In children with cardiac arrest due to a disturbance in rhythm, a defibrillation energy dose of 4 J/kg (of monophasic or biphasic waveform) was recommended by the European Resuscitation Council (ERC) in 2005,1 but in 2006 the International Liaison Committee on Resuscitation (ILCOR)2 recommended a dose of 2 J/kg (monophasic or biphasic), with subsequent doses of 4 J/kg, both using a manual defibrillator following cardiopulmonary resuscitation (CPR). Both accept the use of automated external defibrillators (AEDs) from the age of 1 year, originally recommended by ILCOR in 2003.3

Automatic AEDs, used by “lay responders”, offer proven advantages in reducing the critical time between out-of-hospital arrest and shock administration. Semi-automatic AEDs, if preferred to manual defibrillators, can reduce inappropriate and potentially harmful shocks when used out-of-hospital by paramedics, or in-hospital where most ventricular fibrillation (VF) in children occurs. The efficiency of both devices is limited by increased “no flow time” during rhythm analysis. Further development of AEDs, including concurrent rhythm analysis with CPR, should improve defibrillation success.

Insufficient clinical research in children has led to many recommendations concerning defibrillation being extrapolated from adult or animal studies.

THE ELECTROPHYSIOLOGY OF VENTRICULAR FIBRILLATION

Ventricular fibrillation (and pulseless ventricular tachycardia, hereafter both referred to as VF) is a self-perpetuating cycle of uncoordinated ventricular depolarisation resulting in disordered and ineffective contraction of the ventricles. During normal cardiac function, propagation of an action potential “wave” through the ventricles is responsible for the coordinated depolarisation that stimulates contraction of the ventricles. This wave can be broken, a situation known as “wavebreak”, by a variety of phenomena that involve re-entry of “wavelets” into areas of myocardial tissue that are non-refractory. Two hypotheses for the maintenance and propagation of fibrillation have been proven experimentally: that of multiple wavelets cascading through myocardial tissue4 5 and a stationary, or wandering,6 “mother rotor” that propagates waves outwards from a core.7 The two processes are not mutually exclusive.8

Random premature ventricular extrasystoles are frequently present in normal hearts9 but are rarely able to overcome the refractory threshold for the initiation of VF, even in diseased hearts,10 due to the natural barrier of tissue homogeneity. Rotors, once established, can initiate monomorphic tachycardia11 which breaks down to establish VF12 through the influence of secondary factors, such as intracellular calcium cycling,13 varying electrical currents in cardiac muscle14 and the properties of the membrane action potential.14^–16

As the heart rate increases the action potential duration shortens, with secondary effects of decreasing time for ventricular filling and coronary artery flow. The restitution curve of the action potential steepens, which reduces inertia against the development of VF.16 Finally, the action potential wavelength is critically reduced and the conduction velocity of the wave is so slow that propagation of the wave is no longer possible and fibrillation is inevitable.10 17 Fluctuating action potential wavelength and conduction velocity, “discordant repolarisation alternans”, as expressed by t-wave variability on the ECG, both favour wavebreak.18

Tissue heterogeneity is present in the normal stratification of muscle sheets, branching or twisting of fibres or bundles of fibres14 and the presence of fibres of differing conduction velocities (such as Purkinje fibres).19 Infarction and post-infarction fibrosis20 can have a profound effect on homogeneity of the natural inhibitory mechanisms and serves as an anchor for rotor formation and wavebreak.10

Myocardial ischaemia uncouples gap junctions and reduces sodium channel availability which impairs longitudinal conduction and is pro-arrhythmogenic.21 Pre-oxygenation by 90 s of cardiopulmonary resuscitation prior to defibrillation has been proven statistically to improve survival and neurological outcome in adults (table 1).22

THE EPIDEMIOLOGY OF VENTRICULAR FIBRILLATION

Out-of-hospital VF in studies of cardiac arrest in children have reported rates of 4% to 9%.23^–28 The reported frequency of in-hospital diagnoses of VF is more variable, ranging from 1% to 27%.29^–32 Two studies in Spain demonstrated frequencies of 9% to 10% VF for both in- and out-of-hospital cases.33 34 Between 54% and 65% recorded episodes of VF are documented during cardiac arrest in a paediatric intensive care unit.30 35 VF has a cardiac origin in 21% to 74% of cases.23 24 33 35 Other frequent causes of VF include trauma (17%)36 and drowning (11% to 20%).23 34 37

