Circulation, Vol 90, 2308-2314, Copyright © 1994 by American Heart Association
C Hwang, CD Swerdlow, RM Kass, ES Gang, WJ Mandel, CT Peter and PS Chen
BACKGROUND--The upper limit of vulnerability is the stimulus strength above
which electrical stimulation cannot induce ventricular fibrillation even
when the stimulus occurs during the vulnerable period of the cardiac cycle.
The purpose of this study was to test the hypothesis that the upper limit
of vulnerability can accurately predict the defibrillation threshold in
patients undergoing implantable cardioverter-defibrillator (ICD)
implantation using nonthoracotomy lead systems. METHODS AND RESULTS--We
studied 77 patients at the time of ICD implantation. Multiple
endocardial-endocardial and endocardial- subcutaneous shock pathways were
used. Two different protocols were used to test the upper limit of
vulnerability. In protocol 1 (n = 17), the upper limit of vulnerability was
tested with two shocks on the peak or the up-slope of the T wave of paced
rhythm. The shocks were given randomly either at the peak and 20
milliseconds before the peak of T wave (n = 7) or at 20 and 40 milliseconds
before the peak of T wave (n = 10). In protocol 2 (n = 60), the upper limit
of vulnerability was tested with three shocks delivered at 0, 20, and 40
milliseconds before the peak of the T wave. The weakest shock that failed
to induce ventricular fibrillation by a 5-J step-down or step-up method was
defined as the upper limit of vulnerability. The defibrillation threshold
was also determined by a 5-J step-down or step-up method. In protocol 1,
the upper limit of vulnerability (9 +/- 6 J) was significantly lower than
the defibrillation threshold (13 +/- 7 J) with a correlation coefficient of
.87 and P < .001. In protocol 2, the upper limit of vulnerability (13
+/- 6 J) was not significantly different from the defibrillation threshold
(13 +/- 6 J) with a correlation coefficient of .85 and P < .001. In 45
of the 60 patients, the upper limit of vulnerability was < or = 15 J;
all had a defibrillation threshold of < or = 20 J. In 51 of the 60
patients, the upper limit of vulnerability was within 5 J of the
defibrillation threshold. The upper limit of vulnerability overestimated
the defibrillation threshold by > 10 J in 8 patients and underestimated
the defibrillation threshold by > 10 J in only 1 patient. The
overestimation and underestimation occurred only in patients with the upper
limit of vulnerability > 15 J. CONCLUSIONS--When tested with three
shocks on and before the peak of the T wave, the upper limit of
vulnerability accurately predicted the defibrillation threshold in patients
undergoing ICD implantation using nonthoracotomy lead systems. This method
required either one or no episodes of ventricular fibrillation in most
patients.
ARTICLES
Upper limit of vulnerability reliably predicts the defibrillation threshold in humans
Division of Cardiology, Cedars-Sinai Medical Center, Los Angeles, CA 90048.
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