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

也许最常见和最简单的医学检查是“摸脉”。我们通过手指感受到的搏动感，是我们的身体由柔性管道组成的直接结果，这些管道随着心脏泵出的血液而扩张和收缩。当我们年轻的时候，我们的血管非常有弹性和“顺从”。随着年龄的增长，动脉会自然硬化，如果我们过着不健康和不运动的生活方式，“动脉硬化”可能会危及生命。糖尿病是西方世界最常见的疾病之一，与这种“血管顺应性”的丧失有关。因此，能够测量体内所有重要血管的顺应性具有重要的医学价值。如果可以测量容器内的压力，就可以很容易地确定是否符合要求。在这种情况下，我们可以测量已知压力增加所带来的血管直径的增加。这在已经从身体上移除的血管上效果很好，但很难应用于活体。因此，生理学家开发了另一种方法：测量脉搏离开心脏的速度。众所周知，声音在像钢铁这样的固体中比在空气中传播得快得多。这是因为钢的可压缩性比空气小得多。因此，声速是固体介质弹性特性的直接量度。血管也是如此，如果血管僵硬，它们从心脏传递动脉脉冲的速度要比血管柔顺时快。这个想法已被成功地用于测量体内大血管的顺应性。然而，了解体内最小的血管也很有趣，即直径小于0.2毫米的血管。这些血管形成了微循环，并发挥着将氧气和其他营养物质输送到需要它们的细胞的重要作用。在这个应用中，我们想探索一种测量这种小容器弹性特性的新概念。我们将采用一种新技术，一种光学雷达，以前所未有的分辨率（每毫米几千像素）对这些血管中的血流进行成像。通过仔细测量动脉脉冲期间血流速度和血管直径的变化，将有可能首次在活人身上确定这些血管的弹性特性。在这项初步研究中，我们想在一个简单的理想系统中测试这一想法的基础；如果鼓励试点数据将为更全面的调查奠定基础。我们以这种方式收集的信息在很多方面都很重要。例如，在糖尿病中，血管顺应性的丧失导致充血反应的丧失，而通常情况下，身体对暂时的血流量减少的反应是新鲜血液的增加。然而，目前尚不清楚血管顺应性的改变是这类疾病的原因还是结果。我们所知道的是，糖尿病患者必须忍受一系列非常不愉快的症状，这些症状的根本原因与微循环对细胞的血液供应受到干扰的概念非常吻合。当足部神经死亡导致感觉缺失和相关的疼痛性压疮爆发时，就会出现糖尿病足。糖尿病性视网膜病变是由视网膜中对光有反应的神经死亡引起的逐渐失明。我们的研究项目是由光学物理学家、流体动力学家和微血管生理学家共同合作完成的。它旨在应用尖端的光学物理学和流体流动模型来提高我们对微循环疾病的理解。最终，这可能会导致开发出更好的药物来控制微血管疾病的症状，从而改善数百万患者的生活质量。

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.