Continuous non-invasive blood pressure estimation.

血压是人体五大生命体征之一，是衡量患者健康状况的重要指标。高血压（hypertension）是心血管疾病（CVD）的最大危险因素之一，而心血管疾病是世界范围内导致死亡的主要原因。目前测量血压的方法是使用血压计，这是一种可充气的袖带，放在病人的手臂上，充气直到血液流动被阻塞。这提供了病人的收缩压（心脏收缩时的最高血压）和舒张压（心脏放松时的最低血压）的单一读数。然而，血压是一个全天变化的信号，因此一次读数不能提供足够的信息来准确诊断心血管疾病或其他与血压相关的疾病。此外，“白大褂”效应的存在，即患者的血压因临床医生测量而升高，表明许多患者在进行血压检查时可能被误诊。这证明了持续无创血压测量的必要性。目前在连续无创血压测量领域的研究主要集中在脉冲传递时间（PTT），即血液从一个动脉部位移动到另一个动脉部位所需的时间。经适当校准后，PTT的变化与血压的变化相关。为了记录两个不同位置的血流开始，大多数使用PTT来估算血压的设备都是通过在患者身上放置两个或多个设备来实现的。这些例子包括一个心电图（ECG）和一个光电容积脉搏波（PPG）传感器连接到同一个系统，或两个PPG传感器。由于需要两个传感器，该设备不太可能一直使用，因为患者会发现它们不舒服，并且会妨碍他们的日常生活；因此，很难说这些装置是真正连续的。本博士项目旨在探索一种新的连续血压测量方法。目前这个项目正在考虑两种方法（然而，由于这个项目还很年轻，很可能会有更多的方法需要探索）。第一种方法是使用放置在患者手腕上的单个PPG传感器来获得类似ptt的信号，或者使用机器学习技术来估计PPG的血压。第二种方法将研究基于相机的血压估算方法。已经证明，患者的视频摄像机流能够导出患者的病理信号，这项工作可以扩展到考虑患者许多位置的血液流动，因此PTT分析可以用于从这些位置之间的时间差中导出血压估计。这个项目的一个理想应用是在夜间连续监测血压。最近的研究表明，50岁以上的住院患者夜间收缩压升高，而不是年轻人通常的下降模式。该研究还发现了明显的性别差异，以及根据一周中的一天而变化。这些结果表明，跟踪血压随时间的变化不仅可以帮助评估患者的临床状况，如高血压，而且还可以提供心血管健康的新信息。使用血压计在夜间反复测量血压变化是困难的，因为有吵醒患者的风险，可能导致血压升高。因此，持续的无创血压评估（或遵循相同趋势的衍生血压替代品）将使血压不下降的患者易于识别并进行进一步治疗。

Blood pressure is one of the five vital signs and a major indicator of a patient's health. High blood pressure (hypertension) is one of the strongest risk factors for Cardio Vascular Disease (CVD), which is the leading cause of death worldwide. Current methods for measuring blood pressure use a sphygmomanometer, an inflatable cuff placed around the patients arm and inflated until the blood flow is occluded. This provides a single reading of a patient's systolic (highest blood pressure when the heart contracts) and diastolic (lowest blood pressure when the heart relaxes) blood pressure.However, blood pressure is a signal that changes throughout the day and so a single reading cannot provide sufficient information for accurate diagnosis of CVD or other blood pressure related diseases. In addition, the presence of a "white coat" effect, in which a patient's blood pressure increases due to the clinician taking the measurement, has shown that many patients will have been misdiagnosed when having their blood pressure examined. This has proven the need for a continuous non-invasive blood pressure estimation.Current research efforts in the field of continuous non-invasive blood pressure estimation are mostly focussed on Pulse Transit Time (PTT) which is defined as the time it takes for blood to travel from one arterial site to another. Changes in PTT are shown to correlate with changes in blood pressure after appropriate calibration. In order to record the onset of the blood flow at the two different locations most devices that use PTT to derive an estimate of blood pressure do so by using two or more devices placed on a patient. Examples of these include an electrogardiogram (ECG) and a photoplethysmography (PPG) sensor linked to the same system, or two PPG sensors. Due to the need for two sensors it is very unlikely that the device will be used at all times as patients will find them uncomfortable and will get in the way of their daily life; and therefore, it is difficult to call the devices truly continuous.This DPhil project aims to approach a novel implementation of continuous blood pressure estimation. There are two methods that are currently being considered for this project (however, as the project is still young it is very likely that there will be more methods to be explored). The first is to use a single PPG sensor placed on the wrist of a patient to derive either a PTT-like signal or use machine learning techniques to estimate blood pressure from the PPG. The second method will study camera-based methods of deriving a blood pressure estimation. It has already been shown that a video camera stream of a patient is able to derive pathological signals of the patient, this work can be extended to consider the flow of blood in many locations on the patient and thus PTT analysis can be used to derive a blood pressure estimation from the time differences between these locations.An ideal application for this project is in continuous monitoring of blood pressure during the night time. Recent research has shown a nocturnal rise in systolic BP for in-hospital patients above the age of 50 as opposed to the usual dipping patterns found in younger individuals. The research has also identified significant gender-specific differences, as well as variations according to the day of the week. These results suggest that tracking BP changes over time could not only help the assessment of patients with clinical conditions such as hypertension, but also provide new information about cardiovascular health. It is difficult to use a sphygmomanometer to repeatedly measure blood pressure changes during the night for risk of waking the patient which would result in a rise in blood pressure. Therefore, continuous non-invasive blood pressure estimation (or a derived surrogate of blood pressure that follows the same trends) will allow for a patient whose blood pressure does not dip to be easily identified for further treatment.