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47 Cardiac CycleJanuary 28 2008 ElectrocardiogramECGECGA record of the overall spread of electrical activity through the heart during depolarization and repolarization conducted through the body fluidbetween electrodes at two different points on the body surface not the o The recording represents comparisonactual action potentialPQRST Wave Formnot detected because not enough electrical energySA nodal depolarizationis produced to reach the surface of the bodyP waveatrial depolarization not detected because electrical activity occurs almostAtrial repolarizationsimultaneously with ventricular depolarizationQRS complexventricular depolarizationo Ventricle has larger muscle mass than the atriumhave more electrical activity reaching the surfaceT waveventricular repolarization o Relaxation phasemore gradual and longer duration than the contraction to reduce the pressure so blood can enter the ventricles from the atriumbaselineno current is flowing o PR segmentAV nodal delay o ST segmentfrom time that ventricular activation is completed and when ventricle is contracting and emptying o TP intervalheart is completely at rest and have ventricular fillingPR interval 018s 012020satrial depolarization and conduction through the AV nodeQRS duration008s to 0sventricular depolarizationQT interval 04s to 043sventricular depolarization and ventricular repolarizationST interval OTORS032sventricularrepolarizationCardiac CycleThe period from the beginning of one heartbeat to the beginning of the nexto ie the period from the end of one heart contraction to the end of the next contractionAlternate periods of systole contraction and emptying and diastole relaxation and fillingEach cycle is initiated by spontaneous generation of the action potential at the SA node o AV potential travels from both atria through AV bundle into the ventricles th s to allow atria to contractdelay of 110first to empty their contents into the ventricles Wiggers diagram Shows the relation between the ECG heart sound and pressure and volume changes in the left side of the heart and aorta Sequence of Events During a Cardiac Cycle right atrial systole precedes left atrial systole ie right atrium contracts before the left atrium o pulmonary arterial pressure is less than the aortic pressureo left atrium has to pump against a high resistance and high pressure systemic circulatory systemeasier for right atrium to pump the blood out first left ventricle contracts before right ventricle contracts1
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