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DIFFERENTIAL EFFECTS ON ACTION POTENTIAL DURATION OF CLASS IA, B AND C ANTIARRHYTHMIC DRUGS: MODULATION BY STIMULATION RATE AND EXTRACELLULAR K+ CONCENTRATION

1. Standard microelectrode techniques were used to study the effects on the action potential duration (APD) of canine Purkinje fibres of a therapeutic concentration of nine Class I antiarrhythmic drugs. At an extracellular K+ concentration of 5.6 mmol/L all nine agents reduced APD at all drive rates studied (range of interstimulus intervals = 200–1000 ms). At lower levels of K+, quinidine (5 μmol/L) and disopyramide (10 μmol/L) (Class Ia agents) revealed dual effects on APD. At the lowest levels of K+ (2 mmol/L) and the longest interstimulus interval used (2000 ms), both agents significantly prolonged APD. Under all other conditions, APD was either unchanged or reduced. Lignocaine, 15 μmol/L (Class Ib agent) reduced APD at all rates and all K+ concentrations and this effect was greatest at the slowest rates.