Implantable Impedance Measurement Using FIR Filter Demodulation
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Solution Overview
Problem
Implantable medical devices face challenges in accurately measuring impedance while minimizing noise interference, which can affect the reliability of physiological signals such as respiration and cardiac stroke volume, and impact therapy control.
Innovation Solution
An implantable impedance measurement circuit using a four-point FIR filter demodulator to demodulate a two-phase current excitation waveform, allowing for noise measurement and adjustment of the test current to optimize impedance measurement, including the option to cancel or indicate measurements based on noise levels, and increase current in noisy conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional impedance measurement methods are used in implantable devices, then the device can obtain physiological signals, but noise interference degrades measurement accuracy and reliability
Solution Approach 1:
The patent segments the impedance measurement process into multiple phases (excitation phase, measurement phase, noise measurement phase) and uses separate electrode pairs for current injection and voltage measurement. This segmentation allows the device to isolate and measure noise components separately from the actual physiological signal, thereby improving measurement precision by enabling noise cancellation.
Solution Approach 2:
The patent introduces an intermediary noise measurement step between excitation and final impedance calculation. By measuring noise during the excitation phase and using it as a reference, the system can subtract this noise component from the final impedance measurement, effectively eliminating noise interference and improving measurement accuracy.
2Reliability
If higher test current levels are used to improve signal-to-noise ratio, then measurement reliability improves, but device power consumption increases and longevity decreases
Solution Approach 1:
The patent dynamically adjusts the test current level based on measured noise conditions. When noise levels are low, the device uses lower current to conserve power. When noise levels exceed thresholds, the device automatically increases current to maintain measurement reliability. This dynamic adaptation resolves the contradiction by optimizing the trade-off between reliability and power consumption in real-time.
Solution Approach 2:
The patent implements a feedback mechanism where noise measurements from each excitation phase feed into subsequent measurement decisions. The system continuously monitors noise levels and adjusts test current amplitude accordingly, creating a closed-loop control system that maintains measurement reliability while minimizing power consumption by avoiding unnecessarily high current levels.
3Measurement precision
If noise measurement and adjustment protocols are implemented, then impedance measurement accuracy improves, but device complexity increases
Solution Approach 1:
The patent designs the excitation circuit to serve multiple functions: it provides the test current for impedance measurement, generates the excitation waveform for noise measurement, and enables both physiological signal acquisition and noise characterization. This multi-functionality reduces device complexity by eliminating the need for separate dedicated noise measurement hardware, while still achieving improved measurement precision through noise cancellation.
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
This approach enhances the accuracy and reliability of impedance measurements by effectively managing noise, thereby improving the extraction of physiological signals and prolonging device longevity through reduced current consumption.
Implementation Method 1
an exciter circuit (202) configured to apply a test current between at least two of the electrodes
Implementation Method 2
a preamplifier circuit (206) configured to sense a response signal resulting from the test current using the same or different at least two of the electrodes
Implementation Method 3
a demodulator circuit (210) communicatively coupled to the preamplifier circuit (206) to demodulate the response signal to the test circuit into a measured signal indicative of an impedance measurement
Data Source
AI summary
An implantable medical device performs impedance measurement and demodulation, such as for obtaining lead impedance measurements, or thoracic impedance measurements, such as for extracting respiration, cardiac stroke, or fluid status information. A 4-point FIR filter demodulator can be used to demodulate a two-phase current excitation waveform. The demodulator can also be used to measure noise for triggering a noise response. Among other things, an increased excitation current level can be used when noise is deemed to be present.