Improved pulse transit time estimation by system identification analysis of proximal and distal arterial waveforms

Abstract

Xu D, Ryan KL, Rickards CA, Zhang G, Convertino VA, Mukkamala R. Improved pulse transit time estimation by system identification analysis of proximal and distal arterial waveforms. Am J Physiol Heart Circ Physiol 301: H1389 -H1395, 2011. First published July 29, 2011 doi:10.1152/ajpheart.00443.2011.-We investigated the system identification approach for potentially improved estimation of pulse transit time (PTT), a popular arterial stiffness marker. In this approach, proximal and distal arterial waveforms are measured and respectively regarded as the input and output of a system. Next, the system impulse response is identified from all samples of the measured input and output. Finally, the time delay of the impulse response is detected as the PTT estimate. Unlike conventional foot-to-foot detection techniques, this approach is designed to provide an artifact robust estimate of the true PTT in the absence of wave reflection. The approach is also applicable to arbitrary types of arterial waveforms. We specifically applied a parametric system identification technique to noninvasive impedance cardiography (ICG) and peripheral arterial blood pressure waveforms from 15 humans subjected to lower-body negative pressure. We assessed the technique through the correlation coefficient (r) between its 1/PTT estimates and measured diastolic pressure (DP) per subject and the root mean squared error (RMSE) of the DP predicted from these estimates and measured DP. The technique achieved average r and RMSE values of 0.81 Ϯ 0.16 and 4.3 Ϯ 1.3 mmHg. For comparison, the corresponding values were 0.59 Ϯ 0.37 (P Ͻ 0.05) and 5.9 Ϯ 2.5 (P Ͻ 0.01) mmHg for the conventional technique applied to the same waveforms and 0.28 Ϯ 0.40 (P Ͻ 0.001) and 7.2 Ϯ 1.8 (P Ͻ 0.001) mmHg for the conventional technique with the ECG waveform substituted for the ICG waveform. These results demonstrate, perhaps for the first time, that the system identification approach can indeed improve PTT estimation.arterial blood pressure; arterial stiffness; foot-to-foot detection; impulse response; pulse wave velocity ACCORDING TO THE MOENS-KORTEWEG equation, pulse wave velocity (PWV) increases as the arteries stiffen. Indeed, PWV is the most popular index of arterial stiffness because of the ease of its measurement and its proven independent value in predicting cardiovascular events and mortality in hypertensive patients (2,4,12,14). In addition, because arterial stiffness increases with arterial blood pressure (ABP), PWV and ABP often show positive correlation, suggesting that PWV could provide a means to achieve continuous, noninvasive, and cuffless ABP monitoring (18).Conventionally, PWV is determined from the distance and pulse transit time (PTT) between proximal and distal arterial sites (5). PTT is, in turn, estimated by acquiring arterial waveforms from the two sites and then detecting the foot-tofoot time delay between the waveforms. The premise is that the foot of the proximal waveform represents a time before the return of the reflected wave to its measureme...

A Gaussian Model-Based Probabilistic Approach for Pulse Transit Time Estimation

“…Although these assumptions may not be valid in vivo, they describe well the global aspects of the pulse propagation [25]. In particular, since it clearly represents the relationship between the pulse propagation speed and the arterial compliance, the Moens-Korteweg equation is appropriate to describe the relationship between the PTT, which is inversely proportional to the pulse propagation speed, and the arterial compliance [3], [24], [25].…”

“…Equation (1) states that the PWV is proportional to the square root of the incremental elastic modulus (E) of the vessel wall given a constant ratio of wall thickness to vessel diameter and blood density under the assumptions that the artery wall is isotropic and experiences isovolumetric change with the pulse pressure [26]. Since the PTT is inversely proportional to the PWV [3], we can see that the elastic property of the vessel walls plays an important role in pulse transit time estimation.…”

“…The ongoing research defines the PTT as various time intervals depending on the equipment used and applications: (a) the time interval between the R-wave of the electrocardiogram (ECG) and the following pulse peak of the photoplethysmogram (PPG) [1], [2]; (b) the time interval between the R-wave of the ECG and the pulse onset of the PPG [3]; (c) the time interval between the R-wave of the ECG and the partial amplitude (e.g., 25 % and 50 %) on the upslope of the PPG [4]; (d) the time interval between the R-wave of the ECG and either the maximum peak of the first or second derivative of the PPG [5]; and the time interval between pulse waveforms measured from one or two distinct locations [6], [7]. The PTT is mainly used not only for estimating both systolic and diastolic blood pressures continuously [1], [8], but also for calculating the pulse wave velocity (PWV), which is a measure of arterial stiffness [9], [10].…”

“…Furthermore, they do not require any a priori knowledge of the measurement locations and of the physiologic condition of the cardiovascular system. However, since these approaches cannot accurately track each variation of the PTT and is restricted to arterial blood pressure and flow waveforms [3], it is not useful for analyzing local characteristics of the PTT, such as PTT variability.…”

4D reduced TE (RTE) spiral phase contrast NRI for rapid quantification and visualization of blood flow and hemodynamics.

“…Detected peak signal in pulse oximetry has a time shift relative to ECG signal. Taken from [5]. 29 Figure 22: Schematic of ECG-gated Cine acquisition.…”