Part 5: Electrical Therapies 1v-41 Electrode size information on current dosage for biphasic waveform shocks In 1993 the Association for the advancement of medical Instrumentation recommended a minimum electrode size of 50 cm- for individual electrodes 100 However. advances in “ Occult” Versus“ false” Asystole electrode design and chemical composition may soon require There is no evidence that attempting to"defibrillate" asystole modification of this recommendation is beneficial. In 1989 Losek23 published a retrospective For adult defibrillation, both handheld paddle electrodes review of initial shock delivery for 49 children (infants and self-adhesive pad electrodes 8 to 12 cm in diameter through 19 years of age)in asystole compared with no shock perform well, although defibrillation success may be higher delivery for 41 children in asystole and found no improve with electrodes 12 cm in diameter rather than with those 8 cm ment in rhythm change, ROSC, or survival in the group that in diameter. 90.95 Small electrodes (4.3 cm) may be harmful received the shocks. In 1993 the Nine City High-Dose d may cause myocardial necrosis. 10l When using handheld Epinephrine Study Group published an analysis of 77 asys paddles and gel or pads, rescuers must ensure that the paddle tolic patients who received initial shock compared with 117 is in full contact with the skin. Even smaller pads have been who received standard therapy. 24 There was no benefit from found to be effectivel02 in VF of brief duration. Use of the shock delivery for asystole. In fact, in all outcomes studied, mallest(pediatric) pads, however, can result in unacceptably including ROSC and survival, the group that received shocks high transthoracic impedance in larger children. o3 It is best to showed a trend toward a worse outcome than the group that use the largest pads that can fit on the chest without overlap. did not receive shocks. With recent recognition of the importance of minimizing interruptions in chest compres Fibrillation Waveform Analysis sIons it is difficult to justify any interruption in chest Several retrospective case series, animal studies, and theoret compressions to attempt shock delivery for asystole. ical models (LOE 429,30, 104-l10 and LOE 611-12)suggest that It is possible to predict, with varying reliability, the success Fire hazard attempted defibrillation by analyzing the VF waveform. If Several case reports have described fires ignited by sparks prospective studies can select optimal defibrillation wave- from poorly applied defibrillator paddles in the presence of an forms and optimal timing of shock delivery (eg, before or oxygen-enriched atmosphere (LOE 5). 25-130 Severe fires after a period of CPR), shock delivery may be more likely to have been reported when ventilator tubing is disconnected result in return of spontaneous perfusion, and the delivery of from the tracheal tube and then left adjacent to the patient's unsuccessful high-energy shocks may be prevented. At pre head, blowing oxygen across the chest during attempted ent there is insufficient evidence to recommend for or against defibrillation ( LOE 5). 265, 28 analysis of VF ECG characteristics(Class Indeterminate) The use of self-adhesive defibrillation pads is probably the At issue is whether analysis of the VF waveform is useful best way to minimize the risk of sparks igniting during in predicting therapeutic outcome and modifying therapy defibrillation. If manual paddles are used, gel pads are prospectively. Potential applications include prediction of preferable to electrode pastes and gels because the pastes and success of cardioversion, selection of appropriate waveform gels can spread between the 2 paddles, creating the potential type, and optimization of timing of defibrillation relative to for a spark( Class Ilb) Do not use medical gels or pastes with CPR and medication delivery poor electrical conductivity, such as ultrasound gel. Rescuers should take precautions to minimize sparking Current-Based Defibrillation during attempted defibrillation; try to ensure that defibrilla- Because it is accepted that defibrillation is accomplished by tion is not attempted in an oxygen-enriched atmosphere the passage of sufficient current through the heart, the Class Ila). When ventilation is interrupted for shock deliv concept of current-based defibrillation is appealing. Energy is ery, rescuers should try to ensure that oxygen does not flow a nonphysiologic descriptor of defibrillation despite its en- across the patient's chest during defibrillation attempts trenchment in traditional jargon. Current-based defibrillation has been assessed92 122 but has not yet been used clinically as Synchronized Cardioversion a better physiologic descriptor of defibrillation dose. This Synchronized cardioversion is shock delivery that is timed concept merits exploration in light of the variety of biphasic (synchronized) with the QRS complex. This synchronization waveforms available that deliver current in different ways. avoids shock delivery during the relative refractory portion of Peak current amplitude, average current, phasic duration, and the cardiac cycle, when a shock could produce VF. 3I The phasic current flow need to be examined as determinants of energy(shock dose) used for a synchronized shock is lower shock efficacy. Another difficulty with than that used for unsynchronized shocks(defibrillation) descriptor was described earlier with regard to differences These low-energy shocks should always be delivered as between operator-selected energy and that delivered with the synchronized shocks because if they are delivered as unsyn- rectilinear biphasic waveform. Transition to current-based chronized shocks they are likely to induce VF. If cardiover- description is timely and should be encouraged sion is needed and it is impossible to synchronize a shock(eg Clinical studies using MDS waveform shocks have tried to the patient 's rhythm is irregular), use high-energy unsynchro the range of current necessary to achieve defibrilla- nized shocks d cardioversion. The optimal current for ventricular Delivery of synchronized shocks(cardioversion) is indi- defibrillation appears to be 30 to 40 A MDS. 92 Comparable cated to treat unstable tachyarrhythmias associated with anElectrode Size In 1993 the Association