The relationship between refractory periods and cycle length in man

The effects of cycle length (CL) on the functional and effective refractory period (FRP and ERP) of atrium (AT), A-V node (AVN), and right bundle branch (RBB) were studied in 21 patients (pts) using His bundle recording and extra stimulus technique. Refractory periods (RP) were analyzed at CL ranges of >600 msec (CL₁), 500-599 msec (CL₂), and <499 msec (CL₃). Mean ±SEM atrial ERP, measured in 19 pts was: 248±13 CL₁, 231±16 at CL₂, 213±9 at CL₃. Mean AVN FRP (msec) measured in 18 pts was 421±10 at CL₁, 405±20 at CL₂, and 377±13 at CL₃. Mean AVN ERP (msec), measured in 15 pts, was 297±11 at CL₁, 313±19 at CL₂, and 314±16 at CL₃. In 4 of these 15 pts, ERP was decreased or unchanged with decreasing CL. These 4 had a mean A-H of 110±11 msec at CL₃. In contrast, the remaining 11 pts had a mean A-H of 177±13 msec at CL₃, (P<0.01). In 11 pts both AVB FRP and ERP could be measured at more than 2 CLs. The difference between FRP and ERP averaged 131±15 at CL₁, lll±14 at CL₂, and 69±10 at CL₃. RBBB ERP (msec) could be measured in 6 pts and was 476±17 at CL₁, 384±20 at CL₂. and 339±7 at CL₃. In conclusion, decrease in CL results in decrease in AT and RBB RP. Decrease in CL shortens AVN FRP and lengthens AVN ERP, narrowing the gap between FRP and ERP. Fast AVN conduction time may obviate the increase in AVN ERP with decreasing CL.