Elsevier

Journal of Electrocardiology

Volume 44, Issue 2, March–April 2011, Pages 201-207
Journal of Electrocardiology

Feasibility of cuff-free measurement of systolic and diastolic arterial blood pressure

https://doi.org/10.1016/j.jelectrocard.2010.11.019Get rights and content

Abstract

We validated a prototype cuff-free device for noninvasive estimation of blood pressure (BP). The system assumed a linear relation between BP values and the inverse of arterial blood pulse transit time, measured as time interval between the R wave on the electrocardiograph and the onset of the peripheral pulse wave on a finger plethysmogram. Thirty-three healthy subjects were analyzed at rest and during increasing stress exercise. To estimate subject-specific linear model parameters, the system was calibrated ad personam with reference to BP measures obtained by a cuff sphygmomanometer. High correlation values (R2 = 0.89 and 0.78 for systolic and diastolic BP, respectively) and differences consistent with clinical requirements (mean discrepancy of −0.058 and −0.25 mm Hg; 95% confidence interval of −13.0 to +12.9 mm Hg and −11.3 to +10.8 mm Hg, for systolic and diastolic BP, respectively) were observed between device and reference measurements. Calibration parameter stability and accuracy level were confirmed in a midterm evaluation, 30 days after calibration. These findings suggest the suitability of the device for noninvasive BP monitoring and its potentiality for clinical applications. Improvements can be achieved by further investigation of the calibration procedure and sensor placement.

Introduction

A reliable method for a continuous measure of blood pressure (BP) variations would be essential in many clinical and research medical applications, such as monitoring of anesthetized or intensive care patients, home care of cardiac patients, evaluation of hypertension, diagnostic test for syncope, and analysis of cardiovascular variability.1 Thus far, most of the available beat-to-beat BP measurement systems make use of inflation cuffs (ie, finger cuffs), as the devices based on Peñaz method. Nevertheless, the use of these cuff-based devices has some important drawbacks in terms of the large device size (needing a pump to inflate the cuff) and in terms of the cuff itself that, obstructing the arteries, hinders long-lasting BP monitoring. Alternative approaches, such as tonometric and vascular unloading methods, have been proposed,2 but the problem seems far to be solved.

Thus, a growing interest in noninvasive, cuff-free monitoring of BP is stimulating research studies intended to introduce new measurement systems providing indirect estimates of BP values. Among these, the most promising approach suggests to estimate arterial BP from pulse wave velocity, or pulse transit time (PTT).3 The PTT is defined as the period spent by the pulse pressure wave to travel from the heart to the peripheral circulation and is indeed expected to be in inverse relation with BP.3 Up to now, studies dealing with the estimation of BP from PTT have been mostly limited to experimental evaluations,4, 5 whereas quantitative testing of actual devices are still lacking. In addition, experimental studies have led to conflicting results as concerns the reliability of this methodology.4, 6, 7 In particular, some authors6 considered PTT monitoring not sufficiently accurate to replace direct invasive measurement of arterial BP, although similar inaccuracies have been previously described, when comparing invasive and noninvasive measurements.8

In this study, we evaluated the feasibility of a PTT-based noninvasive measurement of BP, by testing a prototype device and grading its performance according to standardized accuracy requirements.9, 10 The prototype system acquired synchronous recordings of electrocardiographic (ECG) and photoplethysmographic (PTG) signals, from which PTTs could be accurately measured. Blood pressure values were calculated assuming a linear dependence between these and the inverse of PTTs. The parameters of the linear model were estimated in each subject by a calibration step, in which PTT values were associated with reference BPs measured by an automatic sphygmomanometer. A specific protocol involving cycle ergometer exercise at increasing stress was applied to record PTT and BP values over a wide range in each subject. The short-term (ST) and midterm (MT) accuracy of the device were quantitatively evaluated by performing serial comparisons between estimated and reference BP values and computing measurement discrepancies, as suggested by standardized validation protocols.9, 10

Section snippets

Device description

The device tested in the study was a prototype instrument designed for ambulatory and home care monitoring (et medical devices, Cavareno, Italy). It acquired synchronous recordings of 2 ECG leads from 4 chest electrodes and of a finger PTG signal from a PTG sensor on the device body (see Fig. 1). Signals were acquired in recording frames of 10-second length at a sampling frequency of 500 Hz.

Study population and experimental protocol

Thirty-three healthy subjects were enrolled for the study. The demography and resting BP values of the

Calibration

Calibration curves could be accurately estimated in all patients, using an average number of 9.8 ± 1.3 measurements in each subject. The experimental protocol consented to obtain calibration curves, covering a wide pressure and PTT range. Indeed, as exemplified in Fig. 1B and confirmed in the overall subject population, graded exercise elicited a significant decrease in heart period (797.9 ± 143.9 milliseconds at rest vs 468.7 ± 102.5 milliseconds at maximal load exercise, P < .001) and a

Discussion

The purpose of this study was to test in a prototype device the feasibility of a noninvasive estimation of BP by linear modeling of PTT measurements. The results evidenced the following:

  • 1.

    the reliability of the linear model approximation over a wide BP-PTT range and

  • 2.

    the accuracy of the prototype device with respect to reference automatic BP measurements.

Previous studies6, 12 have doubted the reliability of the PTT as a surrogate marker of BP. However, these studies were performed comparing BP

Conclusions

We evaluated in a prototype device the feasibility of a noninvasive estimation of BP values from PTT changes. The results obtained confirmed the validity of the linear model over a wide pressure range and assessed the good accuracy of the approach. Although some issues, such as optimization of the calibration step and/or sensor placement, need to be further investigated, our preliminary findings suggest that the system has the potential to be used in clinical applications, such as remote

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