Implantable Cardiac Lead Circuit With Switchable MRI Heating Protection
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Solution Overview
Problem
Active implantable medical devices, such as cardiac pacemakers and defibrillators, face heating issues and unpredictable behavior during MRI examinations due to the interaction of their leads with the strong magnetic fields, leading to potential tissue damage and device malfunction.
Innovation Solution
A lead configuration with a protection circuit at the distal end, featuring a series dipole with a resistive component and a controlled active switch, which selectively modifies impedance to block MRI-induced currents while allowing stimulation pulses, eliminating the need for additional conductors or sensor detection and ensuring compatibility with existing generators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the lead conductors are isolated and connected to ground to prevent parasitic induced currents during MRI examination, then heating of surrounding tissues is reduced, but the device loses its ability to detect depolarization potentials and deliver stimulation pulses to the myocardium
Solution Approach 1:
The protection circuit dynamically switches between two states: during MRI examination, the active switch connects the resistive component in series with the electrode conductors to block parasitic currents; during normal operation, the active switch disconnects the resistive component to restore full device functionality. This dynamic reconfiguration allows the same circuit to provide protection when needed and full functionality when safe
Solution Approach 2:
The protection circuit introduces an intermediary switching mechanism controlled by the generator's control unit. This intermediary switch acts as a mediator between the harmful MRI field environment and the sensitive electrode conductors, selectively enabling or disabling the protective resistive component based on operational context without requiring permanent modification to the lead structure
2Temperature
If a protection circuit is added to the lead to block MRI-induced currents, then heating is prevented, but the device complexity increases and additional conductors or sensors are required
Solution Approach 1:
The protection circuit utilizes existing components and conductors in the lead system. The resistive component and active switch are integrated into the existing electrode conductor path, and the control unit that already manages device operation is extended to also control the protection switching. This multi-functional approach provides both protection and maintains functionality without requiring separate dedicated protection conductors or sensors
Solution Approach 2:
The protection circuit merges the protective function with the existing lead structure by integrating the resistive component and active switch into the existing conductor path. The control unit combines both pacing/defibrillation control and protection circuit control into a single integrated system, eliminating the need for separate protection wiring harnesses or additional sensor systems
3Temperature
If the protection circuit remains active continuously to prevent heating, then tissue safety is maintained, but energy losses increase and device autonomy is reduced
Solution Approach 1:
The protection circuit operates periodically rather than continuously. The active switch closes the protective resistive component only during the brief periods when MRI examination is detected and activates, and opens it during all other times. This periodic activation ensures tissue safety when needed while minimizing energy consumption during normal device operation, preserving device autonomy
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively prevents heating and maintains device functionality during MRI scans, reducing energy losses and extending the device's autonomy without requiring hardware modifications or additional conductors, thus ensuring safe and seamless operation.
Implementation Method 1
series dipole with a selectively modifiable impedance including a resistive component permanently mounted between the two terminals of the dipole
Implementation Method 2
resistive component permanently mounted between the two terminals of the dipole
Implementation Method 3
In the closed state, the controlled active switch provides a short circuit across the resistive component
Implementation Method 4
Leads placed in an MRI imager act like antennas and collect the radio frequency field (RF) emitted by the imager. Induced currents circulate in the conductors of the leads immersed in the RF field, therefore generating heat
Implementation Method 5
Induced currents circulate in the conductors of the leads immersed in the RF field, therefore generating heat which in turn heats the surrounding blood and tissue
Data Source
AI summary
A lead for an implantable cardiac prosthesis having an integrated protection against the effects of magnetic resonance imaging (“MRI”) fields. A protection circuit (26) may be placed at the distal end of the lead comprises a resistive component (28) interposed between the electrode (E1, E2) and the distal end of the conductor (22, 24) associated with this electrode. A normally-open controlled active switch (34, 36) may allow in its closed state to short-circuit the resistive component. A control stage (32) may be coupled to the conductors and detect the voltage of a stimulation pulse applied on the conductor(s), and selectively control by this voltage the closing of the active switch for a duration at least equal to the duration of detected stimulation pulse.