Cardiac Cycle PDF

Cardiac Cycle Definition: It is the study of the changes which occur in the heart during each cardiac beat (one systole and one diastole) 1 Duration: With a heart rate 70 - 75 beats / min. each cardiac cycle has duration of (0.8...

Cardiac Cycle Definition: It is the study of the changes which occur in the heart during each cardiac beat (one systole and one diastole) 1 Duration: With a heart rate 70 - 75 beats / min. each cardiac cycle has duration of (0.8 sec). Atrial systole 0.1 sec Ventricular systole = 0.3 sec Atrial diastole = 0.7 sec Ventricular diastole =0.5 sec 2  The duration of the cardiac cycle is inversely related to the heart rate. So, when the heart rate increases its duration is decreased. The decrease in the duration is on the expense of diastolic time which becomes short. 3 4 Events of cardiac cycle Phases of cadiac cycle (3 events) (5 phases) 1. Atrial systole. I- Atrial systole phase. 2. Ventricular systole. II- Isometric contraction phase. 3. Ventricular diastole. III- Ventricular ejection phase. Maximum ejection. Reduced ejection. IV- Isometric relaxation phase. V- Ventricular filling phase. Maximum filling. Reduced filling. 5 6 Ventricular pressure changes during cardiac cycle 7 1. Atrial systole phase; - The atria contract → flow of blood from the contracted atria to relaxed ventricles through the opened A-V valves, leads to ↑ in ventricular pressure. - Then, the ventricles relaxed to accommodate the pumped blood → ¯ in ventricular pressure. 8 2. Isometric contraction phase; - The A-V valves close at the beginning of this phase and the semilunar valves were already closed, so the ventricles turn into closed chambers filled with blood. - The ventricles contract isometrically (i.e without change in volume or size) → ↑ in ventricular pressure to reach (80 mmHg in Lt. ventricle or 10 mmHg in Rt. Ventricle). At this pressure, the semilunar valves opened at the end of this phase. 9 3. Maximum ejection phase; - The ventricles contract (with decrease in size) leading to ejection of large amount of blood rapidly to the big arteries through the opened Semilunar valves → ↑ in ventricular pressure to 120 mmHg in Lt. ventricle or 25 mmHg in Rt. Ventricle (as the ↑ in pressure due to ventricular contraction > the ¯ in pressure due to flow of blood to aorta or pulmonary artery). 4. Reduced ejection phase; - The blood is continued to be ejected from the ventricles to the big arteries but slowly and with less force → ¯ in ventricular pressure (as the ↑ in pressure due to ventricular contraction < the ¯ in pressure due to flow of blood to aorta or pulmonary artery). 10 5. Isometric relaxation phase; - The semilunar valves close at the beginning of this phase and the A-V valves were already closed, so the ventricles turn into closed chambers filled with blood. - The ventricles relax isometrically (i.e without change in volume or size) → ¯ in ventricular pressure. - When the intraventricular pressure becomes less than atrial pressure → opening of the A-V valves at the end of this phase. 11 6. Maximum filling phase; - The ventricles relax (with increase in size) leading to sudden rush of blood (which is accumulated in atria during atrial diastole) by gravity from atria to ventricles through the opened A-V valves → ¯ in ventricular pressure (as the ¯ in ventricular pressure due to ventricular relaxation > the ↑ in ventricular pressure due to ventricular filling), 7. Reduced filling phase; - The ventricular filling continues but becomes slow → ↑ ventricular pressure (as the ¯ in ventricular pressure due to ventricular relaxation < the ↑ in ventricular pressure due to ventricular filling). 12  Ventricular volume changes during cardiac cycle 1. Atrial systole phase; Increase 2. Isometric contraction phase; Constant 3. Maximum ejection phase; Decrease 4. Reduced ejection phase; Decrease 5. Isometric relaxation phase; Constant 6. Maximum filling phase; Increase 7. Reduced filling phase; Increase 13  State of valves during cardiac cycle 1. A-V valves: Opened during maximum filling phase, reduced filling phase and atrial systole phase. 1. Semilunar valves: Opened during maximum ejection phase and reduced ejection phase. 14  Atrial pressure changes during cardiac cycle It passes through 3 waves (a, c, and v), each consists of a positive part during which atrial pressure increases and a negative part during which atrial pressure decreases. 15 16 1. A wave: Coincides with atrial systole phase, composed of; - +ve a: (ascending limb of wave) is due to atrial contraction. - -ve a: (descending limb of wave) is due to passage of blood from atria to ventricles. 