Introduction:
Since the introduction in the western world, of the first Biphasic External Defibrillator back in 1996, there has been a growing acceptance that this technology offers an opportunity to increase the success of the defibrillation process.

Whilst this may well be true, the use of a Biphasic waveform, in itself, cannot meet the challenge of truly customising a shock for the unique characteristics of each individual patient.

The story sold at the time, was that Biphasic provided "two shocks for the price of one" , a story that whilst easy to 'sell', is nevertheless totally incorrect.

Whilst there are indeed two phases (compared to one phase in a Monophasic waveform) the second phase of a Biphasic waveform reverses the pathway of the first 'shock' phase current and effectively removes that current, thereby leaving the myocardial (heart) cells in true resting phase.

This is easily seen when viewing an actual waveform that show the 2nd phase as negative, see below:

It is therefore unrealistic to expect that a fixed low energy defibrillator (150joules offered by the Heartstart FR & FR2) can be effective in a patient population with vastly varying resistance levels across the chest.

Indeed, any fixed energy protocol whether high or low level, can only go so far in providing a tailored shock.

Survivalink Corporation has addressed this dilemma by becoming the first defibrillator manufacturer in the world to introduce a Biphasic Defibrillator with Variable, Escalating Energies.

This unique technology ensures that sufficient energy is delivered to each individual patient (based on the principle of exceeding patients defibrillation threshold - TTI) whilst at the same time ensuring they do not receive too much.

The Phillips Heartstart FR2 (formerly the Heartstream Forerunner) was the first AED to use a Biphasic waveform, launched back in 1996.

This device deviated from the Resuscitation Guidelines of the time in two significant ways:

1. It dramatically lowered the energy setting to a nominal fixed 150 joules for all shocks, compared to the established 200, 200, 360joule protocol.
2. It eliminated the established principle of escalating energy settings for the third and subsequent shocks.

The potential benefits of Biphasic defibrillators (which we all believe in), was the potential for improving the success of defibrillation, whilst at the same time, being able to reduce the amount of current delivered to the heart, thereby minimising the possibility of damaging the myocardium.

In defibrillation however, we are faced with two challenges:

a. To deliver enough energy to effectively defibrillate the Heart, bearing in mind that there are significant differences across the patient population as to how much energy is required to achieve this successful defibrillation.
b. To minimise the amount of current is passed through the Heart, thereby minimising the chance of excess damage to the heart muscle.

In considering these challenges, I quote from the International Resuscitation Guidelines, published in August 2000:

"Defibrillation depends on the successful selection of energy to generate sufficient current flow through the heart (transmyocardial current) to achieve defibrillation while at the same time causing minimal electrical injury to the heart. A shock will not terminate the arrhythmia if the energy and current are to low. Selection of appropriate current also reduced the number of repetitive shocks and limits myocardial damage".

"A more predictable increase in current occurs when shock energy is increased. This supports second shocks at higher energies".

It would seem appropriate, that when considering the two challenges mentioned above, that we must favour systems that ensure sufficient energy and current flow is achieved for all patients.

The PowerHeart AED with STAR Biphasic waveform therefore is supported by these latest guidelines, as it can escalate energy levels for subsequent shocks.

Indeed, PowerHeart and Survivalink FirstSave AEDs with STAR Biphasic technology are the ONLY AEDs that can achieve this, by assessing each patients individual characteristics and in response, varying the energy level sufficiently to meet their needs.

By assessing each patient in this way, we also are able to ensure that only sufficient energy is delivered, thereby minimising the potential for myocardial damage.

As the Heartstart FR2 is a fixed low energy defibrillator (i.e. there is no increase in energy available), then we must ask how this can be achieved.

It is also worthy of note that all modern defibrillators using a Biphasic waveforms introduced from 1999 onwards have employed escalating energy protocols, and this includes other more recent defibrillators introduced by Phillips / Laerdal.

It is interesting to note that only two devices were chosen for the first phase of the 'Defibrillators in Public Places' initiative, both units chosen were Biphasic (one being the Survivalink 'FirstSave AED), however both were also set up to deliver escalating energies!

Clinical Data

In support of the above information, we are able to clearly show clinical data that confirms our methodology, whilst inevitably demonstrating the weakness of a fixed low energy protocol.

For every minute a heart is in sudden cardiac arrest, the chance of survival decreases by 10%. The extra time it takes to defibrillate 6 times versus 2.3 times reduces the chance of survival for the patient by more than 20%.

1. Bain AC, et al., Annals of Emergency Medicine. Accepted for publication, January 2001.
2. Bardy GH, etal., Circulation, 1996; 94:2507-2514.
3. Bain, et al., Pre-hospital Emergency Care, 2000; Vol14, No.1 (Updated).
4. Gliner BE, etal., Biomedical Instrumentation and Technology, 1998; 32:631-644.
5. Adgey AA, et al., American Heart Journal, 1988; 112:745-751.
6. Wilson, et al., European Heart Journal, 1998; 9:1257-1265.
7. Herlitz J., et al., British Heart Journal, 1994; 72:408-412.
8. American Heart Association, etal., Circulation, 2000; 102(supppl I): I60-I76.