Changes in myocardial threshold. Physiologic and pharmacologic factors in patients with implanted pacemakers

Seventy-one controlled threshold studies were performed on 14 patients with implanted cardiac pacemakers. Eating and sleeping raised myocardial threshold significantly, whereas exercise had a reverse effect. A solution of insulin and potassium in dextrose, 3 per cent NaCl, and aldosterone also increased threshold. Epinephrine, methylprednisolone, and potassium chloride in Ringer's solution lowered threshold. Isoproterenol caused an initial fall in threshold, followed by a marked rise. No significant changes were demonstrated with 10 per cent dextrose, normal saline, calcium gluconate, lanatoside C, procaine amide, atropine, or morphine at those dosages that were arbitrarily chosen for this study. The results demonstrate that myocardial threshold in man is labile, and that it can be manipulated by physiologic activities and pharmacologic agents.