BACKGROUND "Inappropriate" sinus tachycardia (IST) is an uncommon and poorly defined atrial tachycardia characterized by inappropriate tachycardia and exaggerated acceleration of heart rate with "normal" P wave. The mechanism of this tachycardia is unknown. The purpose of the present study was to determine the role of autonomic balance in the genesis of IST. METHODS AND RESULTS Six female patients aged 23 to 38 years with IST and 10 age- and sex-matched control subjects were assessed with the following autonomic function tests: (1) sympathovagal balance to the sinus node assessed by calculating the LF/HF (low frequency/high frequency) ratio using power spectral analysis both in the supine position and after 10 minutes of head-up tilt to 60 degrees, (2) cardiovagal reflex assessed by cold face test (CFT), (3) beta-adrenergic sensitivity as determined by calculating isoproterenol dose-response curves and isoproterenol chronotropic dose 25 (CD25), and (4) intrinsic heart rate (IHR) assessed after autonomic blockade with atropine 0.04 mg/kg and propranolol 0.2 mg/kg administered as an intravenous bolus. No significant differences in the LF/HF ratio both in the supine position (2.8 +/- 0.3 versus 2.6 +/- 0.4) and during upright tilt (8.7 +/- 1.3 versus 8.5 +/- 0.5) were observed between control subjects and IST patients. Cardiovagal response to CFT was markedly depressed in all patients (6.3% IST patients versus 24.2% control subjects, P < .001). beta-Adrenergic hypersensitivity to isoproterenol was noted in all patients (mean CD25, 0.29 +/- 0.10 microgram IST patients versus 1.27 +/- 0.4 microgram control subjects; P < .001), and high IHR was noted in all cases. The patients were treated with high doses of beta-blockers with adequate short-term control. Radiofrequency catheter ablation of the sinus node area was performed in one drug-refractory patient. CONCLUSIONS These findings suggest that the mechanism leading to IST is related to a primary sinus node abnormality characterized by a high IHR, depressed efferent cardiovagal reflex, and beta-adrenergic hypersensitivity.

Previous reports of the follow-up of patients with atrial fibrillation have been confusing because of the variety of clinical presentations, heterogeneity of underlying pathology, and the initiation of follow-up at various stages of the patient's disease. The Canadian Registry of Atrial Fibrillation (CARAF) is a non-interventional, follow-up study of patients enrolled at the time of their initial diagnosis with atrial fibrillation at seven Canadian centres. At baseline, a comprehensive database recorded clinical, laboratory, and echocardiographic variables. No specific intervention was initiated and care was left to the attending physicians. Follow-up was performed at 3 months, 1 year, then annually. Echocardiograms were repeated every 2 years. Recurrence of atrial fibrillation, medical intervention, stroke, death, and other significant events have been specifically recorded. To date, 967 patients have been enrolled. Seven hundred and sixty-seven patients have been followed for 1 year, 468 for 2 years, and 217 for 3 years. Several studies have been undertaken on these patients. One study compared the variables of patients who were symptomatic with those who were asymptomatic. This study demonstrated that symptoms were more likely to occur if the patient were younger, had high blood pressure and high ventricular response during atrial fibrillation, and were female. These all achieve statistical significance and a formula was developed to predict the probability of symptoms in different subgroups of patients. Antiarrhythmic drug use was evaluated. Sotalol and propafenone were the most commonly used drugs and their use increased when atrial fibrillation was recurrent. Many patients initially received no antiarrhythmic drugs. Trends suggest that therapy is more aggressive with recurrence of the arrhythmia. The prevalence of thyroid abnormalities was investigated utilizing sensitive TSH measurements. This showed that overt hyperthyroidism is rare (1%) but laboratory abnormalities and history of thyroid dysfunction occurred more frequently, in 19% of patients. Another study evaluated antithrombotic therapy. Factors known to increase stroke risk, including congestive heart failure, previous stroke, and large left atrium all increased the use of anticoagulants. Anticoagulants were used more frequently in patients over the age of 65 and in patients with recurrent or chronic atrial fibrillation. There was concern that hypertension, shown to be a high predictor of stroke, did not result in a significant use of warfarin. Aspirin use was common in patients not placed on anticoagulants. Further studies are being undertaken with the ultimate goal to utilize baseline data to predict clinical outcomes.

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We studied the electrophysiologic characteristics of atrioventricular (AV) nodal conduction in patients with reciprocating tachycardia (RT) without ventricular preexcitation, and the relation of these characteristics to RT cycle length (CL). Thirty-five symptomatic patients who had a normal PR interval (0.13-0.20 second) during sinus rhythm underwent detailed intracardiac electrophysiologic study during which ventricular preexcitation was excluded, and the RT mechanism was determined. RT was due to reentry using an accessory AV pathway capable of conduction only in the retrograde direction (concealed AP) in 13 patients (37%) and to reentry within the AV node in 22 (63%). Dynamic properties of AV conduction (assessed by degree of AH prolongation during progressive increase in atrial pacing rate) were normally distributed (p less then 0.005); 12 patients (34%) fulfilled the criteria for enhanced AV conduction (EAVC). The patients with EAVC had a shorter RT CL than did patients without EAVC (294 +/- 43.3 msec vs 360 +/- 68.1 msec, p less than 0.01). However, CL differences were primarily due to the influence of EAVC in the subgroup of patients with RT using a concealed AP (EAVC CL, 274 +/- 35.1 msec; without EAVC, 326 +/- 15.7 msec, p less than 0.005). The RT CL in patients with reentry within AV node was not measureable influenced by concomitant EAVC (EAVC, 314 +/- 43.8 msec; without EAVC, 376 +/- 76.8 msec) (NS). This study suggests that despite the presence of a normal PR interval during sinus rhythm, dynamic AV conduction responses can vary widely in patients with RT. In patients with RT using a concealed AP, but not in those with reentry within the AV node, coexisting diminished physiologic AV conduction slowing may be associated with more rapid tachycardia rates.