Ask the Clinical Instructor: A Q&A column for those new to the cath lab

In a first-degree heart block, why is there such a long delay in the PR interval? What is the pathology of this occurrence?

– Beverly Eskdale, Winter Haven, FL A first-degree heart block can sometimes be dramatic and gathers a lot of attention when it is pronounced. It can also be a source of focus for individuals reviewing a rhythm strip to the point that it is distracting from other issues. While it is nice to know what this abnormality is, we should also realize that this particular rhythm is relatively benign and not needing treatment in the cath lab. A first-degree AV block is a rhythm in which there is a delay in conduction through the AV node. This shows up on the ECG through the interval between the “p” wave and the “r” wave, or the “p-r interval.” The normal range for this interval is 0.12 to 0.20 seconds. Anything longer than 0.20 seconds is considered a first-degree AV block. Figure 1 shows a first-degree heart block.1 A blue box has been placed over numerous sections of this ECG so you can see the delay in the PR interval. It is important to recall that this rhythm would occur on every beat. If you find that you have more p waves than QRS complexes, you should investigate for presence of a true atrioventricular (AV) heart block (our discussion next month!) There are many causes of a first-degree heart block, and some we can tie directly back to our patients that we are seeing in the cath lab. As we mentioned, in a first-degree heart block, there is a delay of the conduction through the AV node, or possibly the HIS-Purkinje system, or a combination of both. This can be caused by heart diseases, or could be a natural occurrence in people with an enhanced vagal tone, such as marathon runners or other well-conditioned athletes. If the QRS complex is of normal width and morphology on the ECG, then the conduction delay is almost always at the level of the AV node. If, however, the QRS demonstrates a bundle-branch morphology, then the level of the conduction delay is often localized to the HIS-Purkinje system.2 One of the causes of first-degree heart block can be a myocardial infarction (MI), particularly an acute inferior MI. In a large majority of the patients, the AV node is supplied off of the right coronary artery. If there is a blockage of this artery, the blood flow to the AV node could be adversely affected, which can cause bradycardias and heart blocks, including a first-degree heart block. A new presentation of a first-degree heart block requires a quick assessment of the patient to assure that an MI is not in progress. Other than the MI, the causes of first-degree heart block are relatively subtle and benign. There can be medications that can decrease the AV conduction by increasing the refractory time of the AV node (an action potential discussion can occur another time) such as calcium channel blockers, beta blockers, and digitalis preparations. Electrolyte imbalance could possibly cause first-degree heart blocks, and could warrant further investigation. However, our patients should already have labs drawn and analyzed before they come to our department anyway. This would likely be identified well before the procedure begins. A history of an infectious disease, such as tick-borne Lyme disease, may be present. Asymptomatic first-degree heart block is part of the spectrum of presentation of Lyme carditis in children. Lyme carditis is most likely in children with Lyme disease who are older than 10 years of age, those with arthralgias, and those with cardiopulmonary symptoms.3 Isolated first-degree heart block has no direct clinical consequences. There are no symptoms or signs associated with it. It was originally thought of as having a benign prognosis. In the Framingham Heart Study (https://www. framinghamheartstudy.org; This is a very interesting ongoing study — check it out if you are not aware of it), however, the presence of a prolonged PR interval or first-degree AV block doubled the risk of developing atrial fibrillation (irregular heart beat), tripled the risk of requiring an artificial pacemaker, and was associated with a small increase in mortality. This risk was proportional to the degree of PR prolongation.2 The first-degree heart block, once acute MI has been ruled out, is benign and does not require any treatment. Secondary conditions may need to be treated that could eventually eliminate the first degree block. As a general rule, we don’t need to worry about the PR interval delay in the cath lab. Next month, we’ll present an alternative process to identify the different heart blocks. Contact Todd Ginapp with your questions at tginapp@rcisreview.com or on www.facebook.com/RCISReview

1. Garcia TB, Holtz NE. 12-Lead ECG: The Art of Interpretation. Sudbury, MA: Jones and Bartlett; 2001.

2. Cheng S, Keyes MJ, Larson MG, et al. Long-term outcomes in individuals with prolonged PR interval or first-degree atrioventricular block JAMA 2009; 301 (24): 2571–2577. doi:10.1001/jama.2009.888.

3. Costello JM, Alexander ME, Greco KM, et al. Lyme carditis in children: presentation, predictive factors, and clinical course. Pediatrics May 2009;123(5):e835-e841.

4. Sherron P, Torres-Arraut E, Tamer D, et al. Site of conduction delay and electrophysiologic significance of first-degree atrioventricular block in children with heart disease. Am J Cardiol May 1 1985;55(11):1323–1327.