Defibrillators & Arrhythmias

Defibrillator Basics

• An external defibrillator is an electrical stimulator that sends an electrical current through the thorax to treat fibrillation
• Modern defibrillators also measure, display, and analyze cardiac electrocardiograms

Electrical Activity & Arrhythmias

• Normally, action potentials travel unidirectionally, creating a series of depolarizations and repolarizations
• During reentry, a premature impulse can get trapped in a tissue branch, causing it to retrigger instead of moving downward
• Tachyarrhythmias are distinguished by a high rate and include paroxysmal tachycardia, flutter, and fibrillation

Paroxysmal Tachycardia

• Paroxysmal means the arrhythmia episodes begin and end abruptly
• Tachycardia means the heart beats abnormally fast, generally 150-250 bpm
• Paroxysmal atrial tachycardia involves rapid, regular heartbeats originating in the atria
• Paroxysmal ventricular tachycardia is a fast but regular rhythm, which can lead to ventricular fibrillation
• Ventricular fibrillation involves fast and irregular heartbeats, so fast that the heart stops pumping blood

Atrial Flutter

• Characterized as a supraventricular tachycardia with a rate of 250-350 bpm
• It results from a reentrant circuit, typically in the right atrium
• Atrial flutter can be spotted on an ECG as typical sawtooth flutter waves
• Ventricles beat more slowly than the atria because the AV node acts as a funnel
• Atrial flutter can persist for months or years, but may deteriorate into atrial fibrillation

Ventricular Flutter

• Primarily caused by re-entry, with electrical current looping in tissue at 300 bpm
• An ECG shows a sinusoidal pattern

Atrial Fibrillation

• Atrial fibrillation is an arrhythmia where the atria do not contract normally, contracting too fast and irregularly instead
• Multiple foci fire signals at the AV node simultaneously
• Impulses in the atria usually vary between 300 and 600
• The heart continues to pump during atrial fibrillation
• Reduced pumping can lead to blood pooling and clotting in the atria, increasing stroke risk
• ECG tracing shows tiny, irregular 'fibrillation' waves making the rhythm irregular and erratic

Ventricular Fibrillation

• One or more ventricular ectopic foci fire continuously with reentry, causing erratic, rapid twitching
• The rate range is described as 350 to 450 bpm
• Patients require immediate attention
• Characterized by an uncoordinated, fast, and irregular rhythm, along with an ineffective heart pump
• Unconsciousness, no breathing or pulse is experienced
• Death is certain without defibrillation

Sudden Cardiac Arrest (SCA)

• Ventricular fibrillation (Vfib) is the most common cause of out-of-hospital sudden cardiac arrest (SCA)
• SCA occurs when the heart stops beating
• Other causes include coronary heart disease, myocardial infarction ("heart attack"), electrocution, drowning, or choking
• SCA happens in approximately 350,000 people annually in the United States
• From 1980 to 2003, 8.4% of all SCA rhythms and 17.7% of Vfib patients survived

Cardiopulmonary Resuscitation (CPR)

• Lay rescuers may perform CPR in an out-of-hospital SCA setting
• Chest compressions and rescue breaths enable circulation to vital organs
• Begin with two rescue breaths for an unconscious, non-breathing person
• Compress the victim's chest 1.5 inches 30 times within 18 seconds using both hands on the lower sternum, administering two rescue breaths within 2 seconds
• A large current shock, or electrical countershock, is administered across the victim's thorax to stop fibrillation, or defibrillation.
• Each minute defibrillation is delayed reduces survival by about 10%, as stated by the American Red Cross
• The time to defibrillation is the major predictor of outcome

Defibrillation Process

• Electric shock is delivered trans-thoracically using large-area electrodes against the anterior thorax via an external defibrillator
• Internal defibrillation through a surgical procedure uses lower current levels with electrodes placed directly on the heart

Electrodes

• Spoon-shaped internal electrodes are applied directly to the heart
• Paddle-type electrodes are applied against the anterior chest wall
• Disposable defibrillation electrodes (pads), stick to the patient's skin as an alternative to paddles
• Pads replaced paddles since they have better skin contact, minimizing resistive losses and are safer
• Anterio-posterior placement involves one electrode on the left precordium and the other on the back between the scapula to be used best for non-invasive pacing and lowest impedance
• Sterno-apical placement involves the sternum electrode on the upper right chest and the apex electrode on the lower left chest
• The anterior-apex scheme can be used when the anterior-posterior scheme is inconvenient or unnecessary

