The canine and feline heart has four chambers. The top two are called the atria. The bottom two are called the ventricles. 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 the heart beat. For the heartheart 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 (AV) node.
On an electrocardiogram (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 AV 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.
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. Heartrate and rhythm are normal, and there may be nothing wrong with the heart. In this stage of heart block, the canine or feline patietn mya not show clinical signs. In fact, in very fit or active dogs and cats, it may be a normal finding due to low resting heart rate. As such, most cases of 1st degree AV block are found incidentally, requiring no treatment and offering a good prognosis.
In 2nd degree AV block, 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 mild to moderate weakness, disorientation, or excercise intolerance. This type of AV block is typically responds well to the drug atropine. Generally prognosis is fair to good with treatment.
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. Signs are similar to Type I AV block, but can be more pronounced. Some of these cases respond well to treatment with atropine, but some require a pacemaker.
3rd degree AV 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 dogs and cats by heart disease. Complete heart block may be a medical emergency with potentially severe symptoms and a serious risk of cardiac arrest (sudden cardiac death). Treatment for third degree AV block is to implant a pacemaker as soon as possible. Prognosis with pacemaker implantation is guarded, however, as pacemaker technology in dogs and cats further advances, prognosis will continue to improve.
Roger L. Welton, DVM
Founder and Chief Editor, Web-DVM.net
President, Maybeck Animal Hospital
Article updated 9/6/2012