The sinoatrial (SA) node is innervated by the parasympathetic and the sympathetic nervous systems; the balance between these systems controls the pacemaker rate. Parasympathetic input via the vagus nerves decreases the SA nodal pacemaker and is the dominant input at rest, wheras sympathetic nerve input, as well as the adrenal medullary release of catecholamines, increases the sinus rate during exercise and stress.
Sinus node dysfunction (SND) is often secondary to senescence of the SA node and surrounding atrial myocardium. Medications may also contribute to, and can often unmask, subclinical SA dysfunction.
The epidemiology of SND is difficult to study, but patients with symptomatic SND are generally older (in the seventh or eighth decade of life) with frequent comorbid diseases.SND occurs as a result of disorders in automaticity, conduction, or both. SN fibrosis is the most common cause of SND.
The natural history of SND typically involves intermittent but progressive cardiac rhythm disorders, which have been associated with higher rates of other cardiovascular events and higher mortality. There is a tendency for the rhythm disturbances associated with SND to evolve over time, along with a higher likelihood of thromboembolic events and other cardiovascular events.
The treatment of sinus bradycardia and pauses starts with investigating/identifying any reversible causes, of which the most common are medications. Permanent pacemakers are often implanted in symptomatic patients with SND.
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This 12-lead electrocardiogram (ECG) is from an asymptomatic girl aged 10 years, 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 (SN) because they have a positive axis (upright) in leads I, II, and aVF. The PR interval is 104ms, and the QRS is narrow at 86ms, with a normal axis of 64°. The corrected QT (QTc) interval measures 402ms. Therefore, this is a normal ECG.
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Below is an electrocardiogram (ECG) of a girl aged 2 years who was referred to the clinic by a 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 ms, and the QRS is narrow at 90 ms. Right ventricular (RV) conduction delay is shown and is best seen in the precordial leads V1 and V2. The QRS frontal axis shows right axis deviation (reference range for a child aged 2 years is 0-110°). The patient does not have RV 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.
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This is a 12-lead electrocardiogram (ECG) from a boy aged 12 years with a 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 ms. Two junctional escape beats are present after a prolonged pause. The QRS is narrow at 85 ms, with a normal frontal axis of 70°. The corrected QT interval (QTc) is 411 ms. A later electrophysiologic study showed prolonged sinus node recovery time (SNRT) and sinoatrial conduction time (SACT). Because of the patient's symptoms and his sinus node (SN) 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 defined sick sinus syndrome, with the patient requiring pacemaker therapy.
