AC Defibrillator: Parallel After-Discharge Circuit for Sustained Muscle Contraction

An alternating current (AC) defibrillator utilizes a parallel after-discharge circuit to deliver a sustained current waveform that produces prolonged myocardial depolarization and muscle contraction. This design contrasts with monophasic/biphasic truncated exponential waveforms used in modern defibrillators.

Historical Context

• Early AC defibrillators (1950s-60s) applied 50/60 Hz mains current directly to the chest.
• The parallel after-discharge circuit stored energy in capacitors and discharged through a parallel resistor-inductor network to shape the waveform.
• Goal: maintain current above defibrillation threshold longer to overcome myocardial refractoriness.

Limitations & Evolution

• High energy requirements, myocardial damage, poor efficacy for ventricular fibrillation.
• Replaced by DC capacitor-discharge defibrillators (Lown, 1962) → modern biphasic truncated exponential (BTE) waveforms.
• BTE: lower energy, higher first-shock success, less post-shock dysfunction.

Current Relevance

AC defibrillators are obsolete clinically. Modern AEDs and manual defibrillators use biphasic waveforms. This concept remains of historical/engineering interest in biomedical device evolution.