Mechanism of mechanical alternans induced by ischemia and
reperfusion: role of impaired and incomplete relaxation of the strong
twitch.
Nwasokwa, Obi N.
Division of Cardiology, Harris Chasanoff Heart Institute, Long
Island Jewish Medical Center, New Hyde Park, N.Y. 11042
APStracts 2:0088A, 1995.
We tested the hypothesis that impaired and incomplete relaxation of
the strong twitch of mechanical alternans causes the peak force
deficit of the weak; and that by decreasing the relaxation deficit of
the strong twitch, dobutamine would diminish the peak force deficit.
We studied isometric twitches of the in-situ blood-perfused canine
papillary muscle (n=8). To produce mechanical alternans, we paced the
heart at 110 - 155/min and decreased mean coronary perfusion pressure
(MCPP) stepwise to produce ischemia and then increased it to produce
reperfusion. We measured the relaxation deficit and peak force
deficit and fit each curve of isometric force, F(t), with the
relation: ( is force at twitch onset) to obtain the parameters A, B
and C. B is a dimensionless index of myocardial relaxation; it
decreases with impaired (delayed) relaxation. At each MCPP, we
averaged B for the strong and weak twitches. The peak force deficit
showed a positive correlation with the relaxation deficit. At each
MCPP, mean B for the strong twitch was lower than for the weak
indicating impaired relaxation of the strong twitch. Dobutamine
increased B from 1.83+0.14 to 2.12+0.16 (p=0.00002) in the strong
twitch; and decreased B from 4.15+2.42 to 2.19+0.18 (p=0.05) in the
weak. Dobutamine thus equalized the relaxation of the strong and weak
twitches. Consequently it decreased the relaxation deficit from
2.57+2.14 g to 0.16+0.24 g (p=0.01) and the peak force deficit from
5.50+3.67 g to 1.04+1.15 g (p=0.01). Thus incomplete relaxation of
the strong twitch accounts for the peak force deficit of mechanical
alternans.
Received 19 December 1994; accepted in final form 28 February
1995.
APS Manuscript Number A1111-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 21 March 1995.