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Often an extra-systole [pronounced sis-tol-ee] has no significance. If you have more than 8-10 per minute then a visit to a doctor for a heart tracing would be advised.
ventricular systole
systole
Contraction of the ventricles and atria is called systole. Relaxation is called diastole.
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Extrasystole is an extra ventricular systole that happens during the begging of relaxation (repolarization). Since the cardiac is able to depolarize only after repolarization, any stimulus upon the repolarization period created an increased ventricular contraction or which is also called extrasystole but not a new contraction.
The cardiac cycle of the heart has two phases - the diastole phase and systole phase. In the systole phase, the ventricles contract and pump blood into the arteries.
repolariztion of the cardiac cells. ie after they have depolarized so quickly it takes a while for them to get back to a normal state
Atrial systole -- The atrium contracts, then the ventricle.
Systole is the period of heart contraction within the cardiac cycle.
This can be seen by clicing on the link below ( The ventricular systole )