10.2 Defibrillators, Biphasic Waveforms, Cardioversion & Pacemakers
Key Takeaways
- Defibrillator stored energy follows E = 0.5 · C · V²; modern biphasic units charge a 32–100 µF high-voltage capacitor to 1500–2500 V to deliver 120–200 Joules.
- Biphasic Truncated Exponential (BTE) and Rectilinear Biphasic (RBW) waveforms reverse current polarity mid-discharge via an H-bridge of high-voltage IGBTs, achieving superior defibrillation efficacy at lower total energy with reduced myocardial stunning compared to monophasic waveforms (200–360 J).
- Synchronized cardioversion employs precision R-wave peak detection to deliver shock discharge within <60 ms of the R-wave, strictly avoiding the vulnerable ventricular repolarization phase (T-wave peak) that causes lethal R-on-T ventricular fibrillation.
- Internal safety bleeder circuits and high-voltage dump relays automatically discharge the storage capacitor through an internal high-wattage bleeder resistor (not the 50 Ω analyzer test load) if the shock is not delivered within 15–30 seconds.
- Transcutaneous pacemakers operate in Demand (VVI) or Fixed/Asynchronous (VOO) modes, delivering 20–40 ms constant-current pulses (0–200 mA) at 30–180 bpm; capture thresholds are verified visually and hemodynamically.
Last updated: August 2026
Defibrillators, Biphasic Waveforms, Cardioversion & Pacemakers
Cardiac defibrillators and pacemakers are life-critical electrotherapy devices designed to terminate lethal arrhythmias and sustain cardiac output during severe conduction blocks. For the Biomedical Equipment Technician (CBET), mastery of high-voltage generation, pulse-forming networks, semiconductor switching bridges, electrophysiological synchronization mechanisms, and quantitative energy verification (IEC 60601-2-4, AAMI DF80) is essential for maintaining life-support readiness.
1. Electrophysiology of Defibrillation & Energy Physics
During Ventricular Fibrillation (VF) and rapid Pulseless Ventricular Tachycardia (pVT), the coordinated syncytial electrical activation of the ventricles breaks down into chaotic, high-frequency re-entrant micro-wavelets. Cardiac output drops to zero, resulting in clinical death within seconds.
Therapeutic Mechanism:
Defibrillation delivers a controlled electrical shock across the myocardium through transthoracic electrode pads or internal paddles. The high-energy electrical field simultaneously depolarizes a critical mass (>75–90%) of refractory myocardial cells. This terminates all chaotic re-entrant wavelets, allowing the heart's natural pacemaker—the Sinoatrial (SA) node—to regain pacing control.
+-----------------------------------------------------------------------------+
| CAPACITOR ENERGY STORAGE PHYSICS |
| |
| ENERGY FORMULA: |
| E = 1/2 · C · V² |
| |
| WHERE: |
| E = Stored Energy in Joules (Watt-seconds) |
| C = Capacitance in Farads (typically 32 µF to 100 µF) |
| V = Potential Difference in Volts (typically 1,500 V to 5,000 V) |
| |
| EXAMPLE CALCULATION: |
| A 32 µF capacitor charged to 5,000 V (Monophasic circuit): |
| E = 0.5 · (32 × 10⁻⁶ F) · (5000 V)² = 400 Joules |
| |
| A 100 µF capacitor charged to 2,000 V (Biphasic circuit): |
| E = 0.5 · (100 × 10⁻⁶ F) · (2000 V)² = 200 Joules |
+-----------------------------------------------------------------------------+
2. High-Voltage Charging, Storage & Safety Dump Architecture
Defibrillators convert low-voltage DC from internal rechargeable lithium-ion batteries (12 to 16.8 V DC) or AC mains power into high-voltage DC (>2000–5000 V) in less than 10 seconds:
+---------------------------------------------------------------------------------------------------+
| DEFIBRILLATOR HIGH-VOLTAGE SYSTEM SCHEMATIC |
| |
| +------------------+ +-------------------+ +------------------+ +-----------------+ |
| | 14.4V Li-ion | | FLYBACK INVERTER | | HIGH-VOLTAGE | | ENERGY STORAGE | |
| | BATTERY PACK / |---->| OSCILLATOR (MOSFET|---->| STEP-UP DISS |---->| CAPACITOR (C) | |
| | AC POWER SUPPLY | | Chopper 25-50 kHz)| | TRANSFORMER | | (32-100 µF Film)| |
| +------------------+ +-------------------+ +--------+---------+ +--------+--------+ |
| | | |
| +--------v---------+ | |
| | HV DIODE RECT | | |
| | BRIDGE (Ultra-Fast) | |
| +------------------+ | |
| | |
| +----------------------------------+ |
| | | |
| v v |
| +-------------------+ +------------------+ |
| | SAFETY DUMP RELAY | | H-BRIDGE ACTIVE | |
| | & BLEEDER RESISTOR| | SWITCHING MODULE | |
| | (10 kΩ 50W Dump) | | (4x HV IGBTs) |
| +-------------------+ +--------+---------+ |
| | |
| v |
| [PATIENT THERAPY PADS]|
+---------------------------------------------------------------------------------------------------+
Subsystem Components & Operations:
- Flyback Switching Inverter: A pulse-width-modulated (PWM) push-pull or flyback MOSFET oscillator chops low-voltage DC at 25 to 50 kHz.
