Different Waveforms, Different Results

Although defibrillators are set by selecting energy, it is important to understand that successful defibrillation actually depends on the delivery of adequate current to the heart.

The flow of electrical current has a shape that can be drawn as a waveform. The waveform shows how the flow of current changes over time, during a defibrillation shock. The highest part of the current waveform is called the peak current and the total amount of current delivered during the shock, divided by the duration is called the average current.

The goal of a defibrillation shock is to deliver the appropriate amount of average current while minimising the peak current.

There are different types of waveforms, traditionally Monophasic shocks were administered; these shocks travel in one direction only. In addition to the monophasic shock there are also biphasic shocks. These shocks administer current traveling in one direction for a portion of the shock and then reversed the direction of the current for the remainder of the shock. There are many variations of biphasic waveforms. The most commonly used is the Biphasic Truncated Exponential.

So what about energy?

Energy is comprised of 3 components: Current, Voltage and Time.

You can think of current as the flow strength of electricity and the voltage as the potential amount of electricity.

Energy can be represented as:
Energy = Current x Voltage x Time

Energy can be affected by changing current, voltage and/or time. You can see that increased energy doesn't necessarily mean an increase in current; it can mean that the shock is delivered over a longer period of time.

The amount of current that can be delivered is also limited by the natural resistance (or impedance) of a patient to the current.

Current = Voltage / Resistance (also known as Ohms Law)

So for defibrillation, Ohms law (equation above) tells us that for a given voltage, the amount of current that gets to the heart is controlled by resistance. The resistance of the patient will affect the shape of the waveforms. The curve of the waveform of a patient with high resistance will show a sharper drop, making it difficult to maintain a good level of average current, this is why high impedance patients are often difficult to defibrillate.

Don't be fooled by energy! A higher energy setting of 360J does not necessarily mean an increase in current.

Issues with increasing time and current

If patient outcomes were all about energy, increasing the duration of a shock would improve patient outcomes, because it would increase the energy of a shock. But, what we actually see is the longer the shock continues for, the more decay there is in the current delivered, and lower average current results. So defibrillators that use variable time of shock, to generate a given energy setting, may be short-changing you on vital current.

So higher current means better outcomes, right?

Wrong. There are two types of current, Peak and Average. As we know it is actually average current that dictates whether a shock will be successful.

High peak currents actually damage the heart and are associated with post shock dysfunction. This is part of the reason that Monophasic shocks result in higher post shock dysfunction, they rely solely on high peak currents to generate the required average current for a shock.

So what we need is a Rectilinear Biphasic Waveform, that delivers high average current, without damaging high peak current, that can cope with a wide range of patient impedances without extending the shock duration.