Snapshot A 20-year-old man presents to his primary care physician for an annual exam. He is currently feeling well but has some concerns since he occasionally has palpitations. Medical history is unremarkable and he takes a daily multivitamin. He denies any alcohol or smoking history. Family history is significant for his paternal grandfather expiring from a fatal arrhythmia. He is a college student and part of the school's basketball team, which he continued since he was a student in high school. He says that his caffeine intake has increased in these past few weeks due to upcoming final examinations. He is requesting an electrocardiogram (ECG) to ensure his heart is healthy since this worry is interfering with his school work. An ECG demonstrates a PR interval of 0.25 sec and is otherwise unremarkable. (First-degree heart block) Introduction Clinical definition login to view 1 more bullet Etiology login to view 10 more bullets Differential login to view 2 more bullets Atrioventricular (AV) Block AV BlocksTypesElectrocardiogram FindingsTreatmentCommentsFirst degree AV block↑ PR interval (> 0.2 secs)1:1 ratio of P waves and QRS complexesNo treatment is required unless other ECG changes (prolonged QRS)Typically seen in young patients with a ↑ vagal toneWell-trained athletesSecond degree AV block (Mobitz type I)Progressive↑ PR interval until a P wave is not followed by a QRS complex (Wenckebach phenomenon)the sequence then repeatsQRS complex is narrowTreatment is unnecessary unless the patient is symptomaticuse atropine or a temporary pacemaker in symptomatic casesPatients are typically asymptomaticCan be seen in patients withdrug intoxication (e.g., β-blockers and digitalis)↑ vagal toneSecond degree AV block (Mobitz type II)Fixed PR intervals with occasional dropped QRS complexesQRS complex is typically wideAtropine and pacing for unstable patientsPermanent pacemakerMay progress to a third-degree heart blockThird degree (complete) AV blockAtria and ventricles depolarize independentlyP waves and QRS complexes are not rhythmically synchronizedAtropinePacingPermanent pacemaker unless the cause is reversible (such as medications)Can be a complication of Lyme diseaseBifasicular block can progress to complete heart block