History

• In 1947, surgeon Claude Beck revived a Vfib patient during a procedure when the chest cavity was open using 110 V/1.5 A alternating current (AC) delivered from an experimental defibrillator after massaging the heart to continue circulation for 35 min
• Cardiologist Paul Zoll extended external pacing to external defibrillation in 1956
• An AC 60 Hz/240-720 V sine wave was delivered through copper electrodes smeared with paste against the chest wall for 0.1-0.5 seconds
• Zoll founded ZMI Corporation in 1983, and later ZMI became the first manufacturer to market a combined external pacemaker and external defibrillator in a compact unit
• Cardiologist Bernard Lown demonstrated that DC countershock is more effective, less lethal, and less likely to induce Vfib or AC
• Cardiovascular surgeon Karl Edmark experimented with the optimum capacitor/inductor combination for a DC Waveform
• Edmark founded Physio-Control Corporation in 1955, which produced the Lifepak series defibrillators as a standard of care for many years

Technical Aspects of Defibrillation

• Defibs in use today send about 3,000 volts (V) – up to 20 amperes (A) – across a patient’s chest
• This energy causes all heart cells to depolarize at once allowing the SA node to resume function and generate a normal electrical pattern

Waveform

• The resulting damped sinusoid waveform is known as the Lown-Edmark waveform
• The capacitor, C, is charged with a DC voltage, VDC, before it switches and discharges
• The current is shaped by the capacitor, the inductor, L, the inductor resistance, Rl, and patient resistance, Rp, typically 50 Ω
• External electrodes require may require as high as 360 J energy
• Internal defibrillators limit maximum output energy to 50 J

2010 Pediatric Guidelines

• Use 2 J/kg for the first manual defibrillator attempt followed by 4 J/kg for subsequent attempts

Waveform Types

• External defibrillation waveform involves a 50 Ω patient resistance for Monophasic Lown-Edmark
• There are damped sinusoidal and truncated exponential for Basic and Specific Monophasic Lown Waveforms
• The biphasic waveform is standard in defibrillators today with a positive and negative component producing better results more quickly
• Biphasic waveforms deliver results with less energy and heart damage

Automatic External Defibrillator (AED)

• AEDs analyze cardiac status and deliver therapeutic shock as indicated using a pre-set algorithm and voice commands
• Designed for non-clinical settings, intended for use by non-clinicians, the device is powered by a non-rechargeable lithium ion battery
• Electrocardiograms are digitized, analyzed by the Vfib detection module, then the signals go back to the processor module
• Shockable rhythms prompt the user to depress the defibrillate button to discharge the waveform circuit which causes the waveform circuit, holding with a charged capacitor, to discharge across the electrodes
• ECGs and detection data can be downloaded through a data downloader module separate from therapy

Cardioversion

• A small energy pulse from a defibrillator converts a non-lethal arrhythmia (e.g., atrial flutter) to normal sinus rhythm
• Delivers a pulse about 30ms after the peak of the R-wave, synchronized by the defibrillator
• It is crucial not to cardiovert across the T-wave
• The cardioverter consist of both a cardiac monitor and defibrillator

Cardioverter Function

• Signals pass from ECG electrodes through a switch (amplifier and cardioscope), the output goes through a threshold detector, filters and then the R wave is detected
• A 30 ms signal delay activates a trigger circuit, which opens the switch and connects to the ECG electrodes, protecting the amplifier
• Simultaneously, it closes a switch that discharges the defibrillator capacitor through defibrillator electrodes
• This R-wave-controlled switch discharges the defibrillator once activated
• After discharge, the switch connecting the ECG electrodes closes, so the operator can check the cardiac rhythm

Defibrillators as External Pacemakers

• Used for patients with bradycardia or other issues
• Pacing is either 'synchronous' or 'asynchronous'
• Synchronous pacing ("Demand" mode) fires only when no complex is sensed for a predetermined time
• Asynchronous pacing fires at a fixed rate independent of cardiac activity
• Typical external pacemakers deliver a 40-millisecond impulse for each beat
• The average current is from 45-100 mA
• The same defibrillator electrodes are used for pacing