 In this video I will list and describe the phases of the cardiac cycle, describe the functions of the heart valves, describe the pressure changes associated with blood flow through the heart, describe the physiological basis of the production of the two principal heart sounds, describe how deflection of the normal ECG corresponds to electrical and mechanical activity of the heart through the cardiac cycle, define fibrillation, tachycardia, and bradycardia. The cardiac cycle is the period of time for one heart beat, beginning with contraction of the atria, ending with relaxation of the ventricles. The word for contraction of a heart chamber is systole, and the word for relaxation of a heart chamber is diastole. Now remember that contraction of cardiac muscle is excited by intrinsic pacemaker cells found in the SA node within the right atrium of the heart. And so we start with that action potential that's generated in the SA node and is spreading throughout the atria causing atrial depolarization. That depolarization is the stimulus that triggers contraction of the atria known as atrial systole. So atrial contraction or atrial systole forces blood from the atria down into the ventricles. Then the electrical signal, the action potential, spreads from the atria down through into the ventricles. So it's spreading through the intrinsic conduction system from the AV node through the AV bundle and bundle branches into the purkinje fibers that branch throughout the myocardium of the ventricles. So following atrial depolarization, the atria enter repolarization, so the end of the action potential is occurring in the atria as the action potential spreads down from the AV node through the purkinje fibers and then causes ventricular depolarization. Ventricular depolarization is then the stimulus that activates ventricular systole. And then ventricular repolarization follows at the end of the action potential and this ventricular repolarization leads to ventricular diastole. Now you probably noticed that ventricular systole and ventricular diastole were subdivided into early and late stages. This is because during early ventricular systole the volume of blood inside of the ventricles does not change. So early ventricular systole is also known as isovolumetric relaxation. Then during late ventricular systole, blood is forced out of the ventricles into the arteries. So late ventricular systole is also known as ventricular ejection. Following ventricular ejection during early ventricular diastole, the volume of blood inside of the ventricles does not change. But then during late ventricular diastole, blood starts to flow from the atria down into the ventricles, increasing the volume of blood inside of the ventricles. The cardiac cycle begins with contraction of the atria, also known as atrial systole, which is then followed by isovolumetric contraction, which is the early phase of ventricular systole. During isovolumetric contraction, the blood pressure inside of the ventricles rises, but the blood volume inside of the ventricles remains constant. Then the late phase of ventricular systole is also known as ventricular ejection. Ventricular ejection occurs when the blood pressure inside of the ventricles is greater than the blood pressure inside of the arteries, and blood flows out of the ventricles into the arteries. Following ventricular ejection is isovolumetric relaxation, the early phase of ventricular diastole. During isovolumetric relaxation, the blood pressure inside of the ventricles is decreasing, but the blood volume inside of the ventricles remains constant. Then the last phase of the cardiac cycle in the beginning of the next cardiac cycle is ventricular filling. During the late phase of ventricular diastole, this occurs when the pressure of blood inside of the ventricles falls below the blood pressure inside of the atria, causing blood to flow from the atria into the ventricles. This illustration shows us an ECG, also known as an EKG. ECG is the abbreviation for the word electrocardiogram, electrocardiogram abbreviated ECG or EKG. So EKG is just as common of an abbreviation as ECG to here, and EKG comes from the spelling of the word in German. So the electrocardiogram is a recording of the electrical activity of the heart taken with electrodes placed on the surface of the skin. There is a P wave of the electrocardiogram that we can see here with the purple color, that deflection is known as the P wave, and the P wave results from atrial depolarization. The P wave occurs immediately before atrial systole because atrial depolarization is the stimulus that triggers atrial systole. The next deflection of the ECG is the QRS complex. So the QRS complex results from atrial repolarization and ventricular depolarization, and the QRS complex occurs immediately before isovolumetric contraction because ventricular depolarization is the stimulus that triggers ventricular systole. The last deflection of the ECG that we see here is the T wave, and the T wave occurs as a result of ventricular repolarization, and so the cardiac cycle is the amount of time between the beginning of the P wave and the end of the T wave transitioning into the beginning of the next P wave, although we often measure the distance between one QRS complex and the next QRS complex to measure a cardiac cycle, to measure the length of the cardiac cycle, because the QRS complex is a large deflection