(Circulation. 1999;100:413-418.)
© 1999 American Heart Association, Inc.
Basic Science Reports |
From the Cardiac Arrhythmia Service, New England Medical Center, Tufts University School of Medicine, Boston, Mass (M.S.L., P.J.W., B.A.V., E.A., N.G.P., N.A.M.E.), and the Cardiovascular Research Division, Minneapolis Heart Institute Foundation, Minneapolis, Minn (B.J.M.).
Correspondence and Reprint Requests to Mark S. Link, MD, NEMC Box #197, New England Medical Center, 750 Washington Street, Boston, MA 02111. E-mail Mark.Link{at}es.NEMC.org
| Abstract |
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Methods and ResultsIn the initial experiment, 6 juvenile swine were given 0.5 mg/kg IV glibenclamide, a selective inhibitor of the K+ATP channel, and chest impact was given on the QRS. The results of these strikes were compared with animals in which no glibenclamide was given. In the second phase, 20 swine were randomized to receive glibenclamide or a control vehicle (in a double-blind fashion), with chest impact delivered just before the T-wave peak. With QRS impacts, the maximal ST elevation was significantly less in those animals given glibenclamide (0.16±0.10 mV) than in controls (0.35±0.20 mV; P=0.004). With T-wave impacts, the animals that received glibenclamide had significantly fewer occurrences of ventricular fibrillation (1 episode in 27 impacts; 4%) than controls (6 episodes in 18 impacts; 33%; P=0.01).
ConclusionsIn this experimental model of commotio cordis, blockade of the K+ATP channel reduced the incidence of ventricular fibrillation and the magnitude of ST-segment elevation. Therefore, selective K+ATP channel activation may be a pivotal mechanism in sudden death resulting from low-energy chest-wall trauma in young people during sporting activities.
Key Words: ventricular fibrillation death, sudden chest trauma sports
| Introduction |
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In our previously described model of commotio cordis,9 we observed ventricular fibrillation with 30-mph impacts occurring 15 to 30 ms before the peak of the T-wave. In addition, ST-segment elevation was produced by impacts on the QRS and ST segments. However, the cellular mechanisms responsible for these striking electrophysiological consequences of blunt chest impact are unknown. Because many of the profound electrophysiological consequences observed in our experimental model of commotio cordis are similar to those of myocardial ischemia, we hypothesized that the cellular mechanisms for ST-segment elevation and ventricular fibrillation in these 2 circumstances may also be comparable.
The cardiac K+ATP channel is normally inactive,10 as it is inhibited by physiological concentrations of ATP. The opening of the channel is associated with a reduction in the cellular ATP/ADP ratio, such as occurs in myocardial ischemia.11 12 It is thought that activation of this channel is responsible for the ST-segment elevation observed in myocardial ischemia.11 12 13 14 15 Furthermore, activation of this channel increases the likelihood of ventricular fibrillation occurring during myocardial ischemia.13 16 17 18 19 Glibenclamide is a sulfonylurea that primarily acts by blocking the K+ATP channel.20 21 22 Preventing K+ATP channel activation with glibenclamide decreases the magnitude of ST-segment elevation14 15 and the incidence of ventricular fibrillation in myocardial ischemia.13 16 17 18 19
To test the hypothesis that abrupt mechanical activation of the K+ATP channel may be a mechanism by which virtually instantaneous sudden death results from chest-wall impact, glibenclamide (a selective blocker of the K+ATP channel) was administered to animals subsequently subjected to low-energy chest-wall impact.
| Methods |
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A 5% solution of glibenclamide was constituted by dissolving 5
mg of glibenclamide in 1 mL of 1 mol/L NaOH, 1 mL of 70%
ethanol, and 0.7 mL of dimethyl sulfoxide in 100 mL of normal
saline.17 After infusing the active agent, animals were
placed prone in a sling to approximate
physiological blood flow and cardiac
hemodynamics (Figure 1
).
Chest-wall impact was produced by a spherical wooden object (150 g),
identical in size and weight to a baseball, propelled at 30 mph. The
impact object was directed, using echocardiographic
guidance, to strike the animal perpendicular to the chest wall directly
overlying the heart.
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In the first series of experiments, 0.5 mg/kg glibenclamide was administered intravenously to 6 animals, after which 1 to 3 chest impacts on the QRS segment of the cardiac cycle were delivered. The results of these experiments were compared with those previously performed without the prior administration of glibenclamide, in which 10 impacts were delivered to the QRS in 9 animals.9 The maximum ST-segment elevation in both experimental groups was measured in the first 2 beats after chest impact (0.08 s after the J-point).
In the second phase of the study, either the active agent (glibenclamide) or the control vehicle was administered to a total of 20 animals. Each animal received 1 to 3 chest impacts timed from -10 ms to -40 ms before the peak of the T-wave, a period vulnerable to the induction of ventricular fibrillation.9 The peak of the T-wave was determined from a monophasic T-wave on either lead 1 or 2 on surface ECG. If ventricular fibrillation occurred, no further impacts were given. All laboratory personnel were blinded to the identity of the infused solution (active or control) except the laboratory technician responsible for administering it.
Differences between groups of animals were analyzed with the
unpaired Student's t test for continuous variables (ie,
magnitude of ST elevation) and Fisher's Exact Test for nominal
variables (eg, incidence of ventricular fibrillation).
