Measurement of Heart Rate Variability with Optical Homodyne Detection Technique

Abstract

The vibrations on the body surface specially chest wall and wrist are results of deformation of the heart. The pulse wave which is traveling through the body causes vibrations on skin surface. Any respiratory and cardio disorder affects the total chest wall mechanics. There is clinical usefulness of chest wall mechanics information for detection of any respiratory and heart disorder. The optical homodyne detection technique is used for measurement of frequency of modulated optical signal. It works on the principle of Michelson interferometer. An optical signal is split into two beams with beam splitter and combined after traveling equal path lengths. If the path difference between two orthogonal beams becomes zero, the phase difference between these beams becomes zero or constant. In optical homodyne technique, one of the two beams get reflected from a mirror which is subjected to vibration frequency of chest wall. After reflection, the optical signal gets modulated by frequency of vibration of mirror. These two optical signals form an interference pattern which contains the information of vibration frequency. Thin nano-dimensional optical reflectors are used on wrist & chest wall for this detection technique. The nano reflectors provide advantage of transferring of complete vibrations due to their negligible mass. Optical homodyne detection technique provides the accuracy of the order of 0.017 Hz for measurement heart rate variability. Vibrocardiogram (VCG) output for different heart rate values are compared with standard electrocardiogram (ECG) signal and measurement of variability is performed.