Heart block is a disorder in the heart’s rhythm due to a fault in the natural pacemaker. This is caused by an obstruction – a block – in the electrical conduction system of the heart. Sometimes a disorder can be inherited. Despite the severe-sounding name, heart block may cause no symptoms at all in some cases, or occasional missed heartbeats in other cases (which can cause light-headedness, syncope (fainting), and palpitations), or may require the implantation of an artificial pacemaker, depending upon exactly where in the heart conduction is being impaired and how significantly it is affected.
Clinically speaking, the blocks tend to have more serious potential the closer they are to the “end” of the electrical path (the muscles of the heart regulated by the heartbeat), and less serious effects the closer they are to the “start” (at the SA node), because the potential disruption becomes greater as more of the “path” is “blocked” from its “end” point. Therefore, most of the important heart blocks are AV nodal blocks and infra-Hisian blocks.
SA blocks are usually of lesser clinical significance, since in the event of SA block, the AV node contains a secondary pacemaker which would still maintain a heart rate of around 40 – 60 beats per minute, sufficient for consciousness and much of daily life in the majority of individuals.
SA blocks rarely give severe symptoms, because even if an individual had complete block at this level of the conduction system (which is uncommon), the secondary pacemaker of the heart would be at the AV node, which would fire at 40 to 60 beats a minute, which is enough to retain consciousness in the resting state. However SA block is capable of causing problematic symptoms even so, and may also hint at conduction issues elsewhere in the heart, and therefore SA blocks are – despite their lower level of life-threatening risk – still “the most common indication for pacemaker implantation in the US”.