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Cardiac cycle: Cardiac cycle involves coordinated actions of atrial, ventricular contractions, opening and closing of various valves in order to achieve effective pumping of blood in desired direction. Mechanical contraction of heart occurs in three stages alternating each other. First stage is atrial systole during which both left and right atria contracts and pushed blood into respective ventricle through open atrio-ventricular (bicuspid and tricuspid) valves. In single cardiac cycle (a heart beat) of about 0.8 second, atrial systole comprises 0.15-0.20 second. During atrial systole, atria are emptied and ventricles are filled.
The second stage is ventricular systole during which ventricular myocardium contracts causing pumping of blood out from both ventricles into respective arteries through open semi lunar valves. Usually ventricular systole lasts for 0.25-0.30 second. Ventricles are emptied during this stage. The third stage is Ventricular diastole during which ventricular myocardium relaxes and allows blood to fill slowly into both ventricles through opened atrio-ventricles valves. This stage extends from 0.35-0.45 seconds.
All three stages occur in a sequence and constitute one cardiac cycle and each cycle constitute one heart beat.
Heart beat and heart sounds: with progression of each cardiac cycle, apart from pumping of blood, there is sequential closing and opening of various valves situated in the heart. The movement of these valves, especially their closing produced feeble vibrations/sounds which can be recorded with the help of stethoscope placed over left side on chest wall. Ist heart sound, sounded as ‘LUB’ is heard at the time of closing of A-V valves present between atria and ventricles. IInd heart sound, sounded as ‘DUP’ is heard at the time of closing of semilunar valves present between aorta and ventricle. One lub and one dup sounds constitute one heart beat. Number of such sounds in normal subject is 72 in one minute duration which is known as heart rate. Heart rate tends to increase during exercise, fever, excitement etc. and it tends to decrease during sleep, depression etc. These variations in heart rate are brought about by action of various nerves (sympathetic accelerates and Vagus depresses) and hormones like adrenaline from adrenal medulla. Heart beat and heart sounds are very useful clinically to evaluate health status of individuals.
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