The Cardiac Cycle Lecture Note

Before we begin with this lecture I would like to give you some advice, please don’t memorize this lecture and try to understand the information as much as possible. This lecture might seem difficult at first glance, however as soon as you understand it you will realize that it is fairly simple. The Cardiac Cycle Throughout this lecture we will consider that each cardiac cycle has a duration of 0.8 seconds. Why 0.8 seconds? We know that a normal heart rate ranges between 60-100 bpm, let’s take 75 bpm as the usual heart rate , we divide 60 by heart rate to have time of one heart beat (60/75=0.8), so one heart beat (cardiac cycle) takes 0.8 seconds. During these 0.8 seconds we will touch upon the mechanical, electrical, pressure, and volumetric changes that occur in both the atria and ventricles during one cardiac cycle. The mechanical changes that occur during one cardiac cycle are as such: Figure 1 Remember: Each one small square is equal to 0.04 sec, so in 0.8 sec we have 20 small squares. • Atrial contraction (otherwise known as Atrial Systole) has a duration of 0.1 seconds. Therefore, by simple mathematics we can calculate the duration of Atrial relaxation (otherwise known as Atrial Diastole) by subtracting the duration of Atrial Systole from the duration of the whole cycle giving us the following equation: 0.8-0.1=0.7. Therefore, we can conclude that in a cardiac cycle of 0.8 seconds Atrial Systole is 0.1 seconds and Atrial Diastole is 0.7 seconds. 2|P a ge • Due to Atrioventricular node delay, the ventricles do not begin to contract until the atria have concluded their contraction. • According to the previous statement (as shown in figure 1), Ventricular Systole begins at 0.1 seconds and ends at the 0.4 second point meaning that Ventricular Systole has a duration of 0.3 seconds. By means of the same calculation we used for the atria 0.8-0.3=0.5. Therefore, again in a cardiac cycle of 0.8 seconds Ventricular Systole is 0.3 seconds and Ventricular Diastole is 0.5 seconds. This means that Ventricular diastole occurs from 0.4-0.8 seconds in the first cycle and takes up 0.1 seconds from the next cycle. By that we finished the mechanical changes that occur during the cardiac cycle, now we are going to look at the electrical changes and how they coincide with the mechanical changes. Figure 2 • Let’s revise the ECG waves: § P-wave: Atrial Depolarization § QRS-complex: Ventricular Depolarization § T-wave: Ventricular Repolarization • As you can see the electrical activity always proceeds the mechanical activity of the heart: § P-wave is recorded prior to Atrial Systole § QRS-complex is recorded prior to Ventricular Systole § T-wave is recorded prior to Ventricular Diastole § Remember the Atrial repolarization wave is masked by the QRS-complex causing no visible wave to occur before Atrial Diastole. 3|P a ge Following the relationship between the electrical activity and mechanical activity in the cardiac cycle we are now going to look at the relationship between the electrical activity and the action potentials that occur during a cardiac cycle. Figure 3 • W e stated previously that the P-wave depicts Atrial Depolarization therefore, it coincides with Phase 0 of the action potential as this is the phase of rapid depolarization in the Atria. • Atrial Systole coincides with Phase 2 (plateau) • As for the ventricles we stated that the QRS-complex depicts Ventricular Depolarization; therefore, it also occurs with Phase 0 but now it is Phase 0 of the Ventricular Action Potential. • Ventricular Systole and Phase 2 of the Ventricular action potential occur simultaneously • The T-wave depicts Ventricular Repolarization thus the T-wave and phase 3 coexist. The doctor said he will ask about the previous points, so make sure you understand everything written. We will now move onto the Volume Changes that occur during each successive cycle. You should be aware of the fact that the volume of blood that is found/stored in the right ventricle is equal to the volume of blood found/stored in the left ventricle. Also, both ventricles eject the same volume of blood during each cardiac cycle. The only difference between the two ventricles in the aspect of cardiac cycle is the pressure of blood ejected from the two (this will be discussed later in the lecture). 4|P a ge Knowing this fact, we will be studying the left ventricular volume changes of the heart. Figure 4 This figure is very important as it illustrates everything that will be explained till the end of the lecture. Let’s take for example that before Atrial Systole, the left ventricle contains about 100 ml of blood. At this instance, the pressure in the Atria is relatively higher than in the Ventricles; therefore, the atrioventricular valve is opened. If we were to measure the pressure in the heart chambers we would find that the pressure in the Atria is about 0, and the pressure in the ventricles at the same moment in time is -1 or-2. You might be wondering how the pressure could be 0, this value is taken relative to atmospheric pressure being our baseline. Therefore, a pressure of 0 means that it is equal to atmospheric pressure which is 760 mmHg. As soon as Atrial Systole occurs, the contractions lead to the “pushing” of a certain amount of blood into the ventricles forcefully. Dr. Faisal stated here that after Atrial Systole the volume of blood in the ventricles is about 125 ml. Comparing it to our initial volume (100 ml) we can conclude that only 25 ml of blood enters the ventricles from the atria during Atrial Systole. Thus, Atrial Systole only contributes to 25% of the ventricular 5|P a ge volume (25% is a maximum, it’s even lesser than this). In that sense we can conclude that Atrial Systole is not an essential process for the normal functioning of the cardiovascular system and a person can live normally in the absence of Atrial Systole. I want you to remember that the atrioventricular valve is open and blood flows passively into the ventricles, meaning that the ventricles will fill even in the absence of Atrial Systole, Atrial systole just pushes this blood faster. We will now move to the pressure changes in the left ventricle and during its explanation the remaining volume changes will be explained. Keep in mind once again that the volume between both ventricles is constant, however, the pressure in the left ventricle is significantly higher than the right (we will be explaining pressure in the left ventricle). As stated previously when we were discussing volume, before Atrial Systole the pressure in the Atria was 0 and the pressure in the ventricles was -1 or -2. Now during Atrial Systole, the blood is ejected into the ventricle causing a slight increase in pressure up to about +5. Logically after Atrial Systole, Ventricular Systole follows. When the ventricle contracts the pressure within it increases rapidly causing the pressure in the ventricle to become higher than the pressure inside the atrium. This increased pressure forces the atrioventricular valve to close. At this instant in time, the AV valve is closed and the pressure has not risen to a point where it is higher than the aortic pressure to lead to the opening of the semilunar aortic valve. This instant is what we call Isovolumic contraction. Why is this so? It is due to the fact that at this instance we have contraction of the ventricular walls; however, due to the status of both valves being closed there is no change in volume and thus isovolumetric. In the previous paragraph I said that the pressure has not risen to a point higher than the aortic pressure to open the aortic semilunar valve, what does this statement mean? Similar to the atrioventricular valve, for it to open, the pressure in the atrium needs to be higher than the ventricular pressure. Therefore, for the aortic semilunar valve, the pressure in the contracting ventricle needs to exceed the pressure of the Aorta which is +80. Therefore, as soon as the ventricular pressure exceeds this value the semilunar valve is forced open and the blood is ejected into the Aorta. 6|P a ge This applies to most of the ejection period except towards its end, at this moment the aortic pressure exceeds the ventricular pressure. With everything we have stated you might say that the Aortic semilunar valve should close, and the blood should stop flowing. However, due to the blood flowing at very high speeds from the ventricle into the Aorta, the only way to stop the flow is to overcome its momentum by stopping the flowing current and this is difficult, so blood keeps flowing forward. This is depicted by Ohm’s Law which states: Flow= ∆𝑃 𝑅 , where ∆𝑃 is the change in pressure and 𝑅 is the resistance. Meaning that when the pressure in point A is higher than the pressure in point B blood will flow from point A to point B with no difficulty however, when there is high resistance to the flow the blood will simply stop flowing. Now in order for the blood to continue to flow from the left ventricle to the Aorta, the pressure in the ventricle needs to remain higher than the pressure in the Aorta. The pressure in the ventricles reaches its maximum which is 120 mmHg, but the Aortic pressure is always less than that (around 118-119) during ejection, again to ensure efficient ejection. After Ventricular Systole ends, Ventricular Diastole begins. When Ventricular Diastole begins the pressure in the ventricles begins to decrease, when it reaches a value below the pressure of the Aorta (approximately at 100 mmHg) the Aortic Semilunar valve is forced shut. W e have now reached a phase in the Cardiac Cycle known as Isovolumetric Relaxation, in this phase both the semilunar valve and the atrioventricular valve are closed, therefore, there is no change in volume in the ventricles. This is very similar to Isovolumetric contraction, but the only difference is that one involves contraction of the ventricular walls and one involves relaxation. Now the ventricular pressure keeps decreasing until its pressure returns to its minimum pressure (-2-0) which leads to the opening of the atrioventricular valve and the filling of blood from the atrium to the ventricles and the next cycle begins. 7|P a ge Figure 5 Figure 5 is the top portion of figure 4 zoomed in, you should now be able to know what happens in each stage of the figure and what causes these fluctuations in pressure. The pressure in the Aorta has a maximum of 120 (systolic) and a minimum of 80 (diastolic), while in the ventricles the pressure ranges from 0 (diastolic) to 120 (systolic). That being said, when we use a sphygmomanometer to measure the blood pressure we are measuring arterial blood pressure. Now we understand why the normal values for systolic pressure (in both the left ventricle and Aorta) are equal or slightly below 120 and diastolic (only in Aorta) are equal or slightly below 80. Now let’s go back to the volume changes that occur during a cardiac cycle: • Following Atrial Systole, we have around 125 ml of blood in the ventricles • As Ventricular Systole begins we have Isovolumetric Contraction • When the pressure in the ventricles exceeds the pressure in the Aorta we enter the Rapid Ejection phase, meaning that the volume of blood in the ventricles decreases • W e then move into the Slow Ejection phase as the pressure begins to decrease • The last phase of ventricular relaxation before the atrioventricular valve opens is Isovolumetric Relaxation as both valves are closed. 