Sick Sinus Syndrome
William A. Scott
Sick sinus syndrome is a clinical entity that has been associated with a variety of arrhyth- mias. One of the earliest descriptions by Lown stated: “a defect in the elaboration or con- duction of sinus impulses characterized by chaotic atrial activity, changing P-wave con- tour, bradycardia, interspersed with multiple and recurrent ectopic beats with runs of atrial and nodal tachycardia.” While there are nu- merous etiologies described in the literature, sinus node dysfunction in pediatric patients most often occurs secondary to injury to the node itself, its arterial supply and/or its auto- nomic innervation in the course of cardiac sur- gical interventions. This injury is usually not limited to the sinoatrial node, and abnormali- ties in atrial automaticity and conduction are included in the sick sinus syndrome (Table 1).
There is evidence that the abnormal variation in heart rate has a destabilizing effect on the atrial tissue, contributing to the progression of the disease. While not specifically part of si- nus node disease, these coexisting atrial and, to a lesser degree, atrioventricular conduc- tion disturbances, support the concept that the pathology affecting the sinus node is diffuse.
Conditions associated with a high preva- lence of sick sinus syndrome include atrial repair of d-transposition of the great arter- ies, complete repair for anomalous pulmonary venous drainage and atrial septal defect, and
single ventricle palliation with the Fontan op- eration, although almost any open heart repair may result in sinus node impairment. Con- genital sinus node abnormalities are less fre- quent. Sinus node malformation is associated with left atrial isomerism and sinus node dys- function may coexist with congenital com- plete heart block. Familial clustering has been reported in the absence of structural heart dis- ease. Extrinsic causes of sinus node dysfunc- tion include autonomic imbalance and med- ications. Heavily conditioned athletes have bradycardia and sinus pauses of greater than 2 seconds due to prominent vagal influence.
Recently, sleep apnea has been related to si- nus node dysfunction. Many antiarrhythmia medications can further impair the sinus node function, particularly for patients with pre- existing abnormalities.
ECG CHARACTERISTICS
The ECG characteristics of sick sinus syndrome are variable and can change within a single patient. Not all patients will manifest all of the electrocardiographic findings noted be- low. Potential sources of electrocardiographic data include: standard 12–15 lead electrocar- diograms, Holter monitoring, event recorders, and rhythm strips from physiologic monitors.
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TABLE 1. Sick Sinus Syndrome
Electrocardiographic Manifestations of Sick Sinus Syndrome
Sinus Bradycardia Extreme Sinus Arrhythmia Tachycardia-Bradycardia Syndrome Sinoatrial Exit Block
Sinus Pause/Sinus Arrest Sinoatrial Reentrant Tachycardia
Electrocardiographic Findings Associated with Sick Sinus Syndrome
Intra-atrial Reentrant Tachycardia (IART) Atrial Fibrillation
AV Nodal Dysfunction
Clear documentation is not always present in short-term recordings. Multiple leads should be recorded to detail P-wave morphology.
Long-term recordings with Holter pro- vide an overview of overall heart rate variation (Figure 1) and the prevalence of abnormally slow and fast rhythms. Ideally three orthog- onal leads are recorded to optimize P-wave recognition (i.e., Lead I, Lead II or III, and a precordial lead). Event recorders, particularly those with continuous storage capabilities, allow for correlation of symptoms to electro- cardiographic changes. Newer devices have programmable parameters for automated recording of tachycardia and bradycardia episodes. An implantable device is available for special circumstances that preclude wear- ing an external monitor.
Exercise testing is useful to assess the chronotropic response. All children and ado- lescents should be able to attain a heart rate of
FIGURE 1. Full disclosure printout from a Holter monitor. Individual complexes are too small for detailed evaluation, but abnormal pattern of heart rate variation is apparent.
180 bpm. Patients with sinus node dysfunc- tion also may have exaggerated slowing of heart rate or pauses in the recovery period.
Sinus Bradycardia
Bradycardia is the most common feature of sinus node dysfunction. Bradycardia may be sustained or paroxysmal. Escape rhythms (Figure 2) arise from the atrium, atrioventricu- lar (AV) node and ventricles, but these tissues are often dysfunctional and the escape rates are frequently lower than expected for age.
Healthy children may have brief periods of sinus bradycardia with atrial or junctional es- cape rhythm in the absence of any sinus node pathology. The normal heart rate is age de- pendent. The lowest values observed during Holter recording are less than those from ECG recording.