- High-Voltage Step-Up Transformer: Multi-secondary ferrite transformer steps the chopped AC voltage up to >2500 V AC RMS.
- High-Voltage Rectifier Bridge: Ultra-fast recovery, high-voltage diode array (>10 kV reverse breakdown rating) rectifies the AC output into high-voltage DC to charge the storage capacitor.
- Energy Storage Capacitor: High-energy-density, self-healing metallized polypropylene film capacitor (32 to 100 µF, rated for >5000 V). Electrolytic capacitors are never used due to high equivalent series resistance (ESR) and catastrophic leakage failure modes.
- Internal Safety Dump Relay & Bleeder Network: A high-voltage mechanical relay held normally open during charging. If a charged shock is aborted by the user, if the therapeutic pads disconnect, or if a programmed timeout expires (15 to 30 seconds), the relay de-energizes, shunting the capacitor charge through a ceramic power bleeder resistor (10 kΩ, 50 W) to discharge the unit safely inside the chassis.
3. Waveform Topologies: Monophasic vs. Biphasic & H-Bridge Switching
Early defibrillators used monophasic waveforms requiring high energy (200 to 360 Joules). Modern clinical standards mandate biphasic waveforms (120 to 200 Joules) due to superior clinical efficacy and reduced cellular injury.
+-----------------------------------------------------------------------------+
| MONOPHASIC VS. BIPHASIC WAVEFORM TOPOLOGIES |
| |
| MONOPHASIC DAMPED SINUSOIDAL (MDS / Lown): |
| CURRENT (A) |
| +40 + ^ |
| | / \ |
| | / \ |
| 0 +-----+-----+---------------------------> TIME (ms) |
| | \___/ |
| |
| BIPHASIC TRUNCATED EXPONENTIAL (BTE): |
| CURRENT (A) |
| +30 + +-----\ |
| | | \ (Phase 1: Forward Current ~6 ms) |
| | | \ |
| 0 +-----+--------+-----+---------------+--> TIME (ms) |
| | | / |
| -15 + +-----/ (Phase 2: Reverse Current ~4 ms) |
+-----------------------------------------------------------------------------+
The Biphasic Advantage:
- Membrane Capacitance Discharging: During Phase 1, the forward current pulse depolarizes cardiac cells. However, residual electrical charge remains stored in the cell membrane capacitance (Cm), leaving cells vulnerable to post-shock re-fibrillation. Phase 2 reverses current polarity, stripping away this residual charge and restoring normal membrane resting potential.
- Reduced Peak Current & Myocardial Stunning: High peak currents (>50 A in 360 J monophasic shocks) cause electroporation of cell membranes, thermal micro-burns, and post-resuscitation myocardial stunning (acute left ventricular ejection fraction depression). Biphasic waveforms maintain lower peak currents (<25–30 A) while delivering equivalent or superior transmembrane action potentials.
- Impedance Compensation: Transthoracic impedance varies from 25 Ω to 180 Ω across patients. Biphasic systems measure patient impedance dynamically during initial pulse onset, adjusting Phase 1 and Phase 2 durations (T1, T2) and truncation voltages to ensure consistent energy delivery.
The High-Voltage H-Bridge Inverter:
To deliver a biphasic pulse from a single unipolar capacitor, modern defibrillators route the capacitor discharge through an H-Bridge composed of four Insulated Gate Bipolar Transistors (IGBTs) or Silicon Controlled Rectifiers (SCRs):
+-----------------------------------------------------------------------------+
| DEFIBRILLATOR H-BRIDGE SWITCHING CIRCUIT |
| |
| HIGH-VOLTAGE (+) RAIL (From Storage Capacitor) |
| o |
| | |
| +--------------+--------------+ |
| | | |
| +---+---+ +---+---+ |
| | SW1 | (IGBT 1) | SW3 | (IGBT 3) |
| +---+---+ +---+---+ |
| | | |
| +--------[PATIENT PADS]-------+ |
| | (Apex) (Sternum) | |
| +---+---+ +---+---+ |
| | SW2 | (IGBT 2) | SW4 | (IGBT 4) |
| +---+---+ +---+---+ |
| | | |
| +--------------+--------------+ |
| | |
| o |
| HIGH-VOLTAGE (-) RETURN RAIL |
| |
| SWITCHING LOGIC: |
| - PHASE 1 (Forward): SW1 and SW4 CLOSED; SW2 and SW3 OPEN. |
| (Current flows Apex -> Sternum for 6 ms) |
| - INTER-PHASE GAP: ALL SWITCHES OPEN for 500 µs (Prevents shoot-through)|
| - PHASE 2 (Reverse): SW3 and SW2 CLOSED; SW1 and SW4 OPEN. |
| (Current flows Sternum -> Apex for 4 ms) |
+-----------------------------------------------------------------------------+
4. Synchronized Cardioversion & The R-on-T Phenomenon
Synchronized Cardioversion is indicated for organized, perfusing tachydysrhythmias including Atrial Fibrillation (AFib), Atrial Flutter, and monomorphic Ventricular Tachycardia with a pulse.
