Bradycardia Causes/Bradycardia is defined as a resting heart rate below 60 beats per minute (b.p.m.). A slow heart rate can occur naturally under various circumstances and is not necessarily associated with a medical condition. For example, some highly trained athletes have bradycardia. However, there is also pathological bradycardia, which is caused by conditions that affect the electrical conduction system of the heart, including sick sinus syndrome (SSS) and/or atrioventricular (AV) block.[rx] Bradycardia does not necessarily require treatment unless it causes symptoms. People suffering from symptomatic bradycardia can present with dizziness, confusion, palpitations, breathlessness, exercise intolerance and syncope (blackout or fainting). However, bradycardia and symptoms related to it may be intermittent or may be non-specific, particularly in the elderly.
Bradycardia is a condition typically defined wherein an individual has a resting heart rate of under 60 beats per minute (BPM) in adults.[rx] Bradycardia typically does not cause symptoms until the rate drops below 50 BPM. When symptomatic, it may cause fatigue, weakness, dizziness, sweating, and at very low rates, fainting.[rx] During sleep, a slow heartbeat with rates around 40–50 BPM is common and is considered normal. Highly trained athletes may also have athletic heart syndrome, a very slow resting heart rate that occurs as a sport adaptation and helps prevent tachycardia during training.
A heart has numerous pacemaker sites within its conduction system which are independently able to keep the heart beating. And the rate of a heartbeat depends upon the pacemaker site, and as we go down its conduction system, the rate of spontaneous depolarization at pacemaker sites decrease. [rx][rx][rx]
Pacemaker sites and their depolarization rates are listed below
Sinoatrial node – 60 to 100 beats per minute
Atria – Less than 60 beats per minute
Atrioventricular node – 40 to 60 beats per minute
Ventricles – 20 to 40 beats per minute
Types of Bradycardia
- Sinus – Atrial bradycardias are divided into three types. The first, respiratory sinus arrhythmia, is usually found in young and healthy adults. Heart rate increases during inhalation and decreases during exhalation. This is thought to be caused by changes in the vagal tone during respiration.[rx] If the decrease during exhalation drops the heart rate below 60 bpm on each breath, this type of bradycardia is usually deemed benign and a sign of good autonomic tone.
- Atrioventricular nodal – Atrioventricular nodal bradycardia or AV junction rhythm is usually caused by the absence of the electrical impulse from the sinus node. This usually appears on an EKG with a normal QRS complex accompanied with an inverted P wave either before, during, or after the QRS complex.[rx]An AV junctional escape is a delayed heartbeat originating from an ectopic focus somewhere in the AV junction. It occurs when the rate of depolarization of the SA node falls below the rate of the AV node.[rx] This dysrhythmia also may occur when the electrical impulses from the SA node fail to reach the AV node because of SA or AV block.[rx]
- Ventricular – Ventricular bradycardia, also known as ventricular escape rhythm or idioventricular rhythm, is a heart rate of less than 50 BPM. This is a safety mechanism when a lack of electrical impulse or stimuli from the atrium occurs.[rx] Impulses originating within or below the bundle of His in the atrioventricular node will produce a wide QRS complex with heart rates between 20 and 40 BPM. Those above the bundle of His, also known as junctional, will typically range between 40 and 60 BPM with a narrow QRS complex.[rx][rx] In a third-degree heart block, about 61% take place at the bundle branch-Purkinje system, 21% at the AV node, and 15% at the bundle of His.[rx] AV block may be ruled out with an EKG indicating “a 1:1 relationship between P waves and QRS complexes.
- Infantile – For infants, bradycardia is defined as a heart rate less than 100 BPM (normal is around 120–160). Premature babies are more likely than full-term babies to have apnea and bradycardia spells; their cause is not clearly understood. The spells may be related to centers inside the brain that regulate breathing which may not be fully developed. Touching the baby gently or rocking the incubator slightly will almost always get the baby to start breathing again, which increases the heart rate. Medications (theophylline or caffeine) can be used to treat these spells in babies if necessary.
Causes of Bradycardia
Common causes of sinus bradycardia in children include increased vagal tone, hypothyroidism, hypothermia, adrenal insufficiency, and increased intracranial pressure. Inherited arrhythmias and cardiomyopathies may also cause bradycardia. Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare inherited disorder.
