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Sinus bradycardia is a sinus node dysfunction giving a
heart rate Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and e ...
that is lower than the normal 60–100
beats per minute Beat, beats or beating may refer to: Common uses * Patrol, or beat, a group of personnel assigned to monitor a specific area ** Beat (police), the territory that a police officer patrols ** Gay beat, an area frequented by gay men * Battery ( ...
(bpm) in humans.
Bradycardia Bradycardia (also sinus bradycardia) is a slow resting heart rate, commonly under 60 beats per minute (BPM) as determined by an electrocardiogram. It is considered to be a normal heart rate during sleep, in young and healthy or elderly adults, ...
is generally defined to be a heart rate of under 60 bpm.


Signs and symptoms

The decreased heart rate can cause a decreased cardiac output resulting in symptoms such as
lightheadedness Lightheadedness is a common and typically unpleasant sensation of dizziness or a feeling that one may faint. The sensation of lightheadedness can be short-lived, prolonged, or, rarely, recurring. In addition to dizziness, the individual may feel ...
,
dizziness Dizziness is an imprecise term that can refer to a sense of disorientation in space, vertigo, or lightheadedness. It can also refer to disequilibrium or a non-specific feeling, such as giddiness or foolishness. Dizziness is a common medical ...
,
hypotension Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the systolic blood pressure (the top number) and the dia ...
,
vertigo Vertigo is a condition where a person has the sensation of movement or of surrounding objects moving when they are not. Often it feels like a spinning or swaying movement. This may be associated with nausea, vomiting, sweating, or difficulties ...
, and syncope. The slow heart rate may also lead to atrial, junctional, or ventricular ectopic rhythms. Bradycardia is not necessarily problematic. People who practice sports may have sinus bradycardia, because their trained hearts can pump enough blood in each contraction to allow a low
resting heart rate Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...
. Sinus bradycardia can also be an adaptive advantage; for example, diving seals may have a heart rate as low as 12 beats per minute, helping them to conserve oxygen during long dives. Sinus bradycardia is a common condition found in both healthy individuals and those who are considered well-conditioned athletes. Heart rates considered bradycardic vary by species; for example, in the house cat, a rate of under 120 beats per minute is abnormal. Generally, smaller species have higher and larger species lower rates.


Pathophysiology & etiology

* Sinus bradycardia is commonly seen in normal healthy persons and athletes in the absence of pathophysiological diseases or conditions. * Different factors or etiologies could lead to the dysfunction of the sinus node, causing a malformation or prolongation of the impulse. * In terms of pathophysiological diseases, sinus rhythm may be caused by: ** Diseases/Conditions: ***
Acute myocardial infarction A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow decreases or stops to the coronary artery of the heart, causing damage to the heart muscle. The most common symptom is chest pain or discomfort which may ...
, carotid sinus syndrome, eating disorders (such as
anorexia nervosa Anorexia nervosa, often referred to simply as anorexia, is an eating disorder characterized by underweight, low weight, Calorie restriction, food restriction, body image disturbance, fear of gaining weight, and an overpowering desire to be thi ...
), rhodotoxin poisoning,
hypothermia Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe h ...
,
hypothyroidism Hypothyroidism (also called ''underactive thyroid'', ''low thyroid'' or ''hypothyreosis'') is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as ...
, infections (such as
diphtheria Diphtheria is an infection caused by the bacterium '' Corynebacterium diphtheriae''. Most infections are asymptomatic or have a mild clinical course, but in some outbreaks more than 10% of those diagnosed with the disease may die. Signs and s ...
, acute
rheumatic fever Rheumatic fever (RF) is an inflammation#Disorders, inflammatory disease that can involve the heart, joints, skin, and brain. The disease typically develops two to four weeks after a Streptococcal pharyngitis, streptococcal throat infection. Sign ...
, or viral
myocarditis Myocarditis, also known as inflammatory cardiomyopathy, is an acquired cardiomyopathy due to inflammation of the heart muscle. Symptoms can include shortness of breath, chest pain, decreased ability to exercise, and an irregular heartbeat. The ...
), intrinsic disease of the SA node (such as sick sinus syndrome), Roemheld syndrome,
sleep apnea Sleep apnea, also spelled sleep apnoea, is a sleep disorder in which pauses in breathing or periods of shallow breathing during sleep occur more often than normal. Each pause can last for a few seconds to a few minutes and they happen many tim ...
** Physiological causes: *** Increased vagal tone, increased
intracranial pressure Intracranial pressure (ICP) is the pressure exerted by fluids such as cerebrospinal fluid (CSF) inside the skull and on the brain tissue. ICP is measured in millimeters of mercury ( mmHg) and at rest, is normally 7–15 mmHg for a supine adult. ...
** Medications, most commonly: digitalis glycosides, beta-blockers, quinidine,
adenosine Adenosine (symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9-glycosidic bond. Adenosine is one of the four nucleoside buildin ...
,
calcium channel blockers Calcium channel blockers (CCB), calcium channel antagonists or calcium antagonists are a group of medications that disrupt the movement of calcium () through calcium channels. Calcium channel blockers are used as antihypertensive drugs, i.e., ...
, class I antiarrhythmic agents, ivabradine, clonidine, reserpine, cimetidine, lithium, amitriptyline


Diagnosis

Diagnosis of sinus bradycardia can be confirmed by an electrocardiogram that shows the following characteristics: * Rate: Less than 60 beats per minute. * Rhythm: Regular. * P waves: Upright, consistent, and normal in morphology and duration. * PR interval: Between 0.12 and 0.20 seconds in duration. *
QRS complex The QRS complex is the combination of three of the graphical deflections seen on a typical electrocardiogram (ECG or EKG). It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the r ...
: Less than 0.12 seconds in width, and consistent in morphology. Taking a thorough medical history and physical exam by healthcare providers can also help with narrowing differential diagnosis. Any recent changes to patient's medication history, new symptoms such as chest pain, shortness of breath and palpitations, family history of sinus bradycardia, physical exam that reveals cyanosis, peripheral edema, altered mental status, dyspnea, rales and crackles are all relevant information to consider for differential diagnosis.


See also

*
Sinus tachycardia Sinus tachycardia is an elevated sinus rhythm characterized by an increase in the rate of electrical impulses arising from the sinoatrial node. In adults, sinus tachycardia is defined as a heart rate greater than 100 beats per minute (bpm). Th ...
*
Long QT syndrome Long QT syndrome (LQTS) is a condition affecting repolarization (relaxing) of the heart after a heartbeat, giving rise to an abnormally lengthy QT interval. It results in an increased risk of an irregular heartbeat which can result in fainting, ...


References


External links

{{DEFAULTSORT:Sinus Bradycardia Cardiac arrhythmia