Role of harmonics and subharmonics in peripheral pulse analysis

Elastic compliance of the blood vessels contributes high-frequency components in the peripheral pulse, which are analyzed with the help of the power spectral density of the signal. The frequency corresponding to heart rate in a subject is called his fundamental harmonic or first harmonic, frequency twice the heart rate is called second harmonic, and so on. The contribution of these harmonics is assessed by performing Fourier transform on the peripheral blood volume or flow pulse signal for sufficient duration, which yields power spectral density of the peripheral pulse. Dedicated software, known as pulse harmonic analyzer, has also been developed for performing harmonic analysis of the peripheral signal. It not only yields a contribution of higher harmonics in the pulse but also subharmonics (frequencies smaller than the heart rate of the subject). Researchers have observed suppression of second harmonics in subjects aging more than 30 years, suppression of high-frequency components in the power spectral density of the pulse in the coronary artery disease and enhancement of the first harmonic in hypertensives and subjects susceptible to diabetes during the past four decades. Subharmonic components are observed to be related to variability in heart rate, pulse volume, and pulse morphology, which has the potential to become a method of choice for continuous real-time variability monitoring in intensive care units. These observations are reviewed in this paper briefly.