This article explores the inner workings of defibrillator systems, including components, block diagrams, and more.
Defibrillator systems play a critical role in emergency cardiovascular care. They deliver a high-energy electric shock to the heart to restore a normal rhythm in patients experiencing sudden cardiac arrest (SCA) caused by arrhythmia such as ventricular fibrillation (VF) or ventricular tachycardia (VT). These systems are used in both clinical and out-of-hospital environments and are vital in bridging the time gap until advanced cardiac life support (ACLS) is initiated. Modern defibrillators combine rapid rhythm recognition, smart shock delivery algorithms, and user-friendly interfaces to maximize survival rates and minimize response time. Key types include automated external defibrillators (AEDs), manual defibrillators, implantable cardioverter defibrillators (ICDs), and wearable defibrillators — each designed to meet specific clinical use cases.
Key Features
- Automated shock delivery: Intelligent analysis and response without user intervention
- Integrated self-testing: Continuous hardware diagnostics to ensure device readiness
- ECG-based decision making: Real-time rhythm detection and shock determination
- High-voltage control: Safe energy discharge using capacitor and H-bridge switching
- Wireless data transmission: Secure data sync and remote monitoring via cloud dashboards
Figure 1: Types of defibrillator systems
Figure 2: System block diagram of an automated external defibrillator (AED).
Power management
Modern defibrillator systems require robust and reliable power management to ensure the rapid delivery of high-voltage shocks during critical emergencies. These systems are powered by rechargeable lithium-ion batteries ranging from 2000 to 5000mAh, offering portability and uninterrupted use in prehospital and transport scenarios. In clinical settings, defibrillators also support 110–240V AC supply through a medical-grade AC-DC converter, compliant with IEC 60601-1 safety standards. A high-efficiency DC-DC boost converter is employed to elevate the voltage from the battery (e.g., 12V) to the required high-voltage levels (up to 5000V) necessary for charging the shock capacitor. To safeguard the device and patient, integrated protection mechanisms such as overvoltage protection (OVP), overcurrent protection (OCP), reverse polarity protection, and thermal shutdown circuits are implemented in accordance with IEC 60601-1-2 (EMC requirements). The system typically includes a PMIC that coordinates battery charging, power sequencing, energy monitoring, and USB Type-C input for efficient recharging and data access. The system also integrates a coin cell to maintain RTC (real-time clock) backup.
ECG monitoring and rhythm analysis
The defibrillator incorporates a high-fidelity ECG acquisition and signal processing module to continuously monitor the patient’s heart rhythm. This module captures real-time cardiac signals using chest electrodes or sensor pads and applies analog and digital filtering to remove noise and artifacts. The processed signal is then analyzed by a high-performance MCU/MPU, which uses embedded algorithms to detect life-threatening arrhythmia such as VF and VT. Figure 2 shows multiple ADC_IN and GPIO inputs linked to ECG processing channels, enabling continuous monitoring. A dedicated DAC is used for analog signal reconstruction or feedback purposes. The system also supports ECG waveform transmission to the display block and a wireless dashboard/app, facilitating remote viewing and analysis by clinicians.
Advanced software modules may include AI-based classification and arrhythmia detection for automated AED operation.
Energy storage and shock delivery
At the heart of the defibrillation mechanism lies the charging circuitry that energizes a high-voltage capacitor, which stores the energy required to deliver a therapeutic shock. Once a shockable rhythm is detected, the capacitor is rapidly charged to the selected energy level, typically between 50 and 360 joules within 5 to 10 seconds. The stored energy is then discharged through a high-voltage switch circuit, commonly implemented using insulated-gate bipolar transistors (IGBTs) or high-speed MOSFETs. The discharge path includes both an inner switch and an outer switch, governed by an H-Bridge circuit, ensuring precise timing and safety. The shock is delivered via adhesive pads or paddles placed on the patient’s chest. A key feature shown in the block diagram is the inclusion of internal discharge circuitry, which safely discharges residual energy if the shock is aborted, enhancing patient and operator safety with compliance to IEC 60601-2-4 (particular requirements for defibrillators).
