血压和心音的测量（英文医学.ppt

The Measurment of,Blood Pressure,and Heart Sound,What is blood pressure Blood is carried from the heart to all parts of your body in vessels called arteries. Blood pressure is the force of the blood pushing against the walls of the arteries. Each time the heart beats (about 60-70 times a minute at rest), it pumps out blood into the arteries. Your blood pressure is at its highest when the heart beats, pumping the blood. This is called systolic pressure. When the heart is at rest, between beats, your blood pressure falls. This is the diastolic pressure.,B L O O D P R E S S U R E,Blood pressure is always given as these two numbers, the systolic and diastolic pressures. Both are important. Usually they are written one above or before the other, such as 120/80 mmHg. The top number is the systolic and the bottom the diastolic. Blood pressure can vary based on many factors, including age, gender, weight and physical condition. In general, a persons blood pressure is lower during sleep and higher when he or she is active. Blood pressure can change easily in response to physiological changes. The setting in which a persons blood pressure is measured may also affect the results.,B L O O D P R E S S U R e,Blood pressure changes during the day. It is lowest as you sleep and rises when you get up. It also can rise when you are excited, nervous, or active. Still, for most of your waking hours, your blood pressure stays pretty much the same when you are sitting or standing still. That level should be lower than 120/80. When the level stays high, 140/90 or higher, you have high blood pressure. With high blood pressure, the heart works harder, your arteries take a beating, and your chances of a stroke, heart attack, and kidney problems are greater.,B L O O D P R E S S U R E,Categories for Blood Pressure Levelsin Adults (In mmHg, millimeters of mercury),B L O O D P R E S S U R E,Blood Pressure as a Tool Blood pressure is an important diagnostic index, especially of circulatory function. It is an important diagnostic index for many reasons. Firstly, any condition that dilates or contracts the blood vessels or affects their elasticity, affects the blood pressure. Secondly, any disease of the heart that interferes with its pumping power, affects the blood pressure. In a healthy animal, the blood pressure normal for its species is maintained within a certain range with great constancy. Thus if blood pressure is abnormally low or high, it usually indicates that greater health problems are present.,B L O O D P R E S S U R E,Blood pressure measurement What is a blood pressure measurement? Blood pressure is measured by inflating a