Circulation, Vol 77, 1266-1275, Copyright © 1988 by American Heart Association
P Taggart, PM Sutton, T Treasure, M Lab, W O'Brien, M Runnalls, RH Swanton and RW Emanuel
Mechanical dysfunction is the strongest predictor of sudden cardiac death
due to arrhythmia. Contraction-excitation feedback whereby changes in
myocardial length/tension influence the time course of repolarization and
excitability would provide a possible mechanism. Such a relationship has
been shown in animals but has yet to be demonstrated in man. A useful model
for studying this relationship is provided by the process of weaning off
cardiopulmonary bypass after routine coronary artery surgery. During this
weaning period of approximately 1 min, the heart is converted from being
partially empty and flaccid (i.e., a "nonworking" state) to being filled
and stretched to support the circulation (i.e., a "working" state).
Monophasic action potentials (MAPs) were recorded from the left ventricular
epicardium as a measure of repolarization time in 16 patients at
discontinuation of cardiopulmonary bypass. Systolic pressure was recorded
from the radial artery line. Measurements were made at three stages that
related to different dynamic states of the heart: (1) starting to come off
bypass ("minimally working"), defined as the time of first appearance of an
inflection on the arterial pressure trace indicating the start of left
ventricular ejection and valve opening, when arterial pressures represent
left ventricular pressure, (2) half off bypass ("partially working"), and
(3) off bypass ("wholly working"). During the process of discontinuing
bypass MAP duration shortened, while systolic pressure increased. MAP
duration at 90% and 60% repolarization (MAP D90, MAP D60) decreased from
288.0 +/- 29.5 msec (mean +/- SEM) and 235.0 +/- 27.9 msec in the minimally
working heart to 274.5 +/- 30.2 msec and 224.2 +/- 27.3 msec in the
partially working heart (p less than .001), with a subsequent decrease to
261.0 +/- 28.8 and 214.0 +/- 28.7 when the heart was wholly working (p less
than .001). Systolic pressure increased from 54.1 +/- 9.3 mm Hg in the
minimally working heart to 65.9 +/- 13.8 mm Hg in the partially working
heart (p less than .001) and subsequently increased to 75.5 +/- 13.3 mm Hg
when the heart was wholly working (p less than .001). Mean heart rates did
not change significantly. A strong correlation was obtained between
absolute MAP duration and systolic pressure. Regression analysis revealed:
MAP D90 vs systolic pressure (p less than .001) and MAP D60 vs systolic
pressure (p less than .01).(ABSTRACT TRUNCATED AT 400 WORDS)
ARTICLES
Monophasic action potentials at discontinuation of cardiopulmonary bypass: evidence for contraction-excitation feedback in man
Department of Cardiology, Middlesex Hospital, London, England.
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