Electrophysiological consideration on sick sinus syndrome.

1) Factors influencing abnormally prolonged Sinus Recovery Time in Sick Sinus Syndrome were investigated with a microelectrode technique. 2) Although the action of released acetyl choline was confirmed by the effect of atropine and neostigmine on the SRT, an abnormally long SRT like that in SSS was not produced by vagal stimulation in the in situ heart. 3) When the drive rate exceeded a certain value (e.g. two times the spontaneous rate in normal preparations), SA entrance block resulted in less overdrive suppression. When the SA junction is easily blocked with relatively low drive rates, the maximum SRT must be determined within a wide range of drive rates. 1) Verapamil in a concentration of 10(-7) g/ml produced an abnormal SRT suggesting that ionic derangement is one of the factors influencing SRT. 5) A preparation from an aged rabbit with an abnormal SRT revealed fibrosis and reduction in the number of SA nodal cells. The latter seems to be one of the main factors causing a reduction in the potency of the pacemaker and a prolonged SRT in SSS.