VF is more widely observed in adults and older children at the time of cardiac arrest.24 38 39 Bradycardia is the most common terminal cardiac rhythm in children prior to death40 and has a significantly worse survival.27 34 A study in Sweden describing out-of-hospital cardiac arrest recorded the relative frequencies of VF as 41% in adults >35 years old, 13% in young adults between 18 and 35 years old and 8% in children <18 years old.24 No survival to discharge was recorded for the youngest group, 31% of young adults survived and 19% of the older adults survived.24

In one Spanish study, 57% of children with VF had a return of spontaneous circulation (ROSC), but only 6.8% lived to 1 year from discharge.33 Two American studies describing the use of manual defibrillators have reported 33%41 and 0%25 survival to hospital discharge. Survival figures of 33% using an AED42 and 63% using an AED with an energy attenuator43 are discussed below.

The generally poorer survival in children probably reflects differences in precipitating pathologies compared to adults and unknown variables in the treatment of paediatric defibrillation.

THORACIC IMPEDANCE

The energy delivered by all defibrillators, whether monophasic or biphasic, is affected by thoracic impedance. There is no linear relationship between weight and thoracic impedance. Impedance depends on the shape of the thorax, water and fat content, pulmonary volume, body weight, the type of conductor used (gel), the surface area of the paddles (electrodes), their separation and the physical force used in their application.44^–47 The force on the paddles necessary to produce an identical impedance in a 4-kg piglet is only 30% less than that required for a 24-kg pig.48

Thoracic impedance has been measured with precision in a study of 80 anaesthetised children under going routine anaesthesia, impedance as being inversely proportional to the compression applied to the paddles.45 The optimal force on 16 cm² paediatric paddles is 2.9 kg for children weighing <10 kg, and 5.1 kg for a child weighing >10 kg using 82 cm² adult paddles.45

The force applied by a group of medical personnel on dummies representing an infant of 9 months and a child of 6 years has been measured.49 Only 47% applied the paddles with sufficient pressure on the infant dummy to reduce thoracic impedance to a minimum and as few as 24% for the child dummy.49

A retrospective study of thoracic impedance during cardioversion in children has recommended using adult-sized paddles for children weighing >10 kg and infant paddles of 16 cm² for those <10 kg.44 Paddles may be positioned sternal-apex or anterior-posterior in small children to avoid overlap, defibrillation being equally effective with either position.50 Larger paddles reduce impedance but, importantly, current may bypass the heart because of its smaller size, thereby reducing current density leading to sub-threshold energy doses.44 In 2005, the ERC made recommendations concerning paddle size and force for manual defibrillators (table 2).

ENERGY DOSE

Early experiences with cardioversion have recently been eloquently described by Lown.51 The optimal energy dose is controversial and critical, being that which eliminates all re-entrant sources without initiating new ones or damaging the myocardium (table 3).19 An intra-operative study in adults has shown that after an insufficient energy dose significantly higher energy is necessary for subsequent “rescue” shocks.52 It remains unknown whether peak energy is dangerous or whether it is the accumulated dose of several shocks that is damaging.

Postmortem pathological examination of the myocardium of defibrillated children has revealed bands of necrosis, epicardial coagulation and areas of haemorrhage.53 Direct mitochondrial damage has been demonstrated experimentally using relatively high energy doses in dogs.54 Resuscitation therapies, such as the use of ionotropes, can simultaneously damage the myocardium.53

A study of 100 dogs with artificially induced VF has shown a wide margin between therapeutic and toxic doses.55 Dose regimens using a monophasic shock of 0.5–512 J/kg revealed that the dose necessary to convert 50% of dogs was 1.5 J/kg, the dose at which myocardial damage appeared in 50% of the dogs was 30 J/kg and the 50% lethal dose was 470 J/kg.55

Porcine animal models used to extrapolate the correct energy dose for defibrillation can imply artificially elevated success rates, in comparison to humans, because thoracic impedance in pigs is about half that in humans.56^–59 A low energy shock, instead of ablating fibrillation, can create focal epicardial activation stimulating re-entry which causes defibrillation to fail.58 A biphasic shock of 4 J/kg is largely effective56 and an adult dose in child-sized pigs unsurprisingly is harmful to cardiac function, neurological outcome and survival.57

