--- title: "Ventricular fibrillation and DC defibrillator" subject: "Biomedical Instrumentation" chapter: 8 key: "semester_7/biomedical_instrumentation:ventricular-fibrillation-dc-defibrillator" source: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/ventricular-fibrillation-dc-defibrillator site: BCT Engineering --- # Ventricular fibrillation and DC defibrillator Biomedical Instrumentation · Chapter 8: Electrical Safety of Medical Equipment - Asked in: 2075 Chaitra, 8 marks - Video: [Defibrillator (circuit and concept explained)](https://www.youtube.com/watch?v=7cU34eWAYUQ) — Anniappan Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/ventricular-fibrillation-dc-defibrillator - Whole chapter: https://www.bctengineer.com/md/semester_7/biomedical_instrumentation/chapter-8 ## Question What is ventricular fibrillation? Is it caused by physiological effects of electric current then how? Give some reasons. Design a DC Defibrillator machine to restore the normal cardiac rhythm and explain its function briefly. ## Short answer Ventricular fibrillation is unsynchronized ventricular quivering, causing no cardiac output. Currents of 75–400 mA through the chest or ~100 µA on the heart can induce it, particularly during the T-wave. A DC defibrillator charges a capacitor (32 µF, ~4.7 kV) to ~360 J and delivers a damped sine shock via paddles to depolarize all myocytes, resetting the rhythm. ## Answer #### Ventricular fibrillation (VF) VF is a condition in which the ventricular muscle fibres contract rapidly and out of step, so the ventricles quiver and do not pump blood. The cardiac output falls to zero and the patient dies within minutes unless the heart is defibrillated. The ECG shows irregular, chaotic waves without QRS complexes. #### Is it caused by electric current? Yes Electric current acts on the heart by stimulating the excitable cardiac cells. If a current passes through the chest, it can cause disordered excitation of these cells: - A current of about **75 to 400 mA (50/60 Hz) through the body** (macroshock) can send the ventricles into fibrillation. - A current as low as about **100 µA applied directly to the heart** (microshock, through a catheter or pacing wire) can cause VF. - The danger is highest if the current falls in the **vulnerable period** (T-wave), when some cells have recovered and others have not, so re-entrant, uncoordinated circuits start. - A frequency of 50/60 Hz is the most dangerous; higher frequencies (>1 kHz) are less so. - Reasons for danger: leakage from faulty equipment, a broken ground, wet skin (low resistance), contact with the mains, or a catheter in the heart. #### DC defibrillator It gives a large DC pulse through the chest to depolarise all the heart cells at once, so that the natural pacemaker (SA node) can take control again. ```text Mains -> [Step-up ]-> [Rectifier] -> [HV capacitor C] [transformer] ^ | [variable auto- | [Inductor L ~100 mH] transformer] charge control | [Discharge switch/relay] | Paddles (apex/sternum) -> chest [ECG monitor + synchroniser]--(R-wave trigger for cardioversion) ``` **Function:** 1. The AC mains is stepped up (to about 5 kV) and rectified to DC; the voltage control (autotransformer) selects the energy (about 50 to 360 J). 2. The capacitor stores energy E = ½CV². Example: C = 32 µF; for E = 360 J, V = √(2E/C) = √(720/32×10⁻⁶) ≈ 4.7 kV. 3. The paddles are placed on the chest (sternum and apex) with conductive gel, and the operator presses the discharge buttons. 4. The capacitor discharges through the inductor (which shapes a smooth damped sine pulse, the Lown waveform, lasting about 5 to 10 ms) and through the patient (resistance about 50 Ω). 5. A relay isolates the patient from the HV during charging; a safety discharge resistor dumps the stored energy if it is not used. 6. In **synchronised mode (cardioversion)**, the shock is triggered by the R-wave to avoid the vulnerable period. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What happens in ventricular fibrillation? A. The ventricles contract strongly and in step B. Ventricular fibres contract rapidly and out of step, so cardiac output falls to zero C. The atria stop and ventricles beat normally D. The heart rate slows to 40 bpm Answer: B. In VF the ventricles quiver without pumping, and the ECG has chaotic waves without QRS complexes. (Key point: VF) 2. Why is current falling in the T-wave (vulnerable period) especially dangerous? A. All cells have recovered together B. Some cells have recovered and others have not, so re-entrant uncoordinated circuits can start C. The heart is in a long absolute refractory period D. The T-wave has the lowest amplitude Answer: B. Uneven recovery of cells allows re-entry and fibrillation. (Key point: Vulnerable period) 3. A defibrillator capacitor of 32 µF must deliver 360 J. What voltage is needed? A. About 470 V B. About 4.7 kV C. About 47 kV D. About 150 V Answer: B. V = sqrt(2E/C) = sqrt(720/32e-6) which is about 4.7 kV. (Key point: Defibrillator energy) 4. What is the purpose of synchronised (cardioversion) mode? A. To increase the energy B. To trigger the shock from the R-wave and avoid the vulnerable period C. To charge the capacitor faster D. To remove the inductor Answer: B. R-wave triggering prevents delivering the shock during the T-wave, which could cause VF. 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