25 Medical Products Inspired by Biological Oscillators …
389
endothelial response. The local control of vascular resistance is primarily achieved
through vascular smooth muscle activity changing the arteriolar diameter to regulate
local blood flow, arterial pressure and capillary filtration. However, vascular resistance may also be altered through vascular smooth muscle activity in venules and the
potential dilate of capillaries by actively relaxing pericytes [31]. The dynamic interaction of pressure-dependent vasoconstriction, flow-dependent endothelium-mediated
vasodilation, metabolic vasodilation and spontaneous myogenic activity acting on
basal vascular tone produces local changes in blood volume and blood flow. Oscillations can be observed as fluctuations in blood volume, blood flux (flowmotion)
and oxygenation in microcirculation when studied with optical techniques such as
laser Doppler fluximetry and reflectance spectroscopy. It is hypothesised that these
oscillations are generated at different frequencies depending upon the regulatory
control mechanism. In quantifying the amplitude and proportion of these oscillations by Fourier and wavelet analysis it is believed that it is possible to monitor the
changes in myogenic, neurogenic and endothelial activity regulating blood flow and
oxygen delivery. Pioneering work by Stefanovska using LDF in forearm skin has
revealed five characteristic frequencies of oscillation in blood flux in microcirculation at around 1 Hz (heart rate), 0.3 Hz (respiration), 0.1 Hz (myogenic activity),
0.04 Hz (sympathetic activity) and 0.01 Hz (endothelial activity) [12–39].
Why vasomotion occurs is still very much a matter of debate but it has been
suggested that vasomotion is beneficial for tissue oxygenation [50, 15–73]. Tissue
oxygenation can be improved by increasing the surface area available for exchange
i.e. capillary recruitment or by optimising oxygen delivery. How vasomotion might
ensure adequate oxygen delivery to all tissues has come primarily from theoretical
modelling [75, 30, 37]. These models suggest that pO 2 transients caused by vasomotion can oxygenate tissue domains which under steady-state conditions would
remain anoxic [75]. Therefore vasomotion appears to play a role in improving oxygen
perfusion especially to hypoxic tissue. It has been shown that ischaemia elicits an
amplification of skin blood flow oscillations besides an increment of skin perfusion
[57, 58].
25.3 Medical Products Inducing Oscillations Through
Intermittent Impulse Compression
For over two hundred years external applied pressure systems have been wrapped
around limbs with the aim of improving the circulation of blood. A historical review
of these methods designed to improve peripheral blood flow can be found in The
Venous System in Health and Disease by A.M.N. Gardner and R.H. Fox (IOS Press,
The Netherlands, 2001). In the past, compression bandages and hosiery have been
the most effective tool to treat impaired blood flow due to venous insufficiency. These
techniques continue to provide clinical benefits in conditions such as venous hypertension and chronic venous insufficiency [24] and have been found to improve ulcer
389
endothelial response. The local control of vascular resistance is primarily achieved
through vascular smooth muscle activity changing the arteriolar diameter to regulate
local blood flow, arterial pressure and capillary filtration. However, vascular resistance may also be altered through vascular smooth muscle activity in venules and the
potential dilate of capillaries by actively relaxing pericytes [31]. The dynamic interaction of pressure-dependent vasoconstriction, flow-dependent endothelium-mediated
vasodilation, metabolic vasodilation and spontaneous myogenic activity acting on
basal vascular tone produces local changes in blood volume and blood flow. Oscillations can be observed as fluctuations in blood volume, blood flux (flowmotion)
and oxygenation in microcirculation when studied with optical techniques such as
laser Doppler fluximetry and reflectance spectroscopy. It is hypothesised that these
oscillations are generated at different frequencies depending upon the regulatory
control mechanism. In quantifying the amplitude and proportion of these oscillations by Fourier and wavelet analysis it is believed that it is possible to monitor the
changes in myogenic, neurogenic and endothelial activity regulating blood flow and
oxygen delivery. Pioneering work by Stefanovska using LDF in forearm skin has
revealed five characteristic frequencies of oscillation in blood flux in microcirculation at around 1 Hz (heart rate), 0.3 Hz (respiration), 0.1 Hz (myogenic activity),
0.04 Hz (sympathetic activity) and 0.01 Hz (endothelial activity) [12–39].
Why vasomotion occurs is still very much a matter of debate but it has been
suggested that vasomotion is beneficial for tissue oxygenation [50, 15–73]. Tissue
oxygenation can be improved by increasing the surface area available for exchange
i.e. capillary recruitment or by optimising oxygen delivery. How vasomotion might
ensure adequate oxygen delivery to all tissues has come primarily from theoretical
modelling [75, 30, 37]. These models suggest that pO 2 transients caused by vasomotion can oxygenate tissue domains which under steady-state conditions would
remain anoxic [75]. Therefore vasomotion appears to play a role in improving oxygen
perfusion especially to hypoxic tissue. It has been shown that ischaemia elicits an
amplification of skin blood flow oscillations besides an increment of skin perfusion
[57, 58].
25.3 Medical Products Inducing Oscillations Through
Intermittent Impulse Compression
For over two hundred years external applied pressure systems have been wrapped
around limbs with the aim of improving the circulation of blood. A historical review
of these methods designed to improve peripheral blood flow can be found in The
Venous System in Health and Disease by A.M.N. Gardner and R.H. Fox (IOS Press,
The Netherlands, 2001). In the past, compression bandages and hosiery have been
the most effective tool to treat impaired blood flow due to venous insufficiency. These
techniques continue to provide clinical benefits in conditions such as venous hypertension and chronic venous insufficiency [24] and have been found to improve ulcer