Recovery Curve and Concealed Conduction in the His‐Purkinje System of the Rabbit Heart: Effects of Radiofrequency Modification of the Low AV Junction
Authors: SANCHIS JUAN , CHORRO FRANCISCO J. , SUCH LUIS , ARTAL LEANDRO , BODÍ VICENTE , ATIENZA FELIPE , LLAVADOR ESTEBAN , LLAVADOR JUAN , MERINO VICENTE LÓPEZ 
Published: July 2006

The aim of this study was to analyze the recovery curve and concealed conduction in the normal His‐Purkinje system and after delivering radiofrequency current in the low AV junction, in the perfused rabbit heart. Twenty‐one rabbit hearts were studied. Radiofrequency current (5 W) was delivered in the low AV junction to induce an incomplete His‐Purkinje AV block (HV prolongation with 1:1 AV conduction); this was achieved in 9 experiments (Croup I), while 12 experiments developed a complete block (Group II). Atrial stimulation was performed in both Groups at baseline, and in Group Softer radiofrequency delivery, as follows: (1) pacing at increasing rates to determine the His‐Purkinje AV block cycle length; (2) atrial extrastimulus test (A1 A2)J to calculate the His‐Purkinje effective refractory period and the fitting of the recovery curve (H1H2 vs H1V2) to the exponential equation ΔHV=a.e‐b.(H1H2);(3) concealed conduction protocol (in 15 experiments) consisting of an atrial extrastimulus test with an interposed beat (A1‐A0‐A2) at a fixed A1A2 coupling interval. The baseline recovery 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%). Radiofrequency application prolonged the HV interval (25 ± 6 ms vs 46 ± 16 ms: P = 0.001) and His‐Purkinje effective refractory period (167 ± 28 ms vs 217 ± 57 ms; P = 0.02). The percentage increment was greater 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 fitted an exponential equation in all experiments, showing a rightward shift 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 experiments 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 H0 was conducted, the curve rightward shift and In a increase (26 ± 7.5; P = 0.0001) were greater. Conclusions: (1) His‐Purkinje physiology, as in A V nodal physiology, can be described by a recovery curve that fits an exponential equation, especially if conduction becomes depressed with radiofrequency current. (2) Radiofrequency application in the low AV junction modifies His‐Purkinje conduction more than refractoriness, though the refractoriness increase determines the degree of block at fast atrial rates. (3) Concealed conduction is uncommon in the normal His‐Purkinje system during atrial pacing, but very frequent after modifying the low AV junction with radiofrequency current.