for the Advancement of Medical Instrumentation recommended a minimum electrode size of 50 cm2 for individual electrodes.100 However, advances in electrode design and chemical composition may soon require modification of this recommendation. For adult defibrillation, both handheld paddle electrodes and self-adhesive pad electrodes 8 to 12 cm in diameter perform well, although defibrillation success may be higher with electrodes 12 cm in diameter rather than with those 8 cm in diameter.90,95 Small electrodes (4.3 cm) may be harmful and may cause myocardial necrosis.101 When using handheld paddles and gel or pads, rescuers must ensure that the paddle is in full contact with the skin. Even smaller pads have been found to be effective102 in VF of brief duration. Use of the smallest (pediatric) pads, however, can result in unacceptably high transthoracic impedance in larger children.103 It is best to use the largest pads that can fit on the chest without overlap. Fibrillation Waveform Analysis Several retrospective case series, animal studies, and theoret￾ical models (LOE 429,30,104–110 and LOE 6111–121) suggest that it is possible to predict, with varying reliability, the success of attempted defibrillation by analyzing the VF waveform. If prospective studies can select optimal defibrillation wave￾forms and optimal timing of shock delivery (eg, before or after a period of CPR), shock delivery may be more likely to result in return of spontaneous perfusion, and the delivery of unsuccessful high-energy shocks may be prevented. At pres￾ent there is insufficient evidence to recommend for or against analysis of VF ECG characteristics (Class Indeterminate). At issue is whether analysis of the VF waveform is useful in predicting therapeutic outcome and modifying therapy prospectively. Potential applications include prediction of success of cardioversion, selection of appropriate waveform type, and optimization of timing of defibrillation relative to CPR and medication delivery. Current-Based Defibrillation Because it is accepted that defibrillation is accomplished by the passage of sufficient current through the heart, the concept of current-based defibrillation is appealing. Energy is a nonphysiologic descriptor of defibrillation despite its en￾trenchment in traditional jargon. Current-based defibrillation has been assessed92,122 but has not yet been used clinically as a better physiologic descriptor of defibrillation dose. This concept merits exploration in light of the variety of biphasic waveforms available that deliver current in different ways. Peak current amplitude, average current, phasic duration, and phasic current flow need to be examined as determinants of shock efficacy. Another difficulty with using energy as a descriptor was described earlier with regard to differences between operator-selected energy and that delivered with the rectilinear biphasic waveform. Transition to current-based description is timely and should be encouraged. Clinical studies using MDS waveform shocks have tried to identify the range of current necessary to achieve defibrilla￾tion and cardioversion. The optimal current for ventricular defibrillation appears to be 30 to 40 A MDS.92 Comparable information on current dosage for biphasic waveform shocks is under investigation. “Occult” Versus “False” Asystole There is no evidence that attempting to “defibrillate” asystole is beneficial. In 1989 Losek123 published a retrospective review of initial shock delivery for 49 children (infants through 19 years of age) in asystole compared with no shock delivery for 41 children in asystole and found no improve￾ment in rhythm change, ROSC, or survival in the group that received the shocks. In 1993 the Nine City High-Dose Epinephrine Study Group published an analysis of 77 asys￾tolic patients who received initial shock compared with 117 who received standard therapy.124 There was no benefit from shock delivery for asystole. In fact, in all outcomes studied, including ROSC and survival, the group that received shocks showed a trend toward a worse outcome than the group that did not receive shocks. With recent recognition of the importance of minimizing interruptions in chest compres￾sions, it is difficult to justify any interruption in chest compressions to attempt shock delivery for asystole. Fire Hazard Several case reports have described fires ignited by sparks from poorly applied defibrillator paddles in the presence of an oxygen-enriched atmosphere (LOE 5).125–130 Severe fires have been reported when ventilator tubing is disconnected from the tracheal tube and then left adjacent to the patient’s head, blowing oxygen across the chest during attempted defibrillation (LOE 5).126,128,130 The use of self-adhesive defibrillation pads is probably the best way to minimize the risk of sparks igniting during defibrillation. If manual paddles are used, gel pads are preferable to electrode pastes and gels because the pastes and gels can spread between the 2 paddles, creating the potential for a spark (Class IIb). Do not use medical gels or pastes with poor electrical conductivity, such as ultrasound gel. Rescuers should take precautions to minimize sparking during attempted defibrillation; try to ensure that defibrilla￾tion is not attempted in an oxygen-enriched atmosphere (Class IIa). When ventilation is interrupted for shock deliv￾ery, rescuers should try to ensure that oxygen does not flow across the patient’s chest during defibrillation attempts. Synchronized Cardioversion Synchronized cardioversion is shock delivery that is timed (synchronized) with the QRS complex. This synchronization avoids shock delivery during the relative refractory portion of the cardiac cycle, when a shock could produce VF.131 The energy (shock dose) used for a synchronized shock is lower than that used for unsynchronized shocks (defibrillation). These low-energy shocks should always be delivered as synchronized shocks because if they are delivered as unsyn￾chronized shocks they are likely to induce VF. If cardiover￾sion is needed and it is impossible to synchronize a shock (eg, the patient’s rhythm is irregular), use high-energy unsynchro￾nized shocks. Delivery of synchronized shocks (cardioversion) is indi￾cated to treat unstable tachyarrhythmias associated with an Part 5: Electrical Therapies IV-41