User interface and control
The user interface is driven through a combination of manual selectors, touch-based controls, and physical buttons such as CHARGE, ABORT, ENERGY SELECTOR, and POWER ON. The system includes a touchscreen display, interfaced via LVDS/ MIPI and I2C touch controllers, to show ECG waveforms, energy level, countdown timers, and user prompts. Manual defibrillators used by healthcare professionals feature high-resolution color LCDs or OLED displays, soft keys, tactile buttons, and rotary encoders for parameter adjustments such as energy level selection, synchronization mode (for cardioversion), and shock initiation. The user interface design adheres to IEC 62366 (usability engineering for medical devices) and supports multilingual prompts and volume-adjustable voice guidance. Medical alarms are provided by an AUDIO AMP, enabling IEC 60601-1-8-compliant alerting. Inputs from the user selection unit are routed through GPIOs and ADCs to the control processor, enabling seamless mode switching and real-time updates to the display.
Microcontroller unit and data logging
The core control logic of the defibrillator is managed by a high-speed MCU/ MPU (typically an ARM Cortex-M4/M7), operating at 100–400 MHz. This unit is responsible for executing rhythm analysis algorithms, managing capacitor charging and discharging sequences, handling user interactions, and performing system diagnostics. This controller receives inputs from ADCs, touch interfaces, GPIOs, and external switches. It also manages the display output, the wireless transmission, and the shock initiation logic. A dedicated memory block stores patient data, ECG logs, and event records, which can be accessed via the dashboard/app wirelessly. The system’s RTC, backed by a coin cell, ensures accurate event timestamps for regulatory and clinical review. Lastly, this part of the system includes an internal watchdog timer and secure boot mechanisms for software reliability and cybersecurity.
Sensors and safety mechanisms
To ensure optimal performance and patient safety, defibrillators are equipped with multiple sensors. ADC channels monitor capacitor voltage, patient impedance, and pad contact. The inner and outer switches in the H-Bridge ensure redundant control of the shock path. The internal discharge path enables safe energy release in the event of shock cancellation. The temperature sensors embedded in the capacitor module help prevent overheating during repeated shocks. Built-in self-test routines run during device startup and periodically in standby mode to verify the integrity of the battery, charging circuit, electrodes, memory, and system logic. Firmware-level diagnostics continuously validate hardware health and system readiness. These safety features comply with IEC 60601-1 and IEC 60601-2-4 standards, ensuring the device functions within defined clinical limits.
Wireless communication and connectivity
Defibrillator systems integrate wireless communication capabilities to facilitate real-time monitoring, device management, and data transfer. Bluetooth Low Energy (BLE) and Wi-Fi (IEEE 802.11) modules enable secure communication with external systems, allowing emergency medical services (EMS) or hospital staff to review ECG data and usage logs. Some models support cloud connectivity for fleet management, battery health monitoring, and remote diagnostics. USB Type-C ports provide a physical interface for firmware upgrades and log downloads. Additionally, near-field communication (NFC) and RFID modules may be included for user authentication, inventory tracking, or integration with hospital asset management systems. All data transmissions comply with HIPAA and IEC 80001-1 standards to ensure patient data security and system interoperability.
Advanced features in next-generation defibrillators
Modern defibrillators incorporate AI-driven analysis, remote monitoring, and real-time CPR feedback. The architecture supports modular integration with patient monitoring systems and cloud-connected dashboards. Cloud-enabled dashboards facilitate centralized device management, remote status tracking, and predictive maintenance alerts. Combination devices that integrate vital sign monitoring with defibrillation functionality provide a compact solution for advanced life support in critical care and emergency settings. Future innovations are focusing on wearable and miniaturized defibrillators featuring adaptive therapy delivery and AI-driven event prediction models.
For any engineering questions regarding defibrillator systems, please contact Arrow Engineering.
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