Lyme carditis

 Q: Which finding on EKG is the best predictor of impending AtrioVentricular (AV) block in patients with Lyme carditis? 

Answer: PR interval greater than 300 milliseconds (ms) 

It takes about one to two months after infection for disseminated Lyme disease to show cardiac effects. Other systemic effects may occur at the same time including neurological symptoms. Once the heart gets involved patient may complain of palpitations and shows signs of conduction abnormalities, myocarditis, pericarditis, and CHF. Impending AV conduction abnormality sets off quickly and can be life-threatening. First-degree AV block quickly progresses to second or complete AV block. 

A PR interval greater than 300 milliseconds is a good predictor of impending AV block. Fortunately, it is reversible with treatment in few weeks. The drug of choice is IV ceftriaxone, which should be continued till the PR interval drops below 300 ms. Oral antibiotic maintained with doxycycline is required for few weeks.

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References:

1. Steere AC, Batsford WP, Weinberg M, et al. Lyme carditis: cardiac abnormalities of Lyme disease. Ann Intern Med 1980; 93:8.

2. Wormser GP, Dattwyler RJ, Shapiro ED, et al. The Clinical Assessment, Treatment, and Prevention of Lyme disease, Human Granulocytic Anaplasmosis, and Babesiosis: Clinical Practice Guidelines by the Infectious Diseases Society of America. Clin Infect Dis 2006; 43:1089.