium with a fall in the pressure, so called X descent.

    05:12 Then you see the… the V wave, which is ventricular systole pushing the valve back up a little bit into the atrium causing the pressure to rise and then we start again with the A, C and V wave. This kind of pattern is seen in patients with normal right atrial and left atrial pressures. In the next two slides, you will see the description of what I just told you - what the events are that are leading to the A, C and V waves? Now, you can imagine in a patient that doesn’t have atrial systole. Let’s say there is a period where the atrium becomes paralyzed, the A wave would disappear, you wouldn’t see it in the tracing. So, these slides should be read over a little bit carefully so that you have a full understanding of what causes the A, C and V waves. Now, here we see the A, C and V waves in the right atrial pressure tracing against the electrocardiogram.

    06:08 We are going to talk much more about the electrocardiogram later, but just to give you a little introduction, the electrocardiogram has a small wave that starts in a very small deflection, that starts before the big deflection. The small deflection is called the P wave that is atrial systole.

    06:27 Then there is the large deflection that’s QRS, that’s the ventricular depolarization and then finally, following the QRS, there is a T wave which is repolarization.

    06:36 In other words, getting ready for another electrical wave to pass through the heart. You’re going to ask - how come it’s P QRS and T, what happened to A B C and D? In the very beginning of electrocardiography, there were a number of waves that turned out to be artifacts, and they were A and B and C. So, all of those waves disappeared and the really the ones that remained where the real ones the P, the QRS and the T. And you can see the timing of the electrocardiogram on the top against the right atrial pressure tracing below.

    07:12 Here it is magnified once more for you. You can see the A wave, the C wave and the V wave.

    07:16 The X descent follows the A wave and the Y descent follows the V wave.

    07:22 Now, our catheter has moved across the tricuspid valve and into the right ventricle and instead of low level pulsations, low level changes in the A, C and V waves, we see large fluctuations in the diagram below corresponding to the generation of pressure during systole and then relaxation during diastole. Again, remember that the pressure in the right ventricle is much lower than in the left ventricle because it’s pumping to a low pressure system- the pulmonary artery, in a normal person, of course. When there is disease, sometimes the right ventricle pumps to higher pressures because the resistance in the lung goes up and we are going to talk about what determines pressure in the cardiovascular system at a later point.

    08:12 As we move the catheter through the pulmonic valve and into the pulmonary artery, you again see the fluctuations as the pressure rises during ventricular systole and then falls during diastole. We don’t get down as low as we do in the right ventricle because when the pulmonic valve closes, pressure doesn’t go any further down. In the right ventricle, of course, pressure continues to fall with the pulmonic valve closed and that, of course, initiates the opening of the tricuspid valve and blood flows in to the right ventricle during diastole. If we move the catheter out and wedge it in a small blood vessel in the lung, the… the opening of the catheter points downstream and actually records the pressure in the pulmonary capillaries. The pressure in the pulmonary capillaries is approximately the same as the pressure in the left atrium and the pulmonary veins. So, what we are really seeing is, we are seeing a measure of the diastolic pressure in the left ventricle when we measure the pulmonary capillary wedge pressure. So, with the right heart catheterization, we get right atrial pressure, we get right ventricular pressure, we get pulmonary artery pressure and we get a pulmonary capillary wedge pressure which is a reflection of pulmonary vein and left atrial pressure which is reflection of left ventricular pressure during diastole.

    09:37 And this measure, pulmonary capillary wedge pressure is frequently used as a measure of how well the left ventricle is functioning. Is it functioning with a nice filling pressure or is the filling pressure very high because the left ventricle is functioning abnormally? Here we see the four pressure tracings, four chambers that we just talked about.

    10:04 In the upper left hand corner, you see the A, C and V waves of the right atrium. Right next to it, you see the right ventricle with a high systolic pressure and a low diastolic pressure.

    10:15 In the right... in the left lower quadrant, you see the pulmonary artery pressure which during systole has the same pressure as peak pressure in the right ventricle, but in diastole, doesn’t fall as far, and then in the final right hand lower quadrant, you see the pulmonary capillary wedge pressure. Again, showing some of the A, C and V waves that you see in the right atrium. They are a little muted because you are seeing A, C and V waves at a distance from the left atrium, but nevertheless, very similar to what you see in the right atrium.

    10:49 And here we see the catheter being advanced from the right atrium all the way on your left hand side into… into the right ventricle with the high pressure and then followed by a low pressure that’s approximately the same as right atrial pressure followed by pulmonary artery pressure just as high as right ventricular systolic pressure, but never gets as low and then finally, pulmonary capillary wedge pressure which is approximately the same as pulmonary artery diastolic pressure. And again, as we said before, is a reflection of filling pressures on the left side of the heart. In fact, during the cardiac cycle, there are other things that affect the pressures in the heart besides the squeezing or relaxing of the ventricles and the atria. What is that? That is, in fact, respiration. When you take a deep breath, you are actually creating negative pressure inside your chest that allows the lungs to expand. When you exhale, you are actually creating a small amount of positive pressure that causes the lungs to collapse. That pressure is transmitted to the heart.

    12:01 So, during inspiration, pressures are slightly lower and during expiration, pressures are slightly higher. And you can see that wave form here as the patient breaths in and out.

    12:14 Now normally, this is inconsequential. It's 1 or 2 mmHg. However, when patients have lung disease and they make great efforts to breathe, the negative intrathoracic pressure and positive intrathoracic pressure may be significant and actually cause significant changes in the intracardiac pressures of the atria and the ventricles.

    12:37 Let’s talk a little bit about ventricular systole. What happens during ventricular systole , is that the ventricles push blood out into the pulmonary artery and into the aorta.

    12:50 How much blood do they push out? What’s pumped out with each beat is called the stroke volume.

    12:56 It’s a little bit like the piston in the car, right? Each time the piston moves up, it pushes out a certain amount of… of gas out of the piston chamber and that is of certain volume and that is the... each stroke has a certain volume. So that the cardiac output has two determinants - stroke volume and heart rate. We are going to talk a little bit more about that in just a moment. But it’s important to understand each squeeze pushes out a certain amount of blood. Normally, in most people, it’s about 80 cc of blood with each squeeze.

    13:33 Obviously, we can measure that and if it’s lower, that means the heart’s not functioning as well as it ought to and there’s a number of reasons for that which we will also discuss in a moment. If it’s more than that, it’s because the heart is being stimulated by something. For example, being stimulated by adrenaline or by an overactive thyroid or a whole bunch of different reasons. During the filling phase, of course, the ventricle has to fill with the same amount of the blood that it squeezes out. So, 80 cc of blood have to go into the two ventricles so that during the next systole, they can push out 80 cc .

    14:09 And they have to be balanced, right? If one ventricle pumps more than the other, you are soon going to have all the blood end up on one side of the circulation or the other.

    14:17 So, the two have to be balanced, of course. Now, what determines cardiac output- which

    About the Lecture

    The lecture Phases and Waveforms – Physiology of the Heart by Joseph Alpert, MD is from the course Introduction to the Cardiac System.

    Author of lecture Phases and Waveforms – Physiology of the Heart

     Joseph Alpert, MD

    Joseph Alpert, MD

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