Nearly, all AEDs use monophasic waveforms that essentially direct energy through the heart in one direction. There are several types of monophasic waveforms. All monophasic devices are programmed to deliver increasing levels of energy as needed to achieve conversion of the heart to a normal rhythm.

In recent years, AEDs began using biphasic waveform technology, the same technology that is used in implantable defibrillators. Biphasic technology essentially directs energy through the heart in one direction and again in the reverse direction. Usually less energy is required when biphasic waveforms are used. Some devices with biphasic waveforms maintain a constant energy level for all shocks; others use escalating levels of energy.

Both monophasic and biphasic waveforms are safe and effective. Both types of devices in the market have been approved by the FDA and meet American Heart Association guidelines for emergency cardiovascular care.

Monophasic Vs Biphasic technology

The optimal energy level for successful defibrillation has not yet been determined. The AHA’s ACLS protocols stipulate successive shocks of 200 J, 300 J, and 360 J energy levels extrapolated from clinical trials in the 1970s that used monophasic defibrillation. These levels represent mid- range values sufficient to cause defibrillation without risk of unnecessary injury to the myocardium.

Monophasic waveforms with escalating energy yielded better success rates in some patients. Escalating the energy level at intervals helps ensure that ample energy reaches the heart in patients who have high chest impedance.

Originally, defibrillators used only a monophasic damped sine waveform; a single-directional form of energy passed through the heart. Biphasic damped sine waveforms, in which current travels in one direction and then reverses polarity to travel in the opposite direction, can achieve defibrillation using less energy than the monophasic waveform Lower energy requirements could lead to miniaturized devices with smaller capacitors and batteries. To deliver a 360 J monophasic damped sine wave, 5200 volts of electricity are needed. Only 1600 to 1750 volts are needed to deliver a 150 J biphasic truncated waveform.

In clinical trials, biphasic shocks of 150 J converted VF in 93 per cent of cases in 1 or 2 attempts. Also, low-energy, impedance-compensating biphasic waveforms are more successful than high-energy shocks at terminating VF of long duration in out-of-hospital cardiac arrest. Biphasic technology is emerging as the standard in newer AED models