Bradycardia and Blocks

CHAPTER


12


Bradycardia and Blocks


SINUS BRADYCARDIA AND SINOATRIAL (SA) NODE DYSFUNCTION


General Information


Impairment of the sinoatrial (SA) node ability to generate propagated impulses


Consider a 2:1 sinus exit block.


Epidemiology and Clinical Features


Sinus bradycardia occurs in 1/600 patients over 65 years of age.


Variable symptoms:


Asymptomatic to fatigue, exercise intolerance, dyspnea, presyncope, or syncope.


May lead to angina or heart failure.


Often these patients are highly sensitive to cardiac medications (non-dihydropyridine calcium-channel blocker [ND-CCB], β-blockers, and antiarrhythmic drugs [AAD]).


Frequently coexists with atrial tachyarrhythmia (atrial fibrillation [AF] and atrial flutter [AFL]).


Etiology


Intrinsic


Degenerative: Idiopathic age-related fibrosis (most common)


SA node fibrosis: Inappropriate sinus bradycardia and exaggerated overdrive suppression


Atrial myocardial fibrosis: Propensity to AF or AFL


Atrioventricular node (AVN) fibrosis: atrioventricular (AV) block


Myocardial ischemia


SA node: Supplied by right coronary artery (60%) or left circumflex artery (40%)


Myocardial infiltration


Sarcoidosis, amyloidosis, hemochromatosis


Inflammatory


Pericarditis, myocarditis


Familial diseases


Myotonic dystrophy, Friedrich ataxia, Na+ channel mutations


Collagen vascular disease


Systemic lupus erythematosus, rheumatoid arthritis, scleroderma, ankylosing spondylitis


Trauma or surgery


Valve replacement, ablation, atrial septal defect (ASD) repair, heart transplantation


Extrinsic


Drugs


AADs (class I, class III), β-blockers, diltiazem/verapamil, digoxin, ivabradine


Sympatholytic (reserpine, methyldopa, clonidine), alcohol, lithium


Electrolyte imbalances


Potassium, calcium, or magnesium


Metabolic


Hypothyroidism, hypothermia


Myocardial ischemia


Inferior myocardial infarction (MI): Neural reflex


Autonomic-mediated syndromes


Neurocardiogenic syncope, carotid-sinus hypersensitivity


Situational: Coughing, micturition, defecation, vomiting


Infection


Chagas, endocarditis, Salmonella, diphtheria, rheumatic fever, viral myocarditis


Classification


Inappropriate sinus bradycardia


Sinus bradycardia in the absence of an appropriate cause or that results in symptoms


Tachycardia-bradycardia syndrome


Atrial tachycardia (AT; usually AF) alternating with sinus bradycardia, sinus pauses, and/or an AV block


Intermittent sinus pauses include:


Sinus arrest: Transient cessation of SA node firing (>2 seconds)


Sinoatrial (SA) exit block: Depolarization fails to exit the SA node (the RR interval is unchanged).


Type 1 (Mobitz): The time between sinus firing and when the atrial capture progressively prolongs leading to gradual shortening in the PP intervals before the pause (pause PP < two preceding PP).


Type 2 (Mobitz): The PP interval is constant before and after the pause and is a multiple (2×, 3×…) of the basic PP interval.


Chronotropic incompetence: Inappropriately low heart rate response during exercise.


Absolute: Inability to increase heart rate to 60% age-predicted target (220 – age) or to >100–120 bpm.


Relative: Able to reach target heart rate but at a significant delay with reduced exercise tolerance.


12-Lead ECG


image


Heart rate <60 bpm with characteristics of sinus rhythm


1:1 P:QRS relationship


Every P wave is followed by a QRS complex and every QRS complex is preceded by a P wave.


Sinus P-wave morphology and axis


Upright in I, II, aVF, and V2–V6 (best seen in II)


Inverted in aVR


Upright or biphasic in V1 and V2; upright in V3–V6


Other Investigations


Carotid sinus massage or tilt table testing to identify neurocardiogenic causes


Treadmill stress test


Assesses the chronotropic response to exercise


Ambulatory electrocardiogram (ECG) monitoring (Holter, event monitor, ILR)


Correlates symptoms with the electrical disorder


Assessment of intrinsic heart rate (IHR: Rarely performed)


IV atropine 0.04 mg/kg + IV propranolol 0.2 mg/kg


Predicted IHR = 118.1 – [0.57 × age]:


Low IHR = intrinsic sinus node dysfunction (SND)


Normal IHR = autonomic imbalance


Electrophysiology study (see Assessment of SA Node Function section)


Management


Acute Management


Atropine or isoproterenol


Chronic Management


Stop medications that suppress the SA node (e.g., β-blockers, verapamil/diltiazem, digoxin).


Pacemaker indications:


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Feb 28, 2017 | Posted by in CARDIOLOGY | Comments Off on Bradycardia and Blocks

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