Recording artifacts and other rhythm abnormalities should be excluded. An abnor- mally low heart rate may be due to second- or third-degree AV block, as well as atrial and junctional extrasystoles. A sudden loss of sig- nal may appear similar to a sinus pause. There are often clues to this type of artifact includ- ing resumption of recording with a T-wave or a different time of drop out on another channel (Figure 3). Older Holter recorders using a tape drive mechanism can suddenly speed or slow, mimicking bradycardia or tachycardia.
With this type of artifact, all components of
the recording are expanded or compressed
(Figure 4).
FIGURE 2. Sinus bradycardia with alternating atrial and junctional escape rhythms in a patient 15 years after Mustard repair of d-transposition of the great arteries.
Extreme Sinus Arrhythmia
Sinus arrhythmia is a normal pattern of heart rate acceleration and decelera- tion present in most children. A signif- icant component of this variation is due
FIGURE 3. Simultaneous lead I (upper panel) and lead III (lower panel) from Holter recording. Apparent sinus pause is actually due to artifact. Signal loss is not simultaneous on both channels and lead I resumes rhythm with a T-wave, which is not possible.
to respiratory influences on the autonomic
nervous system. Abnormal or extreme si-
nus arrhythmia is defined as greater than
100% variation in PP intervals. Sinus node
exit block may also cause variation of this
magnitude.
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FIGURE 4. Simultaneous lead V1 (upper) and V5 (lower) from Holter recording. Apparent bradycardia and tachy- cardia due to artifact from changing tape speed. Note that all components of the rhythm (P-wave, QRS complex, and T-wave) are compressed with tachycardia.
Tachycardia–Bradycardia
The tachycardia–bradycardia syndrome is diagnosed in the presence of recurrent prolonged pauses or sustained bradycardia following paroxysms of tachycardia. The tachycardia causes exaggerated overdrive suppression of already impaired automaticity, worsening the bradycardia (Figure 5).
Sinoatrial Exit Block
Sinoatrial exit block is failure of an im- pulse generated in the sinus node to propagate normally to the atrium. The degree of block is classified just as for the AV node. A di- rect recording of the sinus node electrogram is necessary to diagnose first degree sinoa- trial block. The pattern of PP intervals prior to a pause can be used to infer second-degree block. Mobitz I sinoatrial block (Figure 6) is
FIGURE 5. Tachycardia–bradycardia with junctional escape in a 2-year-old.
manifest by a gradual shortening of PP in- tervals followed by a pause of less than two times the resting cycle length. Mobitz II is pre- sumed to be the mechanism when a sudden pause of two times the resting cycle length is encountered (Figure 7). Both of these are difficult to recognize in the presence of sinus arrhythmia and both patterns may be due to abnormal impulse formation in the sinus node.
Neither of these conduction abnormalities can be absolutely proven without direct recording of the sinus node electrogram. Complete or third-degree sinoatrial block is indistinguish- able from sinus node arrest on the surface electrocardiogram.
Sinus Pauses and Sinus Arrest
Prolonged pauses are often present due to
either sinoatrial node exit block or sinus arrest
(Figure 8). Sinus pauses of less than 2 seconds
FIGURE 6. Lead I recording suggestive of Mobitz I type sinoatrial exit block. There is a gradual shortening of PP intervals, with a consistent P-wave morphology, followed by a pause of less than two times the resting cycle length.
Validation of this phenomenon requires invasive recording of the sinus node electrogram.
FIGURE 7. Simultaneous lead I and II recording suggestive of Mobitz II type sinoatrial exit block. There is a pause with a PP interval that is twice the resting cycle length. Validation of this phenomenon requires invasive recording of the sinus node electrogram.
are normal in young children and adolescents.
A non-sinus rhythm at a rate lower than ex- pected for sinus rhythm is referred to as an escape rhythm and is suggestive of sinus node arrest.
Sinoatrial Node Reentrant Tachycardia
Sinoatrial reentrant tachycardia is uncommon. It results from abnormal con- duction within the sinus node or immediate perinodal tissue. The P-waves are identical to those seen with sinus rhythm, but associated
FIGURE 8. Sinus arrest during a breath-holding spell. This represents transient sinus node dysfunction due to a sudden increase in parasympathetic activity.
with a paroxysmal increase and decrease in rate. This rhythm cannot be distinguished from a focal atrial tachycardia in close proximity to the sinoatrial node.
Atrial Flutter/Fibrillation
Typical atrial flutter is uncommon with sinus node dysfunction. More commonly atrial muscle reentry or intra-atrial reentrant tachycardia (IART) is observed (Figure 9).
