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THE CARDIAC CYCLE At the end of lecture, the student should be able to: • • • • • • Define what is cardiac cycle? Describe the phases of cardiac cycle. Describe the relationship of the electrocardiogram to the Cardiac Cycle. Tell us the Pressure Changes in the Atria. Explain the Period of Isovolumic (Isometric) Relaxation. Relationship of the Heart sounds with cardiac cycle. Lecture Outline Definition: 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. Atria act as primer pumps for the ventricles. Ventricles in turn provide the major source of power for moving blood through the body’s vascular system. Diastole: A period of relaxation called diastole, during which the heart fills with blood. Systole: A period of contraction called systole. Phases of Cardiac Cycle: It can be divided into four distinct phases: (I) Contraction Phase and (Ii) Ejection Phase, Both Occurring In Systole; (Iii) Relaxation Phase and (Iv) Filling Phase, Both Occurring In Diastole. At the end of phase IV, the atria contract. ATRIAL SYSTOLE Definition: Atrial systole occurs in late part of ventricular diastole, Main force driving blood from the atria to the ventricles is the decrease in ventricular pressure that occurs during ventricular diastole. The drop in ventricular pressure that occurs during ventricular diastole allows the atrioventricular valve to open, emptying the contents of the atrium into the ventricle. Contraction of the atrium confers a relatively minor, additive effect toward ventricular filling. Function of the Atria as Primer Pumps: About 80 per cent of the blood flows directly through the atria into the ventricles even before the atria contract. Then, atrial contraction usually causes an additional 20 per cent filling of the ventricles. Therefore, the atria simply function as primer pumps that increase the ventricular pumping effectiveness as much as 20 per cent. Relationship of the Electrocardiogram: The P wave is caused by spread of depolarization through the atria, and this is followed by atrial contraction, which causes a slight rise in the atrial pressure curve immediately after the electrocardiographic P wave. • • The PR segment is electrically quiet as the depolarization proceeds to the AV node. This brief pause before contraction allows the ventricles to fill completely with blood. Pressure Changes in the Atria—The a Wave. The a wave is caused by atrial contraction. right atrial pressure increases 4 to 6 mm Hg during atrial contraction, and Left atrial pressure increases about 7 to 8 mm Hg. Heart Sounds: • • Fourth heart sound (S4) is abnormal and is associated with the end of atrial emptying after atrial contraction. It occurs with hypertrophic congestive heart failure, massive pulmonary embolism, tricuspid incompetence, or cor pulmonale. Ventricular systole: Atrio-ventricular delay in impulse conduction. Phases: 1. Isovolumic contraction. 2. Rapid ejection - 70% emptying. 3. reduced/slow ejection - 30% emptying. Period of Isovolumic (Isometric) Contraction: The ventricular pressure rises abruptly after the beginning of ventricular contraction causing the A-V valves to close. 0.02 to 0.03 second is required for the ventricle to build up sufficient pressure to push the semilunar valves open against the pressures in the aorta and pulmonary artery. Therefore, contraction is occurring in the ventricles, but there is no emptying so called the period of Isovolumic or isometric contraction, meaning that tension is increasing in the muscle but little or no shortening of the muscle fibers is occurring. Pressure Changes: The c wave occurs when the ventricles begin to contract; it is caused partly by slight backflow of blood into the atria at the onset of ventricular contraction but mainly by bulging of the A-V valves backward toward the atria because of increasing pressure in the ventricles. The v wave occurs toward the end of ventricular contraction; it results from slow flow of blood into the atria from the veins while the A-V valves are closed during ventricular contraction. Relationship of the Electrocardiogram: About 0.16 second after the onset of the P wave, the QRS waves appear as a result of electrical depolarization of the ventricles, which initiates contraction of the Ventricles and causes the ventricular pressure to begin rising, as also shown in the figure. Therefore, the QRS complex begins slightly before the onset of ventricular systole. Heart Sounds • The first heart sound (S1, "lub") is due to the closing AV valves and associated blood turbulence. Period of rapid ejection: The first third of the period of ejection is called the period of rapid ejection because 70 per cent of the blood emptying occurs during this period. • • • The semilunar (aortic and pulmonary) valves open at the beginning of this phase. While the ventricles continue contracting, the pressure in the ventricles (red) exceeds the pressure in the aorta and pulmonary arteries (green); the semilunar valves open, blood exits the ventricles, and the volume in the ventricles decreases rapidly (white). As more blood enters the arteries, pressure there builds until the flow of blood reaches a peak. The Period Of Slow Ejection: The last two thirds is the period of slow ejection The remaining 30 per cent emptying occurs during the next two thirds of the period of ejection. • • • At the end of this phase the semilunar (aortic and pulmonary) valves close. After the peak in ventricular and arterial pressures (red and green), blood flow out of the ventricles decreases and ventricular volume decreases more slowly (white). When the pressure in the ventricles falls below the pressure in the arteries, blood in the arteries begins to flow back toward the ventricles and causes the semilunar valves to close. This marks the end of ventricular systole mechanically. Relationship of the Electrocardiogram: The ventricular T wave in the electrocardiogram represents the stage of repolarization of the ventricles when the ventricular muscle fibers begin to relax. Therefore, the T wave occurs slightly before the end of ventricular contraction. Ventricular diastole: Phases: -isovolumic relaxation - rapid filling - slow filling - atrial systole. Period of Isovolumic (Isometric) Relaxation: Ventricular relaxation begins suddenly. At the end of systole, allowing both the right and left intraventricular pressures to decrease rapidly. The elevated pressures in the distended large arteries that have just been filled with blood from the contracted ventricles immediately push blood back toward the ventricles, which snaps the aortic and pulmonary valves closed. For another 0.03 to 0.06 second, the ventricular muscle continues to relax, even though the ventricular volume does not change, giving rise to the period of Isovolumic or isometric relaxation. The intraventricular pressures decrease rapidly back to their low diastolic levels. Then the A-V valves open to begin a new cycle of ventricular pumping. • The second heart sound (S2, "dup") occurs when the semilunar (aortic and pulmonary) valves close. S2 is normally split because the aortic valve closes slightly earlier than the pulmonary valve. RAPID VENTRICULAR FILLING • • Once the AV valves open, blood that has accumulated in the atria flows rapidly into the ventricles. Ventricular volume (white) increases rapidly as blood flows from the atria into the ventricles. SLOW REDUCED VENTRICULAR FILLING: • • Rest of blood that has accumulated in the atria flows slowly into the ventricles. Ventricular volume (white) increases more slowly now. The ventricles continue to fill with blood until they are nearly full. References: Text book of medical Physiology. GUYTON & HALL. Eleventh Edition. Color Atlas of Physiology. Stefan Silbernagel, Agamemnon Despopoulos. 6th Edition.