Premature ventricular contraction

The heart has two lower chambers, called the ventricles, so a premature ventricular contraction is when the ventricles contract earlier than normal in the cardiac cycle. This happens because an abnormal contraction signal, called a depolarization, originates from somewhere in the ventricles rather than coming from the pacemaker cells.

So, if we simplify this heart a little bit, normally, the sinoatrial node or SA node sends an electrical signal that propagates out through the walls of the heart and contracts both upper chambers. Then that signal moves to the atrioventricular node or AV node, where the signal is delayed for a split second, and then it goes down into the ventricles or lower chambers where it moves down the bundle of His and into the left and right bundle branches and into each ventricle’s Purkinje fibers, causing them to contract as well. So, in a healthy heart the upper chambers contract first, then shortly after, the lower chambers contract. On an electrocardiogram or ECG, which measures the electrical activity of the heart via electrodes that are placed on the skin, the atrial depolarization, and therefore its contraction, is seen as a p-wave; the ventricular contraction is seen as a QRS complex; and the ventricular repolarization, and therefore its relaxation, is seen as a T-wave. 

This empty space here is called the PR segment, and it corresponds to the pause in the AV node, and this one is called the ST segment, and it corresponds to the interval between ventricular depolarization and repolarization, and this one is called the TP segment, which represents the heart’s quiet time when the cells are finished repolarizing and are ready for another signal. 

Now, if we just look at the QRS complex, which normally lasts less than 100 milliseconds or 2-and-a-half little boxes, it’s usually made up of three smaller waves, also called deflections. If the first wave after the p-wave is downwards, or negative, it’s called a Q wave - which you can remember by the letter Q having a downward tail. If the next deflection is upward, or positive, then it’s called the R wave. If, though, the first wave after the p-wave is upwards, instead, or positive, you basically skip the Q and just call it an R wave. Finally, any downward deflection after the R wave is called the S wave.