17 2. C wave: Composed of; - +ve c: Coincides with isometric contraction phase. It is due to bulge of the cusps of A-V valves inside the atrial cavity. - -ve c: Coincides with maximum ejection phase. It is due to descent of cusps of A-V valves. 18 3. v wave: Composed of; - +ve v: Coincides with reduced ejection phase & isometric relaxation phase. It is due to accumulation of blood in atria. - -ve v: Coincides with ventricular filling phases. Passes into 2 steps; o Decrease at first (during maximum filling phase) due to passage of blood from atria to ventricles. o Increase gradually (during reduced filling phase) due to accumulation of venous return into atria. 19 Aortic pressure changes during cardiac cycle Composed of: 20 Aortic pressure changes during cardiac cycle 21 1. Ascending (anacrotic) limb; - Coincides with maximum ejection phase. - During which aortic pressure increases to 120 mmHg due to the amount of blood that enters the aorta is > the amount of blood that leaves the aorta. 22 2. Descending (catacrotic) limb; It is divided into 4 stages: - During the reduced ejection phase, the aortic pressure ¯ because the amount of blood enters the aorta is < the amount of blood that leaves the aorta. - Dicrotic notch: Sudden drop in aortic pressure by the end of ejection phase due to sudden closure of the aortic valve. - Dicrotic wave: slight increase in aortic pressure due to elastic recoil of wall of aorta during isometric relaxation phase. - Gradual ¯ in aortic pressure as the blood leaves aorta to the peripheral vessels during the filling phases, atrial systole and isometric contraction phase. 23  Heart sounds during cardiac cycle 1. First heart sound: Produced by sudden closure of A-V valves at the beginning of isometric contraction phase. 2. Second heart sound: Produced by sudden closure of semilunar valves at the beginning of isometric relaxation phase. 24  ECG during cardiac cycle It is the summed electrical activity of all cardiac muscle fibers, which occur prior to the onset of mechanical activity. The ECG shows the P wave, the QRS waves, and the T wave. The P wave is caused by the spread of depolarization through the atria and is followed by atrial contraction. The QRS waves appear as a result of electrical depolarization of the ventricles, which initiates contraction of the ventricles. 1. P wave: Starts 0.02 sec. before atrial systole phase. 2. QRS complex: Starts 0.02 sec. before isometric contraction phase. 25 26 vRemarks on the cardiac cycle Ventricular diastole constitutes about 2/3 of the duration of cardiac cycle, while ventricular systole constitutes about 1/3 of the duration of cardiac cycle and this is very important as during ventricular diastole the following occur. a. Coronary blood flow occurs during it. b. Ventricular filling occurs (especially in early diastole). c. Ventricular rest. 27  Ventricular diastole can be divided into 3 stages; o Early diastole: includes; - isometric relaxation phase. - maximum filling phase. o Mid diastole: includes; reduced filling phase. o Late diastole: includes; atrial systole phase. 28 Ventricular filling occurs during 3 phases (maximum filling, reduced filling and atrial systole). 70-80 % of the blood passes from the atria to the ventricles during the filling phases by its weight (especially the rapid filling phase) and the remaining part 20-30% passes as a result of atrial systole, so the action of the atria as a pump is not important as it acts mainly as a blood reservoir. 29  The pressure changes during the cardiac cycle are as follow: Lt. ventricle Rt. ventricle Aorta Pulmonary artery Systolic 120 25 120 25 pressure (mmHg) Diastolic 0 0 80 10 pressure (mmHg) (i.e The systolic pressure in each ventricle is the same as that in its corresponding vessel, while the diastolic pressure in both ventricles is equal at 0 mm Hg). 30 Blood flows to the tissues both during systole and diastole. During systole it flows by ventricular contraction and ejection of blood, while during diastole it flows by elastic recoil of the aorta, as it produces sufficient pressure during ventricular diastole (diastolic pressure) that maintain blood flow to the tissue during diastole. 31  Summary 1- Cardiac cycle (definition & duration). 2- phases of cardiac cycle: 1) Atrial systole 2) Ventricular systole: a- Isometric contraction phase b- Maximum ejection phase c- Reduced ejection phase 3) Ventricular diastole: a- Isometric relaxation phase b- Rapid filling phase c- Reduced filling phase 32 33 34