cuff placed around the arm. Most people have had their blood pressure taken at some time or other. It is a simple and painless examination that gives a lot of useful information about the heart and the condition of the blood vessels.,B L O O D P R E S S U R E,What is measured? The doctor measures the maximum pressure (systolic) and the lowest pressure (diastolic) made by the beating of the heart. The systolic pressure is the maximum pressure in an artery (which carries blood from the heart) at the moment when the heart is beating and pumping blood through the body. The diastolic pressure is the lowest pressure in an artery in the moments between beats when the heart is resting. Both the systolic and diastolic pressure measurements are important - elevation of either one (or both) constitutes high blood pressure,B L O O D P R E S S U R E,临床上血压测量技术,直接法：将一跟导管经皮插入欲测部位的血管或心脏内，通过导管内的液柱同放在体外的应变式传感器、线性可变电感式差动变压器、电容式传感器等相连，从而测出导管端部的压力。另一种形式是把传感器放在导管的末端，直接测出端部所在点的血压值。这种方法的优点是测量值准确，并能进行连续测量。但它必须经皮将导管放入血管内，所以是一种创伤性的方法。 间接法：利用脉管内压力与血液阻断开通时刻所出现的血流变化间的关系，从体表测出相应的压力值。由于这种方法不需要剖切的外科手术 ，同时测量简便，因此在临床上得到广泛的应用。其缺点是测量精度较低，不能进行连续测量以及不能用以测定心脏、静脉系统的压力。,B L O O D P R E S S U R E,血压直接测量法导管术,导管法不仅可用来测量动脉压，而且可以测量和监护中心静脉压、肺动脉和肺毛细血管楔入压和左心房、右心室的压力。 中心静脉压（CVP,central venous pressure)一般是指右心房、上腔静脉或锁骨下静脉血液所给出的压力。静脉压正常范围：右心房0490Pa；上腔或锁骨下静脉5881176Pa。肺动脉压力(PAP,pulmonary artery pressure)为3.3/2kPa，平均压小于2.4kPa。肺毛细血管压力(PCWP pulmonary capillary wedge pressure)平均值小于22.4kPa. 用不透射线的聚乙烯导管通过皮肤插入或通过臂中的大静脉或锁骨下静脉，有时也可将颈静脉割开插到欲测部位。测量肺动脉楔入压采用漂浮导管，是一根在末端带有一个可膨胀气球的双腔导管。使用时先抽去气球内空气，当进入到锁骨下静脉时，用空气或二氧化碳将气球充气到一半容积（0.40.6mL），血液顺流将导管推送到右心房，然后使气球完全膨胀（0.8mL），在血流的推动下，通过三尖瓣而进入右心室，最后通过肺主动脉瓣漂流到肺动脉，挤进末端的动脉枝，从而测出PCWP值。,B L O O D P R E S S U R E,血压直接测量法导管术,在循环休克期间，由于低血压值和低心输出量，这时不可能使用间接测压法。当需观察主动脉瓣机能，常需采用动脉压力监护。它把一根细而短的导管插入外周动脉，如股动脉逆行进入至左心室。这种手术特别要注意避免出现由导管顶端血液凝块所引起的栓塞。 心导管术要在装由X线机的专门实验室里进行，以便观察心脏的结构和压力导管所处的位置。此外，导管法测压也可通过导管管腔对血液取样，分析血液的血氧饱和度、代谢产物、心输出量、血气和呼吸气体等许多参数的测量，并能检出所在位置的异常血流通路。通过导管管腔注入放射性的染料，医生就可确定心室或主动脉的功能。给冠状动脉注射不透射线的X线染料，对冠状动脉造影，诊断心脏疾病。通过压力测量还可以提取心内心音。,B L O O D P R E S S U R E,血压标定方法,测量血压用的各种类型传感器，与血压值相对应的传感器输出必须经过放大和处理后才能显示和记录。由于传感器特性的离散性，不同传感器配用相同测量电路时，所得结果不可能一致。因此必须对传感器的灵敏度加以标定，并使不同灵敏度的传感器与同一测量电路相配时，仍可得到同样的显示结果。 解决标定问题通常有三种方法：一是严格要求制造厂的出厂指标；二是采用传感器内部的灵敏度调节，三是每只传感器使用定标系数，以便和已知的压力放大器一起工作。目前大多采用第三种方法。,B L O O D P R E S S U R E,血压标定 电原理图,压力放大器,根据血压传感器激励电压的不同，有四种基本形式的压力放大器：直流（DC）、直流隔离、脉冲激励和交流（AC）载波放大器。DC放大器只适用于电阻应变式传感器；AC载波放大器既可用于电阻应变式传感器，也可用于电感式传感器；脉冲激励放大器可以与电感式传感器相匹配，但一般用于电阻应变式传感器。,B L O O D P R E S S U R E,直流压力放大器 实际上是直流放大器，首先应考虑温漂和时漂问题。关键在挑选第一级运放，选择高稳定度、低漂移的运放。这种形式的放大器对第一级 运放要求高，电路虽然简单，但不能解决零漂问题，所以只能用在测量精度不高的场合。,B L O O D P R E S S U R E,脉冲激励压力放大器 激励信号是一双相短间期脉冲，A1为直流放大器，A2为单位增益的相减级。