+-----------------------------------------------------------------------------+
| SYNCHRONIZED CARDIOVERSION TIMING DIAGRAM |
| |
| R |
| / \ |
| / \ T |
| / * \ <-- [SYNC MARKER TICK] / \ |
| / | \ / \ <-- VULNERABLE PERIOD |
| / | \ / \ (Do NOT shock here!) |
| -+ | +- +-------+-------------------> |
| P | S |
| v |
| [SHOCK DELIVERED] |
| (Delay < 60 ms from R-wave peak) |
+-----------------------------------------------------------------------------+
The Vulnerable T-Wave Period & R-on-T Hazard:
- The upstroke and peak of the T-wave represent inhomogeneous ventricular repolarization (the cardiac relative refractory period). Some ventricular myocytes have fully repolarized, while neighboring cells are still refractory.
- If an unsynchronized shock is delivered during this vulnerable window (the R-on-T phenomenon), the electrical wavefront fragments around refractory zones, immediately inducing lethal Ventricular Fibrillation.
- Synchronization Mechanism: When the user arms SYNC mode, the monitor's DSP continuously tracks the patient's surface ECG, placing a visual flag or dot directly over each detected R-wave peak. When the operator presses and holds the shock button, the device inhibits discharge until the exact peak of the next valid R-wave is detected.
- Timing Specification: IEC 60601-2-4 mandates that the shock must discharge within <60 milliseconds (ideally <25 ms) following the detected R-wave peak.
5. External Transcutaneous Pacemaker (TCP) Electronics
External transcutaneous pacing provides temporary, emergency cardiac pacing for patients with hemodynamically unstable bradycardia or high-grade atrioventricular (AV) heart block.
+-----------------------------------------------------------------------------+
| EXTERNAL TRANSCUTANEOUS PACER PARAMETERS |
| |
| CURRENT (mA) |
| 0 - 200 mA +------------------------+ |
| | | |
| | | |
| | | |
| 0 mA +-----+ +--------------------------> |
| |<--- PULSE DURATION --->| TIME |
| (20 to 40 ms) |
| |
| PACING MODES: |
| 1. DEMAND (VVI / Synchronous): Senses intrinsic R-waves; inhibits |
| pacer pulse if patient's own rate exceeds programmed pacing rate. |
| 2. FIXED / ASYNCHRONOUS (VOO): Delivers pulses at set rate regardless |
| of intrinsic beats (Warning: High risk of R-on-T if intrinsic beat |
| coincides with fixed pulse). |
+-----------------------------------------------------------------------------+
Technical Characteristics:
- Constant-Current Output Generator: Operates as a regulated constant-current source supplying 0 to 200 mA into patient impedances ranging from 200 Ω to 1000 Ω (current is adjusted in 5 mA increments).
- Wide Pulse Width (20–40 ms): Internal implantable pacemakers use narrow pulses (0.5 to 1.5 ms) delivered directly to endocardial tissue. Transcutaneous pacing requires a wide pulse (20–40 ms) to overcome skin impedance and depolarize the chest wall musculature and myocardium with minimal pain.
- Electrical vs. Mechanical Capture:
- Electrical Capture: Appears on the monitor as a distinct pacer spike immediately followed by a wide QRS complex and prominent, broad T-wave.
- Mechanical Capture: Must be verified independently by assessing hemodynamic response: checking a right femoral pulse, arterial line pulsatility, or SpO2 plethysmograph waveform. (Do not check carotid pulse due to confounding neck muscle twitching).
