Coexisting mechanical and electrical alternation in drug-induced pulsus alternans in dogs

Alternation can be evoked in the dog heart by i.v. injections of moderately large doses of methoxamine hydrochloride. In a study of responses to 123 doses in 50 dogs, ‘true’ pulsus alternans without measurable electrical alternation occurred most frequently. Coexisting electrical alternation was present in only one-fourth of the cases in which mechanical alternans was induced by the drug. Only once did electrical alternans develop in the absence of mechanical alternans. Electrical alternation, when present, was usually limited to changes in amplitude of T; more rarely, of R. The usual pattern was one in which the electrogram associated with the weaker ventricular contraction showed greater negativity than that for the stronger heart beat. The mechanism of mechanical alternans is probably independent from that of electrical alternation. When T wave changes are the only electrical evidence of alternation, beat-to-beat changes in time of onset of repolarization in localized areas of the myocardium are thought to be involved. Changes in R probably indicate that conductional delays have also contributed, but since alternation in R was rare in drug-induced alternans, it is doubtful that delayed conduction is of great general significance.