A BIOTELEMETRIC IMPLANTABLE HEART-SOUND RATE MONITORING SYSTEM

Luís Torres-Pereira*, Carla Torres-Pereira**, Paulo C. Ruivo* and Carlos Couto*
*Dept. of Engineering and **Dept. of Biological and Environmental Engineering,
University of Trás-os-Montes and Alto Douro, Apt. 202, 5000 Vila Real, Portugal

INTRODUCTION We aim to develop a reliable, implantable, biotelemetric, heart-sound rate monitoring system for studies of heart rate variability, causing minimal constraint to subjects. This concept of biotelemetric phonocardiography searches for advantages over electrocardiography in heart rate monitoring: it only requires an encapsulated single sensor probe and pursues the objective of obtaining reliable biotelemetric transmission from subjects moving.

METHODS Heart sounds arise from heart muscle and valve activity or heart blood flow. Piezoelectric transducers allow the detection on the human chest surface of the acoustic vibration produced by the heart mechanical activity. Taking this into account, we developed a small dimension intelligent capsule with sensor, signal processing and biotelemetric capabilities for heart rate monitoring. The intelligent probe detects each heart cycle, counts the number of cardiac cycles during 7.5 s and sends these data by radio frequency to a personal computer which traces a graph or saves a file of patient heart rate evolution. Due to size constraints, the analog signal processing module was projected to execute as many fuctions as possible with a minimal number of parts. In fact, this module is based on a single four operational amplifier chip, implementing a respiration and movement noise filter, a fullwave signal rectifier, an amplifier with automatic gain control, a second order active low-pass filter, a peak detector, a threshold circuit and a comparator. The AM transmitter module has small size (13 mm x 13 mm x 5 mm) and 418 MHz working frequency.

RESULTS The digital pulse heart rate signal is obtained comparing the envelope of the heart sound signal with an adjustable threshold. A preset threshold is difficult to optimize and a too low threshold makes the detector sensitive to noise, although it may detect the signal well. This is the case of using an average signal as threshold. The automatic gain control allows the amplifier to self adjust heart sound signal amplification to a level allowing processing. In the circuit used, a change in signal amplitude not only varies amplifier gain but also its threshold. The amplitude of the previous heart S1 pulse determines the new threshold level, thus shortening the adaptation time. Good results were achieved in respiration and movement noise reduction using a passive high pass filter at the first stage of the heart rate detection module.

DISCUSSION This new heart rate monitoring system relies on the identification of heart sound S1, the most proeminent natural sound signal that occurs during each cardiac cycle. S1 detection is complicated by the wide variation in heart sound morphologies and rhythms, as well as by noise originating from respiration, vocalization, and movement, that may corrupt phonocardiograms. These constraints imply further developments towards a self-adjustable probe selecting and processing heart sounds only.