8|P a ge • When the Atrioventricular valve opens we enter a phase called Rapid Filling • Then the filling gets slower, thus we enter the phase of slow filling, otherwise known as diastasis • At the end of a single cycle (at the end of a ventricular systole), we have about 55 ml of residual blood in the ventricles this is otherwise known as the End Systolic Volume • At the beginning of the cycle (at the end of a ventricular diastole), we have 125 ml, this is known as the End Diastolic Volume By subtracting the End Systolic Volume from the End Diastolic Volume, we get the Stroke Volume. The stroke volume is defined as the amount of blood ejected from each ventricle in one cardiac cycle, which is equal to 70mL (125mL – 55mL). This brings us to another value which is the cardiac output. Cardiac output is defined as the amount of blood ejected from the heart from each ventricle in one minute. For it to be calculated we simply multiply the stroke volume by the heart rate. Let’s take an example: A 45-year-old male came to the cardiology clinic for a normal check-up. His ECG showed unremarkable rhythm with an R-R interval measuring at 0.857 seconds. His echocardiography studies also show unremarkable gross appearance of the heart. His EDV was calculated at 125 ml and his ESV was calculated at 55. From the results please calculate the Cardiac Output. A. B. C. D. E. 5100 ml 5000 ml 4900 ml 4800 ml 4700 ml Answer: 1. To calculate the cardiac output, we need the heart rate and the stroke volume. 2. To calculate heart rate, we have the R-R interval: 60 HR= 𝑅−𝑅 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙 60 HR= 0.857 9|P a ge HR= 70 bpm 3. To calculate Stroke Volume we use ESD and EVD SV=EVD-ESD SV=125-55 SV=70 ml 4. Now we can calculate Cardiac Output Cardiac output= Stroke Volume * Heart Rate Cardiac Output= 70*70 Cardiac Output= 4900 ml (The answer is C) Keep in mind that the total volume of blood in a normal human is between 4-5 litres, therefore in one minute your body can pump the total volume of your blood. You should also keep in mind that the stroke volume in the right ventricle is the same as the stroke volume in the left ventricle. Let’s say for example that they are not the same and that the stroke volume in the right ventricle is 70 ml but the stroke volume in the left ventricle is 69 ml. Will this 1 ml make a difference? Per beat the extra residual blood in the left ventricle is 1 ml. Per minute 1ml *HR (for simplicity let’s say HR=70) so the extra residual blood in the left ventricle is 70 ml in one minute. Therefore, in an hour the residual blood will be 70ml*60 min= 4200 ml of blood in the left ventricle. Don’t worry this doesn’t occur and the stroke volume is equal bilaterally. Actually, the stroke volume of the left ventricle could be a little higher; because the left side receives more blood than the right side (from the Pulmonary veins) so it is normal to have a higher cardiac output in the left side compared to the right side. (More will be discussed in RS) * I know that at first, I said that the stroke volume is the same bilaterally then I said that the left can be higher than the right, but this is what Dr. Faisal said in the lecture. We discussed the mechanical, electrical, volumetric, and pressure changes that occur in one cardiac cycle. W e are now going to discuss the heart sounds that occur in a cardiac cycle. In the clinic we use a stethoscope to auscultate the heart. What we listen to is the sounds of the closing of the valves. W e have two valves that are of importance to us currently which are the atrioventricular valve and the aortic valve. 10 | P a g e As stated previously the atrioventricular valve closes when the pressure in the ventricle is higher than that of the atrium it even tries to “prolapse” into the atrium, but the Papillary Muscles and Chorda Tendinea do not allow this to happen. When the valve closes the blood starts to “hit” on the valve causing us to hear what is known as turbulence. The turbulence caused by the closed atrioventricular valve is known as Heart Sound Number 1 (S1). In a similar manner the closure of the semilunar valve causes turbulence which is the Heart Sound Number 2 (S2). The time between S1 and S2 is approximately 0.3 seconds (corresponding to the time of Ventricular Systole), and the time between S2 and the next S1 is 0.5 seconds (corresponding to the time of Ventricular Diastole). Therefore, while listening to hearts sounds you need to be very concentrated due to the small difference in time between them. We talked previously about the blood pressure in the Aorta and the left ventricle, now what about the right ventricle? The graphs (Figure 4 and Figure 5) are the same, the only difference is the scale. The maximum pressure in the Pulmonary Artery is 25 mmHg and the minimum pressure is 8 mmHg. Again, what about the right ventricle? The maximum is 25 mmHg and the minimum pressure is 0 mmHg. Coming to an end we are going to discuss the pressure changes in the atria (the dotted line in Figure 4): • In Atrial Systole, the pressure increases giving us the first wave called the A-wave • W hen the atrioventricular valve closes, due to the high pressure in the ventricles, the valve pushes back on the atria causing a slight increase in pressure known as the C-wave. • Now blood starts to fill into the atrium again in creasing the pressure leading to what is known as the V-wave • Thus, the waves in atrial pressure are A C V 11 | P a g e