Pediatric Sinus Node Dysfunction Workup

• Echocardiography: This study is useful for the evaluation of ventricular function and in patients with coexistent CHD for the assessment of associated anatomic and hemodynamic abnormalities.

• Electrocardiography: Impulses originate from the sinus node (SN), with a P-wave axis between +0º and +90º. A surface ECG may reveal the following:

  • Moderate-to-severe bradycardia (for age) occurs.
  • Severe sinus arrhythmia is indicated by variation from the R-R interval by 100% or more. Bradycardia may alternate with paroxysmal atrial tachyarrhythmias (ie, bradycardia-tachycardia syndrome).
  • Sinus pause or arrest occurs when the SN fails to generate an impulse.
  • Second-degree sinoatrial (SA) block type I is known as Wenckebach SA block and occurs when prolonged conduction of the SN impulse through the atrial tissue is present without actual block in the AV node (all QRS complexes are preceded by P waves). The P-P intervals shorten until block occurs (ie, while the SN-to-SN interval is constant, the SN–to–high right atrium [HRA] interval lengthens until an SN impulse is not followed by a P wave). The surface ECG shows progressively shortening P-P intervals until the SN impulse is dropped; it also shows P-P intervals that are less than twice the preceding (normal) P-P intervals.
  • Long pauses follow PACs. Sinoatrial conduction time can be directly measured with EP testing when PACs are present. In sinus node dysfunction (SND), the prolonged P-P interval is not a multiple of the sinus (normal) P-P interval. Pauses in SND occur at the end of lengthening P-P intervals rather than at the end of shortening P-P intervals (which is observed in second-degree SN exit block type I).
  • In second-degree sinoatrial block type II, the long P-P interval is a multiple of the sinus P-P interval.
  • In third-degree SN exit block, impulse generation in the SN is normal; however, no conduction to the RA occurs. Diagnosis is confirmed only by EP (intracardiac) study. An atrial, junctional, or ventricular escape rhythm is present.
  • Escape rhythm at slow rates occurs after a prolonged sinus arrest.
    • The escape rhythm may originate in the atria, such as an ectopic right or left atrial rhythm, or it may originate from multiple foci in the atria as is observed in wandering atrial pacemaker, in which the P-wave axis changes on the same ECG recording.
    • An escape rhythm may also originate below the atria at the His-Purkinje junction (ie, junctional escape rhythm, with a rate of 60-80 min in infants and 50-70 min in children) or lower if originating in the ventricles (ie, ventricular escape rhythm).
    • The further below the atria the escape rhythm originates, the slower the rate.
    • Escape rhythms, which are those that occur by default, should be distinguished from usurpation rhythms, which are those that occur because of increased automaticity from other pacemakers that fire at a faster rate than the SN.
  • Bradyarrhythmias-tachyarrhythmias occur when bradycardia and tachycardia alternate. The bradycardia may originate in the sinus, atria, AV junction, or ventricle, whereas the tachycardia is usually caused by atrial flutter or fibrillation and is less commonly caused by reentrant supraventricular tachycardia (SVT) in the SN or atrial muscle.
  • See ECG images belowThis 12-lead ECG is from an asymptomatic 10 year-old girl, which was brought to our attention because of the irregularity of the P-P intervals. This ECG shows sinus arrhythmia at a rate of 65-75 beats per minute. The P waves all originate from the sinus node because they have a positive axis (upright) in leads I, II, and aVF. The PR interval is 104 milliseconds, and the QRS is narrow at 86 milliseconds, with a normal axis of 64°. The corrected QT (QTc) interval measures 402 milliseconds. Therefore, this is a normal ECG. Below is an ECG of a 2-year-old girl who was referred to the clinic by the pediatrician for evaluation of a heart murmur. This ECG shows atrial rhythm originating most likely from the lower left atrium (P waves are inverted in lead I and are positive in II and aVF, with a frontal axis of 124°). The PR interval measures 113 milliseconds, and the QRS is narrow at 90 milliseconds. Right ventricular conduction delay is shown, which is best seen in the precordial leads V1 and V2. The QRS frontal axis shows right axis deviation (reference range for a 2-year-old child is 0-110°). The patient does not have right ventricular hypertrophy by voltage criteria. The inverted T waves in V1 are a normal finding at this age. An echocardiogram showed a moderately sized atrial septal defect. Nonsinus atrial rhythm is not a synonym of sinus node dysfunction. This is a 12-lead ECG from a 12-year-old boy with history of syncope. This patient was healthy until 1 month earlier, when he started to experience episodes of lightheadedness. The ECG shows sinus arrhythmia (bradycardia) at a rate of 50-79 beats per minute with a PR interval of 136 milliseconds. Two junctional escape beats are present after a prolonged pause. The QRS is narrow at 85 milliseconds with a normal frontal axis of 70°. The corrected QT interval (QTc) is 411 milliseconds. A later electrophysiologic (EP) study showed prolonged sinus node recovery time (SNRT) and sinoatrial conduction time (SACT). Because of the patient's symptoms and his sinus node dysfunction, he received an atrial pacemaker. If this 12-lead ECG had been recorded from an asymptomatic patient, the findings would be considered within normal limits and no further workup would be indicated. In this case, the lightheadedness and, ultimately, the syncope define sick sinus syndrome, with the patient requiring pacemaker therapy.
• Holter monitoring

