Cardiac Cycle By Dr. Khaled Ibrahim Khalil By

Objectives: By the end of this lecture, you should : By the end of this lecture, you should :  Describe events in cardiac cycle.  Describe atrial, ventricular and aortic pressure changes during cardiac cycle.  Describe the changes in ventricular volume & stroke volume during cardiac cycle.  Relate ECG changes to the phases of cardiac cycle.  Describe the functions of cardiac valves and relate their state to the production of heart sounds during cardiac cycle. Objectives: By the end of this lecture, you should : By the end of this lecture, you should :  Describe events in cardiac cycle.  Describe atrial, ventricular and aortic pressure changes during cardiac cycle.  Describe the changes in ventricular volume & stroke volume during cardiac cycle.  Relate ECG changes to the phases of cardiac cycle.  Describe the functions of cardiac valves and relate their state to the production of heart sounds during cardiac cycle. References: Textbook of Medical Physiology by Guyton 12 th ed. Pages:

THE CARDIAC CYCLE Definition:  It is the cardiac events that occur from the beginning of one heart beat to the beginning of the next beat.  These events consists of periods of contraction called "systole" and a period of relaxation called "diastole". Duration:  Assuming a heart rate of 75 beat/min. the average duration of each cycle is 0.8 (60 / 75) second.

I- Atrial systole. (during which the ventricle is relaxed) II- Ventricular systole, (during which the atrium is relaxed) III- Ventricular diostole, (during which the atrium is relaxed) (i.e., relaxation of the whole heart). Phases of the cardiac cycle:

I- Atrial systole. (during which the ventricle is relaxed) II- Ventricular systole, (during which the atrium is relaxed) It occurs in 3 phases : a- Isometric (or isovolumetric) contraction phase. b- Maximum ejection phase. c- Reduced ejection phase. III- Ventricular diostole, (during which the atrium is relaxed) (i.e., relaxation of the whole heart). It occurs in 3 phases: a- Isometric (or isovolumetric) relaxation phase. b- Rapid filling phase. c- Reduced filling phase. Phases of the cardiac cycle:

I- Atrial Systole 0.1 second Duration Increases temporarily from zero to 2 mmHg due to atrial contraction. By the end of this phase, the pressure returns back to zero due to relaxation of the atrium and evacuation of blood into the ventricles. The constriction of the circular muscle sleeve present around the orifices (openings) of the superior and inferior venae cavae and pulmonary veins prevents blood regurgitation into these veins. Atrial pressure opened A-V valve ↑slightly due to rush of blood from the atria, then decreases again as the ventricles are still relaxed (it dilates). Intraventricular pressure.

I- Atrial Systole Increases slightly due to entry of blood from the atria into the ventricles. Ventricular volume. closed Semilunar valves Decreases gradually due to continuous flow of blood into the peripheral circulation. Aortic pressure The 4 th heart sound occurs in this phase. This sound is normally inaudible, but can be recorded by the phonocardiogram. Heart sounds. The P wave starts 0.02 second before atrial systole. Electrocardiogram (ECG).

II- Isometric contraction phase 0.05 second Duration Shows slight, but sharp increase due to sudden closure of AV valve and ballooning of its cusps towards the cavity of the atrium by the sudden rise of intraventricular pressure. Atrial pressure Closes suddenly A-V valve As ventricular systole starts, the Ventricular pressure very rapidly exceeds the atrial pressure, leading to sudden closure of the AV valves. Now, all the valves are closed and the ventricle becomes as a closed chamber. So, the ventricle contract isometrically. i.e without change in the length of the muscle fibers. Intraventricular pressure↑ from zero to 80 mm Hg in the left ventricle. Intraventricular pressure

II- Isometric contraction phase No change Ventricular volume. Still closed Semilunar valves Is still decreasing and the aortic valve is still closed. Aortic pressure Early part of the 1 st heart sound is present which is mainly due to sudden closure of AV valves. Heart sounds. The Q wave starts 0.02 second before this phase, and the remaining part of the Q R S complex occurs during it. Electrocardiogram (ECG).

