Estimation of blood pressure variability during orthostatic test using instantaneous photoplethysmogram frequency and pulse arrival time

The beat-to-beat blood pressure and heart rate variability provide information about the dynamic features of cardiovascular control mechanisms. Pulse arrival time can be used for assessment of blood pressure and is inversely related to it. The hypothesis of this study is that pulse arrival time and changes in photo-plethysmogram signal waveform (instantaneous photoplethysmogram signal frequencies) could be used as the surrogate parameters for blood pressure variability estimation. The aims are: (1) to present instantaneous photoplethysmogram signal frequencies extraction algorithm; (2) to quantify variability of heart rate, systolic and diastolic blood pressure, pulse arrival time and instantaneous photoplethysmogram frequencies during orthostatic test and assess their interrelation. The experimental protocol by using orthostatic test was designed to stimulate autonomic nervous system of the body and observe its reaction. The data were recorded from fourteen healthy subjects. During exercise, physiological and motion signals (electrocardiogram, photoplethysmogram, accelerometer, and continuous arterial blood pressure) were recorded. Two systems for synchronous recordings of physiological data were used: Cardioholter 6.2-8E78 and Portapres Model-2. Classical signal processing algorithms were applied to obtain R-R intervals, systolic, and diastolic blood pressure. Hilbert transform was used for pulse arrival time estimation and Hilbert-Huang transform for instantaneous photoplethysmogram signal frequencies extraction. Time-domain and frequency-domain parameters show that heart rate, blood pressure, pulse arrival time, and instantaneous photoplethysmogram signal frequencies have different variability in different postures of the body (vertical and horizontal). Variability of R-R intervals is higher in horizontal posture, meanwhile, variability of blood pressure, pulse arrival time, and instantaneous photoplethysmogram signal frequencies are higher in vertical posture. Pulse arrival time and instantaneous photoplethysmogram signal frequencies have the same trends as blood pressure. Results showed that the differences between vertical and horizontal (supine and standing) positions are statistically significant (p < 0.05). Changes of blood pressure, pulse arrival time, and instantaneous photoplethysmogram signal frequencies variability are interrelated during orthostatic test. The results support the assumption that pulse arrival time and instantaneous photoplethysmogram signal frequencies can be used as the surrogate parameters for blood pressure variability estimation. (C) 2016 Elsevier Ltd. All rights reserved.