Two FeCl3 chloridized silver wire electrodes (wire diameter 0.3 mm, electrode tip diameter 1.5 mm) were placed on the epicardial surface of the heart and were free to move with the contractions. Both electrodes were positioned in the AV valve plane, one anteriorly near to the origin of the interventricular artery and the other posteriorly between the two auricles. The unfiltered signals were amplified by a factor of 100 with an instrumentation amplifier (Anton Pear) with alternating current input (cutoff frequency 0.72 Hz). His bundle activity was visible in the bipolar-recorded ECG signals, which were monitored on a digital storage oscilloscope and stored on a tape recorder at a sampling rate at 5 kHz. This high resolution ECG recording technique has been described elsewhere.
Parameters measured: In spontaneously beating hearts, changes in sinus rate, AV nodal (AH interval), His bundle (HV interval) and intraventricular (QRS interval) conduction were evaluated during control conditions and 15 mins after the addition of each concentration of either tedisamil or D-sotalol (1, 3 and 10 |jM). Additionally, the repolarization period (JT interval) was measured at a constant ventricular rate of 240 ms (Figure 1).
Figure 1 Original recording from the epicardiac surface of isolated guinea pig heart. AH Atrioventricular nodal conduction interval; HV His bundle conduction interval; JT Repolarization period; QRS Intraventricular conduction interval; T T wave
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