IV- Reduced Ejection Phase III- Maximum Ejection Phase 0.1 second0.15 second Duration Shows gradual increase due to continuous accumulation of venous return. Shows a sharp decrease followed by gradual increase. The decrease is due to shortening of ventricular muscle (by systole), pulling down the AV ring, increasing atrial capacity and so decreasing the atrial pressure. The gradual increase of atrial pressure is due to: - Accumulation of venous blood in the atria. - Upward displacement of AV ring to its normal position. Atrial pressure Remain closed A-V valves Still ↓ing.↓rapidly due to ejection of most of ventricular blood into the aorta. Ventricular volume

IV- Reduced Ejection Phase III- Maximum Ejection Phase Slightly ↓due to reduced force of pumping of blood into aorta. Markedly ↑ as a result of continuous contraction of ventricular muscle. The ventricular pressure is slightly higher than the aortic pressure. Intraventricular pressure Still openedopened Semilunar valves Drops slightly because the blood leaving the aorta to the peripheral circulation is greater than the blood pumped into the aorta from the ventricle. Is increased due to ejection of blood from the left ventricle. The amount of blood entering the aorta exceeds the amount leaving it to the peripheral circulation. So, the aortic pressure increases, but remains lower than the ventricular pressure. Aortic pressure

IV- Reduced Ejection Phase III- Maximum Ejection Phase No sounds are produced1 st Heart sound continues, due to continuous flow of blood from the ventricles to the aorta causing vibration of its walls Heart sounds. The top of T waveThe T wave starts in the late part of this phase Electrocardiogram (ECG)

V- Isometric Relaxation Phase 0.06 second Duration Still increasing due to continuous venous return Atrial pressure Remain closed A-V valve ↓rapidly. The ventricle is now a completely closed chamber. So, it relaxes isometrically i.e without changing the length of its muscle fibers. Therefore, there is no change in volume but the pressure rapidly falls towards the zero line. Intraventricular pressure. No change Ventricular volume.

V- Isometric Relaxation Phase Shows an initial sharp decrease due to sudden closure of the aortic valve, called the “diacrotic notch”. This is followed by secondary rise of pressure due to the elastic recoil of the aorta. It is called the “diacrotic wave”. Aortic pressure Closes suddenly semilunar valves The 2 nd heart sound is present, due to sudden closure of the aortic valve. Heart sounds. T wave ends during this phase. Electrocardiogram (ECG).

VII- Reduced Filling Phase VI- Rapid Filling Phase 0.2 second0.1 second Duration Around zero or still increasing due to continuous venous return. At the beginning of this phase, atrial pressure is more than the ventricular pressure leading to opening of the AV valve and rushing of blood by its weight into the relaxed ventricle. This leads to rapid ventricular filling and decrease in the atrial pressure. Atrial pressure Still openedopened (A-V) valve Around zero line but below atrial pressure. Around zero Intraventricular pressure. gradual increaseMarked ↑ due to rapid ventricular filling with blood from the atria. Ventricular volume

VII- Reduced Filling Phase VI- Rapid Filling Phase still decreasing.Gradually decreases due to continuous escape of blood to the peripheral circulation. Aortic pressure Still closed closedState of semilunar valves No sound is present The 3 rd heart sound is present Heart sounds. P wave, for the next cardiac cycle begins. U wave may be present Electrocardiogram (ECG).

N.B.: Systolic B.P. in the left ventricle = 130 mmHg Diastolic B.P in the left ventricle = zero Systolic B. P. in the right ventricle = 35 mm Hg Diastolic B. P. in the right ventricle = zero Systolic B. P. in the aorta = 120 mm Hg Diastolic B. P. in the aorta = 80 mm Hg Systolic B. P. in pulmonary artery = 30 mm Hg Diastolic B. P. in pulmonary artery = 10 mm Hg N.B.: Systolic B.P. in the left ventricle = 130 mmHg Diastolic B.P in the left ventricle = zero Systolic B. P. in the right ventricle = 35 mm Hg Diastolic B. P. in the right ventricle = zero Systolic B. P. in the aorta = 120 mm Hg Diastolic B. P. in the aorta = 80 mm Hg Systolic B. P. in pulmonary artery = 30 mm Hg Diastolic B. P. in pulmonary artery = 10 mm Hg