25 Medical Products Inspired by Biological Oscillators …
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25.1.2 Structure and Function of the Microcirculation
An understanding of in vivo microcirculation in man has predominantly been derived
using non-invasive optical techniques to study both the structure and function of
these vessels [25]. Skin is surprising transparent to visible light and provides access
to ~1.7 m
2 of microcirculation that lies just below an avascular surface of stratum
corneum and epidermis. Unfortunately light attenuation in Caucasian skin is dominated by scattering rather than absorption (transport scattering coefficient µ
’
s = 2.73
± 0.54 mm
−1 versus absorption coefficient µ a = 0.033 ± 0.009 mm
−1 at 633 nm)
thus hindering the direct visualisation of all vessels in the microcirculation. The skin
microcirculation consists of a mesh of vessels <300 µm predominantly laid out as
two parallel horizontal plexi. Due to the high scattering of light in skin these plexi
cannot be visualised directly. However, there are capillary loops rising perpendicular to the surface from the superficial plexus that can be observed simply using a
microscope and a camera or video recorder. These capillary loops supply oxygen
and nutrients to the avascular epidermis. As can be seen in Fig. 25.2a only the tips of
the capillary loops are visualised in the majority of skin and quantitative measures
are predominantly confined to capillary density, this being the number of capillary
tips in one square millimetre. An exceptional site is at the nailfold on fingers and
toes where these capillaries run parallel to the skin surface Fig. 25.2b. In visualising these nailfold capillaries over time it is possible to study the haemodynamics
of red blood cells as they travel through these smallest vessels, demonstrating the
continuous fluctuations in blood flow induced by vasomotion. Similarly, there are
a few uniquely accessible sites in the body such as sublingual tissue in the mouth
(Fig. 25.2c) and the retina of the eye where there is a thin epithelial layer covering
the microcirculation and images of arterioles and venules can be obtained directly
by light microscopy [68]. A measure of capillary density changes in chronic venous
insufficiency has also been made by imaging the “visual” capillaries that come into
focus in supra-malleolar skin 5 cm proximal to the medial malleolus [33].
Access to the microcirculation of deeper tissues including muscle can be achieved
using longer wavelengths of light such a near infrared. With a reduced transport scattering coefficient µ
’
s = 1.63 mm
−1 at 900 nm compared to µ
’
s = 2.73 mm
−1 at 633 nm
[66] it is possible for near infrared light to penetrate up to ~6 cm of tissue. Techniques such as laser Doppler fluximetry (LDF) and near infrared spectroscopy at these
longer wavelengths cannot produce images but can interrogate the haemodynamics
of microcirculation in deeper tissues even with high melanin concentrations [2] and
can derive a measure of the oxygen content of the blood in the microcirculation [3].
25.2 Endogenous Vascular Oscillators
With advances in optical techniques and digital imaging it is now possible to undertake non-invasive, in vivo research in man to study both the dynamic changes in the
387
25.1.2 Structure and Function of the Microcirculation
An understanding of in vivo microcirculation in man has predominantly been derived
using non-invasive optical techniques to study both the structure and function of
these vessels [25]. Skin is surprising transparent to visible light and provides access
to ~1.7 m
2 of microcirculation that lies just below an avascular surface of stratum
corneum and epidermis. Unfortunately light attenuation in Caucasian skin is dominated by scattering rather than absorption (transport scattering coefficient µ
’
s = 2.73
± 0.54 mm
−1 versus absorption coefficient µ a = 0.033 ± 0.009 mm
−1 at 633 nm)
thus hindering the direct visualisation of all vessels in the microcirculation. The skin
microcirculation consists of a mesh of vessels <300 µm predominantly laid out as
two parallel horizontal plexi. Due to the high scattering of light in skin these plexi
cannot be visualised directly. However, there are capillary loops rising perpendicular to the surface from the superficial plexus that can be observed simply using a
microscope and a camera or video recorder. These capillary loops supply oxygen
and nutrients to the avascular epidermis. As can be seen in Fig. 25.2a only the tips of
the capillary loops are visualised in the majority of skin and quantitative measures
are predominantly confined to capillary density, this being the number of capillary
tips in one square millimetre. An exceptional site is at the nailfold on fingers and
toes where these capillaries run parallel to the skin surface Fig. 25.2b. In visualising these nailfold capillaries over time it is possible to study the haemodynamics
of red blood cells as they travel through these smallest vessels, demonstrating the
continuous fluctuations in blood flow induced by vasomotion. Similarly, there are
a few uniquely accessible sites in the body such as sublingual tissue in the mouth
(Fig. 25.2c) and the retina of the eye where there is a thin epithelial layer covering
the microcirculation and images of arterioles and venules can be obtained directly
by light microscopy [68]. A measure of capillary density changes in chronic venous
insufficiency has also been made by imaging the “visual” capillaries that come into
focus in supra-malleolar skin 5 cm proximal to the medial malleolus [33].
Access to the microcirculation of deeper tissues including muscle can be achieved
using longer wavelengths of light such a near infrared. With a reduced transport scattering coefficient µ
’
s = 1.63 mm
−1 at 900 nm compared to µ
’
s = 2.73 mm
−1 at 633 nm
[66] it is possible for near infrared light to penetrate up to ~6 cm of tissue. Techniques such as laser Doppler fluximetry (LDF) and near infrared spectroscopy at these
longer wavelengths cannot produce images but can interrogate the haemodynamics
of microcirculation in deeper tissues even with high melanin concentrations [2] and
can derive a measure of the oxygen content of the blood in the microcirculation [3].
25.2 Endogenous Vascular Oscillators
With advances in optical techniques and digital imaging it is now possible to undertake non-invasive, in vivo research in man to study both the dynamic changes in the