Sinus Node Dysfunction Workup

Because hypothyroidism and electrolyte imbalances can contribute to SND, thyroid function test and serum electrolyte test (Na⁺, K⁺ and Ca²⁺) can be useful. Infiltrative cardiomyopathies (amyloid, sarcoid, and the others mentioned above) can present with evidence of diffuse conduction system disease, but screening is typically reserved for those where specific clinical factors suggest the diagnosis.

• No specific imaging studies are required in the initial workup of SND.
• An echocardiogram should be considered because it can document the presence of underlying valvular or ischemic heart disease and may suggest the diagnosis of amyloid when diffuse conduction system findings are present.

Electrocardiograph recording studies

The electrocardiograph recording is the most important method for the diagnosis of SND. Depending on the level of clinical suspicion, the frequency of symptoms and their symptom relationship to exertion, the following methods are often used in combination to establish the diagnosis. Because SND symptoms are often intermittent, long term monitoring with an event recorder or implantable loop recorder can be very useful.

• ECG - 12-lead ECG recording
• Holter - 24-48 hours continuous 3-lead ECG recording
• Event recorder study – Symptom and autoalgorithm-triggered 1-lead ECG recording for 2-3 months or longer
• Implantable loop recorder study - Predefined parameters that trigger single-lead electrocardiograph recordings for up to 3 years duration (These parameters may include bradycardia, tachycardia, pauses, episodes of atrial fibrillation, and symptoms, usually indicated with an external triggering device.)
• Exercise testing - ECG recordings during exercise to evaluate maximal heart rate at peak exercise

The ECG criteria for SND diagnosis include the following:

• Inappropriate sinus bradycardia: The arbitrary cutoff for a low sinus rate in a person at rest but awake is usually defined as < 55-60 bpm. However, a study in 500 healthy subjects suggests the low afternoon sinus rate for men and women should be around 46 and 51 bpm, respectively.^[16] In the 2008 guidelines, pacemaker therapy is a Class IIb indication for patients with minimal symptoms and a chronic heart rate of less than 40 bpm while awake.^[2]
• Sinus pause or arrest

  • Sinus pause or arrest is defined as absence of sinus P waves on the ECG for more than 2 seconds due to a lack of sinus nodal pacemaker activity. The duration of the pauses should have no arithmetical relationship to the baseline sinus rate (ie, the P-P interval should not be an interval of the pause), otherwise the diagnosis of sinoatrial exit block should be considered. Symptomatic long sinus pauses or arrests in patients with SND often occur after the termination of atrial fibrillation or atrial flutter.
  • A sinus pause of 2 seconds is not unusual in a healthy person.^[17] However, a sinus pause of more than 3 seconds is very uncommon except under certain conditions, such as sleep apnea, hypervagotonia state, or seizure activity.^[17]
• Sinoatrial exit block (SA exit block)

  • First-degree SA exit block reflects a conduction delay between the sinus node and atrium that cannot be recognized on regular ECG recordings.
  • Second-degree SA exit block reflects intermittent conduction block between the sinus node and atrium. It has 2 classic types and likely some atypical types. Only the classic types can be recognized on regular ECG recordings.
  • Type I (Wenckebach type) is manifested as group beating, which is progressive shortening of the P-P intervals, and then a pause that is less than twice the shortest P-P interval.
  • Type II is manifested as a pause that is a multiple of the baseline sinus P-P interval.
  • Third-degree SA exit block reflects complete conduction block from sinus node to atrium. It cannot be definitely distinguished from sinus arrest on regular ECG recordings.
• Chronotropic incompetence: Chronotropic incompetence is defined as failure to achieve 70-80% of maximal predicted heart rate (maximal predicted heart rate = 220-age) at peak exercise. The clinical value of this definition, however, has not been well validated.^[18] The peak exercise heart rate can be influenced by multiple factors.

Pharmacological stimulation tests

Due to their moderate sensitivity and specificity for SND diagnosis, intrinsic heart rate and atropine stimulation tests are occasionally used as accessory tests in selected patients (such as patients with suspected hypervagotonia). The value of isoproterenol, propranolol, and adenosine stimulation tests in SND diagnosis is more controversial.

• Intrinsic heart rate (IHR): Atropine (0.04 mg/kg) and propranolol (0.2 mg/kg) have been used to pharmacologically denervate the sinus node, which is then followed 5-20 minutes later by an evaluation of its IHR. The IHR in a healthy person is approximately equal to 117.2 – [0.53 x Age].^[19] Intrinsic SND is presumed to be present if the sinus rate after medications is below calculated IHR. Patients with mild SND may have a normal or exacerbated response. This test is probably helpful in patients with sinus bradycardia due to suspected hypervagotonia^[20] , in whom the IHR is expected to be normal.
• Atropine test: Atropine alone (up to 0.04 mg/kg) may provide as much information as the combination of atropine (0.04 mg/kg) and propranolol (0.2 mg/kg).^[21] Atropine (1-3 mg) is the most commonly used agent to assess the parasympathetic tone. A normal response is an increase in sinus rate above 90 bpm or an increase of more than 25% above the baseline sinus rate.^[18] Patients with symptomatic sinus node dysfunction usually demonstrate a decrease in IHR. However, patients with only mild SND may have a normal or exacerbated response to atropine.

Electrophysiological study (EPS)

Due to its moderate sensitivity and specificity for SND diagnosis, EPS is only occasionally used when other tests yield ambiguous results.^[21]

Corrected sinus node recovery time (CSNRT): CSNRT is the most commonly used method in EPS for diagnosing SND.

At a site close to the sinus node, sinus rhythm is overdrive paced at different rates for one minute. The overdrive pacing rate starts at a rate just above basal sinus rate, and increases by 10-20 bpm with each successive overdrive pacing cycle until the overdrive pacing rate reaches 200 bpm. At each overdrive pacing cycle, the sinus node recovery time is measured as the time from the last paced atrial beat to the first post-paced spontaneous sinus beat. The corrected sinus node recovery time at each overdrive pacing cycle is calculated as follows:

Corrected sinus node recovery time = Sinus node recovery time – Basal sinus cycle length before the overdrive pacing

For normal sinus nodes, the maximal corrected sinus node recovery time should be less than 450 ms.^[21] In the THEOPACE study, patients with syncope and CSNRT ≥800 ms treated with permanent pacemaker had a 25% reduction of syncope over 4 years as compared with patients treated with theophylline or no therapy.^[22]

Sinus node conduction time (SNCT) is another EPS parameter used in evaluating sinus node function.^[21] It is used less frequently than CSNRT.

Pacemaker implant