Biomedical Engineering Reference
In-Depth Information
between metabolic and microhemodynamic processes in the skin and showed that improved
oxygen uptake and glucose disposal by tissues is accompanied by a significant increase in endothe-
lial rhythm amplitude. Bernjak et al. (2008) showed that congestive heart failure exhibits abnormally
attenuated blood flow oscillations in the frequency interval 0.005-0.021 Hz, and that treatment with
β
1
-blockers (Bisoprolol) can move the spectral amplitude 0.005-0.0095 Hz to that of the healthy
control subjects.
Laser speckle contrast imaging is a recently developed optical noncontact technique that allows
for the continuous assessment of skin perfusion over wide areas. In comparison to laser Doppler
flowmetry, laser speckle contrast imaging can provide real-time monitoring of the microcirculation
with high resolution (Basak et al. 2012). An extensive overview of these optical techniques may
be found in a review of blood flow imaging by Daly and Leahy (2013). A major drawback of these
methods is that the instruments they employ are complicated and may not be suitable for daily use.
The fingertip temperature response to a thermal stimulus or to pressure loading depends on the
amount of blood perfusion, which implies that monitoring alterations in fingertip temperature could
be employed to study microvascular reactivity. Based on a bioheat transfer equation, Haga et al.
(2012) employed an inverse analysis method for examining blood perfusion. The instrument that
was developed from this method can be used to estimate blood perfusion according to the observed
fingertip temperature response under a certain thermal stimulus and shows good measurement
repeatability and sensitivity. Yue et al. (2008) presented a three-point method to measure blood
perfusion, whereby a heater is wrapped around a cylindrical section of living tissue, and three-point
skin temperatures are measured. The characteristic points are located at the center of the heater,
1 cm away from the edge of the heater, and 2 cm away from the heater, respectively. By constructing
an objective function between the measured and calculated temperatures and minimizing the func-
tion, the optimal value for blood perfusion and thermophysical properties can be obtained.
Apart from methods that are needed to load heat sources in living tissues, Nagata et al. (2009)
established a passive method to evaluate blood perfusion only by using thermal information for the
human body, whereby blood perfusion can be expressed in terms of the rate of temperature change
at the contact sensor point and the initial skin temperature. Furthermore, a novel thermal method
has been presented that uses the fingertip temperature, forearm temperature minus rectal tempera-
ture, and their changes across time to predict finger blood flow (Carrillo et al. 2011). Recently, a
new thermal peripheral blood flowmeter has been developed that is integrated with a force sensor
for force-compensated blood flow measurement (Sim et al. 2012). The important feature of this
device is that, apart from the conventional metal resistance of the temperature detector, there is
a membrane fabricated by surface and bulk micromachining techniques that is embedded with a
piezoresistive force sensor. The compensated blood flow can be determined by detecting the rate of
temperature change and the contact force.
Due to simple structures and low costs, many thermal methods for peripheral blood low mea-
surement have been developed. Not only can the peripheral blood flow rate be determined from the
skin temperature, but vasodilated function may also be detected. Fingertip digital thermal monitor-
ing (DTM) during cuff-occlusive reactive hyperemia (RH) is a new, noninvasive method of vascular
function assessment that is based on the premise that changes in fingertip temperature during and
after an ischemic stimulus reflect changes in blood flow and endothelial function. DTM technology
usually requires 2 min of cuff inflation to cause brachial artery occlusion and 5 min of deflation later
to bring about blood reperfusion. During the occlusion stage, a vasodilatory response occurs in the
peripheral arteries and capillaries due to the absence of blood flow. After the brachial occlusion is
finished, blood rushes into the forearm and hand, and thus causes a transient temperature rebound
in the fingertip.
Ley et al. (2008, 2009) presented two mathematical models of heat transfer based on Pennes equa-
tion to estimate the influence of different factors on the dynamic temperature response in the in-
gertip during vascular occlusion and reperfusion. The models are based on different dimensions and
anatomical details. One is a lumped parameter model that neglects tissue composition and the other
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