Effect of drugs on the stimulation threshold of the human heart.

Clinical measurements were made of the effects of various drugs on the stimulation threshold of human heart [right ventricle]. Of antiarrhythmic drugs, a marked and prolonged elevation of the stimulation threshold was produced by procainamide, quinidine, and aimaline. Relatively short-lasting elevation was elicited by local anaesthetics [trimecaine, lidocaine]. Beta-blockers were little effective. No detectable effect on the stimulation threshold was produced by phenytoin and digitalis glucosides from the group of antiarrhythmics. The deepest and most prolonged depression of the stimulation threshold was achieved with anabolics [nandrolone phenylpropionate, nandrolone decanoate] and with high doses of prednisone and of 6-methylprednisolone sodium succinate. Hydrocortisone was ineffective. A slight and brief depression was produced by atrophine. The effects of sympathomimetics were complicated: at stimulation with short pulses the threshold was always raised. At stimulation with long pulses the effects were variable, mostly biphasic [occasionally triphasic]: a transitory depression of the stimulation threshold was followed by a marked elevation. The I-t curve's slope altered. In exit block, sympathomimetics are indicated because of their chronotropic effect, not because of their questionable effect on the stimulation threshold. A number of drugs, often administered to stimulated patients, produced no effect on the stimulation threshold: antibiotics, nitrites, purine compounds, opiates, and others.