Prospective randomized comparison of anodal monophasic shocks versus biphasic cathodal shocks on defibrillation energy requirements

Biphasic versus monophasic waveforms for transthoracic defibrillation in out-of-hospital cardiac arrest

BACKGROUND

Transthoracic defibrillation is a potentially life-saving treatment for people with ventricular fibrillation (VF) and haemodynamically unstable ventricular tachycardia (VT). In recent years, biphasic waveforms have become more commonly used for defibrillation than monophasic waveforms. Clinical trials of internal defibrillation and transthoracic defibrillation of short-duration arrhythmias of up to 30 seconds have demonstrated the superiority of biphasic waveforms over monophasic waveforms. However, out-of-hospital cardiac arrest (OHCA) involves a duration of VF/VT of several minutes before defibrillation is attempted.

OBJECTIVES

To determine the efficacy and safety of biphasic defibrillation waveforms, compared to monophasic, for resuscitation of people experiencing out-of-hospital cardiac arrest.

SEARCH METHODS

We searched the following electronic databases for potentially relevant studies up to 10 September 2014: the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE and EMBASE. Also we checked the bibliographies of relevant studies and review articles, contacted authors of published reviews and reviewed webpages (including those of device manufacturers) relevant to the review topic. We handsearched the abstracts of conference proceedings for the American Heart Association, American College of Cardiology, European Society of Cardiology, European Resuscitation Council, Society of Critical Care Medicine and European Society of Intensive Care Medicine. Regarding language restrictions, we did not apply any.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) that compared biphasic and monophasic waveform defibrillation in adults with OHCA. Two review authors independently screened the literature search results.

DATA COLLECTION AND ANALYSIS

Two review authors independently extracted data from the included trials and performed 'Risk of bias' assessments. We resolved any disagreements by discussion and consensus. The primary outcome was the risk of failure to achieve return of spontaneous circulation (ROSC). Secondary outcomes included risk of failure to revert VF to an organised rhythm following the first shock or up to three shocks, survival to hospital admission and survival to discharge.

MAIN RESULTS

We included four trials (552 participants) that compared biphasic and monophasic waveform defibrillation in people with OHCA. Based on the assessment of five quality domains, we identified two trials that were at high risk of bias, one trial at unclear risk of bias and one trial at low risk of bias. The risk ratio (RR) for failure to achieve ROSC after biphasic compared to monophasic waveform defibrillation was 0.86 (95% CI 0.62 to 1.20; four trials, 552 participants). The RR for failure to defibrillate on the first shock following biphasic defibrillation compared to monophasic was 0.84 (95% CI 0.70 to 1.01; three trials, 450 participants); and 0.81 (95% CI 0.61 to 1.09; two trials, 317 participants) for one to three stacked shocks. The RR for failure to achieve ROSC after the first shock was 0.92 (95% CI 0.81 to 1.04; two trials, 285 participants). Biphasic waveforms did not reduce the risk of death before hospital admission (RR 1.05, 95% CI 0.90 to 1.23; three trials, 383 participants) or before hospital discharge (RR 1.05, 95% CI 0.78 to 1.42; four trials, 550 participants). There was no statistically significant heterogeneity in any of the pooled analyses. None of the included trials reported adverse events.

AUTHORS' CONCLUSIONS

It is uncertain whether biphasic defibrillators have an important effect on defibrillation success in people with OHCA. Further large studies are needed to provide adequate statistical power.

Optimizing defibrillation waveforms for ICDs

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.

Present Understanding of Shock Polarity for Internal Defibrillation: The Obvious and Non-Obvious Clinical Implications

BACKGROUND

Uncertainty about the best electrode configuration has combined with the programming flexibility in modern implantable cardioverter-defibrillators (ICDs) to result in routine polarity reversal during an implant to deal with a high defibrillation threshold (DFT). We feel that this practice is not always supported by the clinical data and the present scientific understanding of defibrillation.

METHOD

A meta-analysis of the clinical studies on ICD shock polarity was performed. Subgroup analyses were also performed to test the impact of high DFTs, various tilts, and the use of the hot can electrode. A review of the basic research surrounding the effects of polarity in defibrillation is also presented.

RESULTS

A total of 224 patients were studied. The use of an anodal right ventricular (RV) coil lowers the mean DFT by 14.8% (P = 0.00001). It provides thresholds equal to or lower than cathodal defibrillation in 83% of patients. The fraction of patients with lower anodal DFTs was 94/224 versus 38/224 for cathodal polarity. This phenomenon may be explained by virtual electrode effects. In particular, anodal electrodes tend to produce collapsing wavefronts while cathodal electrodes tend to produce expanding proarrhythmic wavefronts.

CONCLUSION

In an ICD implant, the RV coil should be the anode. Furthermore, DFT testing beginning with cathodal defibrillation is most likely unnecessary and needlessly extends the procedure's duration and increases the risks for the patient.

Effect of biphasic waveforms on transvenous defibrillation thresholds in patients with coronary artery disease

This study is a prospective, randomized comparison of monophasic and biphasic defibrillation thresholds in 19 patients with a single transvenous lead. Despite using reverse polarity and optimal tilts for the monophasic waveform, the defibrillation threshold was reduced with biphasic shocks from 15.8 +/- 11.3 to 11.5 +/- 6.1 (p <0.05) with comparable reductions of leading edge voltage and current.