Summary of Atrioventricular block
Transcript for Atrioventricular block
Each heartbeat starts with the heart’s pacemaker cells in the sinoatrial node, sometimes just called the SA node, in the right atrium. The SA node sends an electrical signal that propagates out through the walls of the heart and contracts both upper chambers, then moves through the atrioventricular node, or AV node, where the signal stops for a split second. Next, the signal goes down into the lower chambers, where it moves down the bundle of His, into the left and right bundle branches, and into each ventricles’ Purkinje fibers, causing them to contract as well. So, in a healthy heart, the upper chambers contract first, and then shortly after, the lower chambers contract.
On an ECG, the atrial contraction is seen as a “P wave,” and the ventricular contraction is seen as the “QRS complex.” The interval from the start of the P wave to the start of the QRS complex is called the “PR interval,” and is normally between 120 and 200 milliseconds, or 3-5 tiny boxes on the graph paper that it’s usually printed out on, since each box is 40 milliseconds or 0.04 seconds. Heart block describes a type of arrhythmia, or abnormal rhythm, that happens when the electrical signal gets delayed or blocked entirely at some point along the conduction system.
These blocks and delays usually happen because of some sort of damage or fibrosis to the electrical conduction system, the pathways that conduct the electrical signal. Lev’s disease, or Lenegre-lev syndrome, describes the large proportion of cases that are idiopathic and described as progressive cardiac conduction defects. This means it’s not clear exactly what causes it, but over time fibrosis, or scarring, develops in the conduction system which can delay or stop electrical conduction. This is usually a result of the aging process in the heart, and happens most often in the elderly, although some hereditary forms have been identified and can happen in younger people. However, another large proportion of cases are a result of ischemic heart disease, which is when the heart cells don’t receive enough oxygen and can die off, as with a heart attack. This again leaves scar tissue that can block the electrical signal. In fact, it’s estimated that about 20 percent of patients that have a heart attack go on to develop a heart block.
Finally, it’s worth pointing out that the electrical conduction system is kind of like the electrical wiring in the walls of a house, so it makes sense that diseases of the heart muscle walls — or, cardiomyopathies — and inflammation of the heart muscle — or, myocarditis — can both cause heart block.
An atrioventricular, or AV block, describes when the signal is delayed or blocked when it’s trying to move from the atria to the ventricles. First-degree AV block is when the signal is delayed, but still makes it to the ventricles. This type has a PR interval greater than 200 milliseconds. Even though these signals are delayed, in first degree block, they still reach the ventricles. First degree block isn’t usually associated with any symptoms. Treatment or management of first degree block might involve identifying electrolyte imbalances or causes due to medications, but it usually doesn’t require further treatment.
Second degree AV block can be split into two types. Type I, called “Mobitz I,” or sometimes “Wenckebach,” happens when the PR interval gets progressively longer with each beat until a P wave is blocked completely. So, maybe the first PR interval is 200 ms, then the next is 260 ms, then 300 ms, and finally the next one doesn’t make it to the ventricles, and you get what’s called a “dropped beat.”
When a signal doesn’t make it to the ventricles from the atria, and if a long enough time passes by — let’s say about 2 seconds — then the ventricles’ pacemaker cells kick in, sort of as a fail-safe mechanism, called a ventricular escape beat. Most people with this type don’t have symptoms, but occasionally patients feel lightheadedness, dizziness, and syncope.