Reflection-free Pulse Wave Velocity (PWV)

Improved measurement accuracy and precision of the pulse wave velocity (PWV) and its incremental variations within a cardiac cycle. Implimented wave reflections-compensated Pulse Trasit Time (PTT) for PWV computation.

Project Overview

This project was undertaken as part my PhD thesis at Advanced Cardiovascular Technologies Laboratory and Healthcare Technology Innovation Centre at the Indian Institute of Technology (IIT) Madras, Chennai, India.

In both in-vivo studies on humans using dual ultrasound probes and animal studies on pigs using invasive dual pressure catheters, we consistently observed that the repeatability of PWV deteriorates dramatically at mid-to-late systole, often to the point where forward and backward traveling waveforms crossed each other. This corruption is widely attributed to pulse wave reflections, as also supported by literature.

Eliminating reflection effects is therefore hypothesized critical for accurate PWV measurement, particularly when accounting for incremental changes in arterial stiffness (or PWV) across the cardiac cycle.

This project introduces a reflection-free PWV measurement method leveraging the pulse trasnit time (PTT) between simulatenously acquired and processed forward traveling waves from the common carotid artery by applying Wave Separation Analysis (WSA) to improve PWV accuracy and precision. The estimation of carotid pulse pressure (PP) from the refelction-free PWV using the Bramwell-Hill equation also provides additional credibility of the proposed method.

Overview of the incremental nature of arterial stiffness due the the elastin to collagen load bearing shift and plane wave imaging as a means to record this non-linear behaviour.

Why It Matters

  • Wave reflections distort PWV measurement and mask incremental stiffness behavior during systole.
  • Accurate assessment of PWV and PP from central arteries improves prediction of vascular aging and hypertension risk.
  • Non-invasive solution validated with in-house developed dual-channel ultrasound probe integrated with a tonometry sensor for clinical translation.
Effect of arterial pulse wave reflections on simulatenously recorded pulse waves (seperated by 35 mm) from the left carotid artery.

Key Contributions

  • Developed a reflection-compensated PWV measurement algorithm using classical WSA (requires pressure or diameter waveform and flow) and flow-independent methods .
  • Designed a custom dual-channel ultrasond vascular probe with integrated tonometry (pressure) sensor for simultaneous diameter waveform and tonometry waveform acquisition.
  • Demonstrated significant improvement in PWV repeatability (CoV reduced from 25% to 10%) at systole on 60 participants.
  • Verified agreement of reflection-free PWV with theoretical Bramwell-Hill estimates.
  • Achieved improved accuracy in carotid pulse pressure (PP), validated against tonometry-calibrated pressure, SpygmoCor (aortic pulse pressure), brachial pulse pressure from oscilometric devices.
  • Enabled measurement of incremental arterial stiffness (PWV) changes across diastole and systole.

Methodology at a Glance

  1. Acquire tonometry pressure and two diameter waveforms using a custom vascular probe.
  2. Apply Wave Separation Analysis (WSA) to isolate forward traveling components from the diameter-calibrated to pressure waveforms.
  3. Compute pulse transit time and derive PWV across multiple pressure levels.
  4. Validate PWV and PP against reference methods, including Bramwell-Hill and clinical tonometry.
Wave Separation Analysis (WSA) for decomposing forward and backward travelling pulse waves.

Experimental Setup

The in-house dual-channel ultrasound probe integrated with a tonometry sensor used for pressure and diameter waveform acquisition. Overview of the data collected and process.

Key Results

Highlights:

  • Repeatability improved: CoV reduced from 25% to 10% at systole across the cardiac cycle.
  • Accurate PWV mapping: Reflection removal prevented waveform crossing at mid-systole.
  • Incremental stiffness captured: Enabled estimation of pressure-dependent PWV (ΔC) from diastole to systole.
  • Pulse pressure improved: Central PP from carotid artery aligned with tonometry and SphygmoCor after WSA.
  • Statistical significance: Bias reduction and stronger correlation with theoretical estimates.
  • Clinical impact: Combined PWV and PP estimation enhances early cardiovascular risk detection.

Conclusion

This reflection-free method provides reliable method of local PWV, its changes within caric cycle and central pulse pressure, improving intra-cycle analysis of arterial stiffness. The combined approach offers better diagnostic precision for vascular health assessment in non-invasive clinical setups.

📄 Read Full Paper

Measurement of pressure dependent variations in local pulse wave velocity within a cardiac cycle from forward travelling pulse waves
Published in Scientific Reports (Nature)
Read Full Paper Here