RECOVERY CURVE AND CONCEALED CONDUCTION IN THE HIS-PURKINJE SYSTEM OFTHE RABBIT HEART - EFFECTS OF RADIOFREQUENCY MODIFICATION OF THE LOW AV JUNCTION

The aim of this study was to analyze the recovery curve and concealed conduction in the normal His-Purkinje system and after delivering radi ofrequency current in the low AV junction, in the perfused rabbit hear t. Twenty-one rabbit hearts were studied. Radiofrequency current (5 W) was delivered in the low AV junction to induce an incomplete His-Purk inje AV block (HV prolongation with 1:1 AV conduction); this was achie ved in 9 experiments (Group I), while ZZ experiments developed a compl ete block (Group II). Atrial stimulation was performed in both Groups at baseline, and in Group I after radiofrequency delivery, as follows: (1) pacing at increasing rates to determine the His-Purkinje AV block cycle length; (2) atrial extrastimulus test (A(1)A(2)) to calculate t he His-Purkinje effective refractory period and the fitting of the rec overy curve (H1H2 vs H2V2) to the exponential equation Delta HV = a.e( -b.(H?1H?2)); (3) concealed conduction protocol (in 15 experiments) co nsisting of an atrial extrastimulus test with an interposed beat (A(1) -A(0)-A(2)) at a fixed A(1)A(0) coupling interval. The baseline recove ry curve fitted an exponential equation in 17 experiments (with a 93% +/- 42% maximum H2V2 increase at the shortest H1H2), but did not in 4 experiments (the maximum H2V2 increase being only 22% +/- 7%). Radiofr equency application prolonged the HV interval (25 +/- 6 ms vs 46 +/- 1 6 ms; P = 0.001) and His-Purkinje effective refractory period (167 +/- 28 ms vs 217 +/- 57 ms; P = 0.02). The percentage increment was great er for HV than for refractory period (99% +/- 65% vs 35% +/- 32%; P = 0.02); however, the increment of the His-Purkinje block cycle length ( 77% +/- 74%) only correlated with that of the refractory period (r = 0 .95; P = 0.0001). The recovery curve after radiofrequency delivery fit ted an exponential equation in all experiments, showing a rightward sh ift expressed by an increment of the constant In a (2.7 +/- 1.9 vs 5.5 +/- 5.5; P = 0.02). Concealed conduction appeared in only three exper iments at baseline. After radiofrequency, however, it was observed in all experiments, producing a rightward shift of the recovery curve and an In a increase (2.87 +/- 1.2 vs 9.9 +/- 2.7; P = 0.0001). When H-0 was conducted, the curve rightward shift and In a increase (26 +/- 7.5 ; P = 0.0001) were greater. Conclusions: (2) His-Purkinje physiology, as in A Vn odal physiology, can be described by a recovery curve that fits an exponential equation, especially if conduction becomes depress ed with radiofrequency current. (2) Radiofrequency application in the low AV junction modifies His-Purkinje conduction more than refractorin ess, though the refractoriness increase determines the degree of block at fast atrial rates. (3) Concealed conduction is uncommon in the nor mal His-Purkinje system during atrial pacing, but very frequent after modifying the low AV junction with radiofrequency current.