Feasibility of determining small vessel compliance using Doppler optical coherence tomography.

Perhaps the commonest and simplest medical test one can apply is to 'feel for a pulse'. The sensation of pulsation which we feel through our fingers is a direct consequence of the fact that our bodies are 'plumbed' with flexible tubes which expand and contract in response to blood pumped by the heart. When we are young, our vessels are very elastic and 'compliant'. As we age, they naturally stiffen and if we lead an unhealthy and inactive lifestyle, 'hardening of the arteries' can become life-threatening. Diabetes, one of the commonest diseases in the western world, is associated with just such a loss of 'vascular compliance'. It is therefore of great medical value to be able to measure the compliance of all the important blood vessels in the body.Compliance can be determined rather easily if one can measure the pressure inside a vessel. In this case one can measure the increase in vessel diameter brought about by a known increase in pressure. This works well on vessels that have been removed from the body but is very difficult to apply in a living subject. So physiologists have developed another approach: to measure the speed at which the pulse travels away from the heart. It is well known that sound travels much faster in solid objects like steel than it does in air. This is because steel is much less compressible than air. Sound speed is thus a direct measure of the elastic properties of a solid medium. The same is true in blood vessels, they transmit the arterial pulse from the heart faster if the vessels are stiff than if they are compliant. This idea has been used successfully to measure the compliance of large vessels in the body. However it is of great interest to know about the smallest vessels in the body also i.e. vessels that are 0.2 mm or less in diameter. These vessels form the microcirculation and perform the vital function of actually delivering the oxygen and other nutrients to the cells that need them. In this application we want to explore a new concept for measuring the elastic properties of such small vessels. We will apply a new technique, a form of optical radar , to image the blood flow in these vessels with unprecedented resolution (a few thousands of a millimetre). By carefully measuring how the flow velocity and vessel diameter change together during an arterial pulse, it should be possible to determine the elastic properties of these vessels for the first time in living subjects. In this initial study, we want to test the basis of this idea in a simple idealised system; if encouraging the pilot data will lay the foundations for a fuller investigation.The information that we glean in this way will be important in many ways. In diabetes, for example, loss of vessel compliance leads to a loss of hyperaemic response , whereby ordinarily the body responds to a temporary loss of blood flow with an elevated burst of fresh blood. However it is still not clear whether changes in vessel compliance are a cause or an effect of such diseases. What is known is that diabetes sufferers must endure a host of very unpleasant symptoms whose underlying cause fits very well with the concept of a disturbed blood supply to cells by the microcirculation. The condition known as diabetic foot arises when the nerves in the foot die causing a lack of sensation and associated outbreaks of painful pressure sores. Diabetic retinopathy is creeping blindness caused by death of the nerves in the retina that respond to light. Our research project is a joint collaboration between optical physicists, fluid dynamicists and microvascular physiologists. It aims to apply cutting-edge optical physics and fluid-flow modelling to the task of improving our understanding of diseases of the microcirculation. Ultimately this could lead to the development of better drugs to control the symptoms of microvascular disease and a consequent improvement in the quality of life of millions of sufferers.