External Defibrillator

Tachyarrhythmias

• Paroxysmal tachycardia or tachycardia

Paroxysmal tachycardia is identified by its general rate range of 150-250 bpm. It may be caused by irritable ectopic focus firing, with rapid sudden pacing, or by reentry.

– Paroxysmal atrial tachycardia : the ventricular rate is high, and the P wave is present.

Paroxysmal atrial tachycardia printed from Rivertek RSIM-100 Simulator

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Tachyarrhythmias

– Paroxysmal junctional tachycardia: the ventricular rate is high, and the P wave is either inverted or missing. An irritable focus in the AV junction or AV nodal reentry may initiate tachycardia pacing.

Paroxysmal junction tachycardia due to AV nodal reentry. 3

Tachyarrhythmias – Paroxysmal ventricular tachycardia (Vtach): the

ventricular rate is high and looked like PVC complexes. Normally, reentry triggers this arrhythmia. Although the atria are still paced, P waves are hidden in the large complexes. This condition usually indicates coronary artery insufficiency, with inadequate oxygen reaching the heart.

Paroxysmal ventricular tachycardia with right BBB 4

Tachyarrhythmias

• Flutter Flutter is identified by its general rate range of 250-350 bpm.

– Atrial flutter: one extremely irritable atrial ectopic focus rapidly fires in this rate range. Reentry causes the impulse to continuously circle a region of the right atrium. These flutter waves look like sawtooth. Since every atrial depolarization does not reach the ventricles, a slower ventricular rate is observed.

Atrial flutter, with a ventricular rate of 152 bpm. 5

Tachyarrhythmias

– Ventricular flutter: one extremely irritable ventricular ectopic focus rapidly fires in the general rate range of 250-350 bpm. These flutter waves are sinusoidal in nature. Because the rate is so rapid, the ventricles hardly have time to fill with blood before contraction. The resulting lower cardiac output and hypoxia rapidly deteriorate into deadly ventricular fibrillation.

Ventricular flutter at 175 bpm.

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Tachyarrhythmias

– Torsades de pointes: a peculiar arrhythmia in the general rate range of flutter, 250-350 bpm. Translating into “twisting of points,” this arrhythmia is caused by low potassium, medications that block potassium channels, or congenital abnormalities that length the QT segment. Torsades de pointes results from two competitive, irritable ventricular foci, which cause an undulating amplitude of sinusoids.

Torsades de pointes

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Tachyarrhythmias

• Fibrillation Fibrillation is identified by its general rate range of 350-450 bpm.

– Ventricular fibrillation (VF or Vfib), one or more ventricular ectopic foci continuously fire with reentry, producing an erratic, rapid twitching of the ventricles. Although the rate range of Vfib is described as 350-450 bpm, in reality, it may be difficult to calculate. Because no blood is being pumped by the heart, a Vfib patient requires immediate attention (Dubin, 2000)

Ventricular fibrillation

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Tachyarrhythmias

– Atrial fibrillation, one or more atrial ectopic foci continuously fire, with reentry. Thus, the complete depolarization of the atria is prevented, with only occasional depolarization through the AV node to stimulate the ventricles.

9 Atrial fibrillation, with a ventricular rate of 53 bpm.

Sudden cardiac arrest and cardiopulmonary resuscitation

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Ventricular fibrillation is the most common cause of out-of-hospital sudden cardiac arrest (SCA). Other causes include coronary heart disease, myocardial infarction (a “heart attack”), electrocution, drowning, or choking.

When SCA occurs outside a hospital, rescuers may perform cardiopulmonary resuscitation (CPR) until EMS arrives. CPR involves chest compressions at least 5 cm deep and at a rate of at least 30 times per 18 s. The rescuer then administers two rescue breaths within 2 s, enables oxygenated blood to circulate to vital organs. This cycle of 30 compressions and two rescue breaths is repeated until help arrives or the rescuer is too exhausted to continue (ARC, 2006).

DEFIBRILLATION MECHANISM AND THRESHOLD

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When EMS arrives, a large electrical countershock, is administered across the patient’s thorax to stop fibrillation, also known as defibrillation. The sternum electrode is placed below the clavicle and to the right of the sternum. The apex electrode is over the cardiac apex and to the left of the nipple. Diameters of transchest defibrillator electrodes are 8-13 cm. Electrodes manufactured for direct application to the heart (e.g., during a surgical procedure) are smaller (4-8 cm). Large-diameter electrodes are used to achieve a uniform field within the heart and also to avoid high current densities that could burn the skin. The total dry transchest impedance is found to be 25-150 Ω, while the transcardiac impedance is typically 20-40 Ω.

External Defibrillator

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The mechanism behind defibrillation is still unproven. The goal in defibrillation is to interfere, electrically, with the reentry circuits to stop this electric activity. Since the reentrant circuits lie throughout the heart, achieving this goal requires an adequate stimulating field at all points within the heart. The minimum field for successful defibrillation was found to lie in the range of 3-9 V/cm.

Physio-Control Lifepak 5 portable defibrillator

Simplified external defibrillation circuit for a DC waveform

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DC countershock was administered by discharging energy stored in a capacitor. The capacitor, C, is charged with a DC voltage, VDC, before t=0, when the capacitor then switches to discharge. The voltage across the capacitor is Vi (t). The current in the circuit is shaped by the capacitor (C), the inductor (L), the inductor resistance (Rl) and by the patient resistance (Rp). Typically, a patient resistance is assumed to be 50 Ω.