that's easy to identify for measuring that time duration of one cardiac cycle. The heart valves function to prevent backward flow of blood. There are atrio ventricular valves, AV valves that prevent blood from flowing backwards from the ventricles into the atria, and there are semilunar valves, SL valves, that prevent blood from flowing backwards from the arteries into the ventricles. The AV valve on the right side of the heart is known as the tricuspid valve, and so the tricuspid prevents blood from flowing backwards out of the right ventricle into the right atrium. When the right ventricle contracts, it forces the tricuspid valve closed so that blood doesn't flow backwards. The bicuspid valve, also known as the mitral valve, is the AV valve on the left side of the heart, preventing blood from flowing backwards from the left ventricle into the left atrium. So during ventricular filling, the AV valves are open and blood is flowing from the atria down into the ventricles. But during isovolumetric contraction, the AV valves are forced closed, and it is the increasing blood pressure inside the ventricles that causes the AV valves to be forced closed. Then the semilunar valves are the valves that prevent blood from flowing backwards from the arteries into the ventricles. Here we see there's a aortic valve, also known as the left semilunar valve, which prevents blood from flowing backwards from the aorta into the left ventricle. And there's also a pulmonary valve, which is also known as the right semilunar valve, which prevents blood from flowing backwards from the pulmonary arteries into the right ventricle. The semilunar valves remain closed during isovolumetric contraction. Here we see that the semilunar valves open during ventricular ejection. And this is because the blood pressure inside of the ventricles is greater than the blood pressure inside of the arteries. And this blood pressure forces open the semilunar valves, enabling blood to flow out of the ventricles into the arteries during ventricular ejection. Then following ventricular ejection, when the heart enters into isovolumetric relaxation in the early phase of ventricular diastole, the semilunar valves are closed and the AV valves are closed because the blood pressure inside of the ventricles is lower than the blood pressure inside of the arteries, but higher than the blood pressure inside of the atria. The AV valves will remain closed until the blood pressure inside of the ventricles falls lower than the blood pressure inside of the atria. Once the blood pressure in the atria is greater than the blood pressure in the ventricles, the AV valves open, and ventricular filling occurs. Oscultation is the practice of listening to the sounds of the body. Using a stethoscope, we can listen to the heart sounds. The characteristic sound of the heartbeat is lubbed up, lubbed up, lubbed up. The lub is the first heart sound, or S1, resulting from the closing of the atrioventricular valves, the AV valves, then the dub, or the second heart sound, S2, results from the closing of the semilunar valves, the SL valves. When listening to the heart sounds, the placement of the stethoscope will determine the relative volume of the sound coming from individual heart valves. For example, you can see if you were to place the bell of the stethoscope on the left side of the chest at the level of the fifth intercostal space along the midpoint of the clavicle. This is the location near the apex of the heart that allows us to hear the sounds coming from the mitral valve most loudly. Then if we move the stethoscope over to the right side of the sternum, we can hear the sounds coming from the right AV valve, the tricuspid valve. Similarly, moving superiorly to the second intercostal space on the right side of the sternum allows us to hear the sounds coming from the aortic valve, whereas moving to the left side of the sternum at that second intercostal space is positioning that allows us to hear the sound of the pulmonary valve as the loudest of the heart sounds. Moving the stethoscope to listen to the individual valves can enable the diagnosis of any abnormal heart sounds such as a murmur that could result from a heart valve that does not close entirely, allowing some blood to flow backwards, allowing some regurgitation, or a valve that does not open wide enough could also produce an abnormal heart sound. Here we see a graph showing the blood pressure inside of the left atrium in the yellow line, the blood pressure inside of the left ventricle in the green line, and then the pressure inside of the aorta, the largest systemic artery in the red line. And then just below this blood pressure graph we see the heart sounds. So we can see that initially the blood pressure inside of the heart chambers rises as the atria contract during atrial systole. And then the blood pressure inside of the ventricles rises as we enter into isovolumetric contraction, and this occurs at the same point as the first heart sound, the lub, or S1. So S1 results from closing of the AV valves at the beginning of isovolumetric contraction. Then as we enter ventricular ejection the semilunar valves open, the semilunar valves are opening here at the beginning of ventricular ejection when the pressure inside of the ventricles rises greater than the pressure inside of the arteries. So ventricular ejection proceeds until the semilunar valves close, the semilunar