MacWilliam discovered that fibrillar contraction of the heart is due to “a rapid succession of incoordinated peristaltic contractions.” He clearly described auricular and ventricular fibrillation, and showed that ventricular fibrillation could be caused by the injection of certain poisons into the blood stream. His paper is included, with an account of his life, in Willius & Keys, Cardiac classics, 1941, pp. 666-678.

First description of a case of death from ventricular fibrillation.

MacWilliams described experiments in which application of an electrical impulse to the human heart in asystole caused a ventricular contraction, and that a heart rhythm of 60–70 beats per minute could be evoked by impulses applied at spacings equal to 60–70 /minute. Digital facsimile from PubMedCentral at this link.

"Physiologists and physicians had proposed various theories to explain transient and fatal cardiac standstill in animals and humans who were apparently healthy. MacWilliam defined two distinct mechanisms depending on whether recovery was possible. He offered experimental proof that “fibrillar contraction” (later termed ventricular fibrillation) was the cause of irreversible cessation of the heartbeat and sudden death. Writing in 1889, he argued that

"He explained that a variety of pathological conditions could predispose to ventricular fibrillation, including “degenerative changes of a fatty or fibroid nature in the muscular walls” and “diseased conditions...of the coronary arteries.” MacWilliam demonstrated that the fundamental electrophysiologic properties of the hearts of cold-blooded amphibians could be reproduced in mammals. He also extended the time an isolated mammalian heart preparation remained viable by combining artificial ventilation and rhythmic manual compression of the ventricles with the administration of pilocarpine.10 In 1889, MacWilliam published an article in The British Medical Journal announcing his conviction 'that ventricular fibrillation rather than cardiac standstill (asystole) was the cause of many, if not most cases of sudden death in humans' (Silverman & Fye, "John A. MacWilliam: Scotish pioneer of cardiac electrophysiology," Clin. Cardiol, 29 (2006) 90-92.)