Cardiac cycle: Cardiac cycle Ass. Prof./ Manal M. Kamal Physiology Department objectives: objectives At the end of this lecture, the student should be able to: - Understand the phases of the cardiac cycle, duration and events occur in each phase. - Describe the changes occur in each phase in atrial pressure, ventricular pressure, ventricular volume, aortic valve, aortic pressure and A-V valves. - Know the correlation of the cycle events with ECG and heart sounds. Slide 3: Cardiac cycle: The cardiac events that occur from the beginning of one heart beat to the beginning of the next. It is series of electrical, pressure, volume, flow, sound events which occur during each heart beat. Each cardiac cycle begins with spontaneous production of an action potential by the SA node and ends immediately before production of the next action potential. During cardiac cycle action potential travels from SA node to both atria, then through the A-V bundle into ventricles. There is a delay of more than 0.1 second during passage of impulse from atria into ventricles allows the atria to contract ahead of ventricular contraction, pumping blood into the ventricles before ventricular contraction begins. Slide 4: - The length of the cardiac cycle is determined by the heart rate (HR). When HR is 75/ min it is 0.8 sec. increase HR shorten the cardiac cycle mainly during diastole. - A single cardiac cycle involve two basic stages. The first stage is diastole, which represents ventricular filling. The second stage is systole, which represents the time of contraction and ejection of blood from the ventricles. - To analyze these two stages in more detail, the cardiac cycle is divided into seven phases. The first phase begins with the P wave of the ECG, which represents atrial depolarization. The last phase of the cardiac cycle ends with the appearance of the next P wave. The phases of the cardiac cycle: The phases of the cardiac cycle The following phases occur in the left side of the heart and are nearly similar to those in the right side except the lower pressure in the right ventricle and pulmonary artery. I- Atrial systole phase. II- Isometric contraction phase. III- Maximum ejection phase. Ventricular systole IV- Reduced ejection phase. V- Isometric relaxation phase. VI- Maximum filling phase. Ventricular diastole IV- Reduced filling phase. I- Atrial systole phase : I- Atrial systole phase During it atria contract to push about 30% of blood to the ventricles (most of ventricular filling occurs prior to atrial contraction as blood passively flows from atria to ventricles). 1. Duration: 0.1 second. 2. Atrial pressure: increase due to atrial contraction (7-8 mmHg) return to zero by the end of this phase, constriction of cardiac ends of SVC, IVC prevent blood regurgitation to those veins during atrial contraction. 3. Ventricular pressure: increase at first due blood entered from atria and then decrease again as ventricle dilate. Slide 8: 4. Ventricular volume: increase due blood entered from atria At this time, the ventricular volumes are maximal, which is termed the end-diastolic volume (EDV) about 120 ml in the left ventricle. 5. Aortic valve: closed as intra- aortic pressure is higher than Left ventricular pressure. 6. Aortic Pressure: decreases gradually as continuous escape of blood from the aorta to the arteries. 7. A-V valves: open. 8. Heart sounds: Fourth heart sound. 9. ECG: P wave begins 0.02 sec. before the beginning of atrial systole. II- Isometric contraction phase: II- Isometric contraction phase At the beginning of ventricular contraction, the ventricular muscle contracts isometrically , all valves are closed making the ventricles as a closed chamber. 1. Duration: 0.05 second. 2. Ventricular pressure : ventricular pressure rises above atrial pressure with sudden closure of AV valves, making the ventricles as a closed chamber ( the ventricle contract isometrically without change in length of the muscle fibers but pressure rapidly incraeases ) Slide 10: 3. Ventricular volume : No change. 4. Aortic valve: closed 5. Aortic Pressure : decreasing. 6. A-V valve: is suddenly closure, the papillary muscles contraction prevents them from being everted towards the atrium. 7. Heart sounds : First heart sound due to sudden closure of A-V valves. 8. Atrial pressure: increase due to sudden closure of AV valve and ballooning of its cusps towards the atria. 9. ECG: Q wave begins 0.02 sec. before start of this phase and the rest of QRS occurs during it which represents ventricular depolarization. III- Maximum ejection phase.: III- Maximum ejection phase. During this phase blood is ejected at high rate out of ventricles to the aorta. 