Recording of diastolic slope from the junctional area in dogs with junctional rhythm.

Stable junctional rhythm was induced in 14 of 25 anesthetized dogs by exclusion of the sinus node and surrounding atrial tissue. In 12 of the 14 dogs diastolic slope was recorded through a unipolar lead consisting of a terminal of a plunge or catheter electrode placed in the His bundle area coupled with an indifferent superior vena caval electrode. Reversed polarity, high-gain amplification (0.1 mV/cm), and low-pass filters (0.1 to 50 Hz) were used for the unipolar recording. The diastolic slope (0.15 +/- 0.07 mV/sec) was recorded from a localized area close to the His bundle and preceded the bipolar His bundle deflection. Overdrive atrial stimulation was followed by slowing of the junctional rate and decrease in the diastolic slope. A strong negative correlation (r values from -.78 to -.95) was found between the junctional cycle lengths and the diastolic slopes after atrial pacing. Isoproterenol infusion increased the junctional rate and the diastolic slope, whereas vagal stimulation and verapamil injection decreased them. The effects of verapamil were partly reversed by CaCl2 injection. The responses of the diastolic slope to overdrive pacing, vagal stimulation, and isoproterenol suggest that the deflection is indeed due to membrane current responsible for phase 4 depolarization. The responses of the junctional rhythm and the diastolic slope to verapamil and CaCl2 suggest that AV nodal automaticity is responsible for the rhythm. Clinical application of this recording technique should help in the identification and characterization of automatic junctional rhythm.