The Cardiac Cycle

Cardiovascular system lecture 5

The cardiac cycle

Objective:

Definition of cardiac cycle?
Phases of cardiac cycle?

The cardiac cycle

The cardiac cycle is refers to the sequence of mechanical and electrical events that repeats with every heart beat. Each cardiac cycle has two phases: diastole, the time during which cardiac muscle relaxes and ventricles fill with blood, and systole, the time during which the cardiac muscle contracts and blood ejected by ventricles.

. The duration of cardiac cycle at heart rate of 75 beat/min is about 0. 8 second, 0.5 second for diastole and 0. 0.3 second for systole. The long duration of diatsole has two important physiologic and clinical implications.
• most of ventricular muscle perfusion occur during diastole.
• ventricular filling occur during diastole.
.

phases of cardiac cycle

The main phases of cardiac cycle are:
1. Filling Phase
2.Atrial Systole
3. Isovolumetric Contraction Phase
4. Rapid Ejection Phase
5. Reduced Ejection Phase
6. Isovolumetric Relaxation Phase

1.Filling Phase

ventricular pressure decreases below atrial, the A-V valves open and filling occurs rapidly (rapid filling phase), the elastic recoil of the ventricle may aid in drawing blood into the ventricle .As the ventricle fills intraventricular pressure increases slowing the rate of filling(reduced filling phase) or diastasis.

At large volumes, and with ventricles with low compliance, the rapid reduction of filling at the end of this phase may produce the third heart sound. About 70% of ventricular filling occur in this phase.

2.Atrial Systole

The sinoatrial pacemaker complex initiates excitation, which spreads across the atria and is recognised in the ECG as the P-wave atrial contraction follows increasing the pressure within both atria this increase in the RA pressure gives the a-wave of the Jugular Venous pulse, or CVP trace.

NB: With reduced ventricular compliance, atrial contraction may give rise to a fourth heart sound

3.Isovolumetric Contraction Phase

Excitation of the ventricles occurs as the wave of excitation passes through the AV node, bundle of His and Purkinje system. Ventricular contraction coinciding with the peak of the R-wave of the ECG, in response to contraction of the muscle,

, the ventricular pressure rises very rapidly, the increasing intraventricular pressure lead to closure of the mitral and tricuspid valves, producing the first heart sound. the rapid rise in ventricular pressure is transmitted across the semilunar valves and appears as a small rise in the aortic pressure trace.

4.Rapid Ejection Phase

The intraventricular pressure rises rapidly until it exceeds the pressure in the great arteries (aorta and pulmonary), leading to opening of the outflow (Aortic/Pulmonary) valves. There is then a rapid ejection period, where both intraventricular and arterial pressure rise to a maximum.


Because of the high ventricular pressure, unless the papillary muscles perfectly compensate, there ,is a tendency for the A-V valves to bulge into the atria producing the av-wave of the JVP. Immediately after this the atrial pressure fall due to the descent of the base of the heart and stretching of the atria, producing the x descent of the JVP. This reduced atrial pressure aids the return of blood from the periphery.

5.Reduced Ejection Phase

both the contractile forces and the pressure within the ventricles are decreasing during this phase and are less than those within the aorta by several mmHg . This phase coinciding with the Repolarisation of the myocardium occurs during ( T-wave of the ECG).

The ventricular pressure is decreasing very rapidly and there is reversal of flow toward the heart, closing the valves and generating the second second heart sound.

6.Isovolumetric Relaxation Phase

both valve sets are closed, so pressure drops very rapidly as there is "no" change in volume as this is occurring atrial pressure is increasing to its maximum, due to the:
1. movement of blood from the periphery
2. movement of the base of the heart back to its resting position ® v-wave of the JVP

Synchrony of Contraction

1.LV is first to start contraction and the last to start to fill. RV contraction lags due to the anatomy of the conducting system by ~ 15 ms. The pulmonary valve opens first, and pulmonary flow begins around 10 ms before aortic due to the lower pressure in the pulmonary circuit

2. The isovolumetric period for the LV ~ 40 ms and RV ~ 15 ms

3. Because of higher systemic pressure LV outflow ends first with the total LVET being shorter that the RVET
4. the mitral valve opens after the tricuspid due to the greater time for ventricular pressure to drop below atrial pressure.

Q1// The semilunar valves close during.:

• ventricular systole.
• ventricular diastole
• Atrial systole.
• Atrial diastole.
• both atrial and ventricular systole