**2. DFT concept**

DFT testing is performed by inducing ventricular arrhythmia and then finding the minimal amount of energy delivered by the ICD to defibrillate the myocardium back to sinus rhythm. Central to the concept of determining the DFT, was the discovery since 1930, that electrical shocks themselves can induce ventricular fibrillation (VF) [1]. By giving a shock during the vulnerable period of repolarization, VF could be reproducibly induced. Ventricu‐ lar tachycardia (VT) may also be induced, but in a relatively small percentage of patients [2]. As the amount of energy delivered during the vulnerable period is increased, a threshold is reached that does not induce VF [3]. This is called the upper limit of vulnerability (ULV) and is proportional to DFT value [4-5]. This concept can be used as a surrogate for DFT [6-7]. Instead of inducing VF and checking the threshold at which it terminates, a different proto‐ col is used that delivers variable strengths of shocks during the vulnerable T-wave phase and then establishes ULV. As compared to regular DFT test, ULV test is done during sinus

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rhythm and not during VF. Using this approach there is a need for more number of shocks, but less number of times that VF are induced.

To find the exact DFT value, this process has to be repeated several times with either a step up, step down, or a binary fashion until DFT or ULV are found [16-17]. Although the term threshold should indicate a value above which defibrillation is successful and below which shocks fail, in reality DFT is a probabilistic phenomenon with a certain percentage of success rate [17-18]. One shock may be successful while a successive one with the same conditions may fail [5]. Despite the poor reproducibility of DFT, it is still a useful parameter, as this val‐ ue taken with a safety margin above it, gives a high clinical success of VF termination. The

Defibrillator Threshold Testing http://dx.doi.org/10.5772/52594 123

DFT testing with several shocks and titrations may be useful in a research protocol, where the exact value has important clinical significance. A simpler, yet safe approach is the defib‐ rillator safety margin (DSM) test [21]. In this method, a single or two VF inductions may be enough. The device is usually programmed to a value that is expected to restore sinus rhythm following induction. This value depends on the operator preference. It is usually an average value that is at least 10 J below the maximum output of the device [22]. If it is suc‐ cessful, then this value should at least be equal to the DFT value. If the shock fails, then step

> **Study Year N Percent** Kelly et al [23] 1988 94 5.3% Winkle et al [24] 1989 270 2.6% Pinski et al [25] 1991 125 18% Epstein et al [26] 1992 1946 4.6% Gold et al [27] 1997 114 8% Brodsky et al [28] 1999 764 3.1% Shukla et al [29] 2003 968 11% Russo et al [30] 2005 1139 6.2% Theuns et al [31] 2005 127 14% Mainigi et al [32] 2006 121 12% Guenther et al [33] 2012 975 1.4% Cheng et al [34] 2012 243 5.3%

High DFT, defined in most studies as a threshold of < 10 J below the maximum output of the device, is estimated to occur in about 5% of ICD implants (range 1.4 to 18%) (Tables 1) [23-33]. Although the exact value of DFT cannot be predicted in an individual patient prior to testing, there are some clinical parameters associated with increased thresholds (Table 2)

standard safety margin is 10 J, although 5 J may be enough [19-20].

up approach has to be used to find the threshold.

**Table 1.** Incidence of high DFT

[27-29,32-35].

The threshold to terminate ventricular fibrillation with ICD's is usually in the 5 to 30 J range. It was the ability to reduce the energy requirements by 10 folds as compared to external de‐ fibrillator that made the ICD a reality [8]. Otherwise the size of the ICD would have to be much bigger to store the needed energy for defibrillation. Although the threshold is ex‐ pressed in relation to the energy discharged by the ICD, in reality it is the voltage and its duration that is the critical factor in defibrillation [9-10]. Duration has a relatively narrow range to be effective (in the range of few milliseconds). To achieve this precise phase dura‐ tion of defibrillation, the capacitor discharge is truncated in either a tilt based formula which truncates the discharge after a certain percentage of decay in the voltage has been reached, or more simply in a time based manner after a certain time has elapsed. Voltage is usually in the hundreds of volts range. If voltage is too low it may induce rather than terminate fibril‐ lation as explained earlier. High voltage (above 1000 Volts) is also not without its risks, as it may result in stunning of the myocardium and subsequent electromechanical dissociation [11-12]. The voltage wave form can be manipulated to make defibrillation more effective and therefore require less energy. Biphasic wave, which is the standard now in ICD's, has reversal of the initial polarity. The initial wave results in charging of the cell membrane as a result of the voltage gradient. The reversal which is termed "burping" is theorized to absorb the initial energy and therefore avoid proarrhythmia [11-12]. The biphasic wave form can be manipulated by changing the initial voltage, and by changing the duration and the ratio of its waves. Different manufactures use different formulas in their devices, and some allow changing of the parameters by the electrophysiologists in case of high DFT.
