The heart's "natural" pacemaker is called the sinoatrial (SA) node or sinus node. It's a small mass of specialized cells in the heart's right atrium. It produces electrical impulses that make your heart beat. For your heart to beat properly, the signal must travel from the SA node down a specific path to reach the ventricles. As the signal goes from the atria to the ventricles, it passes through specialized conducting tissue called the atrioventricular (A'tre-o-ven-TRIK'u-ler) (AV) node.
On an electrocardiogram (e-lek"tro-KAR'de-o-gram) (ECG), a portion of the graph called the P wave shows the impulse passing through the atria. Another portion of the graph, the QRS wave, shows the impulse passing through the ventricles. As long as the impulse is transmitted normally, the heart pumps and beats at a regular pace.
What is heart block?
Sometimes the signal from the heart's upper to lower chambers is impaired or doesn't transmit. This is "heart block" or "AV block." This does not mean that the blood flow or blood vessels are blocked.
Heart block is classified according to the level of impairment — first-degree heart block, second-degree heart block or third-degree (complete) heart block.
What is first-degree heart block?
First-degree heart block, or first-degree AV block, is when the electrical impulse moves through the AV node more slowly than normal. The time it takes for the impulse to get from the atria to the ventricles (the PR interval) should be less than about 0.2 seconds. If it takes longer than this, it's called first-degree heart block.
Heart rate and rhythm are normal, and there may be nothing wrong with the heart.
Certain heart medicines such as digitalis (DIJ'ih-TAL'is) can slow conduction of the impulse from the atria to the ventricles and cause first-degree AV block. Also, well-trained athletes may have it.
Generally, no treatment is necessary for first-degree heart block.
What is second-degree heart block?
In this condition, some signals from the atria don't reach the ventricles. This causes "dropped beats." On an ECG, the P wave isn't followed by the QRS wave, because the ventricles weren't activated. There are two types:
- Type I second-degree heart block, or Mobitz Type I, or Wenckebach's AV block. Electrical impulses are delayed more and more with each heartbeat until a beat is skipped. This condition is not too serious but sometimes causes dizziness and/or other symptoms.
- Type II second-degree heart block, or Mobitz Type II. This is less common than Type I but generally more serious. Because electrical impulses can't reach the ventricles, an abnormally slow heartbeat may result. In some cases a pacemaker is needed.
What is third-degree or complete heart block?
Complete heart block (complete AV block) means that the heart's electrical signal doesn't pass from the upper to the lower chambers. When this occurs, an independent pacemaker in the lower chambers takes over. The ventricles can contract and pump blood, but at a slower rate than that of the atrial pacemaker.
These impulses are called functional or ventricular scope beats. They're usually very slow and can't generate the signals needed to maintain full functioning of the heart muscle. On the ECG, there's no normal relationship between the P and the QRS waves.
Complete heart block is most often caused in adults by heart disease or as a side effect of drug toxicity. Heart block also can be present at — or even before — birth. (This is called congenital heart block.) It also may result from an injury to the electrical conduction system during heart surgery. Complete heart block may be a medical emergency with potentially severe symptoms and a serious risk of cardiac arrest (sudden cardiac death). If a pacemaker can't be implanted immediately, a temporary pacemaker might be used to keep the heart pumping until surgery can be performed.
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