Heart block

Heart block (HB) 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 this disorder is inherited. Despite the severe-sounding name, heart block may cause no symptoms at all, or mere occasional missed heartbeats and ensuing light-headedness, syncope (fainting), and palpitations. Depending upon where in the heart, exactly, conduction is impaired (and how significantly), the disorder may require the implantation of an artificial pacemaker (a medical device that provides correct electrical impulses to trigger heartbeats, compensating for the natural pacemaker's unreliability); heart block is usually treatable.

Heart block should not be confused with other conditions which may or may not be co-occurring or possibly serious (and which relate to the heart and/or other nearby organs), such as angina (heart-related chest pain), heart attack (myocardial infarction), any heart failure, cardiogenic shock or other types of shock, different types of abnormal heart rhythms (arrhythmias), cardiac arrest, or respiratory arrest.

The human heart uses electrical signals to maintain and initiate the regular heartbeat in a living person. Conduction is initiated by the sinoatrial node ("sinus node" or "SA node"), and then travels to the atrioventricular node ("AV node") which also contains a secondary "pacemaker" that acts as a backup for the SA nodes, then to the bundle of His and then via the bundle branches to the point of the apex of the fascicular branches. Blockages are therefore classified based on where the blockage occurs – namely the SA node ("Sinoatrial block"), AV node ("AV block" or AVB), and at or below the bundle of His ("Intra-Hisian" or "Infra-Hisian block" respectively). Infra-Hisian blocks may occur at the left or right bundle branches ("bundle branch block") or the fascicles of the left bundle branch ("fascicular block" or "Hemiblock"). SA and AV node blocks are each divided into three degrees, with second-degree blocks being divided into two types (written either "type I" or "II" or "type 1" or "2"). The term "Wenckebach block" is also used for second-degree type 1 blocks of either the SA or AV node; in addition, second-degree blocks type 1 and 2 are also sometimes known as " Mobitz 1" and "Mobitz 2".

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 an SA node 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 most cases.