What Is Sinus Node Dysfunction? – Symptoms, Treatment

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What Is Sinus Node Dysfunction?/Sinus Node Dysfunction (SND) refers to a wide range of abnormalities involving sinus node impulse generation and propagation, leading to the inability of the sinus node to generate heart rates that are appropriate for the physiological needs. Causes of SND can be classified as intrinsic (secondary to a pathological condition involving the sinus node proper) or extrinsic (caused by depression of sinus node function by external factors such as drugs or autonomic influences).

The sinoatrial node (SA) is the default pacemaker and therefore a crucial component of the heart’s conduction system. It is located subepicardial and is crescent in shape. In an average adult, a sinoatrial node is 13.5 millimeters in length and is innervated by the vagus and sympathetic nerves. The sinoatrial nodal artery supplies blood to the sinoatrial node, it branches off the right coronary artery in 60% of cases, whereas in 40% of cases it comes off the left circumflex coronary artery. Sinus bradycardia is a cardiac rhythm with appropriate cardiac muscular depolarization initiating from the sinus node generating less than 60 beats per minute (bpm). The diagnosis of sinus bradycardia requires visualization of an electrocardiogram showing a normal sinus rhythm at a rate lower than 60 bpm. Where a normal sinus rhythm has the following criteria:

  • Regular rhythm, with a P wave before every QRS.
  • The p wave is upright in leads 1 and 2, P wave is biphasic in V1.
  • The maximum height of a P wave is less than or equal to 2.5 mm in leads 2 and 3.
  • The rate of the rhythm is between 60 bpm and 100 bpm.

Causes of Sinus Node Dysfunction

Sinus bradycardia has many intrinsic and extrinsic etiologies.

Both could result from abnormal mechanisms, including fibrosis, atherosclerosis, and inflammatory/infiltrative processes.

  • Sinus Node Fibrosis – Replacement of the sinus node tissue by fibrous tissue is the most common cause of sinus node dysfunction, the replacement can also include other parts of the conduction system, including the AV node.
  • Medication – Prescription medications can depress the sinus node function, potentially resulting in sinus node dysfunction include beta-blocker, non-dihydropyridine calcium channel blockers, digoxin, antiarrhythmic medications, and acetylcholinesterase inhibitors.
  • Infiltrative disease – The SA node tissue can be affected during the disease process of some of the infiltrative diseases such as amyloidosis, sarcoidosis, scleroderma, hemochromatosis, and pericarditis leading to sinus node dysfunction.
  • Ischemia – The sinus node is perfused by the sinoatrial nodal artery, which arises from the right coronary artery in 60 % of the time and from the left circumflex artery in 40 % of the time. Narrowing of this artery can lead to impairment of the sinus node function leading to sinus node dysfunction that can be potentially reversible. Almost all such cases are present in inferior myocardial infarction.
  • Familial – Sinus node dysfunction in rare cases can be the result of cardiac sodium channel mutations of SCN5A and HCN4 genes.
  • Miscellaneous – Other disorders that can rarely cause sinus node dysfunction to include hypothyroidism, hypothermia, and hypoxia.
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Inherent Etiologies

  • Chest trauma
  • Ischemic heart disease
  • Acute myocardial infarction
  • Acute and chronic coronary artery disease
  • Repair of congenital heart disease
  • Sick sinus syndrome
  • Radiation therapy
  • Amyloidosis
  • Pericarditis
  • Lyme disease
  • Rheumatic fever
  • Collagen vascular disease
  • Myocarditis
  • Neuromuscular disorder
  • X-linked muscular dystrophy
  • Familial disorder
  • Inherited channelopathy

Extrinsic Etiologies

  • Vasovagal simulation (endotracheal suctioning)
  • Carotid sinus hypersensitivity
  • Beta-blockers
  • Calcium channel blockers
  • Digoxin
  • Ivabradine
  • Clonidine
  • Reserpine
  • Adenosine
  • Cimetidine
  • Antiarrhythmic Class I to IV
  • Lithium
  • Amitriptyline
  • Narcotics
  • Cannabinoids
  • Hypothyroidism
  • Sleep apnea
  • Hypoxia
  • Intracranial hypertension
  • Hyperkalemia
  • Anorexia nervosa

Diagnosis of Sinus Node Dysfunction

Laboratory studies that should be ordered include:

  • Glucose level
  • Electrolytes
  • Calcium, magnesium
  • Thyroid function
  • Troponin
  • Toxicological drug screen

A 12-lead ECG is necessary to make the diagnosis.

Sick sinus syndrome is defined by the presence of ECG findings and symptoms together, ECG finding alone, especially sinus bradycardia, does not indicate the presence of sick sinus syndrome:

ECG Findings

  • Periods of inappropriate and often severe sinus bradycardia.
  • Sinus pauses, sinus arrests and sinus exits block that can happen with and without appropriate escape rhythm.
  • Alternating tachycardia and bradycardia referred to as a tachy-Brady syndrome, which could also be associated with other supraventricular tachycardias.

The key to diagnosing sinus node dysfunction is to establish a correlation between the patient symptoms and the ECG findings at the time of symptoms. It is also beneficial to review previous ECG tracing to check for any changes in the rhythm upon the start of the symptoms.