6. Automated External Defibrillator (AED) Analysis Algorithms
AEDs use microprocessor-based digital signal processing to classify surface ECG rhythms without clinical operator interpretation:
+-----------------------------------------------------------------------------+
| AED RHYTHM CLASSIFICATION |
| |
| ECG ACQUISITION (Bipolar Pads) ---> [DSP Filter / Spectral FFT Analysis] |
| | |
| +--------------------------+ |
| | |
| v |
| [SHOCKABLE RHYTHMS] [NON-SHOCKABLE RHYTHMS] |
| - Ventricular Fibrillation (VF) - Asystole (<100 µV p-p) |
| - Pulseless V-Tach (rate >150-180) - Normal Sinus Rhythm (NSR) |
| Action: Auto-Charge HV Capacitor - Pulseless Elec Activity |
| Prompt Shock Button (PEA) |
| Action: Prompt CPR & Resume |
+-----------------------------------------------------------------------------+
Algorithmic Feature Extraction:
- Waveform Amplitude: Discriminates coarse VF (>200 µV) from fine VF or asystole (<100 µV). Asystole is non-shockable; shocks into flatline damage tissue without therapeutic benefit.
- Frequency Domain Analysis: Evaluates power spectral density via Fast Fourier Transform (FFT). True VF exhibits concentrated power between 3 Hz and 7 Hz with high spectral variability.
- Waveform Regularity & Slope: Monomorphic VT exhibits high regularity and high slew-rates, triggering shock criteria when rate exceeds 150–180 bpm.
7. Biomedical Defibrillator & Pacemaker Testing Protocols
Biomedical technicians perform semi-annual or annual testing of defibrillators using a calibrated energy analyzer (e.g., Fluke Impulse 7000DP, BC Biomedical QA-45).
+-----------------------------------------------------------------------------+
| DEFIBRILLATOR / PACER TEST SETUP WITH IMPULSE 7000DP |
| |
| +--------------------+ |
| | DEFIBRILLATOR / | |
| | PACER UNDER TEST | |
| +---------+----------+ |
| | (Therapy Cable & Direct Connector / Paddle Adaptors) |
| v |
| +-----------------------------------------------------------------------+ |
| | FLUKE IMPULSE 7000DP PRECISION ANALYZER | |
| | [Standard 50 Ω Non-Inductive Load Resistor] | |
| | [High-Speed Voltage/Current Sampling ADC - 100 kHz] | |
| | [Precision ECG / Arrhythmia Rhythm Simulator] | |
| | [Transcutaneous Pacer Test Module] | |
| +-----------------------------------------------------------------------+ |
+-----------------------------------------------------------------------------+
Rigorous Preventive Maintenance Protocol:
- Energy Output Accuracy Test (into 50 Ω Load):
- Defibrillators are calibrated against a precision 50 Ω non-inductive standard load (representing average human transthoracic impedance).
- Test full energy range: Low (2–10 J), Mid (50–100 J), Max (200–360 J).
- Tolerance Standard: Delivered energy must be within ±15% or ±3 Joules (whichever is greater) per ANSI/AAMI DF80 and IEC 60601-2-4.
- Charge Time Verification:
- Measure time required to charge to maximum energy on AC power and fully charged battery. Specification: <10 seconds.
- Test charge time under depleted battery conditions (minimum 15 discharges at maximum energy per battery standard).
- Synchronized Cardioversion Delay Test:
- Connect analyzer ECG simulator (NSR rhythm at 60 bpm). Engage SYNC mode on defibrillator.
- Discharge into analyzer load. Measure time delay between simulated R-wave peak and pulse initiation. Specification: <60 ms.
- Internal Safety Dump & Timeout Test:
- Charge unit to maximum energy; do not discharge. Verify internal bleeder circuit disarms and safely dumps energy within 15 to 30 seconds.
- Transcutaneous Pacemaker Performance Test:
- Current Output Accuracy: Test at 10 mA, 50 mA, 100 mA, 150 mA, 200 mA (Tolerance: ±10% or ±2 mA).
- Pulse Rate Accuracy: Test at 40 bpm, 80 bpm, 120 bpm, 160 bpm (Tolerance: ±5%).
- Pulse Width Duration: Measure waveform width at 50% amplitude points (Specification: 20 to 40 ms ± 2 ms).
- Demand Pacing Sensitivity: Verify pacer pulse inhibition when intrinsic simulated ECG rate exceeds set pacer rate.
Loading diagram...
Test Your Knowledge
What is the primary physiological and clinical rationale for using a Biphasic Truncated Exponential (BTE) waveform rather than a traditional Monophasic Damped Sinusoidal (MDS) waveform in cardiac defibrillation?
A
B
C
D
Test Your Knowledge
A biomedical technician is evaluating a high-voltage defibrillator charging circuit. If a 32 µF energy storage capacitor is charged to an electrical potential of 5,000 Volts DC, what is the total energy stored in the capacitor?
A
B
C
D
Test Your Knowledge
During synchronized cardioversion for a patient in unstable supraventricular tachycardia (SVT), why must the defibrillator discharge strictly coincide with the R-wave and avoid the T-wave?
A
B
C
D
Test Your Knowledge
During routine testing of a defibrillator, the technician charges the unit to 200 Joules but does not press the discharge button. What safety circuit action must occur automatically within 15 to 30 seconds?
A
B
C
D