A heartbeat is one contraction and relaxation action of the heart that takes about a second. It is triggered by electrical impulses generated in the heart. SA node initiates the electrical impulses which are then conducted through the heart muscle to stimulate the atria to contract. The impulses are then conducted via AV nodes à bundle of His à bundle branches à Purkinje fibers to the ventricles causing them to contract.
At rest, normally the heart beats around 60 to 80 times a minute. It can vary according to the body’s physical needs. Exercise, sleep, stress, fever, certain medications, etc. can incite change.
Extra or skipped heartbeat is called ectopic heartbeat. It is caused when the electrical impulses are initiated from regions other than SA node. Common types of ectopic heartbeats are premature ventricular contractions and premature atrial contractions.