valves close at the point when the pressure inside of the ventricle falls lower than the pressure inside of the arteries, and this marks the beginning of isovolumetric relaxation and is marked by S2, the DUP, or the second heart sound. The electrocardiogram is a recording of the electrical activity of the heart. The recording is made with electrodes, recording electrodes placed on the surface of the skin, and there's characteristic deflections resulting from the electrical changes occurring in the heart through the cardiac cycle. The first of these is known as the P wave, and the P wave corresponds to depolarization of the atria. Next we have the QR and S waves, and we can group together the QRS waves and call that the QRS complex. The QRS complex results from repolarization of the atria and depolarization of the ventricles. Then following the QRS complex, the next letter in the alphabet T is used to refer to the T wave, which corresponds to repolarization of the ventricles. The ends of the ECG are regions between two waves, for example the P to R segment. The P to R segment is the region between completion of atrial depolarization and the beginning of atrial repolarization. You'll notice that the line of the electrocardiogram is flat during the P to R segment, and this corresponds to the plateau phase of the action potential in contractile cells of the atria. There's no repolarization until the QRS complex. Depolarization produced the P wave, and these cells are in the plateau phase where they are depolarized and producing contraction through this P to R segment. Here we see the ST segment. Similarly, the ST segment is a flat region on the line of the electrocardiogram. The ST segment corresponds to the time between completion of ventricular depolarization and the beginning of ventricular repolarization. During this ST segment, the contractile cells in the ventricles are in the plateau phase of the action potential. All of these contractile cells of the ventricle are fully depolarized and have not begun to repolarize yet and are therefore in the contraction. This is the plateau of the action potential occurring while those cells are contracted. We can also measure time intervals of the ECG, which are larger than the segments. The P to R segment and the ST segments are flat lines corresponding to the plateau phase of the action potentials for the atria and ventricles respectively. We can see the P to R interval includes the P to R segment as well as the entire P wave. The P to R interval is the time between the beginning of atrial depolarization and the beginning of ventricular depolarization. That's the amount of time that it takes for the action potential to travel from the SA node into the ventricles. Then the Q to T interval includes the ST segment as well as the QRS complex and T wave. So the Q to T interval is the time between onset of ventricular depolarization until the completion of ventricular repolarization. Then the last interval is the R to R interval. The R to R interval is the time between one cardiac cycle and the same point in the subsequent cardiac cycle. The R wave is the largest signal in the ECG normally. The R to R interval is a convenient way to measure the length of a cardiac cycle. By measuring the R to R interval we've measured the duration of one cardiac cycle and we can then calculate from the R to R interval the heart rate. So if we measure the R to R interval in seconds we can then convert from the R to R interval to heart rate in beats per minute. So R to R interval is the seconds for one beat. We can take the reciprocal of that to get the number of beats per second and then we just need to multiply by a conversion factor to convert from seconds to minutes. So multiply by 60 seconds per minute and that will then give us the heart rate. This figure correlates the electrical activity of the heart shown with the ECG with the mechanical activity of the heart. So the P wave corresponds to atrial depolarization and you can see that immediately following the P wave the cardiac cycle enters atrial systole the first phase of the cardiac cycle. The QRS complex corresponds to atrial repolarization and ventricular depolarization. So immediately following the QRS complex the cardiac cycle enters atrial diastole and isovolumetric contraction the early phase of ventricular systole. And we see that the T wave which corresponds to ventricular repolarization occurs immediately before the beginning of ventricular diastole and so the isovolumetric relaxation the early phase of ventricular diastole occurs immediately following the T wave. So here we can see the electrical activity of the heart through the cardiac cycle and how it corresponds to different regions of the electrocardiogram. So if we're starting off in ventricular filling the heart is completely repolarized and resting in diastole then the SA node generates an action potential that spreads through the atria leading to atrial depolarization which we see the P wave corresponds to atrial depolarization. And as atrial systole is occurring we are in the P to R segment until atrial repolarization occurs and ventricular depolarization occurs. So the QRS complex results from atrial repolarization and ventricular depolarization. So we see the action potential spreading from the AV node down through the AV bundle, bundle branches and purkinje fibers leading to ventricular depolarization