A<unk>  (also spelled<unk>  is a disorder in the normal rhythm of the heart, known as a heart block, that is initiated in the<unk>  node. The initial action impulse in a heart is usually formed in the<unk>  node (SA node) and carried through the atria, down the internodal atrial pathways to the atrioventricular node (AV) node. In normal conduction, the impulse would travel across the bundle of His (AV bundle), down the bundle branches, and into the Purkinje fibers. This would depolarize the ventricles and cause them to contract. In an SA block, the electrical impulse is delayed or blocked on the way to the atria, thus delaying the atrial beat. (An AV block, occurs in the AV node and delays ventricular depolarisation). SA blocks are categorized into three classes based on the length of the delay.
Sinoatrial block
Left bundle branch block
Right bundle branch block
Bundle branch block
Sinoatrial arrest
Atrioventricular block
Ectopic beat
Pre-excitation syndrome