1. Duration: 0.15 second. 2. Ventricular pressure: intraventricular pressures increase from 80 to 120 mmHg due to ventricular systole 3. Aortic valve: opens due to ventricular systole exceed the pressure within the aorta. The contracting ventricle push blood with great force into the aorta. 4. Aortic Pressure: increase from 80 to 120 mmHg due to pumped blood. 5. Ventricular volume: decrease rapidly due to ejection of most of the ventricular blood to aorta. Slide 13: 6. Atrial pressure: Left atrial pressure initially decreases as the atrial base is pulled downward, expanding the atrial capacity. This is followed by gradual increase in atrial pressure due to blood continues to flow into the atria from veins also due to upward displacement of A-V ring. 7. A-V valves: closed. 8. Heart sounds: The first haert sound continue. No heart sounds are ordinarily noted during ejection because the opening of healthy valves is silent. 9. ECG: Origin of T wave is found in this phase. IV- Reduced ejection phase: IV- Reduced ejection phase The ejection of blood continued but at a slower rate. 1. Duration: 0.1 second. 2. Ventricular pressure: decrease due to decrease force of pumping blood, ventricular systole ends by the end of this phase. 3. Ventricular volume: decreasing. 4. Aortic valve: open 5. Aortic Pressure: decrease as blood leave the aorta to peripheral circulation. 6. Atrial pressure: continue increasing. 7. A-V valves: closed. 8. ECG: Top of T wave. V- Isometric relaxation phase: V- Isometric relaxation phase At the end of systole, intraventricular pressures fall sufficiently the aortic and pulmonary valves abruptly close and ventricular relaxation begins, allowing both the right and left intraventricular pressures to decrease rapidly. 1. Duration: 0.1 second. 2. Ventricular pressure: intraventricular pressures decrease till it becomes lower than aortic pressure. 3. Aortic valve: is suddenly closed due to low intra-ventricular pressures makes aortic blood tend to regurgitate to the ventricle. Also due to elastic recoil of the aorta. Slide 18: Valve closure is associated with a small backflow of blood into the ventricles cause a sharp decrease in aortic pressure called ( incisura or dicrotic notch). After valve closure, the aortic and pulmonary artery pressures rise slightly ( dicrotic wave) due to elastic recoil of their wall. 4. Ventricular volume: no change (The volume of blood that remains in a ventricle is called the end-systolic volume and is about 50 ml in the left ventricle. The difference between the end-diastolic volume and the end-systolic volume is about 70 ml and represents the stroke volume). 5. Atrial pressure: gradual increase. 6. A-V valves: still closed. Slide 19: 7. Heart sound: Second heart sound due to sudden closure of Aortic and pulmonary valves. 8. ECG: T wave ends at this phase. VI- Maximum filling phase: VI- Maximum filling phase Blood flow at high rate from the atria to the relaxed ventricles 1. Duration: 0.1 second. 2. Atrial pressure: more than the ventricular pressure. 3. Ventricular pressure: around zero. 4. A-V valves: open with rush of blood but its own weight to the ventricle and then the atrial pressure will decrease. 5. Ventricular volume: increase due to ventricular filling. 6. Aortic valve: closed. Slide 22: 7. Aortic Pressure: decrease as blood leave the aorta to peripheral circulation. 8. Heart sounds: third heart sound due to rapid blood flow from the atria to the relaxed ventricles. 9. ECG: U wave may be present. (Most of ventricular filling (70%) occurs passively in maximum filling and reduced filling phases) VII- Reduced filling phase: VII- Reduced filling phase Blood continues to flow from the atria to the relaxed ventricles but at slower rate. 1. Duration: 0.2 second. 2. Atrial pressure: around zero. 3. Ventricular pressure: around zero but below atrial pressure. 4. A-V valves: open. 5. Ventricular volume: increase due to ventricular filling. 6. Aortic valve: closed. 7. Aortic Pressure: decrease as blood leave the aorta to peripheral circulation. Slide 26: During cardiac cycle changes occur in right ventricle and pulmonary artery are the same as right ventricle and aorta except: 1- Aortic valve opens when ventricular pressure increased to 80 mm Hg. Due to low pressure in the right ventricle pulmonary valve opens when right ventricular pressure reaches 10 mmHg only. 2- The highest pressure inside the left ventricle is 120 mm Hg, (systolic), while the systolic pressure in the right ventricle is 30 mmHg. 3- Right atrial systole precede the left while left ventricular systole precede the right.