S1S5为CMOS电子开关，所有电路的动作由四相时钟控制，1、2激励传感器，并使放大器漂移抵消电路工作；3使显示表显示最新数据；4相随后使数据复原。所有DC放大器都有因热引起零点漂移的现象，这可以由开关S2和S3、电容C1和放大器A2组成的漂移抵消电路来克服。,传感器只有在1为高电平和2为低电平时受激励，而其余时间都不受激，这样使传感器的自热最小。,B L O O D P R E S S U R E,隔离式直流压力放大器 由于用直接式直流压力放大器测量血压时，导管传感器必须通过液 柱同体内测试点接触，这样容易遭受微电击，因此为了保障病人的安全， 必须将病人地和仪器地 隔离开（这样由于增加了泄露电流通路的阻抗， 从而减少了通过人体的泄露电流，而确保人身安全）。目前血压测量仪 器中大多数采用这种形式的放大器。,交流载波压力放大器 需用交流电源激励，因此很适应电阻应变仪和电感式传感器。一般压力监护仪自身包含有一个震荡源，而很多集中监护系统或架装的导管实验室则倾向于使用一个功率足以能推动几个载波放大器的公用载波震荡器。 载波放大器由于采用交流激励，当对大气压校零时应力求使交流载波放大器的残余电压尽可能小。但传感器特性的不对称性以及桥路分布电容的影响都会使残余电压增大，因此必须加以平衡。压力载波放大器的解调电路使血压调制信号解调，恢复为原始的血压波形。,B L O O D P R E S S U R E,血压波形的基波频率约为2Hz,为了能不失真地取得血压波形,放大器频带应不小于10倍基波频率,故一般血压放大器的频响为20Hz,这样既可获得满意的结果。但当估计心脏泵血功能时,常用dp/dt微分波形或(dp/dt)/p参数。为了使微分器具有一定的增益，就需加宽放大器频带，在这种情况下带宽采用100Hz。滤波电路可用二阶无源或二阶有源低通滤波器。,B L O O D P R E S S U R E,收缩压、舒张压和平均压检测电路,血压计原理框图,压力信号经放大后，由收缩压和舒张压保持电路检出血压波中的峰值和谷值。平均压值也可通过对血压波求积而得到。这些压力值通过A/D转换器转换成数字的形式来显示，也可通过放大器供CRT或记录器显示记录。,B L O O D P R E S S U R E,收缩压检测电路,定时图,合上S4后的延时电路和波形,B L O O D P R E S S U R E,血压间接测量,间接式血压测量的方法很多，其中最主要的一种的利用袖带充气加压阻断动脉后，随后缓慢放气，在袖带下或动脉的远端检出脉搏的变化或血流的变化作为收缩压和舒张压的判断，也可把袖带内压力波动的形式作为判据。 柯氏音法 1905年苏联医生KOPOMKOB提出：在正常的情况下，完全受压的动脉并不产生任何声响，只有当动脉不完全受阻时才出现声音，因此可用声音来确定人体的血压。,B L O O D P R E S S U R E,柯氏音法测量精度较差的原因：,就心脏血压而言，血压的读数随传感器的部位和高度而变。不在心脏水平高度所取得的读数应补加上以心脏为基准的相应读数。,如用听诊器，则读数将受使用听力的影响，故使用的听诊器应符合 一定的标准。,出现的运动伪迹与引入系统的振动形式有关。若病人在休克状态， 因其脉搏微弱，柯式音振动很低，所以血压测量对移动特别敏感。,无论对正常人还是情绪紧张的人，触摸手臂都能改变读数。另一 方面，换气过度可以有减小压力的效应。,错误的测量方法如末端的位置不适当、袖带放气速度不适当、水 银压力计不垂直、听诊器间隙及袖带放置不适当等。,B L O O D P R E S S U R E,超声法,超声法的原理是利用超声波对血流和血管壁运动的多普勒效应来检测收缩压和舒张压。,B L O O D P R E S S U R E,当袖带压力增加到超过舒张压而低于收缩压，动脉内的血压在高于或低于袖带压力间摆动。在血管被阻断期间，血管壁静止不动，所以无频移产生，无声频输出。当刚巧低于或高于袖带内压力的时刻，由于血流及管壁运动大，所以产生较大的频移信号，因而就能检出声频输出。因此，随着每次心搏血管呈现开 放和闭合，借助于超声系统就可把这种开放和闭合状态检测出来。,在一个心周期内，随着袖带压力的增加，血管的开放和闭合的时间间隔就随之减小，直到开放和闭合两点重合，该点即为收缩压。相反，当袖带压力减低时，开放和闭合之间的时间间隔增加，直到脉搏的闭合信号与下一次的开放信号重合，这一点可确定为舒张压。此时血管在整个心周期内都是开放的。超声测压法的优点是适用范围比较广，同时可用于噪声很强的环境中，另外应用超声法可以完整地再现动脉波。这时以心电图R波作为基准点，当动脉开放时，用袖带压力与心电图R波之间的关系，描绘出动脉波的上升部分；相反，在动脉闭合期间能描绘出动脉波的下降部分。缺点是受试者身体的活动可以引起传感器和血管之间超声波途径的变化。,B L O O D P R E S S U R E,逐拍跟踪法,逐拍跟踪法是先将袖带充气达到预定的收缩压水平，然后根据每一心周期中 柯式音的出现与否来决定袖带压力是增加还是减小。,B L O O D P R E S S U R E,系统工作过程：,心电电极检出II导联,检测到R波，触发单稳态触发器,微音器检出臂动脉上柯氏音,计算机判断在R波后时间,噪音略去不计,袖带压力 增加2mmHg,袖带减压2mmHg,B L O O D P R E S S U R E,脉搏延时法,脉搏延时法是利用血管内压力超过袖带压力时所产生的脉搏波滞后于心电图 R波的原理，测量出R波和脉搏波之间的时间间隔T,找出T的特征点，从而求 出欲测的收缩压和舒张压的数值。它克服了用柯氏音法难于提供明确的舒张 压辨识点的缺点。,上图所示血压波形中的斜线是袖带压力。当袖带压力刚低于血管内压力时， 就出现脉搏波。随这袖带压力的降低，R波和脉搏波之间的间隔T将随袖带 内压力而变，即随袖带压力的降低，间隔T逐渐减小。T间隔最大时所对应 的袖带压力为收缩压；而最小T值对应的袖带压力为舒张压，根据测出的T 值和相应的袖带压力即可求出血压值。,B L O O D P R E S S U R E,延迟时间最短的Tn点所对应的10.7kPa(80mmHg)即为舒张压；而延迟时间 最长的T1点所对应的16kPa(120mmHg)即为收缩压。这种方法其特征点极 易识别，测量精度也较柯氏音法为好。