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Cardiovassular Medicine Sinus Bradycardia and Heart Block Diagnosis Sinus bradycardia is defined as heart rate <50/min. Common causes of inappropriate sinus bradycardia are age related myocar- dial fibrosis, hypothyroidism, and inferior ischemia. The most common extrinsic cause is medication use (p-blockers, donepezil, neostigmine, pyridostigmine). AV nodal block results from functional or structural abnormalities at the AV node or in the His-Purkinje system. Potentially reversible causes include acute or chronic myocardial ischemia, Lyme disease, sarcoidosis, and amyloidosis.

Sinus Bradycardia and Heart Block Diagnosis Sinus bradycardia is defined as heart rate <50/min. Common causes of inappropriate sinus bradycardia are age related myocar- dial fibrosis, hypothyroidism, and inferior ischemia. The most common extrinsic cause is medication use (p-blockers, donepezil, neostigmine, pyridostigmine). AV nodal block results from functional or structural abnormalities at the AV node or in the His-Purkinje system. Potentially reversible causes include acute or chronic myocardial ischemia, Lyme disease, sarcoidosis, and amyloidosis. STUDY TABLE: Heart Block Type Diagnostic Criteria First-degree block PR interval >0.2 s without alterations in HR Second-degree block lntermittent P waves not followed by a ventricular complex; further classified as Mobitz type 1 or type 2 Third-degree block (complete heart Complete absence of conducted P waves (P-wave and ORS complex rates differ, and block) the PR interval differs for every ORS complex) and an atrial rate that is faster than the ventricular rate; most common cause of ventricular rates 30-50/min LBBB Absent O waves in leads l, aVL, and V6; large, wide, and positive R waves in leads l, aVL, and V6; ORS >0.12 s RBBB rsR' pattern in lead V1 ("rabbit ears"), wide negative S wave in lead V6, ORS >0.12 s Bifascicular block Right bundle branch and one of the fascicles of the left bundle branch are involved Trifascicular block Characterized by bifascicular block and prolongation of the PR interval Left anterior hemiblock Left axis usually -60", upright ORS complex in lead l, negative ORS complex in aVF, and normal ORS duration Left posterior hemiblock Right axis usually +120", negative ORS complex in lead l, positive ORS complex in lead aVF, and normal ORS duration

STUDY TABLE: Heart Block Type Diagnostic Criteria First-degree block PR interval >0.2 s without alterations in HR Second-degree block lntermittent P waves not followed by a ventricular complex; further classified as Mobitz type 1 or type 2 Third-degree block (complete heart Complete absence of conducted P waves (P-wave and ORS complex rates differ, and block) the PR interval differs for every ORS complex) and an atrial rate that is faster than the ventricular rate; most common cause of ventricular rates 30-50/min LBBB Absent O waves in leads l, aVL, and V6; large, wide, and positive R waves in leads l, aVL, and V6; ORS >0.12 s RBBB rsR' pattern in lead V1 ("rabbit ears"), wide negative S wave in lead V6, ORS >0.12 s Bifascicular block Right bundle branch and one of the fascicles of the left bundle branch are involved Trifascicular block Characterized by bifascicular block and prolongation of the PR interval Left anterior hemiblock Left axis usually -60", upright ORS complex in lead l, negative ORS complex in aVF, and normal ORS duration Left posterior hemiblock Right axis usually +120", negative ORS complex in lead l, positive ORS complex in lead aVF, and normal ORS duration STUDY TABLE: Second-Degree AV Block: Mobitz Type 1 and Type 2 TyPe Characteristics Significance Mobitz type 1 Constant P-P interval with progressively increased PR interval Rarely progresses to third-degree heart block (Wenckebach block) untilthe dropped beat; grouped beating is classic Mobitz type 2 Usually associated with RBBB or LBBB; constant PR interval in May precede third-degree heart block the conducted beats; R-R interval contains the nonconducted (dropped) beat equalto two P-P intervals

STUDY TABLE: Second-Degree AV Block: Mobitz Type 1 and Type 2 TyPe Characteristics Significance Mobitz type 1 Constant P-P interval with progressively increased PR interval Rarely progresses to third-degree heart block (Wenckebach block) untilthe dropped beat; grouped beating is classic Mobitz type 2 Usually associated with RBBB or LBBB; constant PR interval in May precede third-degree heart block the conducted beats; R-R interval contains the nonconducted (dropped) beat equalto two P-P intervals Treatment Sinus bradycardia requires no treatment for asymptomatic patients. For hemodynamically stable patients with symptoms, treat the underlying condition (e.g., MI, thyroid disease, medications). Initial therapy ofAV block includes correcting reversible causes ofimpaired conduction such as ischemia, increased vagal tone, and elimination of drugs that alter electrical conduction (donepezil, digitalis, calcium channel blockers, p-blockers). Common indications for permanent pacemaker implantation include absence of reversible cause and: . symptomatic bradycardia o permanent AF and symptomatic bradycardia . complete heart block . high-degree AV block (several consecutive nonconducted P waves) o Mobitz type 2 second degree AV block o alternating bundle branch block 14

Cardiovascular Medicine Choose IV atropine and/or transcutaneous or tranwenous pacing for symptoms of hemodlmamic compromise caused by brady- cardia or heart block toblu lypr I Hcaft Blodr: Ihe rhythm strip shows progressive prolongation of the PR intewal until the dropped beat. I , h t i l t r IobNE ltpc 2 lloail Blotk lhe rhy.thm strip shows constant PR interval. The R-R interval containing the nonconducted beat is equal to two P-P intervals' t5