Innovation from the Laboratory of Metrology and Quality: A microphone for blood pressure measurement

Cardiovascular diseases are the leading cause of death worldwide, claiming nearly 18 million lives each year. One of the most significant risk factors for cardiovascular disease is untreated hypertension, or chronically elevated blood pressure, which can only be identified through accurate blood pressure measurement.

Existing blood pressure monitors are considerably less accurate than many people realize. Current standards allow a measurement error of up to 3 mmHg for new devices and up to 4 mmHg for monitors that have been in use for one year. Although these values may appear small, they can make the difference between a healthy and a pathological diagnosis for millions of patients. An inaccurate diagnosis may result in either unnecessary treatment or a failure to provide treatment where it is urgently needed.

There is also an important gap in the field of metrology. Static pressure can be measured and calibrated using well-established methods, whereas dynamic pressure changes are not measured with sufficient accuracy. Yet these dynamic pressure variations are exactly what automated blood pressure monitors use to determine blood pressure values. As a result, devices used in hospitals and homes are not properly calibrated for the quantity they actually measure.

A new measurement system developed at UL FE

Researchers from the Laboratory of Metrology and Quality at the University of Ljubljana Faculty of Electrical Engineering have developed a novel measurement system. To measure dynamic pressure changes inside the cuff of a sphygmomanometer, they employed a condenser microphone. Although using a microphone for pressure measurement may initially seem unusual, the concept is firmly grounded in physics.

A condenser microphone functions as a highly sensitive pressure sensor. Its diaphragm moves in response to air pressure changes, altering its electrical capacitance, which can then be used to determine pressure variations. The researchers faced two major challenges. Most microphones cannot withstand the high static pressures encountered during blood pressure measurements, while conventional microphone systems have limited sensitivity at the low frequencies where blood pressure pulse signals occur.

The solution was found in a radio-frequency modulation technique. Instead of converting changes in the microphone diaphragm’s capacitance directly into a voltage signal, the researchers used these changes to modulate a radio-frequency signal, which was subsequently demodulated. This approach enabled pressure measurements with a response extending down to 0 Hz.

The GRAS 40BH microphone used in the system is normally designed for measuring high-intensity impulse sounds, such as explosions. The researchers integrated it into a custom-designed printed circuit board incorporating an RF oscillator, mixer and amplifier, creating a compact prototype named the RF microphone.

System testing and measurement results

The measurement system was evaluated in several ways:

• using a custom-built low-frequency sound source covering the range from 0.5 Hz to 280 Hz,
• using a SmartArm NIBP blood pressure simulator generating realistic pressure waveforms at different blood pressure values,
• using a UNIK-5000 piezoresistive pressure sensor as the reference instrument.

The results demonstrated that the RF microphone operates reliably over a frequency range of 0.5 Hz to 280 Hz and a pressure range of 0 to 300 mmHg. The system achieved a combined expanded measurement uncertainty of 4.32 mmHg, which is slightly above the current target limit of 3 mmHg. However, the researchers identified several improvements that could enable the required accuracy to be achieved, including a more stable power supply, improved mechanical fixation during calibration, and calibration using the primary reciprocity method according to IEC 61094-2.

In the next phase of the research, the team will compare the proposed system with piezoresistive sensors used in commercial blood pressure monitors. They will perform calibration using the primary reciprocity method and evaluate additional condenser microphones suitable for high-pressure applications. The long-term objective is to establish a methodology and measurement system that could serve as the basis for future international standards for dynamic pressure calibration in blood pressure measurement.

The significance of the RF microphone for society

The key value of the proposed approach lies not only in the development of a new measurement instrument, but also in its potential to establish traceability of dynamic pressure measurements to international acoustic standards. This would enable blood pressure monitors to be calibrated using the same internationally comparable methodology applied to laboratory microphone standards. Such an approach could significantly improve the quality and reliability of hypertension diagnosis worldwide.

The proposed method is particularly valuable because dynamic pressure standards are largely absent from current sphygmomanometer calibration protocols. By its very nature, a condenser microphone is designed to detect precisely the dynamic pressure variations that conventional piezoresistive sensors measure less effectively.

Improved calibration of blood pressure monitors leads to more reliable diagnoses, fewer incorrectly treated patients and lower healthcare costs. Since cardiovascular diseases place a substantial financial burden on healthcare systems around the world, every improvement in diagnostic accuracy has a direct impact on public health.

The RF microphone was developed entirely at the University of Ljubljana Faculty of Electrical Engineering within the Laboratory of Metrology and Quality. The project was supported by the Slovenian Research and Innovation Agency (ARIS), and the results were published in the international journal Sensors.