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Correspondence  |   December 2007
Etomidate Unlikely to Have Induced Pacemaker-mediated Tachycardia
Author Notes
  • University of Texas M. D. Anderson Cancer Center, Houston, Texas.
Article Information
Correspondence
Correspondence   |   December 2007
Etomidate Unlikely to Have Induced Pacemaker-mediated Tachycardia
Anesthesiology 12 2007, Vol.107, 1036. doi:10.1097/01.anes.0000290591.72867.22
Anesthesiology 12 2007, Vol.107, 1036. doi:10.1097/01.anes.0000290591.72867.22
To the Editor:—
Although I agree that the case “Etomidate-induced Pacemaker-mediated Ventricular Tachycardia”1 could represent an important contribution to our literature, I am concerned that the data presented in this article cannot justify the conclusion that myoclonus from the etomidate was actually responsible for this pacemaker-driven tachycardia (this is not actually an arrhythmia). Therefore, this article might be cited by future practitioners when, in fact, etomidate likely made little or no contribution to the ventricular pacing shown at 140 beats/min.
The rate-responsive mechanism in this Medtronic Prodigy pacemaker (St. Paul, MN) is a piezo crystal attached to the case of the pacemaker.2 It is activated by case deformation (pressing on the case). The rate-responsive algorithm has a number of programmable settings that determine how the pacemaker will change the pacing rate (called activity-indicated rate) due to stimulation of this crystal. These settings include the following (not specified in the article): lower pacing rate, upper activity rate, activity threshold, activity rate response, acceleration time, and deceleration time. In the default setting, the acceleration time is 0.5 min, so in 30 s, the pacemaker will achieve 90% of the difference between the current paced rate and the higher (calculated) activity-indicated rate consistent with the activity level.
The default deceleration time is 5 min and is of interest here. Upon abrupt cessation of activity, the pacing rate begins decreasing almost immediately, and it will decrease over 5 min by 90% from the current (high) paced rate to the new, lower activity-indicated rate. In this case, assuming that the lower pacing and upper activity rates had been programmed to 70 and 140 beats/min, respectively, the actual pacing rate would have been approximately 122 beats/min at 30 s, 109 beats/min at 1 min, and 92 beats/min at 2 min (fig. 1). No time intervals were reported in the article, but 30 or more seconds likely elapsed from the last patient movement to the reprogramming of the pacemaker.
Fig. 1. The deceleration operational graph from the Medtronic Prodigy Pacemaker Manual on page 11-9 was adapted to the presumed programmed parameters for this case to predict the pacing rate after abrupt cessation of activity. For this adaptation, the lower rate was assumed to be 70 beats/min, the upper activity rate was assumed to be 140 beats/min, and the deceleration time was assumed to be 5 min, the default setting. The  vertical lines  at 30 s, 1 min, and 2 min were added to show that the expected paced rate would have been 122, 109, and 92 beats/min, respectively, after the cessation of mechanical activity detected by the pacemaker and interpreted as patient activity. Original artwork copyright 1995 by Medtronic Corporation, Minneapolis, Minnesota, and reproduced with permission of Medtronic, Inc. The Medtronic Corporation takes no responsibility for the validity or accuracy of any modification to the illustration. 
Fig. 1. The deceleration operational graph from the Medtronic Prodigy Pacemaker Manual on page 11-9 was adapted to the presumed programmed parameters for this case to predict the pacing rate after abrupt cessation of activity. For this adaptation, the lower rate was assumed to be 70 beats/min, the upper activity rate was assumed to be 140 beats/min, and the deceleration time was assumed to be 5 min, the default setting. The  vertical lines  at 30 s, 1 min, and 2 min were added to show that the expected paced rate would have been 122, 109, and 92 beats/min, respectively, after the cessation of mechanical activity detected by the pacemaker and interpreted as patient activity. Original artwork copyright 1995 by Medtronic Corporation, Minneapolis, Minnesota, and reproduced with permission of Medtronic, Inc. The Medtronic Corporation takes no responsibility for the validity or accuracy of any modification to the illustration. 
Fig. 1. The deceleration operational graph from the Medtronic Prodigy Pacemaker Manual on page 11-9 was adapted to the presumed programmed parameters for this case to predict the pacing rate after abrupt cessation of activity. For this adaptation, the lower rate was assumed to be 70 beats/min, the upper activity rate was assumed to be 140 beats/min, and the deceleration time was assumed to be 5 min, the default setting. The  vertical lines  at 30 s, 1 min, and 2 min were added to show that the expected paced rate would have been 122, 109, and 92 beats/min, respectively, after the cessation of mechanical activity detected by the pacemaker and interpreted as patient activity. Original artwork copyright 1995 by Medtronic Corporation, Minneapolis, Minnesota, and reproduced with permission of Medtronic, Inc. The Medtronic Corporation takes no responsibility for the validity or accuracy of any modification to the illustration. 
