EP Lab Digest - May 2008 - (Page 57)

MAY EP CASE STUDY DEFIBRILLATION ENERGY Continued from page 55 ‘path of least resistance’ for the energy. However, since the creation of the external defibrillator, high transthoracic impedance has been the enemy.“High energy is necessary for MDS [monophasic damped sinusodial] waveforms, since the waveform shape degrades in the face of high impedance… Traditional escalating energy protocols were based on the notion that a failed defibrillation attempt indicated the presence of high transthoracic impedance.”3 A hallmark of the biphasic truncated exponential waveforms is the fact that they are not degraded by high impedance, and some of these defibrillators feature algorithms that detect the impedence and compensate for it through customization of the peak current, rate of current decline, and duration of current delivery to ensure the selected energy is delivered.3 In the electrophysiology lab, our patients are tethered to a myriad of electrical conductors known to lower the natural impedance of the body.An average patient is connected to a hemodynamic monitor, a 12-lead electrocardiographic recording system, and indifferent/return electrodes, in addition to the defibrillator pads and its leads, and any number of intra-cardiac pacing and recording electrodes. In electrophysiology we are also taught to arrange these connections in a manner that helps us reach our goal of generating as little impedance and noise as possible. Some components tested by the engineers were found to have impedences of less than 50 ohms.Although many of these connections are designed to be ‘defibrillation proof’ (as indicated at each point of connection to the patient by the icon of a heart shielded within a box; Figure 2), some are not.We can also be lulled into a false sense of security by the term ‘defibrillation proof,’ since this generally refers to the fact that the receiving product is shielded from damage by an over-voltage situation, such as the inflow of current known to be a concern in the case of defibrillation. It does not guarantee that current is not shunted to the device, only that it “is isolated to such a degree that no current higher than the allowable patient leakage current… flows into it from an application of external voltage source to the patient.”4 Per our hospital policy, this allowable leakage should be no more than 50 microamps per patient connection, which is far less than 1% of the current flow used in defibrillation. The theory of the manufacturer is that the shunting occurred through these multiple low-resistance pathways created by one or more of our external or internal patient connections, including the intra-cardiac pacing and recording electrodes.We believe it is also possible that these alternate paths reduced the functional intra-thoracic impedance at the time of defibrillation below what was detected by the pads, thereby causing those same impedance compensation algorithms that are designed to customize the current delivery pattern to deliver an insufficient current. As seen in Figure 3, “low impedance requires higher peak current to address greater shunting.”3 If an intra-thoracic environment was created by these numerous low-resistance connections where the impedance was 25 ohms lower than what was detected by the pads on the skin surface, the defibrillator’s algorithm would have delivered the incorrect biphasic waveform. Testing by the defibrillator manufacturer stopped short of tracing the actual path of energy diversion, but we have found several pieces of indirect evidence that are suggestive of excessive current leakage. Following their tests of our equipment, in which 150-J was delivered, the personality module that connects our RF generator to our mapping system became inoperable. We have also had system errors following monophasic cardioversion of patients during electrophysiology studies that have required intervention, from hardware re-booting to exchanging whole systems in order to complete the procedure.In one case,a pin fracture occurred in our electrophysiology recording system’s amplifier following a 100-J monophasic shock, rendering the channel unusable without repair at the factory. Since that event, we have temporarily disconnected the EP recording and ablation systems from the patients prior to monophasic cardioversion without further evidence of damage to equipment or procedure interruption. A search of MedSun, the FDA Medical Product Safety Network for adverse event reporting, found only one reported incident of biphasic defibrillator failure. In that case, ICD defibrillator threshold testing was unsuccessful and rescue shocks were attempted using that facility’s biphasic defibrillator. Several shocks were attempted using escalating energy up to 200-J. These shocks failed to terminate VF.The staff had to bring in a monophasic defibrillator from a separate care unit to successfully defibrillate the patient. As was the case with our equipment, the unit was inspected and tested and found to be functioning normally. Concrete clinical information on this issue is scarce, which may be due in large part to the nature of our patients’ conditions and our experiences in cardiac arrest settings where multiple shocks and chest compressions are often necessary to return spontaneous circulation.However,it is important to remember that VT or VF induced during an EP procedure should terminate with relative ease. Defibrillation fails because the initial cause persists, the energy traveling through the myocardium is insufficient, or both. In the case of induced VT or VF, the initial causative factor is transient and immediately removed. It is initiated by pacing as opposed to tissue hypoxia from an acute MI.The mass depolarization of the myocardium should create a sufficient refractory period in all areas of the heart. This will block any re-entrant pathways that might be present, if that mass depolarization is actually achieved. It is for these reasons that incidents of induced VF not cleanly terminated with external defibrillation should be viewed with some concern and critiqued. In any case of multiple shocks, the EP team should ask themselves “is there a reasonable cause for this patient to have required multiple shocks?” If not, the event should be reported. Our lab has filed a report of each incident with the FDA using the MedWatch Form 3500 for voluntary reporting of adverse events or product problems (available at www.accessdata.fda.gov/scripts/medwatch or found by following the links on the FDA’s webpage at www.fda.gov). They encourage you to report these events “even if you are not certain the product caused the event” or if you “do not have all of the details.” The hope in publishing this case study is to increase awareness of this phenomenon within the electrophysiology community so these incidents will be tracked and reported in greater number to the FDA as appropriate. The manufacturers of all these systems we use need to study this further to analyze what role, if any, their particular product plays in this, and to help identify the cause. By doing so, the EP staff and our industry partners can then take whatever steps are necessary to prevent harm to our patients and the tragic loss of life from the elective induction of a lethal arrhythmia without a properly working safety net. References 1. Schneider T, Martens PR, Paschen H, et al. Multicenter, randomized, controlled trial of 150-J biphasic shocks compared with 200- to 360-J monophasic shocks in the resuscitation of out of hospital cardiac arrest victims. Circulation 2000;102:1780–1787. 2. Cummins RO, Hazinski MF Baskett PJF et al. , , Guidelines 2000 for cardiopulmonary resuscitation and emergency cardiac care. Supplement to Circulation 2000;102:I-63. 3. Cooke D.Transthoracic biphasic defibrillation: the case for non-escalating energy protocols. EP Lab Digest 2002;3:1–7. 4. Richardson S.Challenges in testing:automating the tests in IEC 6060-1. Conformity 2008:26-31. http://www.accessdata.fda.gov/scripts/medwatch http://www.fda.gov http://www.modulardevices.com

