Electrophysiological and Clinical Study of Sick Sinus Syndrome Using the Overdrive Suppression Test : Symposium on Sinus Node Function and Its Abnormality

1) Right atrial pacing was performed at rates of 80 to 200 beats/min for 30 seconds. The pacing rate was increased by an increment of 10 beats/min, with a 30 second rest period after each atrial pacing. One hundred three patients with sick sinus syndrome (SSS) and 25 normal subjects were studied. Recovery time (RT) was defined as the longest R-R interval of the five subsequent beats after cessation of overdrive stimulation at each rate, and was expressed as the ratio of RT to resting R-R interval (%RT). Accordding to the origin of the beat following the longest pause, RT was divided into sinus node recovery time (SNRT) and A-V junctional recovery time (JRT). The longest RT in all pacing rates was defined as the maximum recovery time (max RT). 2) In normal subjects, the ratio of SNRT to the resting R-R interval (%SNRT) was only slightly affected by the pacing rate, though showed a two-phase phenomenon. Among cases with SSS, in patients with a maximum %SNRT of more than 200%, %SNRT progressively increased as the pacing rate increased. In patients with a maximum %SNRT less than 200%, however, %SNRT remained essentially constant over a wide range of pacing rates. JRT in cases with SSS showed a two-phase pattern with a maximum pause at a pacing rate of 260-280% with reference to the resting heart rate. 3) Patients with SSS were classified into three different response groups according to the above values and patterns of %RT against the pacing rate, with each group showing different clinical and electrocardiographic features. In the 43-case low-rate response group (%RT already more than 200% below a pacing rate of 200%), sinus arrest was detected in 65% of the cases and paroxysmal tachycardia and syncopal attack in about half. In the 31-case high-rate response group (%RT smaller than 200% at a pacing rate lower than 200% but %RT over 200% at a pacing rate more than 200%), SA block was found in 74% of the cases, paroxysmal tachycardia in 8 l% and syncopal attack in 81%. Maximum suppression of sinus node automaticity occurred at a higher pacing rate of 300-400% in some cases. In the 29-case normal response group (%RT smaller than 200% at all pacing rates and little affected by the pacing rate), sinus bradycardia was found in 72% of the cases, but syncopal attack in 24% of the cases despite occurrence of paroxysmal tachycardia in about half of the cases. The importance of pacing rate effect on RT should be stressed in the overdrive suppression test. Hence, our classification was thought to be useful in unmasking the presence of sinus node dysfunction and in confirming its severity in patients with SSS.