Bradycardia

The heart is a specialized muscle containing repeating units of cardiomyocytes, or heart muscle cells. Like most cells, cardiomyocytes maintain a highly regulated negative voltage at rest and are capable of propagating action potentials, much like neurons. While at rest, the negative cellular voltage of a cardiomyocyte can be raised above a certain threshold (so-called depolarization) by an incoming action potential, causing the myocyte to contract. When these contractions occur in a coordinated fashion, the atria and ventricles of the heart will pump, delivering blood to the rest of the body. The SA node contains pacemaker cells that demonstrate "automaticity" and can generate impulses that travel through the heart and create a steady heartbeat. allowing for appropriate filling of the ventricles before contraction. The SA and AV nodes are both closely regulated by the autonomic nervous system's fibres, allowing for adjustment of cardiac output by the central nervous system in times of increased metabolic demand. Following slowed conduction through the atrioventricular node, the action potential produced initially at the SA node now flows through the His-Purkinje system. The bundle of His originates in the AV node and rapidly splits into a left and right branch, each destined for a different ventricle. Finally, these bundle branches terminate in the small Purkinje fibers that innervate myocardial tissue. The His-Purkinje system conducts action potentials much faster than can be propagated between myocardial cells, allowing the entire ventricular myocardium to contract in less time, improving pump function. ==Classification==

Most pathological causes of bradycardia result from damage to this normal cardiac conduction system at various levels: the sinoatrial node, the atrioventricular node, or damage to conduction tissue between or after these nodes. Sinus node Bradycardia caused by the alterations of sinus node activity is divided into three types. Sinus bradycardia Sinus bradycardia is a sinus rhythm of less than 50 BPM. The heart muscle of athletes has a higher stroke volume, requiring fewer contractions to circulate the same volume of blood. Asymptomatic sinus bradycardia decreases in prevalence with age in middle aged adults, with an increase seen after 65. Sinus arrhythmia Sinus arrhythmias are heart rhythm abnormalities characterized by variations in the cardiac cycle length over 120 milliseconds (longest cycle - shortest cycle). The condition can also be caused by dysfunction of the autonomic nervous system that regulates the node and is commonly exacerbated by medications. 2nd degree AV block is characterized by intermittently lost conduction of impulses between the SA node and the ventricles. 2nd degree block is classified into two types. Mobitz type 1 block, otherwise known by the eponym Wenckebach, classically demonstrates grouped patterns of heartbeats on ECG. Throughout the group, the PR interval gradually lengthens until a dropped conduction occurs, resulting in no QRS complex seen on surface ECG following the last P wave. After a delay, the grouping repeats, with the PR interval shortening again to baseline. Type 1 2nd degree AV block due to disease in the AV node (as opposed to in the His-purkinje system) rarely needs intervention with pacemaker implantation. Junctional rhythms An AV-junctional rhythm, or atrioventricular nodal bradycardia, is usually caused by the absence of the electrical impulse from the sinus node. This usually appears on an electrocardiogram with a normal QRS complex accompanied by an inverted P wave either before, during, or after the QRS complex. This is a protective mechanism for the heart to compensate for an SA node that is no longer handling the pacemaking activity and is one of a series of backup sites that can take over pacemaker function when the SA node fails to do so. This would present with a longer PR interval. An AV-junctional escape complex is a normal response that may result from excessive vagal tone on the SA node. Pathological causes include sinus bradycardia, sinus arrest, sinus exit block, or AV block. AV block may be ruled out with an ECG indicating "a 1:1 relationship between P waves and QRS complexes." ==Causes==

Bradycardia arrhythmia may have many causes, both cardiac and non-cardiac. Non-cardiac causes are usually secondary and can involve recreational drug use or abuse, metabolic or endocrine issues, especially hypothyroidism, an electrolyte imbalance, neurological 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. In general, two types of problems result in bradycardias: disorders of the SA node and disorders of the AV node. With SA node dysfunction (sometimes called sick sinus syndrome), there may be disordered automaticity or impaired conduction of the impulse from the SA node into the surrounding atrial tissue (an "exit block"). Second-degree sinoatrial blocks can be detected only by use of a 12-lead ECG. 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. Beta blockers may slow the heart rate to a dangerous level if prescribed with calcium channel blockers. Desensitization of β-adrenergic receptors has been suggested as a possible cause of this. COVID-19 has been found to be a cause of bradycardia. ==Diagnosis==

A diagnosis of bradycardia in adults is based on a heart rate of less than 60 BPM, although some studies use a heart rate of less than 50 BPM. This is usually determined either by palpation or ECG. If symptoms occur, a determining electrolytes may help determine the underlying cause. Many healthy young adults, and particularly well-trained athletes, have sinus bradycardia that is without symptoms. This can include heart rates of less than 50 or 60bpm or even less than 40bpm. Such individuals, without symptoms, do not require treatment. Temporal correlation of symptoms with bradycardia is necessary for diagnosis of symptomatic bradycardia. This can sometimes be difficult. Challenge with oral theophylline can be used as a diagnostic agent in people with bradycardia caused by sinus node dysfunction to help correlate symptoms. Theophylline increases resting heart rate and improves subjective symptoms in most people with bradycardia due to SND. ==Management==

The treatment of bradycardia depends on whether the person is stable or unstable. Treatment of chronic symptomatic bradycardia first necessitates correlation of symptoms. Other positive chronotropes have also been used to treat bradycardia, including the vasodilator and antihypertensive agent hydralazine, the alpha-1 blocker prazosin, anticholinergics, and sympathomimetic agents like beta-1 agonists. Acute or unstable If a person is unstable, the initial recommended treatment is intravenous atropine. ==Epidemiology==

In clinical practice, elderly people over age 65 and young athletes of both sexes may have sinus bradycardia. ==Society and culture==

Records • Daniel Green holds the world record for the slowest heartbeat in a healthy human, with a heart rate measured in 2014 of 26 BPM. • Martin Brady holds the Guinness world record for the slowest heart rate, with a certified rate over a minute duration of 27 BPM. • During his career, professional cyclist Miguel Indurain had a resting heart rate of 28 BPM. == See also ==