This section outlines steps to allow systematic evaluation of sinus node (SN) function.
Calculating the intrinsic heart rate (IHR)
The IHR is the heart rate in the presence of complete pharmacologic denervation of the SN. This is achieved with the simultaneous use of beta blockers and atropine. The calculation of the IHR following simultaneous administration of beta blockers and atropine is largely of historical interest and is rarely performed in the modern evaluation of patients with suspected SN dysfunction (SND). The IHR in a healthy person is approximately equal to 117.2 – (0.53 × age in years).
Sinus node response to pharmacologic challenge
A number of drugs have been used in to aid in the diagnosis of SND. The pharmacologic protocols are briefly described below.
Atropine and isoproterenol
Atropine (1 or 2 mg) and isoproterenol (2-3 mcg/minute) may be useful, because both agents normally increase the sinus rate. A suggested abnormal response is an increase in the sinus rate of less than 25% above the baseline, or to a sinus rate below 90 beats per minute. [31] Due to fact that in most cases the diagnosis of SND can be achieved by establishing a symptom-rhythm correlation with the use of ambulatory monitoring in conjunction with a comprehensive history and physical examination, testing with these agents is rarely necessary.
Adenosine
Adenosine has been proposed as an alternative to invasive electrophysiology studies, but its routine use is not yet established.
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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.
