Kroll Mark W, Swerdlow Charles D
Department of Biomedical Engineering, University of Minnesota, Minneapolis, MN, USA.
J Interv Card Electrophysiol. 2007 Apr;18(3):247-63. doi: 10.1007/s10840-007-9095-z. Epub 2007 Jun 1.
While no simple electrical descriptor provides a good measure of defibrillation efficacy, the waveform parameters that most directly influence defibrillation are voltage and duration. Voltage is a critical parameter for defibrillation because its spatial derivative defines the electrical field that interacts with the heart. Similarly, waveform duration is a critical parameter because the shock interacts with the heart for the duration of the waveform. Shock energy is the most often cited metric of shock strength and an ICD's capacity to defibrillate, but it is not a direct measure of shock effectiveness. Despite the physiological complexities of defibrillation, a simple approach in which the heart is modeled as passive resistor-capacitor (RC) network has proved useful for predicting efficient defibrillation waveforms. The model makes two assumptions: (1) The goal of both a monophasic shock and the first phase of a biphasic shock is to maximize the voltage change in the membrane at the end of the shock for a given stored energy. (2) The goal of the second phase of a biphasic shock is to discharge the membrane back to the zero potential, removing the charge deposited by the first phase. This model predicts that the optimal waveform rises in an exponential upward curve, but such an ascending waveform is difficult to generate efficiently. ICDs use electronically efficient capacitive-discharge waveforms, which require truncation for effective defibrillation. Even with optimal truncation, capacitive-discharge waveforms require more voltage and energy to achieve the same membrane voltage than do square waves and ascending waveforms. In ICDs, the value of the shock output capacitance is a key intermediary in establishing the relationship between stored energy-the key determinant of ICD size-and waveform voltage as a function of time, the key determinant of defibrillation efficacy. The RC model predicts that, for capacitive-discharge waveforms, stored energy is minimized when the ICD's system time constant taus equals the cell membrane time constant taum, where taus is the product of the output capacitance and the resistance of the defibrillation pathway. Since the goal of phase two is to reverse the membrane charging effect of phase one, there is no advantage to additional waveform phases. The voltages and capacitances used in commercial ICDs vary widely, resulting in substantial disparities in waveform parameters. The development of present biphasic waveforms in the 1990s resulted in marked improvements in defibrillation efficacy. It is unlikely that substantial improvement in defibrillation efficacy will be achieved without radical changes in waveform design.
虽然没有一个简单的电学描述符能很好地衡量除颤效果，但最直接影响除颤的波形参数是电压和持续时间。电压是除颤的关键参数，因为其空间导数定义了与心脏相互作用的电场。同样，波形持续时间也是关键参数，因为电击在波形持续时间内与心脏相互作用。电击能量是最常被提及的电击强度指标以及植入式心律转复除颤器（ICD）的除颤能力指标，但它并非电击效果的直接衡量指标。尽管除颤存在生理复杂性，但将心脏建模为无源电阻 - 电容（RC）网络的简单方法已被证明有助于预测有效的除颤波形。该模型做了两个假设：（1）单相电击和双相电击的第一阶段的目标是在给定存储能量的情况下，使电击结束时膜上的电压变化最大化。（2）双相电击第二阶段的目标是将膜放电回到零电位，消除第一阶段沉积的电荷。该模型预测最优波形呈指数上升曲线，但这种上升波形难以高效产生。ICD 使用电子效率高的电容放电波形，为有效除颤需要进行截断。即使进行了最优截断，电容放电波形相比于方波和上升波形，需要更多的电压和能量来达到相同的膜电压。在 ICD 中，电击输出电容的值是建立存储能量（ICD 尺寸的关键决定因素）与作为时间函数的波形电压（除颤效果的关键决定因素）之间关系的关键中介。RC 模型预测，对于电容放电波形，当 ICD 的系统时间常数 τs 等于细胞膜时间常数 τm 时，存储能量最小，其中 τs 是输出电容与除颤通路电阻的乘积。由于第二阶段的目标是逆转第一阶段的膜充电效应，额外的波形阶段并无优势。商业 ICD 中使用的电压和电容差异很大，导致波形参数存在显著差异。20 世纪 90 年代出现的当前双相波形使除颤效果有了显著改善。如果不彻底改变波形设计，不太可能在除颤效果上取得实质性改进。
