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Biphasic defibrillators

Created: 28/10/2005
 
HeartStart XL DefibrillatorUntil recently, the way in which electrical therapy was delivered by transthoracic defibrillators was essentially the same for all manufacturers. Most commonly, the defibrillation waveforms used were monophasic. Monophasic technology was constrained by the electronics components available during the era it originated (1960s), remained largely unchanged over time, and never had substantive research to support its performance. Further, the waveforms used energy inefficiently and were not able to adjust effectively to a patient’s chest impedance.

Without effective defibrillator impedance compensation, high patient impedance degrades the waveform, a key factor in the relatively poor performance of traditional uncompensated monophasic technologies. Low impedance imposes a different set of potential problems. As will be described in greater detail later, low impedance patients may be more likely to shunt current away from the heart.

Today, modern electronics permit much greater control of therapy generation and delivery, including the ability to compensate for the untoward effects of high and low patient impedance. In the next sections, we examine how a well designed modern defibrillator addresses crucial dosing factors to deliver safe and effective electrical medicine.


Dosing Factor 1: Seconds Count; Calculate the Correct Dose for the First Time


There is ample evidence that speed to an effective first shock matters; even as little as a minute difference in time to first shock affects patient outcome.3,6,17,18 The challenge for the defibrillator, then, is to effectively deliver the right amount of current on the first shock.

Figure 4. Shock Efficacy






It is the pattern of current flow, not energy, that enables defibrillation. Voltage, current and the time course of waveform delivery all affect energy delivered to the patient. It is now possible for the defibrillator to manipulate any of these electrical characteristics to deliver an effective current pattern while using energy efficiently.

Escalating energy is no longer required. The solution to problems imposed by patient impedance is, instead, to design a defibrillation waveform that effectively measures and compensates for patient impedance, delivering the correct dose of current (and energy) on the first shock. One of the challenges to delivering the correct dose of current, however, is to design a waveform that addresses the issue of shunted current, thought to be particularly an issue in low impedance patients.

A defibrillator delivers current across the chest ("transthoracic current"), but it is the proportion of the transthoracic current crossing the heart ("transcardiac current") that is clinically meaningful. Unfortunately, however, only a fraction of the current delivered by the defibrillator flows to the heart.19 The rest is divereted or shunted, to the surrounding areas of the chest. To compensate for this current shunting phenomenon, a well-designed defibrillator provides sufficient transthoracic current to supply effective transcardiac current, even in the presence of shunt pathways, as demonstrated in Figure 8.


Figure 5. Return of Spontaneous Circulation (ROSC)




In summary, by manipulating the defibrillator electronics and optimizing the waveform to address issues such as high impedance and shunting, it is now possible to achieve defibrillation on the first dose using a carefully calibrated fixed low-energy waveform design.

In fact, there is extensive and persuasive evidence that the Philips 150J BTE waveform performs as well as or, in most studies, far better on the first shock than the "gold standard" monophasic defibrillation waveform, without the need to escalate.7,9,17,18,20,25,30

Figure 6. Brain Function in Survivors



In one representative pre-hospital study comparing 150 J SMART Biphasic to monophasic defibrillation,7 the Philips waveform was associated with superior efficacy (96% on the first shock, 98% by the third shock, and 100% patient efficacy), improved return of spontaneous circulation (ROSC), and better neurological outcomes in survivors, despite long call-to-first shock times averaging 8.9 minutes (see Figures 4, 5, and 6). No other defibrillator manufacturer can offer comparable patient outcome and waveform performance data for an ischemic sudden cardiac arrest (SCA) patient population.

Dosing Factor 2: Ensure that the Dose is Measured Accurately and Efficiently, Over the Correct Time Course

Another critical factor in achieving effective defibrillation is to deliver an appropriately measured dose of current for the correct amount of time. The engine of this process is a properly sized defibrillator capacitor. The size of the capacitor("capacitance", measured in microfarads, or µF) is crucial to effective and efficient defibrillator design.

Figure 7. Peak Current Levels




To prepare for a defibrillator shock, a defibrillator's capacitor must be charged to a voltage high enough to drive appropriate current through the resistance of the patient's chest throughout the time course of the shock. Energy is stored in preparation for defibrillation when the capacitor is charged. The larger the capacitor, the larger the amount of energy that must be stored in order to achieve the voltage necessary to initiate an appropriate dose of defibrillation current.

It is possible, however, to design a system in which energy is used efficiently, not requiring as much energy as has been historically the case with traditional monophasic waveforms. Recognizing this, Philips Medical Systems patented an optimal 100 µF capacitor design for its impedance-compensating SMART Biphasic waveform. The Philips capacitor requires little energy during charging, yet achieves the necessary voltage required to create effective defibrillation currents throughout the 150J shock.

In contrast, some other modern defibrillator designs use larger capacitors (200 µF). These designs require twice as much energy in order to achieve the same patient currents available with the Philips low-energy 100µF design.

Figure 7 contrasts the 150J SMART Biphasic waveform, using a 100 µF capacitor, with other high-energy waveform designs. The Philips waveform achieves more current delivering 150J than a 200 µF design delivering 200 Joules. The current of the high-energy biphasic defibrillator becomes comparable to the Philips waveform only when the energy reaches 300 Joules - on the second shock.

It should also be noted from Figure 7 that modern biphasic waveform designs deliver far less current than some historic monophasic waveform systems. The modern biphasic technologies have been designed to be effective at comparatively lower peak currents. None of the commerically available biphasic defibrillators reaches current level regarded as potentially dangerous, as is the case with their monophasic predecessors.

In short, the Philips system uses a proprietary low-capacitance design to efficiently generate a waveform personalized to patient impedance. This approach yields consistently favorable results even in challenging long down-time patient populations.

Dosing Factor 3: Deliver the Current (and Energy) Over the Correct Amount of Time for Each Patient Regardless of Impedance - A Personalized Waveform


The last critical dosing factor involves the design of the waveform, which delivers a changing pattern of current to the patient throughout the duration of the shock to accommodate variations in patient impedance. Since this current pattern is sometimes adversely affected by patient impedance, a well designed waveform must measure patient impedance and adjust the waveform shape and duration accordingly, optimizing waveform performance across the range of anticipated impedance values.

Figure 8. SMART Biphasic Impedance Compensation



A defibrillator waveform should compensate for both high and low chest impedance. Patient impedance in humans has been shown to vary anywhere from 25 to 180 ohms. According to Ohm’s Law (I = V/R), a high impedance patient resists the flow of current and, therefore, the peak current is less; the peak current in a low impedance patient is comparatively higher. This Ohm’s Law relationship is illustrated in Figure 8, which shows energy fixed at 150J and the Philips SMART Biphasic waveform shape and duration adjusting actively based on patient impedance.

The shape and duration variations shown in Figure 8 have been carefully designed based on peer-reviewed evidence specific to the Philips waveform.22 Through this research, the "sweet spot" for waveform shape and duration was determined for the SMART Biphasic waveform using a fixed, 150J adult defibrillation protocol.

Based on this research, the SMART Biphasic waveform is designed to perform across a wide range of anticipated patient impedance values. In the case of high impedance patients, the waveform lengthens to deliver adequate energy. For low impedance patients, the defibrillator delivers somewhat higher peak currents to compensate for the possible effects of shunting.

Because Philips measures impedance and dynamically varies these waveform attributes accordingly on every shock, it is simply not necessary to increase the energy on successive shocks. The optimal therapy is delivered with the first shock, as with every shock.

The performance of the Philips SMART Biphasic waveform has been tested in numerous peer-reviewed manuscripts, the number and breadth of which far exceeds that of any other manufacturer. These published studies reflect waveform performance both in animals 14,22-24 and in humans.

Of the 17 published human studies, to date, three report on experience with in-hospital induced, short-duration ventricular fibrillation (VF)9,18,25 and 14 address performance with the challenging long duration VF relevant to out-of-hospital and other delayed defibrillation settings.4-8,17,18,20,21,26,30 These data reflect performance consistently equal or superior to that of high-energy escalating therapies, regardless of factors such as: patient size, age, or impedance, underlying cause of SCA, including myocardial infarction.

