The sinoatrial node (SA) node is the dominant pacemaker of the heart. It is a complex compact region in the junction of superior vena cava and the right atrium. The SA node depolarizes and produces action potential almost synchronously.
Sinus node dysfunction, also known by its historical name sick sinus syndrome, is the inability of the SA node to produce an adequate heart rate that meets the physiologic needs of the individual. Per definition, sinus node dysfunction should include symptoms and EKG findings simultaneously such as sinus bradycardia, sinus pauses or arrest, sinus exit blocks, or chronotropic incompetence (inadequate heart rate response to activity). Or the failure of the return of SA nodal activity after electrical cardioversion. Sinus node dysfunction can be associated with atrial tachycardia as part of the tachycardia-bradycardia syndrome.
Sinus node dysfunction results from the abnormal automaticity, conduction, or both of the sinoatrial node and surrounding tissues. 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.
The study of epidemiology is difficult in sinus node dysfunction due to the variable disease manifestations and ECG findings. However, according to a pooled analysis from two extensive studies that included 20572 patients with a mean age of 59 years old, 43% were males, who researchers followed for an average of 17 years. The incidence of sinus node dysfunction was 0.8 per 1000 person-years. Age showed the greatest significance as a risk factor for having sinus node dysfunction.
To define sinus node dysfunction, both of the symptoms and the EKG findings has to be present. The symptoms of sinus node dysfunction are non-specific and include lightheadedness, fatigue, presyncope, and syncope. For patients who have tachy-brady syndrome, they can present with palpitations. Patients who have other cardiac comorbidities such as coronary artery disease can present with increasing of their baseline ischemic symptoms such as chest pain and dyspnea.
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:
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%)..
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.
The first step in the management of the sinus node dysfunction is to determine whether the patient is hemodynamically stable or no.
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.
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.
Observation is recommended in asymptomatic patients, there are no society guidelines that recommend permanent pacemaker for asymptomatic patients with bradycardia or pauses.
Symptomatic sinus node dysfunction patients will require 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.
Patients with sinus node dysfunction might have episodes of atrial fibrillation/flutter especially patients with the tachy-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.
Other etiologies of bradycardia and syncope should be part of the differential diagnosis of sinus node dysfunction such as carotid sinus hypersensitivity, neurocardiogenic syncope with a predominant cardioinhibitory component, sinus bradycardia in conditioned athletes (although does not cause syncope but they do have sinus pauses). Sinus node dysfunction and carotid sinus hypersensitivity can often occur together.
Sinus node dysfunction is a progressive noncurable but manageable disease. In a study that followed 52 patients with sinus node dysfunction presenting with sinus bradycardia associated with a sinoatrial block or sinoatrial arrest, it took an average of 13 years to progress to complete the sinoatrial arrest and an escape rhythm. It is unclear if sinus node dysfunction is associated with increased mortality as most patients with sinus node dysfunction have other cardiovascular comorbidities. In a study of 19000 from two cohorts followed for an average of 17 years, 213 persons developed sinus node dysfunction (0.6 events per 1000 person a year). While the development of sinus node dysfunction carried an association with increased mortality, it became attenuated when adjusted for incident cardiovascular disease. Sinus node dysfunction correlates with increased morbidity, but it is unclear if it affects mortality.
Sinus node dysfunction is a noncurable progressive disease. Complications of sinus node dysfunction without a pacemaker include hypotension and syncope.
Like every other disease, patient education plays a vital role in disease management. Since permanent pacemaker placement is the usual mainstay in this disease, patients should receive training about the importance of maintaining regular follow up with their electrophysiologist to ensure proper pacemaker function with frequent interrogation. Patients also should be educated to recognize the signs of pacemaker malfunction, which include the recurrence of symptoms that they initially presented with such as syncope lightheadedness and fatigue.
Sinus node dysfunction previously referred to as sick sinus syndrome is a progressive disease that requires both symptoms and ECG findings to diagnose. The mainstay of this disease management is a permanent pacemaker placement. When sinus node dysfunction is associated with atrial arrhythmia, the syndrome then will be called tachycardia-bradycardia syndrome.
A team approach is necessary for managing patients with sinus node dysfunction. A cardiologist, one who specializes in electrophysiology should be a key member in the team care as almost all patients would require permeant pacemaker placement. (Level I) A primary care provider should also follow the patient along with the cardiologist especially considering that patients with sinus node dysfunction often have other cardiovascular comorbidities that should be managed by the primary care as well as the cardiologist. A specialized nurse also plays a significant role in educating the patient about their disease as well as the functions of their pacemaker.
In summary, the diagnosis and management of sinus node dysfunction require an interprofessional team approach, including physicians, specialists, specialty-trained nurses, and pharmacists, all collaborating across disciplines to achieve optimal patient results. [Level V]
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