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A longer baseline PR interval may allow more efficacious delivery of CRT by allowing programming of physiologic AV delays. A short baseline PR interval may contribute to LV under-filling and CRT non-response.

electrophysiology-clinical, electrocardiogram Case PresentationA 62-year-old man presented with left hemiparesis resulting from a right internal capsule stroke. He reported irregular palpitations over the last month but denied lightheadedness or syncope. Echocardiography revealed normal left ventricular function. Figure 1 shows electrocardiographic recordings during his hospitalization. What are the mechanisms for the findings observed on these tracings? DiscussionThe underlying atrial rhythm is sinus with intermittent atrioventricular (AV) block in the setting of alternating right and left bundle branch block (BBB). The tracings show several repetitive but important patterns: (1) PR intervals are shorter with left BBB (LBBB) than right BBB (RBBB); and PR intervals progressively prolong with RBBB before AV block (Wenckebach), (2) LBBB always follows the long pause terminating AV block and starts each Wenckebach cycle, (3) P waves (*) falling into the T wave of the preceding QRS complex (which itself had followed a long PR interval of 280-420 ms) conduct to the ventricle with a paradoxically shorter PR interval (140-200 ms). Earlier-timed P waves landing on the downstroke of the negative T wave (lead V1) conduct with RBBB, while latertimed P waves falling on its upstroke conduct with LBBB. The PR interval following the next sinus impulse increases but remains shorter than the aforementioned long PR interval.Prolongation of the PR interval accompanying a change in BBB pattern indicates that the site of conduction delay is below the His bundle as the bundle branches are the only structures responsible for both the length of the PR interval and morphology of the QRS complex. 1,2 AV block No financial support was required.Address for reprints: Reginald T. Ho, M.D., in the setting of alternating BBB itself suggests block within the bundle branches although without His bundle recordings, intranodal and intra-Hisian block cannot be excluded. Therefore, PR prolongation preceding AV block during RBBB likely results from Wenckebach conduction over the left bundle (Fig. 2). In contrast, RB conduction after long pauses but its failure otherwise during sinus rhythm implies phase 3 block. In the presence of infra-Hisian AV block phase 3, RBBB alone cannot explain resumption of RB conduction after each pause because the proximal RB is always exposed to the sinus rate whether or not AV block is present. During LB Wenckebach left-toright transeptal conduction and late retrograde penetration of the RB maintains its refractoriness for each subsequent sinus impulse. When AV block occurs, momentary loss of the transeptal link allows recovery of RB excitability.The differential diagnosis of the LBBB QRS complexes terminating each pause include right ventricular escape complexes, ventricular preexcitation over a right-sided accessory pathway, and LBBB aberration. The smooth and rapid downstroke of the QRS complex in lead V1 argues strongly against a ventricular origin and favors aberration. 3 Perhaps the most likely explanation for the ...

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A 31 year-old Caucasian male with no significant past medical history presents with increasing dizziness, chest pain, and shortness of breath of one day duration. On returning from a camping trip to Northern Pennsylvania three weeks prior to admission, he noted high fevers, "joint pains all over," and a pink blotchy rash all over his chest and back. He did not recall any insect, tick or animal bites. His primary physician, who clinically suspected Lyme disease, ordered Lyme serologies and offered a prescription for doxycycline which the patient declined. Two weeks later, the patient presented to the emergency department after the onset of chest pain that he described as sharp, radiating to both arms, occurring with exertion and relieved by rest. The chest pain was associated with worsening shortness of breath and dizziness.The patient denied taking any medications, herbal or other supplements. He admitted to smoking half a pack of cigarettes per day, drinking 20 alcoholic beverages per week and occasionally smoking marijuana. He denied any intravenous drug use. He also denied any recent travel outside the United States or exposures to new chemicals or pets. The patient's mother had leukemia; his father had hypertension, diabetes, and hyperlipidemia; and his brother died from an anaphylactic reaction to shellfish.On physical exam, the patient had a temperature of 99.6° Fahrenheit, heart rate of 50 beats/minute, blood pressure of 155/77 mm Hg, respiratory rate of 14 breaths/minute with an oxygen saturation of 100% on room air. The exam also revealed a regularly irregular rhythm with no murmurs, rubs or gallops. His lungs were clear to auscultation bilaterally. He had no focal neurological deficits nor was he orthostatic. Large, well demarcated macular erythematous lesions were noted on his back and thorax. Initial labs showed a white blood cell count of 10,000 cells/mm3 with a normal differential, normal serum chemistries, and a negative troponin. Peripheral smear was negative for babesiosis. The patient's admission chest radiograph showed clear lungs without evidence of infiltrate or effusion. His initial EKG (Figure 1) showed a sinus rhythm at an atrial rate of approximately 100 beats/minute and a highgrade atrioventricular (AV) block with a variable ventricular response of about 40 beats/minute. The patient was admitted to telemetry, and a transcutaneous pacer was placed. Empiric intravenouis ceftriaxone was begun to treat for a clinical diagnosis of Lyme carditis.