Author: Tanner Marshall, MS
Editor: Rishi Desai, MD, MPH, Tanner Marshall, MS
Each heartbeat starts with the heart’s pacemaker cells in the sinoatrial node, or sometimes just 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 (or AV) node where the signal stops for a split second, and then 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, 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 held up and delayed, or blocked entirely at some point along the conduction system.
These blocks or 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, is used to describe the large proportion of cases that are idiopathic, and described as progressive cardiac conduction defects. Meaning 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. Another large proportion of cases, though, are a result of ischemic heart disease, 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 - cardiomyopathies, and inflammation of the heart muscle or myocarditis, can both cause heart block.
An atrioventricular or AV block is used to describe 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. And treatment or management of first degree block might involve identifying electrolyte imbalances or causes due to medications, although it usually doesn’t require further treatment.
Second degree AV block can be split into two types. Type 1, called Mobitz 1 or sometimes Wenckebach, happens is 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 ventricle’s 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.
Second degree AV block–Type 2, or Mobitz II, is similar to type 1 AV block in that you get intermittent dropped beats, except this time they happen fairly randomly, so you don’t have this progressive lengthening of the PR interval. A lot of times a ratio for the overall number of beats conducted to not-conducted is given, like 2:1 Mobitz II AV block, but again remember that the dropped beats happen fairly randomly so we can’t really predict exactly when the next beat will get dropped. Most people with Mobitz type II do feel symptoms from it like fatigue, dyspnea, chest pain, and syncope, though the severity can vary from patient to patient.
Third degree, or complete heart block, describes when the signal is completely blocked when moving from the atria to ventricles, every time. So in this case, even though the atria might be going along at 60 bpm as if everything was normal, none of those signals make it down to the ventricles, and the ventricles struggle along with escape beats at very slow rates, probably closer to 30 bpm. Patients with complete heart block are usually symptomatic, with symptoms ranging from syncope, confusion, dyspnea, severe chest pain, and these patients are at risk of dying.
. When someone has AV block it’s important to find out the underlying cause and address it, for example it could be an adverse effect from a medication or from an infection. In terms of treating the AV block, typically a medication can be used to increase the heart rate like atropine. Alternatively transcutaneous pacing might be used where electrical pacing of the heart is done through electrodes placed on the skin. If the underlying cause is irreversible and the AV block is severe, patients ultimately might need a permanent pacemaker, which is implanted and continually monitors the patient’s rhythm, and if it detects a delay, it’ll send an electrical impulse into the ventricles, causing them to contract.
Sources edit
http://emedicine.medscape.com/article/161919-treatment
http://emedicine.medscape.com/article/161919-treatment
https://en.wikipedia.org/wiki/Heart_block
https://en.wikipedia.org/wiki/Second-degree_atrioventricular_block