Acute effects of anti-arrhythmic drugs on cardiac pacing threshold

Forty cardiac pacing threshold studies were conducted in 20 patients during temporary pacing prior to permanent pacemaker implantation using the following intravenously administered anti-arrhythmic drugs: Lignocaine 2 mg/kg (10 patients), mexiletine 5 mg/kg (5 patients), ajmaline 1 mg/kg (5 patients), verapamil 0.15 mg/kg (10 patients) and propranolol 0.1 mg/kg (10 patients). Changes in voltage (volts), current (mA), energy (micro-joules) and resistance (ohms), were measured using a pacemaker system analyzer at a constant pulse width (0.6 msec) and pacing rate (70/minute). Lignocaine increased the voltage threshold significantly (p less than 0.05) with insignificant (p greater than 0.05) change in current, energy and resistance. Mexiletine and ajmaline significantly (p less than 0.01 and p less than 0.02) increased the voltage and current thresholds but did not change the energy level and resistance (p greater less 0.05). Verapamil caused a significant increase in the current and energy threshold (p less than 0.05), without changing the voltage threshold significantly (p greater than 0.05). The resistance values showed a consistently decreasing trend. Propranolol on the other hand did not affect any parameter significantly (p greater than 0.05). Anti-arrhythmic drugs appear to produce variable effects on different pacing threshold parameters. This has important clinical implications.