Pacing mode selection in patients with sick sinus syndrome.

INTRODUCTION SICK SINUS SYNDROME The sinus node is composed of a collection of specialised pacemaker cells embedded in a fibrous matrix that lies in the sulcus terminalis between the superior vena cava and the right atrial appendage. The sinus node serves as the primary cardiac pacemaker and controls the heart rate in normal individuals. Recent research indicates, that the sinus node is a heterogeneous tissue with multiple cell types and a complex structure (1, 2). Keith and Flack first described the sinus node in 1907 (3). In 1915-1916, “sino-auricular heart block” was described electrocardiographically for the first time (4, 5), and the frequent coexistence of sinus brady-arrhythmia and paroxysmal atrial tachycardia was described by Short in 1954 (6). The sick sinus syndrome (SSS) was further defined in the 1960’s and 1970’s (7-11), and is characterised by symptoms associated with electrocardiographic findings of sinus node dysfunction. In 1972 Rubenstein et al. proposed a simple classification based on the electrocardiographic manifestations of SSS (12). According to this classification, patients with SSS can be divided into three groups: