|Until 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.
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