  • Recording the ECG for 24-48 hours is useful to assess SND related to the previously explained ECG findings that may be present.
  • The specificity of a direct observation of spontaneous (ie, not provoked by EP study) SND is 100%, and an EP study is not required. Therefore, cardiac monitoring and not EP study is the method of choice to assess SND.
  • A 24-hour Holter study also has the advantage of revealing whether SND produces symptoms such as dizziness, presyncope, or syncope, which cannot be determined during an EP study because the patient is heavily sedated. Therefore, a 24-hour Holter study can help decide if pacemaker therapy is required.
• Exercise stress test

  • Because the SN usually responds to autonomic nervous system input, exercise increases the heart rate in response to increased sympathetic tone.
  • Patients with SND usually have a blunted response. Therefore, an exercise stress test can determine whether chronotropic incompetence is present.
• Autonomic tone assessment

  • The influence of autonomic tone on the SN can be studied by measuring heart rate and assessing heart rate variability responses to changes in autonomic tone. This is done by evaluating the response of the SN to atropine, isoproterenol, and propranolol.
  • Patients with SND usually exhibit a blunted response to atropine (ie, failure to increase the heart rate 20-50% above the control rate).
  • Patients with SND who are given isoproterenol usually fail to produce an SN acceleration of at least 25% more than the basal rate. However, their response to this medication is occasionally normal.
  • Patients with SND respond to propranolol in the same way as patients with normal SNs (ie, increase of 12-22% in sinus cycle length). This suggests that sympathetic response in SND is normal.
  • Pharmacologic denervation with atropine and propranolol is used to determine intrinsic heart rate (IHR). Patients with SND have no increase in IHR, similar to patients with intact SNs. IHR depends on the patient's age and is calculated using the following equation in patients aged 15-70 years: IHR = 117.2 - (0.57 X age) bpm.
• Transesophageal atrial pacing: Esophageal EP study constitutes a relatively safe and inexpensive method to detect SND by determining SN recovery time in patients who present with dizziness or syncope and palpitations.

EP studies are indicated in patients with signs of bradyarrhythmias (mainly syncope) in whom bradycardia could not be documented during Holter monitoring.

• SN recovery time

  • EP studies can document SND when studying SN automaticity by directly recording its electrical activity.
  • One EP catheter, which has 2 proximal electrodes that record the HRA electrogram and 2 distal electrodes to pace the HRA near the SN, is positioned in the RA.
  • A second EP catheter, which is used to record low RA (LRA) electrical activity, is positioned across the tricuspid valve.
  • Measurement of SNRT is achieved by pacing the atrium. Pacing should be performed in the HRA near the SN at the junction of the SVC and the RA for 4 -6 trials of 30 seconds each. Each trial should use successively shorter pacing cycle lengths (eg, 600 ms, 550 ms, 500 ms), beginning with a cycle length just shorter than the resting sinus cycle length.
  • SNRT is the time interval between the last paced captured beat to the first spontaneous sinus beat.
  • Gradual return of SN to its baseline rate occurs over 5-6 beats. Prolonged pauses (ie, secondary pauses) can occur after the initial recovery interval in SND.
  • If the longest interval for the recovery interval or secondary pause exceeds 1500 milliseconds, the SNRT is prolonged.
  • To adjust for HR and before each pacing increase, the resting sinus cycle length (SCL) is measured. This resting SCL is subtracted from SNRT, and the CSNRT is obtained. Its upper reference range limit is 525 milliseconds; if the SNRT exceeds the SCL by more than 525 milliseconds, the SNRT is abnormal. The same occurs if the ratio of SNRT to SCL (ie, SNRT/SCL X 100) is more than 160%.
• Sinoatrial conduction time

  • SACT is another parameter to assess SN function. It is the time interval in milliseconds for an impulse that originates in the SN to conduct through the perinodal tissue to the adjacent RA tissue.
  • The tissue that surrounds the SN or perinodal tissue has similar characteristics as those of the AV node.
  • Eight PACs are fired in the HRA at 5-10 bpm faster than the SN rate before they are stopped abruptly.
  • SACT represents the time in milliseconds it takes for the PAC fired in the HRA to enter and reset the SN. It also represents the time for the new spontaneous SN impulse (ie, SCL) to reach the HRA.
  • SACT is measured as the time in milliseconds from the last PAC to the first spontaneous sinus beat.
  • When the time interval between the last spontaneous SN depolarization (ie, before the PAC) and the one that occurred after a PAC is less than twice the value of the 2 previous spontaneous SN depolarizations, reset of the SN by the PAC has occurred.
  • SACT can be calculated as the interval from the PAC to the next spontaneous SN beat, which includes conduction through the perinodal tissue into the SN, resetting the SN, and conduction through the same perinodal tissue back into the HRA (ie, [return interval - SCL]/2). The reference range is 50-125 milliseconds in children and 200-250 milliseconds in adults.
  • If the SN cannot produce a spontaneous impulse following PACs (ie, these have not reset the SN, and, therefore, SACT cannot be calculated), sinoatrial entrance block is present.
  • These could be caused by markedly prolonged sinoatrial conduction and/or increased refractory period of peri-SN or SN fibers, both of which indicate SND.
  • Alternating SN entrance block with reset responses also denotes SND.
  • SN reentry tachycardia occurs when activation of the atrium during the SVT is the same as sinus beats (ie, P-wave axis and morphology are the same as those in sinus rhythm). It is usually indicative of SND.
• Complications: Complications from a diagnostic EP study are rare but may include the following:

  • Hematoma at the puncture site in the groin and or neck
  • Hemorrhage
  • Infection caused by manipulation of catheters inside the heart (theoretical risk)
  • Perforation upon catheter manipulation inside the heart of small patients (most commonly involving the right atrial appendage and the right ventricular outflow tract)