What is the relationship between the cardiac cycle and an ECG? What is isovolumetric contraction? When does it begin and end?

The human heart's cardiac cycle is regulated by key electrical events, namely by muscle cells at points called the sinoatrial and atriovencular (or SA and AV) "nodes." The SA node is calibrated to fire at between 60 and 100 beats per minute (BPM), which is the healthy range for a resting heart rate. When it is excited, it induces atrial contraction. The AV node fires at between 40 and 60 BPM, and slows the electrical impulses sent from the SA node to ensure that each heartbeat terminates, preparing the heart for successive beats by providing the cells in the SA node with an opportunity to return to electrical equilibrium. This also induces ventricular contraction. An electrocardiogram (ECG) is a way of tracing the electrical events of the heart by analyzing the SA and AV nodes by connecting a device that is sensitive to the heart's electrical moments and can display them graphically as waves.
During this heartbeat cycle, there is a tiny window of time (about 0.03 seconds) after ventricular contraction in which both the atrial and ventricular valves are still closed. This moment is isovolumetric contraction, its name referring to the fact that the volume of blood between the valves has stayed constant. The moment of pressure then overcomes the pressure exerted by the aorta and pulmonary trunk, forcing open the semilunar valves and allowing blood to leave the heart. This is when the isovolumetric contraction ends.