Pathological causes of sinus bradycardia
- A congenital defect –or problem you’re born with
- Thyroid disease –an imbalance of hormones in the body
- Obstructive sleep apnea – when your breathing pauses many times throughout the night
- Drugs—for example, β blockers, digoxin, amiodarone
- Problems with the sinoatrial (SA) node – sometimes called the heart’s natural pacemaker
- Problems in the conduction pathways of the heart that don’t allow electrical impulses to pass properly from the atria to the ventricles
- Metabolic problems such as hypothyroidism (low thyroid hormone)
- Damage to the heart from heart disease or heart attack
- Certain heart medications that can cause bradycardia as a side effect
- Acute myocardial infarction
- Obstructive jaundice
- Raised intracranial pressure
- Sick sinus syndrome
Sinus bradycardia has many intrinsic and extrinsic etiologies[rx]][rx][rx][rx][rx][rx].
Ischemic heart disease
Acute myocardial infarction
Acute and chronic coronary artery disease
Repair of congenital heart disease
Sick sinus syndrome
Collagen vascular disease
X-linked muscular dystrophy
Vasovagal simulation (endotracheal suctioning)
Carotid sinus hypersensitivity
Calcium channel blockers
Antiarrhythmic Class I to IV
- Heart tissue damage related to aging
- Damage to heart tissues from heart disease or heart attack
- Heart disorder present at birth (congenital heart defect)
- Infection of heart tissue (myocarditis)
- A complication of heart surgery
- Underactive thyroid gland (hypothyroidism)
- Imbalance of chemicals in the blood, such as potassium or calcium
- Repeated disruption of breathing during sleep (obstructive sleep apnea)
- Inflammatory diseases, such as rheumatic fever or lupus
- Medications, including some drugs for other heart rhythm disorders, high blood pressure and psychosis
- Common in sick sinus syndrome
- Characterized by bursts of atrial tachycardia interspersed with periods of bradycardia
- Paroxysmal atrial flutter or fibrillation may also occur, and cardioversion may be followed by a severe bradycardia
Symptoms of Bradycardia
- Fatigue or feeling weak
- Chest pain, which may signal reduced blood flow to the heart
- Dizziness or lightheadedness
- Fainting (or near-fainting) spells
- Shortness of breath
- Difficulty when exercising
- Cardiac arrest (in extreme cases)
- Confusion or memory problems
- Easily tiring during physical activity
- Heart palpitations or flutters
- Limited ability to exercise
Diagnosis of Bradycardia
- Electrocardiogram (ECG or EKG) – An electrocardiogram also called an ECG or EKG, is a primary tool for evaluating bradycardia. Using small sensors (electrodes) attached to your chest and arms, it records electrical signals as they travel through your heart. Because an ECG can’t record bradycardia unless it happens during the test, your doctor might have you use a portable ECG device at home. These devices include:
- Holter monitor – Carried in your pocket or worn on a belt or shoulder strap, this device records your heart’s activity for 24 to 48 hours. Your doctor will likely ask you to keep a diary during the same 24 hours. You’ll describe any symptoms you experience and record the time they occur.
- Event recorder – This device monitors your heart activity over a few weeks. You push a button to activate it when you feel symptoms so that it records your heart’s activity during that time.
- LINQ insertable cardiac monitor: This wireless, powerful, small insertable cardiac monitor is ideal for patients experiencing infrequent symptoms that require long-term monitoring or ongoing management.
Your doctor might use an ECG monitor while performing other tests to understand the impact of bradycardia. These tests include
- Tilt table test – This test helps your doctor better understand how your bradycardia contributes to fainting spells. You lie flat on a special table, and then the table is tilted as if you were standing up to see if the change in position causes you to faint.
- Exercise test – Your doctor might monitor your heart rate while you walk on a treadmill or ride a stationary bike to see whether your heart rate increases appropriately in response to physical activity.
- A stress test – can be helpful in bringing out either sinus node disease or heart block that becomes apparent only during exertion.