There have been no prospective studies comparing energy doses in children. Four retrospective studies describe monophasic shocks starting at 2 J/kg. In 1996, Gutgesell et al looked at 71 attempts at defibrillation in 27 children. Fibrillation was effectively terminated in 91% of these children using 2 J/kg, while the remaining 9% required 4 J/kg; no data are given for post-shock rhythms, ROSC and survival.35 More recently, Berg et al reviewed 31 shocks given to 13 children. Of seven shocks of 2 J/kg, six resulted in conversion to asystole and one to pulse-less electrical activity, but nine shocks >8 J/kg failed to convert VF; all the children died.25 Rodriguez-Nunez et al looked at 44 children. All shocks were at 2 J/kg, 18% of episodes were terminated with one shock and 55% needed three or more shocks; three children survived to 1 year follow-up.33 Classification of increasing energy intensities used during the defibrillation of 57 children into 2–4 J/kg, 4–6 J/kg and >6 J/kg categories during a total of 185 shocks did not show a statistical difference for survival, which occurred at low and very high energy doses.41

WAVEFORMS

Concerns about myocardial damage related to monophasic (damped sinusoidal and truncated exponential) shocks have led to the development of alternative waveforms with the objective of maximising the effectiveness of the energy delivered60 whilst minimising potential harmful effects.61 Biphasic waveforms are theoretically superior to monophasic waveforms because their first phase hyperpolarises the myocardial membrane, allowing recovery of sodium channels and decreasing the excitation threshold, prior to a simultaneous depolarisation during the second phase (table 4).62

Monophasic damped sinusoidal waveforms have been clinically proven to be superior to monophasic truncated exponential waveforms in adults.63 64 Several shapes of biphasic waveforms have been tested in adults65 and pigs.59 Studies of biphasic versus monophasic waveforms carried out in dogs,61 pigs56 66 and adults67 68 have led to a reduction in total energy requirements whilst reducing the risk of post-first shock asystole or persistent VF.67

Other studies of biphasic waveforms, of equal or lower energy doses, in adults,62 dogs61 and pigs69 have confirmed their beneficial effects in terms of post-shock myocardial oxidative metabolism,61 myocardial function69 and fewer skins burns.70

Despite the less harmful effects and superior efficacy of biphasic over monophasic waveforms, the former have not been proven to affect survival in adults in the long term.71 72 No controlled trials of different waveforms have been undertaken in children.

AUTOMATED EXTERNAL DEFIBRILLATORS

Adult defibrillation has been revolutionised by the introduction of AEDs which interpret shockable rhythms according to their resemblance to a computerised algorithm (table 5). They are extremely reliable in the determination of rhythms due to their high sensitivity (capacity to recognise a shockable rhythm) and high selectivity (capacity to recognise a rhythm where a shock is not indicated).50 73 Some biphasic models calculate impedance automatically.59

Children have a resting heart rate that is more rapid than that of adults and can present with arrhythmias, such as supraventricular tachycardia, whose first line treatment is not cardioversion. In 2001, Cecchin et al interpreted 696 childhood arrythmias and demonstrated a recognition specificity of 96% and sensitivity of 71%.73 The American Heart Association requires AED manufacturers to ensure 96% specificity and 90% sensitivity.74 A study by Atkinson et al of 1561 abnormal rhythms taken from 203 children aged from 1 to 7 years, compared to the rhythms found by a panel of three cardiologists, produced results of 99% specificity and 99% sensitivity.50 The position of the electrodes, sterno-apex or antero-posterior, did not alter the decision to shock. There have been no studies of rhythms of children <1 year old.