Table of Contents Feed for the Digital Edition of EP Lab Digest - May 2008

EP Lab Digest - May 2008 Is There an Under-Referral of Women for Atrial Fibrillation Ablation? A Hybrid Approach to the Cure of Atrial Fibrillation Contents Letter from the Editor Spotlight Interview: University of Michigan Health System 10-Minute Interview: Amit J. Shanker, MD Modular EP Units: Are They Possible? One Hospital’s Experience All About My Job: Technician Supervisor (New Column!) Alcohol Septal Ablation in the Cath Lab: What is it All About? About the Canadian Heart Rhythm Society: Interview with Martin J. Gardner, MD, FRCP(C), FACC Email Discussion Group: May 2008 Second Annual Salary Survey My First Year in Electrophysiology: What Have I Got Myself Into? Highlight on Technology: Video-Audio Integration for the EP Lab On the Horizon: A New Remote Catheter Manipulation System Striving for Excellence in the Care of Cardiac Patients Five Reasons to Participate with a Professional Organization Suspected Shunting of Defibrillation Energy in the EP Lab Can Digital Music Players Cause Interference with Implantable Devices? ECG 101: Ambulatory ECG Monitoring Events Calendar Industry News and Products Classifieds Advertisers Index

EP Lab Digest - May 2008

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