Digital thermal monitoring (DTM) of vascular reactivity closely correlates with Doppler flow velocity

Abstract

The noninvasive measurement of peripheral vascular reactivity, as an indicator of vascular function, provides a valuable tool for cardiovascular screening of at-risk populations. Practical and economical considerations demand that such a test be low-cost and simple to use. To this end, it is advantageous to substitute digital thermal monitoring (DTM) for the more costly and complex Doppler system commonly used for this measurement. A signal processing model was developed to establish the basis for the relationship between finger temperature reactivity and blood flow reactivity following a transient brachial artery occlusion and reperfusion protocol (reactive hyperemia). Flow velocity signals were acquired from the radial artery of human subjects via an 8 MHz Doppler probe while simultaneous DTM signals were acquired from a distal fingertip via DTM sensors. The model transforms the DTM temperature signals into normalized flow signals via a deconvolution method which employs an exponential impulse function. The DTM normalized flow signals were compared to simultaneous, low-frequency, normalized flow signals computed from Doppler sensors. The normalized flow signals, derived from DTM and Doppler sensors, were found to yield similar reactivity responses during reperfusion. The reactivity areas derived from DTM and Doppler sensors, indicative of hyperemic volumes, were found to be within +/- 15%. In conclusion, this signal processing model provides a means to measure vascular reactivity using DTM sensors, that is equivalent to that obtained by more complex Doppler systems.

Figure 1

Vascular reactivity in both flow and temperature domains (a) An ideal reactivity flow signal showing the control amplitude, Q_control prior to cuff occlusion, zero flow during occlusion, and the reactivity response following release of occlusion. The exponential reactivity response has peak amplitude, Q_peak , and decay time constant, τ_reactivity. (b) The reactivity flow curve normalized to the control flow amplitude with normalized reactivity area (shaded green). (c) Temperature domain signal with start temperature, occlusion and temperature rebound, TR, illustrated.

Figure 2

A signal processing map of temperature and flow domain signals.

Figure 3

Comparison of normalized flow signals computed from Doppler and DTM sensors. Top: DTM signals, reactivity finger temperature (solid red), and contralateral reference finger (dashed black); Middle: Doppler low-frequency flow velocity signals, ν₁(t) (green) and ν₂(t) (blue); Bottom: normalized Doppler flow signal, Q_2norm(t) (blue), and normalized DTM flow signal, Q_{DTM norm}(t) (red)

Figure 4

Sample of reactivity area illustrated for normalized flow signals from Doppler (blue) and DTM (red) sensors. This region is defined as the area above the normalized control amplitude immediately following the release of cuff occlusion (shaded green).