Cardiac Cycle Notes PDF

Cardiac cycle The cardiac events that occur from the beginning of one heartbeat to the beginning of the next are called the cardiac cycle. Each cycle is initiated by spontaneous generation of an action potential in the sinus node then the action potential travels from here rapidly through both atria...

Cardiac cycle The cardiac events that occur from the beginning of one heartbeat to the beginning of the next are called the cardiac cycle. Each cycle is initiated by spontaneous generation of an action potential in the sinus node then the action potential travels from here rapidly through both atria and then through the A-V bundle into the ventricles. Diastole and Systole The cardiac cycle consists of a period of relaxation called diastole, during which the heart fills with blood, followed by a period of contraction called systole. The total duration of the cardiac cycle, including systole and diastole, is the reciprocal of the heart rate. For example, if heart rate is 72 beats/min, the duration of the cardiac cycle is 1/72 min/beat—about 0.8 second per beat. Phases of the cardiac cycle 1- Atrial systole (0.1 second) The first event is the depolarization of the atrial muscle cells, which causes the P wave of the electrocardiogram. As the atrial muscle cells develop tension and shorten, atrial pressure rises and an additional amount of blood is forced into the ventricle (about 25%). At normal resting heart rates, atrial contraction is not essential for adequate ventricular filling. Blood normally flows continually from the great veins into the atria; about 80 percent of the blood flows directly through the atria into the ventricles even before the atria contract. Then, atrial contraction usually causes an additional 20 percent filling of the ventricles. This is evident in Figure 1 from the fact that the ventricle has nearly reached its maximum or end-diastolic volume before atrial contraction begins. Atrial contraction plays an increasingly significant role in ventricular filling as heart rate increases because the time interval between beats for passive filling becomes progressively shorter with increased heart rate. 2- Ventricular systole (0.3 second) a- Isovolumetric contraction phase Ventricular systole begins when the action potential passes through the AV node and sweeps over the ventricular muscle-an event represented by the QRS complex of the electrocardiogram. Contraction of the ventricular muscle cells causes intra ventricular pressure to rise above that in the atrium. The increased pressure behind the leaflets in the ventricle causes abrupt closure of the AV valve, and hearing of the first heart sound. Pressure in the left ventricle continues to rise sharply as the ventricular contraction intensifies. When the left ventricular pressure exceeds that in the aorta, the aortic valve passively opens. The period between mitral valve closure and aortic valve opening is referred to as the isovolumetric contraction phase because during this interval the ventricle is a closed chamber with a fixed volume. 1 b- Rapid ejection phase Ventricular ejection begins with the opening of the aortic valve. In early ejection, blood enters the aorta rapidly and causes the pressure there to rise. Pressure builds up simultaneously in both the ventricle and the aorta as the ventricular muscle cells continue to contract in early systole. This interval is often called the rapid ejection period. Fig.1: shows the different events during the cardiac cycle for the left side of the heart. The top three curves show the pressure changes in the aorta, left ventricle, and left atrium, respectively. The fourth curve depicts the changes in left ventricular volume, the eighth depicts the electrocardiogram, and the sixth depicts a phonocardiogram, which is a recording of the sounds produced by the heart—mainly by the heart valves—as it pumps. 2 c- Reduced ejection phase Left ventricular and aortic pressures ultimately reach a maximum called peak systolic pressure. At this point, the strength of ventricular muscle contraction begins to wane. Muscle shortening and ejection continue, but at a reduced rate. Aortic pressure begins to fall because blood is leaving the aorta and large arteries faster than blood is entering from the left ventricle. Throughout ejection, very small pressure differences exist between the left ventricle and the aorta because the aortic valve orifice is so large that it presents very little resistance to flow. Eventually, the strength of the ventricular contraction diminishes to the point where intraventricular pressure falls below aortic pressure. A dip, called the incisura or dicrotic notch, appears in the aortic pressure trace because a small volume of aortic blood must flow backward to fill the space behind the aortic valve leaflets as they close to start the next phase. 3- diastole of the whole heart (0.4 second) a- Isovolumetric relaxation phase Because of the aortic valve structure, the increased pressure behind the leaflets in the aorta causes abrupt closure of the aortic valve, with hearing of the second heart sound. Recoil of the aorta and closure of the aortic valve causes the dicrotic wave (figure 73) in the aortic pressure trace. The ventricle has reached its mini- mum or end-systolic volume at the time of aortic valve closure. After aortic valve closure, intraventricular pressure falls rapidly as the ventricular muscle relaxes. Indeed, the atrial pressure progressively rises because blood continues to return to the heart and fill the atrium. Ultimately, intraventricular pressure falls below atrial pressure, and the AV valve opens. b- Maximum filling phase The mitral valve passively opens when left ventricular pressure falls below left atrial pressure and the period of ventricle filling begins. About 75% of the blood that had previously accumulated in the atrium behind the closed mitral valve empties rapidly into the ventricle, and this causes an initial drop in atrial pressure. Later, the pressures in both chambers slowly rise together as the atrium and ventricle continue passively filling simultaneously the blood returning to the heart through the veins. c- Reduced filling phase As the ventricles continue to fill with blood and expand, they become less compliant and the intraventricular pressure rises. This reduces the pressure gradient across the AV valves so that the rate of filling falls, that is why atrial systole has to start to begin the next heart cycle. Increasing Heart Rate Decreases Duration of Cardiac Cycle. When heart rate increases, the duration of each cardiac cycle decreases, including the contraction and relaxation phases. The duration of the action potential and the period of contraction (systole) also decrease, but not by as great a percentage as does the relaxation phase (diastole). At a normal heart rate of 72 beats/min, systole comprises about 0.4 of the entire cardiac cycle. At three times the normal heart rate, systole is about 0.65 of the entire cardiac cycle. This means that the heart beating at a very fast rate does not remain relaxed long enough to allow complete filling of the cardiac chambers before the next contraction. 3 The aortic pulse curve (figure 2) The aorta distends or balloons out during systole because more blood enters the aorta than leaves it. During diastole, the arterial pressure is maintained by the elastic recoil of walls of the aorta and other large arteries. Nonetheless, aortic pressure gradually falls during diastole as the aorta supplies blood to the systemic vascular beds. The lowest aortic pressure reached at the end of diastole, is called diastolic pressure. The difference between diastolic and peak systolic pressures in the aorta is called the arterial pulse pressure. Typical values for systolic and diastolic pressures in the aorta are 120 and 80 mm Hg, respectively. Fig.2; aortic pulse curve Atrial (jugular venous) pulse curve (Figure 3) The pressure pulsations that occur in the right atrium are transmitted in retrograde fashion to the large veins near the heart. These pulsations, shown on the atrial pressure trace in Figure 75, can be visualized in the neck over the jugular veins in a recumbent individual. They are collectively referred to as the jugular venous pulse and can provide clinically useful information about the heart. Atrial contraction produces the first pressure peak called the “a” wave. The “c” wave, which follows shortly thereafter, coincides with the onset of ventricular systole and is caused by an initial bulging of the tricuspid valve into the right atrium. Right atrial pressure falls after the c wave “x” wave, because of atrial relaxation and a downward displace ment of the tricuspid valve during ventricular emptying. Right atrial pressure then begins to increase toward a third peak, the “v” wave, as the central veins and right atrium fill behind a closed tricuspid valve with blood returning to the heart from the peripheral organs. With the opening of the tricuspid valve at the conclusion of ventricular systole, right atrial pressure again falls as blood moves into the relaxed right ventricle “y” wave. Fig.3: Jugular pulse curve 4 5 6