Arterial compliance probe for cuffless evaluation of carotid pulse pressure

Abstract

Objective: Assessment of local arterial properties has become increasingly important in cardiovascular research as well as in clinical domains. Vascular wall stiffness indices are related to local pulse pressure (ΔP) level, mechanical and geometrical characteristics of the arterial vessel. Non-invasive evaluation of local ΔP from the central arteries (aorta and carotid) is not straightforward in a non-specialist clinical setting. In this work, we present a method and system for real-time and beat-by-beat evaluation of local ΔP from superficial arteries-a non-invasive, cuffless and calibration-free technique.

Methods: The proposed technique uses a bi-modal arterial compliance probe which consisted of two identical magnetic plethysmograph (MPG) sensors located at 23 mm distance apart and a single-element ultrasound transducer. Simultaneously measured local pulse wave velocity (PWV) and arterial dimensions were used in a mathematical model for calibration-free evaluation of local ΔP. The proposed approach was initially verified using an arterial flow phantom, with invasive pressure catheter as the reference device. The developed porotype device was validated on 22 normotensive human subjects (age = 24.5 ± 4 years). Two independent measurements of local ΔP from the carotid artery were made during physically relaxed and post-exercise condition.

Results: Phantom-based verification study yielded a correlation coefficient (r) of 0.93 (p < 0.001) for estimated ΔP versus reference brachial ΔP, with a non-significant bias and standard deviation of error equal to 1.11 mmHg and ±1.97 mmHg respectively. The ability of the developed system to acquire high-fidelity waveforms (dual MPG signals and ultrasound echoes from proximal and distal arterial walls) from the carotid artery was demonstrated by the in-vivo validation study. The group average beat-to-beat variation in measured carotid local PWV, arterial diameter parameters-distension and end-diastolic diameter, and local ΔP were 4.2%, 2.6%, 3.3%, and 10.2% respectively in physically relaxed condition. Consistent with the physiological phenomenon, local ΔP measured from the carotid artery of young populations was, on an average, 22 mmHg lower than the reference ΔP obtained from the brachial artery. Like the reference brachial blood pressure (BP) monitor, the developed prototype device reliably captured variations in carotid local ΔP induced by an external intervention.

Conclusion: This technique could provide a direct measurement of local PWV, arterial dimensions, and a calibration-free estimate of beat-by-beat local ΔP. It can be potentially extended for calibration-free cuffless BP measurement and non-invasive characterization of central arteries with locally estimated biomechanical properties.

Fig 1. The principle of calibration-free cuffless local ΔP measurement from superficial arteries using arterial compliance probe.

Fig 2. Experimental setup used for the phantom study to verify the proposed technique.

Fig 3. In-vivo validation study protocol.

(A) In-vivo study setup; showing brachial BP measurement from the left hand. (B) The operator performing local ΔP measurement from the left carotid artery. (C) Subject performing stair-running exercise.

Fig 4. Summary of phantom-based verification study results.

(A) Ultrasound echo frame from the carotid phantom. (B) A sample of continuous distension wave. (C) A sample of dual MPG waveform acquired along with the distension wave. (D) Correlation analysis between reference ΔP from catheter and ΔP estimated using the MPG-ultrasound arrangement. (E) Bland-Altman analysis showing the degree agreement between the reference and estimated ΔP.

Fig 5. Sample waveforms recorded from human carotid artery using the arterial compliance probe.

(A) An ultrasound echo frame with proximal and distal arterial walls (B) A sample of arterial distention waveform, and (C) simultaneously acquired dual MPG waveforms.

Fig 6. Simultaneously recorded beat-by-beat physiological parameters from the carotid artery.

(A) Local PWV, (B) arterial distension ΔD, and (C) end-diastolic diameter D_D.

Fig 7. Beat-to-beat variations in continuous physiological parameters.

Box-and-whisker diagrams showing beat-to-beat variation in the measured local PWV, ΔD, D_D, and estimated carotid ΔP under physically relaxed condition (data pooled over all the subjects).

Fig 8. Mean carotid local PWV and ΔD of each individual measured from both the physically relaxed and post-exercise conditions.

Fig 9. Reference brachial ΔP versus estimated carotid ΔP.

(A) Box-and-whisker diagrams illustrating the brachial ΔP and carotid ΔP under physically relaxed and post-exercise conditions. (B) Correlation plot of brachial ΔP versus carotid ΔP.

Fig 10. Correlation between measured brachial SBP and estimated carotid SBP (data pooled over all the subjects).