Circulation, Vol 84, 1333-1345, Copyright © 1991 by American Heart Association
DS Rosenbaum, DT Kaplan, A Kanai, L Jackson, H Garan, RJ Cohen and G Salama
BACKGROUND. In single heart cells, abrupt changes in stimulation rate
elicit complex alterations in repolarization. The effects of rate change on
dispersion of repolarization, however, have not been well characterized.
METHODS AND RESULTS. To determine the effects of abrupt cycle length (CL)
shortening on spatial inhomogeneity of repolarization in a syncytium of
ventricular cells, 124 action potentials were simultaneously recorded from
Langendorff-perfused guinea pig hearts using high-resolution optical
mapping with voltage-sensitive dye. The distribution of ventricular action
potential durations (APDs) mapped during each cardiac cycle was used to
calculate mean APD and repolarization dispersion index (DI), defined as the
variance of the distribution. After abruptly shortening CL from 500 to 300
msec, mean APD declined exponentially in normoxic controls (by 23 +/- 3
msec, p less than 0.0001). This response was characterized by beat-to-beat
oscillations of APD that were synchronized at all ventricular sites. After
30 minutes of hypoxia, mean APD decreased from 175.0 +/- 13.3 to 76 +/-
25.7 msec. However, during hypoxia, abrupt CL shortening lowered mean APD
by only an additional 6 +/- 6 msec, and APD oscillations were no longer
synchronized throughout the ventricle. In controls, beat-to- beat DI
decreased significantly (-51.0 +/- 6.8%, p less than 0.01) by the sixth
post-CL shortening beat and then recovered (by 15-20 beats). In contrast,
DI failed to decrease during hypoxia (+7.1 +/- 23%). Two mechanisms for the
transient decline of DI in controls were identified: synchronous APD
oscillations and transient diminution of the apical-to- basal ventricular
APD gradient. CONCLUSIONS. These data demonstrate that inhomogeneity of
ventricular repolarization, as measured by DI, changes dynamically with CL
shortening. Furthermore, the hypoxic ventricle does not attenuate DI after
abrupt CL shortening and thereby lacks a physiological response expected to
diminish vulnerability to arrhythmias.
ARTICLES
Repolarization inhomogeneities in ventricular myocardium change dynamically with abrupt cycle length shortening
Massachusetts General Hospital, Cardiac Unit, Boston.
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