P
0.05 was considered significant.
| Results |
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T-Wave Impact
Ten animals received 0.5 mg/kg glibenclamide, and a total of 27
chest-wall blows were delivered on the upslope of the T-wave;
ventricular fibrillation occurred with only 1 of the 27
impacts (4%; ie, in 10% of the animals) (Table 2
and Figure 3
). Ventricular
tachycardia or transient polymorphic
ventricular tachycardia did not occur, and
premature ventricular contractions were induced by the
chest blow in all animals. In contrast, in the 10 animals who received
the control solution, 18 chest blows delivered at the upslope of the
T-wave produced 6 episodes of ventricular fibrillation
(33%; ie, in 60% of the animals; P=0.01) and 2 episodes of
nonsustained polymorphic ventricular
tachycardia. In those impacts that did not result in
ventricular fibrillation, no significant differences
existed between glibenclamide-treated and control animals with respect
to ST-segment elevation (0.11±0.07 versus 0.07±0.04 mV), bundle
branch blocks (4 of 26 [15%] versus 3 of 12 [25%]), transient
complete heart block (3 of 26 [12%] versus 2 of 12 [17%]), or
isolated premature ventricular beats (26 of 26 [100%]
versus 10 of 10 [100%]). Infusion of glibenclamide did not increase
the QT interval (QTc was 447±33 ms preglibenclamide versus 463±24 ms
postglibenclamide; P=0.26). The ST-segment elevation
observed with T-wave impacts was not significantly different from that
observed in our previously reported study (0.08±0.09
mV).9
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| Discussion |
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In experimental models of myocardial ischemia, ST-segment elevation and ventricular arrhythmias are due, in part, to selective activation of the K+ATP channel.11 12 13 14 15 16 17 18 19 Blocking this channel with glibenclamide reduces these electrophysiologic effects of ischemia.11 12 13 14 15 16 17 18 19 24 25 Because the ST-segment elevation and ventricular fibrillation seen in our experimental model of commotio cordis are similar to those reported in myocardial ischemia, we hypothesized that these entities may share the same or similar cellular mechanisms. Therefore, to test the hypothesis that activation of the K+ATP channel was involved in the profound electrophysiological consequences of modest chest-wall blows (including sudden cardiac death), we designed a double-blind study in which a selective blocker of the K+ATP channel (glibenclamide) or a placebo was administered to swine before delivering a chest blow. Glibenclamide was chosen because its ability to block the K+ATP channel has been well established.20 21 Indeed, in our present experiment with low-energy chest-wall blows, the incidence of ventricular fibrillation was significantly reduced from 33% of impacts (60% of animals) in controls to only 4% of impacts (10% of animals) in glibenclamide-treated animals. These results suggest that it is the immediate mechanical activation of the K+ATP channel by low-energy chest-wall impact that is, in part, responsible for the ventricular fibrillation seen in our experimental model and, by inference, in sudden death on the playing field in young athletes.
Prominent ST-segment elevation was also produced in our model of commotio cordis,9 and it has also been reported in a few survivors of commotio cordis.5 26 27 We found that ST-segment elevation was most pronounced with QRS impacts, but it was observed with chest impacts throughout the cardiac cycle.9 ST-segment elevations were most marked on the beat following the impact; then, they decayed and normalized over the next 30 to 60 s. In the present study, glibenclamide attenuated the ST-segment elevation observed with QRS impacts, substantiating our hypothesis that the K+ATP channel is activated by mechanical trauma to the chest wall in commotio cordis. It is presently unresolved as to why ST-segment elevation was more pronounced with QRS strikes compared with T-wave strikes.9
The present experiment provides convincing evidence that chest-wall
impacts occurring on both T-wave and QRS segments result in activation
of the K+ATP channel (Figure 4
). Whether this activation leads to
ventricular fibrillation or to ST-segment elevation depends
on the timing of the impact. As seen in our previous
experiment,9 only impacts that occurred during the
vulnerable time period of repolarization caused ventricular
fibrillation. That a vulnerable period for the initiation of
ventricular fibrillation exists suggests that a second
trigger or condition is necessary (in addition to
K+ATP channel activation) for
ventricular fibrillation to result from a mechanical force
such as a low-energy chest-wall blow. At the current time, this
additional trigger or condition is unknown, but it is likely related to
the dispersion of repolarization that is present during the
vulnerable period of the cardiac cycle.
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We previously observed marked hemorrhage in the AV bundle and bundle branches in 1 of 2 animals with transient heart block induced by chest-wall impact, suggesting that direct trauma to the conduction system caused the manifestations of heart block.9 In support of this theory, in the present study, we found no difference in the incidence of heart block or left bundle branch block in the glibenclamide-treated or control animals. Thus, these observations, taken together, suggest that the left bundle branch block and transient heart block that was observed in our experiments is primarily a consequence of direct mechanical trauma rather than activation of the K+ATP channel.
In conclusion, in this experimental model of commotio cordis, glibenclamide significantly decreased the incidence of ventricular fibrillation and the magnitude of ST segment elevation. Therefore, direct mechanical activation of the K+ATP channel seems to be an important cellular mechanism by which ventricular fibrillation occurs in commotio cordis.
| Acknowledgments |
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Received December 16, 1998; revision received March 15, 1999; accepted March 31, 1999.
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death caused by low-energy chest-wall impact (commotio cordis),
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observed with T-wave and QRS strikes, respectively. In the current
study, glibenclamide, a selective blocker of the
K+ATP channel, significantly reduced the
incidence of ventricular fibrillation and the magnitude of
ST-segment elevation in this model. Therefore, selective
K+ATP channel activation may be a pivotal
mechanism in sudden death resulting from low-energy chest-wall trauma
in young individuals.
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