Symptomatic Bradycardia in a Young Adult After Camping.

Abstract

Scenario: An 18-year-old man with no significant medical history arrived at the emergency department via ambulance after a sudden syncopal episode of unknown cause. He had just returned from camping a few days prior. At the time of the episode, the patient was performing chores and his mother found him down and disoriented. En route to the hospital, emergency medical services reported a heart rate of 48/min and a blood pressure of 72/33 mm Hg; he was treated with a 500-mL saline bolus. During examination, he recalled the event and stated that he had felt dizzy so he had lain down. At this time, he reported no shortness of breath, chest pain, nausea, vomiting, changes in vision or hearing, or headache. Vital signs upon his arrival at the emergency department were recorded as heart rate 67/min, blood pressure 97/64 mm Hg, body temperature 38 °C (100.7 °F), respiratory rate 18/min, and oxygen saturation (via pulse oximetry) 98% on room air. A 12-lead electrocardiogram (ECG) was obtained after a bradycardic telemetry alarm. The rhythm strip of the 12-lead ECG is shown here.Mobitz II second-degree atrioventricular block at a heart rate of 60/min with bundle branch block (BBB) and a prolonged QTc interval.Atrioventricular (AV) block occurs when the electrical impulses traveling from the sinoatrial (SA) node through the AV node are impaired. The physiological reason for AV blocks is that gap junctions, protein channels that transfer electrical impulses between cells, are scarce in and around the AV node. The scarcity of gap junctions delays electrical conduction to allow the atria enough time to empty blood into the ventricles, but when this delay becomes excessive, it can cause symptoms. There are 3 types of AV block: first-degree AV block implies slowed conduction; second-degree AV block implies partial conduction from the SA node through the AV node; and third-degree AV block implies complete block of conduction from the SA node through the AV node. The rhythm above has a regularly irregular pattern with partial conduction exhibited by every third P wave lacking an associated QRS complex, consistent with second-degree AV block.Second-degree AV blocks are subdivided into Mobitz type I (Wenckebach) and Mobitz type II. Mobitz type I is characterized as progressive lengthening of the conduction time through the AV node until an electrical impulse is completely blocked, appearing on the ECG as progressive lengthening of the PR interval until a P wave appears without an associated QRS complex. Mobitz type I is typically caused by drugs, electrolyte imbalances, or myocarditis. In contrast, Mobitz type II is characterized as a consistent conduction time through the AV node, and a blocked electrical impulse typically in a predictable pattern (every second beat, third beat, etc). On the ECG, Mobitz type II is characterized as a fixed PR interval with a P wave that appears without an associated QRS complex. Mobitz type II is typically caused by idiopathic fibrosis of the conduction system, inflammatory conditions such as rheumatic fever and Lyme disease, and infiltrative myocardial disease.On this strip, the PR interval is fixed at 162 ms and every third P wave is blocked, which is consistent with second-degree AV block (Mobitz II). A BBB (evidenced by the prolonged QRS duration of 138 ms) and a prolonged corrected QT interval (480 ms) also are apparent. The BBB is presumably a left BBB, owing to the broad, monophasic R wave in lead V5.This young adult with no significant medical history had a new onset of unexplained second-degree AV block (Mobitz II), a left BBB, and a prolonged QTc, all of which were confirmed on the 12-lead ECG. Given his recent camping trip, Lyme disease was suspected and later confirmed. Lyme disease is caused by a spirochete (Borrelia burgdorferi), which can infect the myocardium and induce an inflammatory reponse of the transmural and conductive tissues. Notably, AV blocks are the most common cardiac manifestation of Lyme disease; 98% of adults with cardiac manifestations of Lyme disease will develop first-degree AV block, and up to 50% of those patients may transition to a third-degree AV block.In this case, the problem did not advance beyond second-degree AV block (Mobitz II) and the arrhythmia was intermittent. From a nursing standpoint, important actions include preparing for temporary pacing and diligent cardiac monitoring. In this case, the heart block resolved after treatment with antibiotics.