Sinus Node Dysfunction

Sinus node dysfunction is de~ned as inappropriate sinus bradycardia or abnormal pauses in sinus mechanism. Sinus node dysfunction often coexists with atrial ~brillation, _utter, or tachycardia, and in such cases may be termed “tachy-brady syndrome.” Sinus node dysfunction may also be accompanied by abnormalities of AV conduction. Another term for sinus node dysfunction is “sick sinus syndrome.” Patients with sinus node dysfunction are often asymptomatic, so its prevalence has not been well quanti~ed. Furthermore, precise criteria for “inappropriate sinus bradycardia” or “abnormal pauses” have not been established, and are dependent on considerations other than rate or duration alone. For example, whether or not a resting sinus rate of 45 beats per minute (bpm) constitutes sinus node dysfunction depends on the patient’s degree of physical conditioning (which leads to physiologic sinus bradycardia secondary to hypervagotonia) and whether the bradycardia is producing symptoms. One index of the incidence of sinus node dysfunction is the rate with which cardiac pacemakers are implanted for this condition; sinus node dysfunction probably accounted for approximately 50%1 of the estimated 130,000 pacemakers implanted in the United States in 1993. Recently, the de~nition of sinus node dysfunction has been expanded to include patients with “chronotropic incompetence.” Chronotropic incompetence implies the inability to increase heart rate appropriately during exercise, with consequent exercise intolerance. Chronotropic incompetence most commonly is caused by subnormal acceleration of sinus rate during exercise, although less commonly it may be caused by abnormalities of atrioventricular conduction, especially in the setting of atrial ~brillation. While the concept of chronotropic incompetence is straightforward, there is no generally accepted quantitative de~nition. The availability of rate-responsive pacemakers, which have the potential of correcting chronotropic incompetence, has focused attention on the need for more information about this disorder. The American College of Cardiology/American Heart Association criteria for implantation of rate-responsive pacemakers is failure to achieve a maximum heart rate of 100 bpm.2 However, it is clear that this criterion is not an adequate de~nition of chronotropic incompetence, since the vast majority of exercise typically performed by patients is submaximal, so any de~nition must take into account heart rate responses during submaximal exercise. Wilkoff and coworkers3,4 have undertaken a thoughtful analysis of this issue and have proposed a treadmill exercise protocol (“Chronotropic Assessment Exercise Protocol”) focusing on submaximal exercise, along with theoretically-derived and experimentally-validated normal heart rates for each stage of exercise. The use of such analyses in the future may help de~ne the prevalence of chronotropic incompetence in our population. In the last few years there have been several reports of familial forms of heart disease which include among their manifestations sinus node dysfunction. Bharati et al. described a family with congenital absence of sinus rhythm; in one member of the family, the sinus node was found to be markedly atrophic.5 Mehta et al. reported a family with symptomatic sinus bradycardia secondary to hypervagotonia in whom the genetic mode of transmission appeared to be autosomal dominant.6 Olson and Keating studied a family with dilated cardiomyopathy associated with sinus node dysfunction and localized the responsible gene to chromosome 3p.7 Schott et al reported a family with long QT syndrome and marked sinus bradycardia in whom linkage was obtained for markers on chromosome 4q.8 It is likely that in the future there will be a greater appreciation for genetic causes or predispositions to sinus node dysfunction. Sinus node dysfunction following cardiac transplantation continues to be recognized as a clinically signi~cant problem. Sinus node dysfunction is the most common indication for pacemaker implantation following cardiac transplantation; 18 of 20 pacemakers implanted after 453 transplants at Columbia-Presbyterian Medical Center were for sinus node dysfunction.9 Chau et al.10 found that sinus node dysfunction requiring pacemaker implantation was more common in patients receiving hearts from older donors (aged .40 years). However, Heinz et al.11 studied a number of clinical parameters, including donor age, and found none of them to be predictive of pacemaker implantation after transplantation. There is increasing recognition that beyond obvious sinus bradycardia or pauses requiring pace-