Exercise Stress Testing

  • If the diagnosis could not be made based on history, and ECG then exercise stress testing is necessary. Things to look for is the failure of appropriate chronotropic response to exercise, defined as less than 80 percent of the predictable heart rate response to exercise. Also, it will exclude myocardial ischemia and help to program the devices for patients who ultimately receive a permanent pacemaker..

Ambulatory ECG monitoring

  • If all the above failed in making the diagnosis of sinus node dysfunction, then an ambulatory ECG monitoring should be considered. In one study that included 55 symptomatic patients, 24-hour Holter monitor tracking detected the underlying arrhythmia causing the symptoms in 30 patients (55 %).
  • However, longer periods of monitoring might be necessary for patients whom their symptoms are not as frequent. Cardiac event monitors have been shown to be more effective than Holter monitors in detecting the cause of palpitation. In a study involving 43 patients with palpitation, event monitors were twice as likely to provide a diagnostic rhythm strip ECG during symptoms as 48 hours Holter monitors. (67% vs 35%)..
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Review of potentially reversible causes

  • Patients who are taking medications that can be contributing to their symptoms (beta blocker, calcium channel blockers, digoxin, antiarrhythmic) should be monitored off of these medications and on the ECG monitor to assess for symptoms reversibility as well as the resolution of the ECG findings.

Treatment of Sinus Node Dysfunction

The first step in the management of the sinus node dysfunction is to determine whether the patient is hemodynamically stable or no.

Unstable patients

  • Patients with sinus node dysfunction are rarely unstable for prolonged periods of time, however, if that was the case then one should follow the ACLS protocol for symptomatic bradycardia. Symptoms include altered mental status, syncope, ischemic chest pain, and hemodynamic instability.
  • Atropine should be tried first with a dose of 0.5 mg that can be repeated every 3 to 5 minutes with a total dose of 3 mg. However, treatment with atropine should not delay transcutaneous pacing or chronotropic agents.
  • Chronotropic agent infusion should be tried if atropine failure which includes epinephrine, dopamine or isoproterenol infusions.
  • Clinicians should initiate transcutaneous pacing in patients who are hemodynamically unstable but should only be a bridge for transvenous pacing.

Stable Patients

  • As discussed above, one should look for the possibility of a reversible cause first; If an offending medication was identified and could be removed or replaced, the patient should undergo monitoring for the reversibility of symptoms and ECG findings. If the offending medications cannot be removed.
  • Then the patient should be managed the same way as if there is no reversible cause. The next step involves determining whether the patient is symptomatic or asymptomatic.

Asymptomatic Patient

  • Observation is recommended in asymptomatic patients, there are no society guidelines that recommend permanent pacemaker for asymptomatic patients with bradycardia or pauses.

Symptomatic Patients

  • Symptomatic sinus node dysfunction patients will require the placement of a permanent pacemaker. Usually, either single chamber atrial pacemaker (AAI) or dual-chamber pacemaker (DDD) should be used. In patients where there are no AV conduction abnormalities, a single chamber atrial pacemaker (AAI) is a reasonable option, however, patients with AV conduction delay or a branch block would benefit from dual-chamber pacemaker (DDD).
  • Discussing the types and modes of pacemaker is beyond the scope of this activity, however, one of the largest trials that looked into single-lead atrial pacing (AAI) vs. dual-chamber pacing (DDD) in patients with sinus node dysfunction is the DANPACE trail which included 1415 patients with normal AV conduction and the mean follow up was 5.4 years. There was no difference in all-cause mortality between the two groups, however, Single lead atrial pacing was associated with more incidents of paroxysmal atrial fibrillation and a two-fold increase in pacemaker reoperation.
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Anticoagulation

  • Patients with sinus node dysfunction might have episodes of atrial fibrillation/flutter especially patients with the techy-Brady syndrome. Also, patients who received a permanent pacemaker are at a higher risk of developing atrial fibrillation, thus, frequent device interrogation is recommended. Patients with documented atrial fibrillation should be risk-stratified for stroke and bleeding and an informed decision should be made whether to use anticoagulation or no.

Lifestyle and home remedies

You may not necessarily prevent sick sinus syndrome, but you can take steps to keep your heart as healthy as possible and lower your risk of cardiovascular disease:

  • Exercise and eat a healthy diet – Live a heart-healthy lifestyle by exercising regularly. Eat a diet with generous portions of nonstarchy vegetables, fruit, and whole grains, and modest portions of fish, lean meats, poultry and dairy.
  • Maintain a healthy weight – Being overweight increases your risk of developing heart disease. Ask your doctor what your goal weight should be.
  • 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 some conditions, it’s recommended that you completely avoid alcohol. Ask your doctor for advice specific to your condition. 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 use.
  • Don’t use illegal drugs – Talk to your doctor about an appropriate program if you need help ending illegal drug use.
  • Control stress – Avoid unnecessary stress and learn coping techniques to handle normal stress in a healthy way.
  • Go to scheduled checkups – Have regular physical exams and report any signs or symptoms to your doctor.

References

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