Automatic AEDs deliver a predetermined shock dose independently of the operator who is generally a trained lay responder. They potentially reduce the critical time from collapse to shock administration.75 76 Semi-automatic AEDs are used by paramedics and medical personnel and programme a shock that is delivered when, and if, the operator decides. An important advantage of semi-automatic AEDs is the reduction in the number of potentially harmful, inappropriate shocks which occur when a manual defibrillator is used, with a decrease from 26% to 6% in one adult study.77

The potential advantages of both types of AEDs are limited by the detrimental interruption of chest compression before shocks due to AED rhythm analyses during resuscitation.78 79 Pauses are significantly longer for AEDs than for manual defibrillators.77 A median “no flow time” of 51% has been recorded during the resuscitation of 105 adults.80 Future AEDs should be able to suppress CPR artefacts during ECG analysis, significantly reducing pauses, therefore improving myocardial oxygenation, which is essential for defibrillation success.81 82

Two large adult studies of the use of AEDs by lay responders83 84 have not statistically proved that AEDs affect survival in adults, although one of the studies showed improved return of spontaneous circulation.84 Three other adult studies85^–87 have shown increased survival to discharge. However, all five demonstrated statistically shorter times from collapse to shock administration using an AED, which is an important determinant of outcome.88

Out-of-hospital AEDs have been made available to lay responders in areas where people congregate such as sports stadiums,89^–91 businesses,92 airports, railways stations and bus stations93 and have been provided for use by police officers,94 prison officers,95 firemen96 and airline personnel.97 One survey of 409 schools in Washington, US during 2006 showed that of the 118 schools that responded, 64 (54%) possessed one or more AEDs.98

To date, three case reports have been published of the use of AEDs without energy attenuators in children aged 36 months, 6 years and 13 years, during out-of hospital cardiac arrest.99^–101 All children returned to pre-VF function with one showing temporarily raised levels of cardiac enzymes.101

The only study of the use of AEDs in children used a monophasic waveform delivering shocks from 200 to 360 J in seven children aged from 5 to 15 years.43 Sixty seven analyses of cardiac rhythm were produced and 22 episodes of VF were identified and shocks applied, a specificity of 100% and sensitivity of 88%. Three of the children survived without myocardial damage.

ENERGY ATTENUATORS FOR AEDS

Separate AEDs for children have not been marketed so modified cables/patches have had to be produced to attenuate the potentially dangerous adult energy doses of 150–360 J to doses suitable for paediatric patients aged 1–8 years or weighing 10–25 kg (table 6).102

A study using piglets of 4, 14 and 24 kg who received biphasic shocks of 200, 300 and 360 J attenuated by modified paddles to 51, 78 and 81 J compared to a monophasic shock strategy of 2, 4 and 4 J/kg, convincingly demonstrated the advantages of attenuated biphasic shocks.48 Reduced cardiac enzyme levels, better neurological outcome and improved left ventricular ejection fraction were all recorded despite the higher energy dose.48 A similar study, also using modified paddles, of pigs weighing from 13 to 26 kg compared biphasic shocks of 200, 300 and 360 J to attenuated shocks of 50, 75 and 86 J, and produced similar results.57

One case study103 and one clinical study43 have been published demonstrating the use of AEDs with paediatric cables/pads. Between one and four shocks delivered to each of eight children in VF terminated all episodes of VF.43 Six of the nine children who received shocks survived to discharge.43 103

SUMMARY

Pre-oxygenation and chest compressions, according to current recommendations, are vital for defibrillation success. Training is recommended for medical personnel who use manual defibrillators so that they chose the correct paddle size, correctly position the paddles and, particularly, apply sufficient force. In the absence of demonstrated toxicity, the authors agree with the ERC 2005 recommendations that all shocks should be 4 J/kg, being slightly at variance with the ILCOR 2006 statement which recommends shocks of 2 J/kg followed by 4 J/kg. Biphasic waveforms are preferred to monophasic waveforms as they have fewer harmful effects.

Evidence from automatic AED use in children is very limited, and non-existent for those aged <1 year. Automatic AEDs have been proven in adult practice to significantly shorten the critical time from collapse to shock administration, helping tackle one of the most intractable problems in defibrillation, but the benefit to survival is not consistent. Semi-automatic AEDs may be preferred in paramedic or in-hospital settings (where the majority of VF arrests in children occur) and are an important advance with their accurate rhythm analysis which can reduce inappropriate shocks. A reduction in “no flow time” should be possible in the future when AEDs are developed which can incorporate rhythm analysis concurrent with CPR. Where AEDs are available they should be programmed to recognize paediatric arrhythmias and include energy attenuating devices for use in children. The use of amiodarone, not discussed in this article, is recommended for refractory VF.1 2