This arrhythmia most frequently occurs fol-
lowing surgical interventions for congenital
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FIGURE 9. Lead I (upper trace) and esophageal electrogram (lower trace) demonstrating intra-atrial reentrant tachy- cardia with 2:1 conduction. The arrhythmia is not apparent from surface ECG but esophageal recording shows atrial rate twice that of the ventricular rate.
heart disease. The rate is slower than that usu- ally ascribed to typical atrial flutter, usually around 180 to 300 bpm. There is an isoelec- tric interval between atrial activations, and the typical sawtooth pattern is not present, which confounds recognition of this rhythm. Unlike typical atrial flutter, 1:1 conduction to the ven- tricles is frequent. Most commonly 2:1 con- duction is observed, but again at a rate slower than typical for atrial flutter. Variable conduc- tion ratios may be misinterpreted as extrasys- toles. Atrial fibrillation is a late finding with disease progression.
ELECTROPHYSIOLOGIC FEATURES
Electrophysiologic studies have con- tributed greatly to the overall understanding of sinus node physiology. Typically these studies are conducted invasively with the placement of catheters through the vascular system for endocardial stimulation and recording. Lim- ited studies to assess sinus node function may also be performed with temporary epicar- dial or transvenous pacing wires (Figure 10),
esophageal pacing, and with a previously im- planted permanent pacemaker.
Presently, clinical history and non- invasive diagnostic studies primarily guide di- agnosis and management. Electrophysiologic study is not routinely performed solely for the diagnosis of sick sinus syndrome because it is invasive, has relatively low sensitivity and specificity, and noninvasive studies provide adequate information for management. Inva- sive electrophysiologic studies of sinus node function may be performed as an adjunct to hemodynamic study prior to surgical inter- vention or in association with interventional procedures such as radiofrequency ablation.
The primary measures of sinus node function are the sinoatrial conduction time and the corrected sinus node recovery time.
However, atrial refractoriness and intra-atrial conduction times are also prolonged with surgically-induced sick sinus syndrome.
Prolonged Sinoatrial Conduction Time
Sinoatrial conduction time is usually
measured by an indirect method (Chapter 3),
FIGURE 10. Recording of lead I (upper trace), atrial temporary pacing wire (middle trace), and ventricular pacing wire (lower trace). Patient has low amplitude atrial activity with first-degree AV block making rhythm determination difficult. Atrial wire electrogram consistent with sinus arrest or sustained sinoatrial exit block.
Strauss or Narula, which yields a total sinoa- trial conduction time (TSACT). The TSACT reflects conduction into and out of the sinus node. These conduction times are not equal;
therefore, it is more accurate to report a total sinoatrial conduction time rather than divide the TSACT by 2 and assume that the con- duction velocity in both directions is equiva- lent. Normal values for pediatric patients are
<200 msec. With the Strauss method, absence of a zone of reset suggests sinoatrial node en- trance block, which is abnormal as well. Sinus arrhythmia invalidates this measurement.
Directly recorded conduction times from the sinus node electrogram reflect only con- duction out of the sinus node and are usually shorter than the TSACT. As an independent measure of sinus node function, TSACT is relatively insensitive. Atropine has variable effects on TSACT in sinus node dysfunction.
In many patients the measurement normal- izes. Further prolongation of the TSACT is thought to be the result of depressed auto- maticity following enhanced conduction into the sinus node.
Prolonged Sinus Node Recovery Time The maximum corrected sinus node re- covery time (MCSNRT) (Chapter 3) is a measure of recovery of automaticity. Normal
values for MCSNRT are <250−270 msec for children, <445 msec for adolescents, and
<525 msec for adults. When expressed as a ratio, MCSNRT should be less than 150% of the sinus cycle length. At high rates, shorten- ing of the return cycle may occur due to reflex responses to hypotension, sinus node entrance block, or local release of catecholamines.
Abnormal results (Figure 11) may be sec- ondary to excess vagal tone and normalize following autonomic blockade whereas pa- tients with true dysfunction show no improve- ment following autonomic blockade. The specificity of this test may be enhanced by co- administration of disopyramide or autonomic blockade, but this has not been conclusively demonstrated in children. Other findings sug- gestive of sinus node dysfunction include sec- ondary pauses, and a prolonged total recovery time, defined as greater than 5–6 beats to re- turn to the resting cycle length. If MSCNRT occurs at long pacing cycle length, this sup- ports the diagnosis of sinus node disease.
TREATMENT
Prospective, randomized clinical trials
are not available to guide management in pe-
diatric patients. The presence of sick sinus
syndrome does not mandate therapy. Most
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