Criticare - A special feature on Anaesthesiology

Technology used in defibrillators

Aarti Kalra

Survival from sudden cardiac arrest depends on various emergency responses which include early access of the Emergency Medical Services (EMS), early CPR, early defibrillation and early advanced life support.

Numerous scientific studies conducted during the past two decades have proved that rapid defibrillation is the most important factor affecting survival from sudden cardiac arrest in adults. This research, coupled with important technological advances, has driven the international movement to increase access to early defibrillation.

In the past, defibrillators were complicated and cumbersome. Only medical professionals with extensive training in heart rhythm interpretation could use them. Today, defibrillators are automated, portable and easy to use. These Automated External Defibrillators (AEDs)- read heart rhythms- and determine whether a shock should be delivered. In addition, AEDs have audio and visual prompts to guide trained operators.

Monophasic Vs biphasic defibrillators

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.

Rhythm recognition

Cardiac rhythm recognition has also improved greatly in recent years. The first version of the AED used rhythm-recognition protocols that triggered a shock if the heart rate exceeded 150 beats per minute and the QRS amplitude rose above 0.15 mm.

Current products employ a more sophisticated ECG recognition system that analyses rate, amplitude, QRS slope, morphology, power spectrum density, and time away from isoelectric baseline. It then compares this information with a rhythm strip library.

All current AED algorithms have similar features and use impedance-based electrodes and a contact detection sensor. However, the algorithm in each marketed AED is proprietary and distinctive.

ECG-strip segments of two to three seconds are recorded and analysed. If, in three consecutive strip segments, the device senses abnormal QRS complexes at more than twice the frequency of any other QRS complex, it primes itself to deliver a shock. Rhythms are categorised as shockable, nonshockable, or indeterminate.

Current devices usually have a sensitivity for VF greater than 99 per cent. In one series of 100 consecutive patients, the detection sensitivity and specificity were 100 per cent. The real challenge is in discriminating among fine VF, asystole, and artifact. For this reason, CPR must not be in progress during rhythm analysis.

It takes most AED devices on the market 6 to 12 seconds to commit to firing. With fully charged batteries, AEDs require 8 to 15 seconds to reach a level of 360 J. Therefore, for most defibrillators, 15 to 30 seconds between shocks is expected for persistent VF.

Operational issues

Before an AED is applied, care must be taken to ensure that the patient is medically unstable or pulseless. Alert persons with a pulse who have stable VT or rapid but stable bundle branch block patterns are not appropriate candidates for defibrillation.

Emphasising this distinction will become particularly important as laypersons begin using AEDs and as algorithms to shock become increasingly sensitive to fine VF and less specific for artifact detection.

Disposable pads Vs external paddles

Why disposable pads should be used in AEDs and not external paddles? The reason behind this is that when external paddles are used the air space between the paddles and the patient’s chest would require higher energies for defibrillation. Hence, it is preferable to use disposable pads.

FDA approved labeling for AEDs state they are intended for use by first responders or on the prescription of a physician. For AED use to become widespread in public areas, the FDA would have to make amendments in its guidelines to allow laypersons to operate AEDs. Many states have taken up this issue seriously by promulgating good samaritan laws or enacting other legislations.

The writer is a product specialist with Schiller Healthcare India Pvt Ltd.