,血压的自动测量 （无创血压模块NIBP）,NIBP血压模块采用测振法进行血压的无创测量。方法时通过监测因血液流经弹性动脉而引起袖带内压力的波动来实现的。在测量中，首先绕在病人手臂或其他肢端的袖带充气加压使动脉血管阻断；然后袖带以阶梯量逐渐放弃，当袖带内压力下降到一定压力时，血液开始在血管内流动。随着压力的下降，血流量加大，同时引起袖带内压力脉搏波动幅度（pressure pulse)的增大直至达到最大值；当压力进一步下降时，波动幅度开始减小。即袖带压力以阶梯量逐渐下降，压力波动幅度会以先上升后下降的规律下降。根据压力波动幅度的包络曲线就可以计算出平均动脉压、收缩压和舒张压。,B L O O D P R E S S U R E,B L O O D P R E S S U R E,硬件电路,NIBP,硬件电路由CPU、EPROM、RAM、实时时钟、A/D转换器、放大电路和两个压力传感器组成，其中一个压力 传感器用于检测过压并激活看门狗电路，如果压力超过某一设定值，则微处理器复位，从而关闭电磁阀和气泵。,B L O O D P R E S S U R E,NIBP,气动部分,气动部分由一个直流电动机带动的气泵、两个电磁阀和两个传感器等机电元件组成。当电磁阀关闭时，启动气泵给袖带充气。采用两个阀的目的是保证一个阀出现故障时，仍能安全的减压。微处理器根据袖带内的空气容量和压力选择适当的阀进行快速或者精确放气，单向阀的作用是将系统的漏气减至最小，过滤器能够防止灰尘进入系统而引起元器件的故障。,B L O O D P R E S S U R E,NIBP,系统软件,软件模块固化在64K的EPROM中，软件的变化主要是用户的串行通信协议和自检程序。通常软件模块可分为有一定背景的确定状态机构和中断任务处理。初始化之后，等待主命令以便进入测量或校准模式。,有效中断诸如气泵的调节、通信以及各种定时器均有优先权。在检测周期中通过中断来保持模块与主机的通信、A/D转换和阻止看门狗电路复位。,B L O O D P R E S S U R E,NIBP,电路概述,电源,NIBP需要一个+7V的直流电源输入，再由电压调整电路转换为+5V电源。,过压保护电路,由一个压力传感器、一个差动放大器和一个比较器构成。压力传感器根据压力 值产生的电压经差动放大器放大后与一个精密电压相比较。当袖带压力超过 定压力值时，比较器翻转、微处理器复位。,B L O O D P R E S S U R E,袖带压力放大器,与过压保护电路相似，只是没有比较器。,脉动压力放大器,B L O O D P R E S S U R E,看门狗电路,作用是为了保证患者的安全，当模块出现某种情况时，使微 处理器复位，气泵停止工作、电磁阀放气。监测内容有：1. +5V直流电源工作范围。2.主机发来控制信号。3.袖带压力 过高。,A/D转化器,气动元件的控制,B L O O D P R E S S U R E,Digital Blood Pressure Monitor,Stores up to 14 measurements. Prints stored measurements in both digital and bar graph form with time and date. Curved contour cuff fits arms 9” to 13” in circumference. Large digital display measures 1” x 3 1/2”. Includes zippered carrying case, 2 rolls of thermal printing tape and instructions. Operates on 4 ”C” batteries (not included).,IntelliSense technology offers automated level setting and electronic deflation control 12 Memory storage Completely automatic, operates with a push of a button, and has a clear digital display Ultra compact design travels anywhere Quickly and accurately measures blood pressure and pulse Fits writs 5 1/4“ to 8 1/2“ in circumference,OMRON MEMORY, PRINT-OUT & GRAPH BLOOD PRESSURE MONITOR,HEM-705CPN,HEM-609,B L O O D P R E S S U R E,Digital Blood Pressure Monitor,Samsung Blood Pressure Monitor 1000s,Large easy-to-read LCD panel Secure and comfortable arm cuff (fits arms 9“- 13“) Standard cuff fits up to 13“, larger cuff available from manufacturer using a coupon enclosed in box Simple, one button automatic inflation Quiet, rolling motor inflation pump Automatic power conservation Pulse monitor,The Heart Sense Automatic Blood Pressure Monitor This exclusive system uses advanced technology to “sense“ exactly how high to inflate the arm cuff, automatically adjusting for each individual user. That means quick, easy blood pressure and pulse measurements