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Short collapse duration, higher TTI tied to defibrillation success

Stephen Padilla
16 Aug 2017

Biphasic defibrillators should be primed to deliver specific peak currents for best effect in patients with ventricular fibrillation (VF), suggests a Singapore study.

“If our conviction that peak currents rather than energy in joules are the final arbiter of the ability to convert VF holds true, then defibrillators should be primed to deliver specific peak currents, rather than energy levels in joules, for best effect,” researchers said.

“Further research in the use of reliable, specific current delivery mechanisms is required to better appreciate the value of peak current in defibrillation of patients with VF,” they added.

To determine the role of peak current and optimal peak levels for conversion in collapsed VF patients, the authors analysed adult, nonpregnant individuals presenting with nontraumatic VF, as well as all defibrillations that occurred. Impedance values during defibrillation were used to estimate peak current values. Return of spontaneous circulation (ROSC) was the primary endpoint.

A total of 197 patients were included; of these, 105 had ROSC. Patients with and without ROSC had similar characteristics. Short duration of collapse (<10 minutes) was positively associated with ROSC. [Singapore Med J 2017;58:432-437]

In general, those who converted at lower peak currents were patients with average or high transthoracic impedance (TTI): 25 percent with high TTI converted at 13.3±2.3 A, 22.7 percent with average TTI at 18.2±2.5 A and 18.6 percent with low TTI at 27.0±4.7 A (p=0.729). The highest peak conversions were recorded at <15 A and 15 to 20 A.

“The main factors that positively contributed to higher defibrillation success in this study appear to be a shorter duration of collapse and patients with higher TTI receiving lower peak currents,” according to researchers.

There were 44 patients who achieved first-shock ROSC, of whom 33 (75 percent) received <20 A peak current compared with >20 A for the remaining 11 (25 percent) patients (p=0.002).

“In performing electrical defibrillation for patients in VF, one looks for first-shock success. Yet, many patients require more than one shock, and in some instances, higher energy shocks must be applied before electrical conversion occurs,” researchers said. [Open Access Emerg Med 2017;9:9-17]

“In these patients, one may question whether the impedance-compensating mechanisms of the defibrillators have faltered, thus requiring the use of higher energy levels,” they added. [http://www.fda.gov/MedicalDevices/Safety/AlertsandNotices/ucm189259.htm]

In a 2003 study that compared six commercially available biphasic defibrillators using different electrical waveforms, researchers saw significant variations at higher impedance values despite uniform efficacy with such waveforms at low impedance values. As a conclusion, the investigators said that impedance had an impact on defibrillation efficacy despite impedance-compensation schemes in biphasic defibrillators. [Resuscitation 2003;57:73-83]

“In the present study, we could not establish that patients with low TTI values were easier to defibrillate. On the contrary, we found that the success rate in such patients was lower than for those with average or high TTI values, although not significantly so. Even then, the higher currents generated for such patients may not have been optimal, even with the use of low energy shock,” researchers said.

Biphasic energy in the of joules to deliver shock to patients in either VF or pulseless ventricular tachycardia is commonly used in most of the currently available defibrillators. Biphasic energy comes in various forms and shapes, such as biphasic truncated exponential waveform, rectilinear biphasic waveform or pulsed biphasic waveform, according to researchers.

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