Simplified external defibrillation circuit for a DC waveform

Defibrillation Waveforms

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At t=50, the stored energy is related to the initial capacitor voltage, which is VDC, as

𝐸𝑠 𝑡 =1

2𝐶 𝑉𝑖 0

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External for a patient resistance of 50 Ω. Damped sinusoid for 360 J, 16 μF, 0.1 H, and 11 . Resulting a peak amplitude of 2795 V and width of approximately 6 ms

Defibrillation Waveforms

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A BTE waveform of lower voltage and energy was required to produce defibrillation, compared to a damped sinusoid (Gliner et al., 1995). The system needs only a smaller capacitor to deliver a low-energy BTE waveform, which is commonly used in implantable defibrillators.

External defibrillation waveforms for a patient resistance of 50 Ω. Biphasic truncated exponential for 150 J, 100 μF, 51% tilt.

Simplified external defibrillation circuit for a BTE waveform, with switching. The patient is not shown but is connected to the top of the circuit.

Energy Level

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Typically, a the thoracic impedance is assumed to be a purely resistive 50 Ω. However, the tissue response to voltage stimulation may be modeled as a resistor (R) and capacitor (C), in parallel. Transchest defibrillation energy is in the range of 200-360 joules. It needs a current of 24 A, 20 ms, and a voltage of 5 kV, monophasic, or 2 kV, biphasic. An inadequate current for defibrillation can result from the selection of a low energy level while being unaware of a high transchest impedance (inadequate skin preparation). Some devices first sense this impedance and then choose the energy level to ensure an adequate current.

Philips M3713A Heartstart defibrillation pad

Automated External Defibrillator (AED)

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Manual models required a clinician to recognize an ECG that should be defibrillated. For AED, the system relied electrodes attached to the victim’s thorax for sensing ECG. If a shockable rhythm, such as Vfib or Vtach is detected, the processor module prompts the user to depress the defibrillate button. The AEDs take time (generally 10–20 seconds) to diagnose the rhythm, where a professional could diagnose and treat the condition far more quickly with a manual unit.

Combined automated and manual external defibrillator system diagram

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The Vfib detection module not only detects arrhythmias, but also detects the timing of the QRS complex, in order to synchronize countershock with the cardiac cycle. A pacing waveform circuit enables pulses to the patient, typically from 0 to 200 mA, at 40-180 bpm. Optional sensor modules enable measurement of blood pressure, arterial saturation of oxygen (pulse oximetry), or end-tidal carbon dioxide

Key Features from Engineering Standards

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• Battery Charging Time

ANSI/AAMI DF80:2003 requires that manual and AEDs defibrillators be ready for discharge at maximum energy within 25-35 s and 40-50 s of being powered on, respectively, in rechargeable battery mode.

• Rhythm Recognition Detection Accuracy

For AEDS, it is obviously important that ventricular fibrillation, ventricular tachycardia, and other waveforms be accurately detected. ANSI/AAMI DF80:2003 requires minimum sensitivities and specificities of detection. Sensitivity refers to correct classification percentage of a shockable rhythm. Specificity refers to the correct classification percentage of a nonshockable rhythm. the sensitivity for recognizing Vfib at maximum peak-to-peak amplitude of 200 mV or greater must exceed 90% in the absence of artifacts, such as those induced by cardiopulmonary resuscitation. Similarly, for devices detecting Vtach, the sensitivity must exceed 75%. In the absence of artifacts, the specificity for detecting nonshockable rhythms must exceed 95% (AAMI, 2003).

Key Features from Engineering Standards

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• Synchronization

A defibrillator may be used for external cardioversion to treat atrial flutter or atrial fibrillation. To do this, the defibrillator is set in synchronization mode, to ensure that the shock is timed after the peak of the QRS. ANSI/AAMI DF80:2003 specifies that the defibrillator not power on in synchronization mode, and requires 60-ms delay to ensure that the ECG is derived from the defibrillator electrodes or paddles, rather than from a signal input to the defibrillator (AAMI, 2003).

• Recovery after Defibrillation

ANSI/AAMI DF80:2003 requires that at most 10 s pass before an ECG is visible on the monitor display and that the ECG peak-to-valley amplitude not deviate from the original amplitude by greater than 50%.

• Capacitor Discharge Accuracy

ANSI/AAMI DF80:2003 requires that the delivered energy at all settings, for patient impedance of 25, 50, 75, 100, 125, 150, and 175 Ω, be specified. Delivered energy accuracy is then tested according to this specification and must be within 63 J or 615%, whichever is greater (AAMI, 2003).

References

1. Fundamental of Anatomy and Physiology, Frederic H. Martini

2. Biomedical Instrumentation: Application and Design, John G. Webster

3. Medical Device Technologies: A Systems Based Overview Using Engineering Standards, Gail D. Baura

4. The Biomedical Engineering Handbook, Joseph D. Bronzino

• Wikipedia

1. Sinoatrial node

2. Atrioventricular node

3. Bundle of His

4. Left bundle branch

5. Left posterior fascicle

6. Left-anterior fascicle

7. Left ventricle

8. Ventricular septum

9. Right ventricle

10. Right bundle branch

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