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Mechanical Event in the Cardiac Cycle

            Mechanical event of one cardiac cycle can be divided into 7 phases, in which phase 1 is in atrial systole, phase 2-4 is in ventricular systole and phase 5-7 is in ventricular diastole. In phase 1,which is the atrial systole which occurs for 0.15 seconds, the atria contracts, causing the pressure to increase.Atrial contraction produce a small increase in venous pressure that can be noted as the "a-wave." Fourth heart sound, S4 is sometimes noted in atrial systole.This sound is caused by vibration of the ventricular wall during atrial contraction. Generally, it is noted when the ventricle compliance is reduced or "stiff" ventricle as occurs in ventricular hypertrophy and in many older individuals.
             Phase 2 is the isometric ventricular contraction in which the ventricles contract.It occurs for 0.05 seconds.The ventricular pressure is higher than atrial pressure, causing the atrioventricular valves to close, producing first heart sound,S1 or 'lub' sound.All valves are close because ventricular volume is constant.The "c-wave" noted in the left arterial pressure may be due to bulging of mitral valve leaflets back into left atrium. Just after the peak of the c wave is the x'-descent.
             Phase 3 is rapid ventricular ejection which occurs for 0.15 seconds.Ventricles continue to contract until ventricular pressure reaches maximum value of 120mmHg.Since ventricular pressure is higher than aortic pressure,aortic valve opens and causes rapid ejection of blood into the aorta.AV valves remains closed.The aortic pressure increases and reaches a maximum.Most stroke volume is ejected in phase 3. Phase 4 is reduced ventricular ejection, occurring for 0.10seconds.As blood fills up aorta,the ventricular pressure begins to fall. Aortic valve is stil open so blood ejection continues and ventricular volume continues to fall. Left arterial pressure(LAP)gradually increases due to continued venous return. Phase 5 is isovolumetric ventricular relaxation which occurs for 0.


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