and then we have ventricular systole during the ST segment of the ECG and then the T wave corresponds to ventricular repolarization and ventricular diastole the early phase of isovolumetric relaxation occurs immediately following the T wave then ventricular filling will occur at the end of the cardiac cycle to prepare for the next cardiac cycle which will then be initiated with another action potential spreading from the SA node through the atria. A major clinical application of the electrocardiogram is in the diagnosis of cardiac abnormalities such as arrhythmias so an abnormal heart rhythm and arrhythmia or an uncoordinated atrial and ventricular pattern of contractions could be detected with NECG. The first example of an arrhythmia we see here is this second degree block or a partial blockage. So notice that not all of the P waves are followed by a QRS complex and T wave in this ECG. Here we have a P wave that's not followed by a QRS complex and here's another example where there's a P wave that's not followed by a QRS complex or a T wave so this second degree block occurs when atrial depolarization does not spread through the intrinsic conduction system to stimulate ventricular depolarization then the atria contracts without the ventricles contracting and then the atria would be stimulated to contract a second time. Now what do you expect to happen to the heart rate or the pulse rate as a result of this blockage? Well the pulse rate the pulsing of the arteries that's felt when we place a finger onto an artery near the surface that pulsing corresponds to blood entering the arteries causing the arteries to expand with the increasing blood pressure that corresponds to ventricular ejection. But in this second degree blockage the heart is essentially skipping a beat where missing the ventricular systole and getting two atrial systoles without any ventricular systole. So how would this affect the pulse rate? It would cause a decrease in the pulse rate and bradycardia refers to an abnormally low heart rate so any heart rate less than 60 beats per minute at rest is technically classified as bradycardia. So the next example we see here is atrial fibrillation. So fibrillation is irregular chaotic twitching of the myocardium of the heart muscle. So this will occur if one region of the heart starts to depolarize and rather than that depolarization spreading through the entire heart muscle a separate region of the heart generates another action potential and this leads to an uncoordinated pattern of contraction and an uncoordinated pattern of electrical activity in the heart muscle. So this fibrillation will disrupt the efficiency of the heart's ability to pump blood. The atrial fibrillation is specifically the irregular twitching of the myocardium in the atria and we can see there there are QRS complexes still occurring in this ECG and they're occurring at a relatively high frequency. So they're not occurring at a very regular frequency. We see there's a group of three or four and then a space where there's no QRS complex then another two and then a pause and another three and then a pause and another two. While the atria is not contracting as a coordinated fashion to efficiently pump blood down into the ventricles, depolarization can spread down into the ventricles each time it's generated in the atria and so this can lead to an abnormally elevated pulse rate although the blood pressure would be decreased as a result of a loss of efficiency of the heart's ability to pump blood and so tachycardia refers to an abnormally elevated heart rate so a resting heart rate above 100 beats per minute is defined as tachycardia. The next example we see here ventricular tachycardia commonly just abbreviated VTAC. This is occurring when the action potential is being generated by an abnormal region of the heart not being generated in the SA node but instead being generated somewhere in the ventricles so notice this unusual shape of the QRS complex this characteristic shape results from the action potential being generated locally in the ventricles and then spreading through the ventricles causing contraction and then another heart rate another heart beat is generated again inside of the ventricles at a relatively high frequency so this is an abnormal region of the heart functioning as a new pacemaker an ectopic pacemaker a pacemaker that's not found in the SA node but is instead found in the ventricles and so this would lead to an increase in the heart rate indicated by the name ventricular tachycardia this is a form of tachycardia so then the last example we see here ventricular fibrillation commonly just abbreviated V fib is a complete disruption in the electrical activity where the ventricles are having irregular chaotic twitching so one region of the myocardium in the ventricle depolarizes then a different region starts to depolarize and it's it's happening in a uncoordinated chaotic pattern so the heart is not able to function as a pump to really be able to contract and force blood out into the arteries during ventricular fibrillation the pulse rate would go drastically down essentially there would be no pulse as a result of ventricular fibrillation because the heart muscle is not able to efficiently pump blood into the arteries it's just having an irregular chaotic pattern of twitching instead of being able to produce a coordinated contraction necessary to pump blood