References


1. Kerber RE et al. Automatic external defibrillators for public access defibrillation: recommendations for specifying and reporting arrhythmia analysis algorithm performance, incorporating new waveforms, and enhancing safety: a statement for health professions from the AHA Task Force on Automatic External Defibrillation, Subcommittee on AED Safety and Efficacy. Circulation 1997; 95: 1677-82.
2. Cummins RO et al. Low-energy biphasic waveform defibrillation: evidence-based review applied to emergency cardiovascular care guidelines: a statement for healthcare professionals from the American Heart Association Committee on Emergency Cardiovascular Care and the Subcommittees on Basic Life Support, Advanced Cardiac Life Support, and Pediatric Resuscitation. Circulation 1998; 97: 1654-67.
3. Cummins RO et al. Guidelines 2000 for cardiopulmonary resuscitation and emergency cardiovascular care. Supplement to Circulation 2000; 102: I-5, I-63, I-91.
4. White RD et al. Body weight does not affect defibrillation, resuscitation or survival in patients with out-of-hospital cardiac arrest treated with a non-escalating biphasicwaveform defibrillator. Crit Care Med 2004; 32(9 Suppl): S387-92.
5. White RD et al. Transthoracic impedance does not affect defibrillation, resuscitation, or survival in patients with out-of-hospital cardiac arrest treated with a non-escalating biphasic waveform defibrillator. Resuscitation 2005; 64: 63-9.
6. Capucci A et al. Tripling survival from sudden cardiac arrest via early defibrillation without traditional education in cardiopulmonary resuscitation. Circulation 2002; 106: 1065-70.
7. Schneider T et al. Multicenter, randomized, controlled trial of 150-J biphasic shocks compared with 200-360-J monophasic shocks in the resuscitation of out-of-hospital cardiac arrest victims. Circulation 2000; 102: 1780-7.
8. Page RL et al. Use of automated external defibrillators by a U.S. airline. N Engl J Med 2000; 343: 1210-16.
9. Bardy GH, et al. Multicenter comparison of truncated biphasic shock and standard damped sine wave monophasic shocks for transthoracic ventricular defibrillation. Circulation 1996; 94: 2507-14.
10. Xie J et al. High-energy defibrillation increases the severity of postresuscitation myocardial function. Circulation 1997; 96: 683-8.
11. Weaver WD et al. Ventricular defibrillation - a comparative trial using 175J and 320J shocks. N Engl J Med 1982; 307: 1101-6.
12. Reddy RK et al. Biphasic transthoracic defibrillation causes fewer ECG ST-segment changes after shock. Ann Emerg Med 1997; 30: 127-34.
13. Tokano J et al. Effects of ventricular shock strength on cardiac hemodynamics. J Cardiovasc Electrophysiol 1998; 9: 791-7.
14. Tang W et al. The effects of biphasic and conventional monophasic defibrillation on postresuscitation myocardial function. J Am Coll Cardiol 1999; 34: 815-22.
15. Tang W et al. The effects of biphasic waveform design on post-resuscitation myocardial function. J Am Coll Cardiol 2004; 43: 1228-35.
16. Tovar OH et al. Immediate termination of fibrillation at 50% probability of overall success correlates with defibrillation dose-response curve width. J Cardiovasc Electrophysiol 2004; 15: 1207-11.
17. White RD et al. Patient outcomes following defibrillation with a low energy biphasic truncated exponential waveform in out-of-hospital cardiac arrest. Resuscitation 2001; 49: 9-14.
18. White RD. Early out-of-hospital experience with an impedance-compensating low-energy biphasic waveform automatic external defibrillator. J Interv Card Electrophysiol 1997; 1: 203-8.
19. Lerman BB et al. Relation between transcardiac and transthoracic current during defibrillation in humans. Circ Res 1990; 67: 1420-6.
20. Gliner BE et al. Treatment of out-of-hospital cardiac arrest with a low-energy impedance-compensating biphasic waveform automatic external defibrillation. Biomed Instrum Technol 1998; 32: 631-44.
21. White RD, Russell JK. Refibrillation, resuscitation and survival in out-of-hospital sudden cardiac arrest victims treated with biphasic automated external defibrillators. Resuscitation 2002; 55: 17-23.
22. Gliner BE et al. Transthoracic defibrillation of swine with monophasic and biphasic waveforms. Circulation 1995; 92: 1634-43.
23. Tang W et al. A comparison of biphasic and monophasic waveform defibrillation after prolonged ventricular fibrillation. Chest 2001; 120: 948-54.
24. Tang W et al. Fixed energy biphasic waveform defibrillation in a pediatric model of cardiac arrest and resuscitation. Crit Care Med 2002; 30: 2736-41.
25. Bardy GH et al. Truncated biphasic pulses for transthoracic defibrillation. Circulation 1995; 91: 1768-74.
26. Gliner BE, White RD. Electrocardiographic evaluation of defibrillation shocks delivered to out-of-hospital sudden cardiac arrest patients. Resuscitation 1999; 41: 133-44.
27. Poole JE et al. Low-energy impedance-compensating biphasic waveforms terminate ventricular fibrillation at high rates in victims of out-of-hospital cardiac arrest. J Cardiovasc Electrophysiol 1997; 8: 1373-85.
28. Caffrey SL et al. Public use of automated external defibrillators. N Engl J Med
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29. Gurnett CA, Atkins DL. Successful use of a biphasic waveform automated external defibrillator in a high-risk child. Am J Cardiol 2000; 86: 1051-3.
30. Martens PR et al. Optimal response to cardiac arrest study: defibrillation waveform effects. Resuscitation 2001; 49: 233-43.
31. Anon. External biphasic defibrillators. Should you catch the wave? 
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32. Achleitner U et al. Waveform analysis of biphasic external defibrillators. Resuscitation 2001; 50: 61-70.
33. van Alem AP et al. A prospective, randomized and blinded comparison of first shock success of monophasic and biphasic waveforms in out-of-hospital cardiac arrest. Resuscitation 2003; 58: 17-24.
34. Jones JL et al. Predictions from misleading pig model are potentially harmful to humans. Resuscitation 2003; 59: 365-7.
35. Snyder DE et al. External series resistors accurately model waveform time course, but not cardiac dose in animal models of defibrillation. Resuscitation
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ArticleDate:20051028
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