Treatment of Bradycardia
The treatment of bradycardia is dependent on whether or not the person is stable or unstable.[rx] If oxygen saturations are low, supplemental oxygen should be provided.
- Stable – Emergency treatment is not needed if the person is asymptomatic or minimally symptomatic.[rx]
- Unstable – If a person is unstable, the initial recommended treatment is intravenous atropine.[rx] Doses less than 0.5 mg should not be used, as this may further decrease the rate.[rx] If this is not effective, intravenous inotrope infusion (dopamine, epinephrine) or transcutaneous pacing should be used.[rx] Transvenous pacing may be required if the cause of the bradycardia is not rapidly reversible.[rx]
- In children – giving oxygen, supporting their breathing, and chest compressions are recommended.[rx][rx]
- While atropine can be used independently for anti-salivation effects, it most commonly is secondary to anticholinergic or antimuscarinic poisoning which is discussed below. It is not formally recommended for routine use in controlled airways, though can be used off-label for minimizing secretions in the intubated patient.
- Acetylcholine works on three different receptors that must be addressed in nerve agent poisonings. Atropine is only useful to counter muscarinic effects (pralidoxime and benzodiazepines address the others). If there are local symptoms to the eyes or respiratory tract, atropine is not indicated.
- Intravenous (IV) atropine is indicated for patients with hypersalivation, bronchial secretions, or bradycardia. Large doses and repeat doses may be required. Ingestions especially require higher doses (up to 20 mg). Titrate to effect by monitoring the patient’s ability to clear excess secretions. Pupils and heart rate are poor indications of appropriate dosing in these patients.
Rapid Sequence Intubation (RSI) Pretreatment
- Although not recommended as a routine agent – atropine may be used 3-5min prior to initiation of RSI to prevent bradycardia. In the setting of post-intubation related bradycardia, atropine is indicated. Post induction bradycardia is seen more commonly in the pediatric population due to the predominance of vagal response, even without the use of succinylcholine.
- Atropine/Diphenoxylate (Lomotil) – is an antimotility agent that can be used in the treatment of diarrhea as second-line therapy. This allows the central acting opioid effect of diphenoxylate and capitalization on its anticholinergic side effect of constipation to slow motility. Subtherapeutic amounts of atropine are included in the dosage form to discourage diphenoxylate abuse. It may potentiate barbiturates, alcohol, or tranquilizers and therefore must be used with caution.
- Under current ACLS protocols – atropine is indicated for symptomatic bradycardia and not in a pulseless patient. All indications for atropine in the pulseless patient have been removed by the AHA.
- Atropine is not indicated in beta-blocker-induced bradycardias or hypotension – though its use is unlikely to be harmful. Glucagon is first-line to treat beta-blockade-induced symptoms. Pregnancy Class B: It does cross the placenta and may lead to fetal tachycardia; however, it does not cause fetal abnormalities. [rx]
Atropine can be administered by intravenous (IV), subcutaneous, intramuscular, or endotracheal (ET) method; IV is preferred. For ET administration dilute 1 mg to 2 mg in 10 mL of sterile water or normal saline prior to administration. For pediatric ET, double the usual IV dose and dilute in 3 to 5 mL.
Antisialagogue/antiviral – 0.5 mg to 1 mg every 1 to 2 hours
Organophosphate or muscarinic poisoning – 2 mg to 3 mg every 20 to 30 minutes (may require doses up to 20 mg, titrate to the effect of secretion control, not VS)
Bradycardia – 1 mg every 3 to 5 minutes (3 mg max), repeat until desired heart rate is obtained, most effective for sinus and AV nodal disease.
Pediatric – 0.01 mg/kg to 0.03 mg/kg every 3 to 5 minutes. Pediatric minimum dose 0.1 mg, maximum dose 0.5 mg (child) and 1.0 mg (adolescent); maximum cumulative dose 1 mg (child) and 2 mg (adolescent).
Rapid Sequence Intubation Pretreatment – 0.01 mg/kg IV for adults with bradycardia secondary to repeat dosing of succinylcholine. Pediatric 0.02 mg/kg IV, minimum dose 0.1 mg. Not recommended as a routine.
