1st Degree AV Block
As the signal travels from the SA node to the AV node and on through the junction, it gets slowed down. Try to think of it like traffic, when a freeway closes a lane and traffic is funneled through a smaller opening, traffic slows down and you are late getting to work. With First degree, the P-R interval is lengthened because the signal is delayed getting to the ventricles. The problem usually occurs in the AV node. Normally, the AV node already slows down the signal (.12 - .20) to allow the ventricles time to fill with blood. First degree AV block just delays it further. First degree AV Block in of itself is not a dangerous rhythm. More often, many people don't have symptoms and are unaware they have it. First degree AV block has a PR interval greater than .20, (ranging from .21 up to .50). Other than having a wide PR, the rhythm is regular and the SA node still paces the heart. You would call it "SR with 1st degree AV block". In the graph above, the heart rate is 65 beats per minute and the PR interval is .27 seconds. In addition, the QRS is .13 seconds which means this patient also has a bundle branch block. It is not uncommon to have an AV block and a bundle branch block. It may occur in healthy as well as diseased hearts. The PR interval can vary with heart rate. As the rate decreases, the PR can get longer and as the rate increases, it can get shorter. First degree AV block is not uncommon in well-conditioned athletes with slow resting heart rates. It is also common in elderly patients without heart disease. It is a transitional rhythm which can lead to 2nd degree AV block, and if undetected, 3rd degree AV block. Causes The most common causes of first-degree heart block are an AV nodal disease, enhanced vagal tone (for example in athletes), myocarditis, acute myocardial infarction (especially acute inferior MI), electrolyte disturbances and medication. The drugs that most commonly cause first-degree heart block are those that increase the refractory time of the AV node, thereby slowing AV conduction. These include calcium channel blockers, beta-blockers, cardiac glycosides, and anything that increases cholinergic activity such as cholinesterase inhibitors. Digitalis is a sodium/potassium ATPase inhibitor and also prolongs AV conduction. Treatment Treatment includes identifying and correcting electrolyte imbalances and withholding any offending medications. This usually does not require admission unless there is an associated myocardial infarction. Even though it usually does not progress to higher forms of heart block, it may require outpatient follow-up and monitoring of the ECG, especially if there is a bundle branch block. If there is a need for treatment of an unrelated condition, care should be taken not to introduce any medication that may slow AV conduction. If this is not feasible, clinicians should be very cautious when introducing any drug that may slow conduction; and regular monitoring of the ECG is indicated. Return to Home Page |
