Temporal complexity in photoplethysmography and its influence on blood pressure

Abstract

Objective: The temporal complexity of photoplethysmography (PPG) provides valuable information about blood pressure (BP). In this study, we aim to interpret the stochastic PPG patterns with a model-based simulation, which may help optimize the BP estimation algorithms.Methods: The classic four-element Windkessel model is adapted in this study to incorporate BP-dependent compliance profiles. Simulations are performed to generate PPG responses to pulse and continuous stimuli at various timescales, aiming to mimic sudden or gradual hemodynamic changes observed in real-life scenarios. To quantify the temporal complexity of PPG, we utilize the Higuchi fractal dimension (HFD) and autocorrelation function (ACF). These measures provide insights into the intricate temporal patterns exhibited by PPG. To validate the simulation results, continuous recordings of BP, PPG, and stroke volume from 40 healthy subjects were used.Results: Pulse simulations showed that central vascular compliance variation during a cardiac cycle, peripheral resistance, and cardiac output (CO) collectively contributed to the time delay, amplitude overshoot, and phase shift of PPG responses. Continuous simulations showed that the PPG complexity could be generated by random stimuli, which were subsequently influenced by the autocorrelation patterns of the stimuli. Importantly, the relationship between complexity and hemodynamics as predicted by our model aligned well with the experimental analysis. HFD and ACF had significant contributions to BP, displaying stability even in the presence of high CO fluctuations. In contrast, morphological features exhibited reduced contribution in unstable hemodynamic conditions.Conclusion: Temporal complexity patterns are essential to single-site PPG-based BP estimation. Understanding the physiological implications of these patterns can aid in the development of algorithms with clear interpretability and optimal structures.

Analysis of the Chaotic Component of Photoplethysmography and Its Association with Hemodynamic Parameters

“…Although the Granger causality does not establish true causation, it suggests the presence of a predictive relationship between variables. Our previous work demonstrated that hemodynamic fluctuation, including CO, C 1 , and R, contributes to the hysteresis of cardiovascular responses, leading to the formation of fractal patterns [18]. Along with the Granger causality test, our results provided supportive evidence for future, more rigorous investigations in this area.…”

“…To build a link between PPG complexities and hemodynamics, we employed the classical four-element Windkessel (WK4) model [17,18]. Interestingly, the equations describing this model bear resemblance to the Rössler system, which aligns with the findings reported by Sviridova et al In the context of an unstable state in the cardiovascular closed-loop system, our hypothesis suggests that there is a significant exchange of energy within the temporal patterns.…”

“…In our previous study [18], we observed temporal complexities due to hemodynamic fluctuations but lacked a clear understanding of their nature. Our current study used the IRASA algorithm to confirm the fractal nature of PPG's temporal patterns and quantify their contribution.…”

Robust arterial compliance estimation with Katz’s fractal dimension of photoplethysmography

“…While these methods demonstrate good performance in specific datasets, their internal mechanisms remain unknown, which may limit their ability to generalize to populations with different hemodynamic statuses. Nevertheless, these algorithms and relevant studies identified the temporal fluctuation or complexity of PPG as a promising candidate for AC assessment ( Sviridova et al, 2018 ; Xing et al, 2023a ; Xing et al, 2023b ).…”

“…For example, Esteller et al found that Katz’s FD (KFD) yielded consistent results in discriminating between states of brain function and was less susceptible to noise effects ( Esteller et al, 2001 ). In our previous comprehensive search, we found that Higuchi FD is closely related to mean blood pressure (BP), while KFD is significantly associated with compliance ( Xing et al, 2023a ; Xing et al, 2023b ). It is important to note that the compliance estimation method employed in those studies was imprecise and did not undergo validation against a gold standard.…”