Dr. Amin Jan Assistant Professor (Physiology) NwSM, Peshawar Pakistan Cardiac cycle

Cardiac events that occur from the beginning of one heart beat to the beginning of the next are called… cardiac cycle Sequence of all the electrical and mechanical events occurring in the heart during a single beat and the resulting changes in pressure, flow, and volume in various cardiac chambers… cardiac cycle

Electrical Changes: a)Electrocardiogram (ECG) Mechanical changes: a)Phases of cardiac cycle b)Pressure changes during cardiac cycle c)Volume changes during cardiac cycle d)Heart sounds Components of Cardiac cycle…

Each cycle is initiated by spontaneous generation of an action potential in the sinus node, located in the supero-lateral wall of the right atrium near the opening of the superior vena cava, Action potential travels from here rapidly through both atria and then through the A-V bundle into the ventricles

Because of this special arrangement of the conducting system from the atria into the ventricles, there is a delay of more than 0.1 second during passage of the cardiac impulse from the atria into the ventricles This delay allows the atria to contract ahead of ventricular contraction, thereby pumping blood into ventricles before strong ventricular contraction begins

Thus… the atria act as primer pumps for the ventricles, and the ventricles in turn provide major source of power for moving blood through the body’s vascular system

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, 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 min/per beat or Sec/beat

Phases of Cardiac Cycle…

1. Atrial systole… Prior to atrial systole, blood has been flowing passively from atrium into the ventricle through the open AV valve During atrial systole, the atrium contracts and tops off the volume in the ventricle with only a small amount of blood Atrial contraction is complete before ventricle begins to contract

2. Isovolumetric contraction… Atrio-ventricular valves close at the beginning of this phase Isovolumetric Ventricular contraction is actually defined as the interval between the closing of the AV valves & the opening of the semilunar valves (aortic & pulmonary)

3. Rapid ejection Phase… Semilunar (aortic and pulmonary) valves open at the beginning of this phase While the ventricles continue contracting, the pressure in the ventricles exceeds the pressure in the aorta & pulmonary arteries, the semilunar valves open, blood exits the ventricles, & ventricular volume ↓ses rapidly

At the end of this phase the semilunar (aortic and pulmonary) valves close After the peak in ventricular & arterial pressures, blood flow out of the ventricles decreases and ventricular volume decreases more slowly 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 4. Slow ejection phase…

5. Isovolumetric relaxation… At the beginning of this phase the AV valves are closed Throughout this and the previous two phases, the atrium in diastole has been filling with blood on top of the closed AV valve, causing atrial pressure to rise gradually Ventricular pressure continue to drop Ventricular volume is at a minimum & is ready to be filled again with blood

6. 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

7. Reduced ventricular filling (diastasis)… Rest of blood that has accumulated in the atria flows slowly into the ventricles Ventricular volume increases more slowly The ventricles continue to fill with blood until they are nearly full

Cardiac Cycle…

Pressure Changes…

Ventricular Volume Changes…