In general, dosing of atropine can be repeated every 5 minutes until a maximum of 0.04 mg/kg is reached. Dosing in adults to greater than 0.5 mg and slow IV pushes have been associated with paradoxical bradycardia (though likely transient) and ventricular fibrillation (VF).
- If damage to the heart’s electrical system causes your heart to beat too slowly, you will probably need to have a pacemaker. A pacemaker is an implanted device that helps correct the slow heart rate.
- If another medical problem, such as hypothyroidism or an electrolyte imbalance, is causing a slow heart rate, treating that problem may cure the bradycardia.
- If a medicine is causing your heart to beat too slowly, your doctor may adjust the dose or prescribe a different medicine.
The most effective way to prevent bradycardia is to reduce your risk of developing heart disease. If you already have heart disease, monitor it and follow your treatment plan to lower your risk of bradycardia.
Prevent heart disease
Treat or eliminate risk factors that may lead to heart disease. Take the following steps
- Exercise and eat a healthy diet – Live a heart-healthy lifestyle by exercising regularly and eating a healthy, low-fat, low-salt, low-sugar diet that’s rich in fruits, vegetables, and whole grains. Having a heart-healthy eating plan that includes vegetables, fruits, nuts, beans, lean meat, fish, and whole grains. Limit alcohol, sodium, and sugar.
- Maintain a healthy weight –Being overweight increases your risk of developing heart disease.
- Keep blood pressure and cholesterol under control – Make lifestyle changes and take medications as prescribed to correct high blood pressure (hypertension) or high cholesterol.
- Don’t smoke – If you smoke and can’t quit on your own, talk to your doctor about strategies or programs to help you break a smoking habit.
- If you drink, do so in moderation – For healthy adults, that means up to one drink a day for women of all ages and men older than age 65, and up to two drinks a day for men age 65 and younger. Ask your doctor if your condition means you should avoid alcohol. If you can’t control your alcohol use, talk to your doctor about a program to quit drinking and manage other behaviors related to alcohol abuse.
- Don’t use recreational drugs – If you’re using drugs, talk to your physician about treatment programs.
- Exercise and eat healthy – Get regular physical activity and eat a low-fat diet rich in fruits, vegetables and whole grains.
- Get regular checkups – Have regular physicals and, if you experience new or changing symptoms, see your physician.
- Keep blood pressure and cholesterol under control – Make the lifestyle changes listed above and take medications as prescribed.
- Monitor and treat existing heart disease – Understand your treatment plan. Take medications as directed. And report new or worsening symptoms immediately.
- Manage stress – Avoid unnecessary stress and learn coping techniques to handle normal stress in a healthy way.
- Being active on most – if not all, days of the week. Your doctor can tell you what level of exercise is safe for you.
- Losing weight if you need to – and staying at a healthy weight.
- Managing other health problems – such as high blood pressure, high cholesterol, and diabetes.
- Go to scheduled checkups – Have regular physical exams and report signs or symptoms to your doctor.
Monitor and treat existing heart disease
If you already have heart disease, there are steps you can take to lower your risk of developing bradycardia or another heart rhythm disorder:
- Follow the plan – Be sure you understand your treatment plan and take all medications as prescribed.
- Report changes immediately – If your symptoms change or worsen or you develop new symptoms, tell your doctor immediately.
Risk factors that could contribute to bradycardia include:
- Age – Men and women age 65 and older are most likely to develop a slow heart rate that needs treatment.
- Congenital heart defect – Problems with the structure or function of the heart present at birth can cause a slow heart rate.
- Electrolyte imbalance – Any abnormality in the body’s mineral balance – including calcium, chloride, magnesium, phosphate, potassium, and sodium can lead to a slow or irregular heart rate.
- Infection of the heart – Certain bacteria, viruses, and parasites can infect the heart muscle, causing inflammation and damage leading to an irregular heart rate.
- Previous heart attacks – Heart attacks can weaken the heart muscle or cause problems with its electrical system.
- Low thyroid – An abnormally low level of thyroid hormones can cause a slow heart rate.
- Medications for other heart problems – Some medications for treating high blood pressure or other heart conditions like beta-blockers, antiarrhythmics and digoxin (for heart failure) can cause bradycardia.
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