Myocardial conduction time and antiarrhythmic drugs

Area of Science:

• Cardiovascular Physiology
• Electrophysiology

Background:

• Complete atrioventricular (A-V) block was induced in dogs to study myocardial conduction.
• Ventricular pacing was used to elicit mechanical responses.

Purpose of the Study:

• To develop and validate a new method, delta IT, for measuring myocardial conduction rate.
• To investigate the effects of varying stimulus intensity and antiarrhythmic drugs on delta IT.

Main Methods:

• Complete A-V block was created in mongrel dogs via electrocoagulation of the A-V node.
• Ventricular pacing (S1 and S2 stimuli) was employed, with varying delays between stimuli.
• Myocardial tension was measured using strain gauges at two sites to determine the time lag (delta IT) in mechanical response to S2 stimuli.
• The impact of varying S2 current intensity and drug administration (quinidine, disopyramide, lidocaine, verapamil) on delta IT was assessed.

Main Results:

• Delta IT, the time lag between mechanical responses at two myocardial sites, varied between 10-60 msec.
• Increasing S2 stimulus intensity shortened delta IT, allowing for a reproducible strength-delta IT curve.
• Quinidine and disopyramide shifted the curve upward, indicating slowed conduction.
• Lidocaine had no significant effect, while verapamil lowered delta IT values, suggesting improved conduction.

Conclusions:

• Delta IT is a reproducible and reliable indicator of myocardial conduction rate.
• The delta IT measurement is sensitive to the effects of specific antiarrhythmic drugs, offering insights into their mechanisms of action.