Bradycardia

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Overview
Bradycardia, as applied to adult medicine, is defined as a resting heart rate of under 60 beats per minute, though it is seldom symptomatic until the rate drops below 50 beat/min. Trained athletes tend to have slow resting heart rates, and resting bradycardia in athletes should not be considered abnormal if the individual has no symptoms associated with it.

The term relative bradycardia is used to explain a heart rate that, while not technically below 60 beats per minute, is considered too slow for the individual's current medical condition.

This cardiac arrhythmia can be underlain by several causes, which are best divided into cardiac and non-cardiac causes. Non-cardiac causes are usually secondary, and can involve drug use or abuse; metabolic or endocrine issues, especially in the thyroid; an electrolyte imbalance; neurologic factors; autonomic reflexes; situational factors such as prolonged bed rest; and autoimmunity. Cardiac causes include acute or chronic ischemic heart disease, vascular heart disease, valvular heart disease, or degenerative primary electrical disease. Ultimately, the causes act by three mechanisms: depressed automaticity of the heart, conduction block, or escape pacemakers and rhythms.

Causes
There are generally two types of problems that result in bradycardias: disorders of the sinus node, and disorders of the atrioventricular node (AV node).

With sinus node dysfunction (sometimes called sick sinus syndrome), there may be disordered automaticity or impaired conduction of the impulse from the sinus node into the surrounding atrial tissue (an "exit block"). It is difficult and sometimes impossible to assign a mechanism to any particular bradycardia, but the underlying mechanism is not clinically relevant to treatment, which is the same in both cases of sick sinus syndrome: a permanent pacemaker.

Atrioventricular conduction disturbances (aka: AV block; 1o AV block, 2o type I AV block, 2o type II AV block, 3o AV block) may result from impaired conduction in the AV node, or anywhere below it, such as in the bundle of HIS.

Patients with bradycardia have likely acquired it, as opposed to having it congenitally. Bradycardia is more common in older patients.

Complete Differential Diagnosis of the Causes of Bradycardia
(In alphabetical order)


 * Aceclidine
 * Acepromazine
 * Acetylcholinesterase inhibitor
 * Adenosine
 * Amiodarone
 * Amodiaquine
 * Anorexia Nervosa
 * Apnea of prematurity
 * Atenolol
 * Atrioventricular Block
 * Autonomic neuropathy
 * Barbiturates
 * Beta-blockers
 * Brucellosis
 * Bupivacaine
 * Calcium channel blocker
 * Cardiac arrhythmia
 * Cardiac Dysrhythmias
 * Cardiomegaly
 * Catecholaminergic polymorphic ventricular tachycardia
 * Cerebral Hemorrhage
 * Cerebral venous sinus thrombosis
 * Chagas' disease
 * Clomipramine
 * Colorado tick fever
 * Congenital Long QT Syndrome
 * Cushing reaction
 * Cushing reflex
 * Cushing's triad
 * Detomidine
 * Digitalis
 * Diltiazem
 * Diphenhydramine
 * Donepezil
 * Doxepin
 * Drowning
 * Emery-Dreifuss muscular dystrophy
 * Enema
 * Fetal distress
 * Gamma-Hydroxybutyric acid
 * Gastric lavage
 * Glyceryl trinitrate
 * Grayanotoxin
 * Hantavirus pulmonary syndrome
 * Hellebore
 * Holt-Oram syndrome
 * Hydrocodone
 * Hydroxyethyl starch
 * Hypercalcemia
 * Hyperkalemia
 * Hypokalemia
 * Hypothermia
 * Hypothyroidism
 * Ibuprofen
 * Isosorbide dinitrate
 * Ivabradine
 * Legionella pneumonia and Mycoplasma pneumonia
 * Levobetaxolol
 * Levobupivacaine
 * Lidocaine


 * Lithium
 * Mammalian diving reflex
 * Medetomidine
 * Mefloquine
 * Mepivacaine
 * Methacholine
 * Methoxamine
 * Methyldopa
 * Morphine
 * Moxonidine
 * Myxedema coma
 * Nadolol
 * Nalbuphine
 * Nalmefene
 * Neonatal lupus erythematosus
 * Neostigmine
 * Nesiritide
 * Neurocardiogenic Syncope
 * Neurogenic shock
 * Norepinephrine
 * Oculocardiac reflex
 * Opioid
 * Oxymorphone
 * PCI Complications: Radiocontrast toxicity
 * Phenobarbital
 * Phenylephrine
 * Phenytoin
 * Pilocarpine
 * Propafenone
 * Propranolol
 * Pulseless ventricular tachycardia
 * Quinidine
 * Raised Intracranial pressure
 * Rapid sequence induction
 * Reserpine
 * Right Bundle Branch Block
 * Ropivacaine
 * Second Degree AV Block
 * Sick sinus syndrome
 * Sinoatrial Block
 * Speedball (drug)
 * ST Elevation Myocardial Infarction Complications
 * Subarachnoid hemorrhage
 * Sulpiride
 * Suxamethonium chloride
 * Tacrine
 * Theobromine poisoning
 * Timolol
 * Trazodone
 * Trypanosoma cruzi
 * Tularaemia
 * Typhoid fever
 * Uncaria tomentosa
 * Vagal episode
 * Vasa previa
 * Vasovagal syncope
 * Vertebral subluxation
 * Xylazine

Complete Differential Diagnosis of the Causes of Bradycardia
(By organ system)

Urgent Treatment
Drug treatment for bradycardia is typically not indicated for patients who are asymptomatic. In symptomatic patients, underlying electrolyte or acid-base disorders or hypoxia should be corrected first. IV atropine may provide temporary improvement in symptomatic patients, although its use should be balanced by an appreciation of the increase in myocardial oxygen demand this agent causes.

Atropine 0.5-1 mg IV or ET q3-5min up to 3 mg total (0.04 mg/kg)

Indications For a Temporary Pacemaker
Advanced heart block such as complete heart block is an indication for a temporary pacemaker insertion.

Chronic Management
There are two main reasons for treating any cardiac arrhythmias. With bradycardia, the first is to address the associated symptoms, such as fatigue, limitations on how much an individual can physically exert, fainting (syncope), dizziness or lightheadedness, or other vague and non-specific symptoms. The other reason to treat bradycardia is if the person's ultimate outcome (prognosis) will be changed or impacted by the bradycardia. Treatment in this vein depends on whether any symptoms are present, and what the underlying cause is. Primary or idiopathic bradycardia is treated symptomatically if it is significant, and the underlying cause is treated if the bradycardia is secondary.