In the mid 1970s various authoritative sources recommended initial shock doses of 200 J for all children and 60 to 100 J for all infants in VF.9.10 Use of the same defibrillation dose in both children and adults seemed potentially dangerous despite clinical experience that indicated the effectiveness of such doses. These concerns were supported by only limited animal data, some of which suggested that histopathologic myocardial damage may begin to occur with doses as low as >10 J/kg.11-14 In addition, further animal data suggested that doses of 0.5 to 10 J/kg were generally adequate for defibrillation in a variety of species.14
Gutgesell and colleagues15 conducted the largest clinical study of an effective defibrillation dose for children. They retrospectively evaluated the efficacy of defibrillation attempts at energy doses of 2 J/kg. The authors reviewed 71 transthoracic defibrillation attempts in 27 children whose ages ranged from 3 days to 15 years and who weighed from 2.1 to 50 kg. The authors reported that 91% of shocks within 10 J of the standard 2 J/kg dose successfully terminated VF.
The task force recommendation of 2 J/kg is derived entirely from this study, although it included only 27 children with short-duration VF and the definition of success was electrical defibrillation with no reference to postshock clinical outcomes, such as a sustained stable perfusing rhythm. Although decades of clinical use confirm that 2 J/kg is effective, no research to date has confirmed it as the most effective dose.
Incidence of VF in Children
VF is an uncommon cause of out-of-hospital pediatric cardiac arrest in infants (<1 year of age), but its occurrence increases with growing age. Two studies reported VF as the initial rhythm in 19% to 24% of out-of-hospital pediatric cardiac arrests if sudden infant death syndrome (SIDS) deaths were excluded.16,17
In studies that included SIDS victims, however, the frequency dropped to 6% to 10%.18-20 The rationale for exclusion of SIDS patients is that SIDS is not amenable to treatment, so patients with SIDS should not be included in studies that may influence potential treatment strategies for cardiac arrest. A recent report, however, documented VF in a 3-month-old infant with SIDS who was subsequently diagnosed with prolonged QT syndrome.21
Recent data suggest that VF is not a rare rhythm in pediatric arrest. This is encouraging because VF is the arrest arrhythmia associated with improved survival rate in most studies of children.16,17,22,23 For example, Mogayzel and colleagues16 reported that 5 of 29 children (17%) who presented with VF in a prehospital setting survived with good neurological outcome versus only 2 of 128 (2%) who presented with asystole/pulseless electrical activity (P<0.01).
In-hospital studies of pediatric CPR also indicate that VF is not a rare rhythm among children in cardiac arrest. Two recent comprehensive studies report the incidence of VF as the initial rhythm and the incidence of VF at some time during the arrest. Suominen et al24 reported initial VF in 11% of children in cardiac arrest and VF in 20% of children some time during the arrest. In a much larger study,25 cardiac arrest data submitted to the National Registry of CardioPulmonary Resuscitation reveal initial VF/VT in 12% of children and VF/VT at some time during 25% of the pediatric arrests.
Factors That Affect Effectiveness of Transthoracic Shocks
The success of defibrillation depends on delivery of sufficient current flow (amperes) for a sufficient length of time to depolarize a critical mass of myocardium. In the 1970s, animal studies established that inadequate current through the myocardium led to unsuccessful defibrillation, whereas too much current resulted in postresuscitation myocardial damage.11,14 These studies further established that the density of current through the myocardium* determined the balance between effectiveness of the shock and myocardial damage.
The basic principles of electrical cardiac defibrillation have been reviewed.26For any given waveform, current flow increases with higher shock energy (J) and decreases with higher impedance or resistance (ohms). Several factors increase impedance along the path between defibrillator paddles or electrode pads and decrease current through the myocardium. These factors include a paddle or electrode pad that is too small, large lung volumes, and lack of conducting gel between the skin and defibrillator paddles or electrode pads.
Factors that decrease impedance and thus increase current through the myocardium include the use of electrical conducting gel and increased paddle pressure (reduces impedance by improving skin/electrode contact and squeezing air from the lungs). Impedance may also be reduced by repeated shocks (partly due to increased flow of blood after each shock), although the degree of this reduction is unclear.27Increased paddle or electrode pad size does reduce impedance and therefore increases total current flow. But this does not necessarily increase the amount of current delivered to the myocardium (current density), because if the paddles or electrode pads are larger than the cross section of the heart, much of the current bypasses its target-the myocardium-through extramyocardial pathways.
Studies of transthoracic impedance in animals, children, and adults suggest nonlinear relationships among size, weight, and thoracic impedance.28-31 Additional acceptable evidence is needed to resolve these contemporary inconsistencies. The theme of the evidence suggests that children have higher thoracic impedance than would be expected on the basis of weight alone.
This suggests that the present dose of 2 J/kg may need an upward adjustment in smaller patients, or, equally valid, the chance of myocardial damage from any particular dose is less than previously feared. Another important factor influencing shock effectiveness is the shock waveform. In recent years biphasic waveforms have been introduced into external defibrillators and have been shown in clinical studies to have advantages over conventional monophasic waveforms. With biphasic waveforms, a smaller shock will defibrillate effectively yet larger energies are well tolerated, so that a single energy delivery may be applicable across a wider age or size range.32-34