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Therefore, I believe that the continued pacing at 140 beats/min suggests that something else was affecting the pacemaker. It might have been the weight of the programming head. In fact, the Prodigy manual warns: “Clinical studies of activity rate responsive pacemakers have reported instances of … an increase in the pacing rate up to the programmed Upper Activity Rate … by pressing the programmer head over the pacemaker.”
The stability of the heart rate (determined by the intervals between the QRS complexes) in the author's published figure 2 suggests that the pacemaker was sensing continuous mechanical activity. If the mechanical activity had ceased, the pacing rate would have been slowing, which would resulted in the successive lengthening of the intervals between the QRS complexes.
This article is an important reminder that pacemakers (and implanted cardioverter–defibrillators) are mere computers that respond to electrical and environmental signals in a predictable fashion. The authors should have emphasized that these devices can produce unusual pacing behavior that might be misinterpreted by clinicians (likely true in this case) and, therefore, induce inappropriate treatment, which did not happen here. For example, pressure on the chest over a pacemaker or defibrillator due to instruments or surgical personnel, as well as mechanical activity over a pacemaker or defibrillator from surgical site preparation, can lead to a paced tachycardia approaching the upper activity rate. In fact, misinterpretation of pacemaker (pseudomalfunction) behavior is common,3 and sometimes it leads to disastrous results with patient injury.4 
Finally, the comment in this article that “It is essential that the pacemaker should be programmed to an asynchronous mode” is not justified by any part of the case presentation, the discussion, any literature, or any manufacturer recommendation. Pacemaker and implantable cardioverter–defibrillator manufacturers do recommend a comprehensive interrogation of any device that has “seen” an external cardioversion or defibrillation.
University of Texas M. D. Anderson Cancer Center, Houston, Texas.
References
Altose M, Leon-Ruiz E: Etomidate-induced pacemaker-mediated ventricular tachycardia. Anesthesiology 2007; 106:1059–60Altose, M Leon-Ruiz, E
Medtronic: Prodigy Pacemaker Information and Programming Guide. St. Paul, Minnesota, Medtronic, 1995 St. Paul Minnesota, Medtronic
von Knobelsdorff G, Goerig M, Nagele H, Scholz J: Interaction of frequency-adaptive pacemakers and anesthetic management: Discussion of current literature and two case reports [in German]. Anaesthesist 1996; 45:856–60von Knobelsdorff, G Goerig, M Nagele, H Scholz, J
Lau W, Corcoran SJ, Mond HG: Pacemaker tachycardia in a minute ventilation rate-adaptive pacemaker induced by electrocardiographic monitoring. Pacing Clin Electrophysiol 2006; 29:438–40Lau, W Corcoran, SJ Mond, HG
Fig. 1. The deceleration operational graph from the Medtronic Prodigy Pacemaker Manual on page 11-9 was adapted to the presumed programmed parameters for this case to predict the pacing rate after abrupt cessation of activity. For this adaptation, the lower rate was assumed to be 70 beats/min, the upper activity rate was assumed to be 140 beats/min, and the deceleration time was assumed to be 5 min, the default setting. The  vertical lines  at 30 s, 1 min, and 2 min were added to show that the expected paced rate would have been 122, 109, and 92 beats/min, respectively, after the cessation of mechanical activity detected by the pacemaker and interpreted as patient activity. Original artwork copyright 1995 by Medtronic Corporation, Minneapolis, Minnesota, and reproduced with permission of Medtronic, Inc. The Medtronic Corporation takes no responsibility for the validity or accuracy of any modification to the illustration. 
Fig. 1. The deceleration operational graph from the Medtronic Prodigy Pacemaker Manual on page 11-9 was adapted to the presumed programmed parameters for this case to predict the pacing rate after abrupt cessation of activity. For this adaptation, the lower rate was assumed to be 70 beats/min, the upper activity rate was assumed to be 140 beats/min, and the deceleration time was assumed to be 5 min, the default setting. The  vertical lines  at 30 s, 1 min, and 2 min were added to show that the expected paced rate would have been 122, 109, and 92 beats/min, respectively, after the cessation of mechanical activity detected by the pacemaker and interpreted as patient activity. Original artwork copyright 1995 by Medtronic Corporation, Minneapolis, Minnesota, and reproduced with permission of Medtronic, Inc. The Medtronic Corporation takes no responsibility for the validity or accuracy of any modification to the illustration. 
Fig. 1. The deceleration operational graph from the Medtronic Prodigy Pacemaker Manual on page 11-9 was adapted to the presumed programmed parameters for this case to predict the pacing rate after abrupt cessation of activity. For this adaptation, the lower rate was assumed to be 70 beats/min, the upper activity rate was assumed to be 140 beats/min, and the deceleration time was assumed to be 5 min, the default setting. The  vertical lines  at 30 s, 1 min, and 2 min were added to show that the expected paced rate would have been 122, 109, and 92 beats/min, respectively, after the cessation of mechanical activity detected by the pacemaker and interpreted as patient activity. Original artwork copyright 1995 by Medtronic Corporation, Minneapolis, Minnesota, and reproduced with permission of Medtronic, Inc. The Medtronic Corporation takes no responsibility for the validity or accuracy of any modification to the illustration. 
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