every time. It also means comfortable home monitoring because the cuff will never be inflated higher than necessary. Heart Sense home blood pressure monitors are sensitive to your needs - only from Samsung,B L O O D P R E S S U R E,Digital Blood Pressure Monitor,Panasonic,EW3111W,Precise Logic Upper Arm Blood Pressure Monitor with 3 color light system,Precise Logic Technology Panasonics own patented technology, Precise Logic, checks both the height and width of your pulse to provide you with measurements of clinically-proven accuracy and reliability.,One Touch Operation Take your blood pressure and pulse rates easily with the touch of a button.,Large LCD Display Makes your blood pressure and pulse rate results easy to read.,Blood Pressure/Pulse Rate Display For immediate and reliable results, you can read blood pressure and pulse rate results at the same time.,Measurement Tracking Helps you track your progress by storing the last 60 readings with the date and time indicated right in the monitor.,B L O O D P R E S S U R E,Digital Blood Pressure Monitor,Panasonic,Light and Portable Wherever you travel, this monitor is durable, portable and lightweight. At less than 4“ around and just over 4 ounces, you can check your blood pressure and pulse rate anywhere.,Protective Hard Case Comes with a compact carrying case which protects the unit from damage and doubles as an arm rest for taking proper readings.,Long-Lasting, Energy Efficient The wrist units enable up to 400 readings on two AAA alkaline batteries (included). The arm units enable approximately 200 uses on four AA batteries (included).,Range of Circumference Wrist units fit wrists from 5-1/4“ to 7-3/4“ around. (Available on all wrist models) Arm unit fit upper arms from 7-7/8“ to 15-3/4“.,B L O O D P R E S S U R E,Digital Blood Pressure Monitor,Panasonic,Hypertension(over 140),Normal/High(130-139),Normal(under129),Systolic,Hypertension(over90),Normal/High(over85-89),Normal(inder84),Diastolic,Precise Logic?Technology Yes Method of Measurement Oscillometric system with Precise Logic Technology Measuring Range Pressure: 20-300 mmHg / Pulse: 30-160 beats/minute Measuring Accuracy Pressure: +/- 4mmHg; Pulse rate: +/- 5% Range of Circumference Approx. 7-3/4“ to 15-3/4“ (20cm to 40cm) Power Source Four “AA“ Batteries (included) Operating Environment 50-104 degrees F / 30%-85% RH Accessories Cuff storage holder w/carrying handle Operation Time Up to 200 uses on four AA batteries (included) Memory 42 readings (21 readings x 2 persons) Measurement Tracking Yes Protective Hard Case No Dimensions (H x W x D) 411/32 x 53/32 x 77/8 Weight 1.07 lbs,B L O O D P R e S S U R E,Sphygmomanometers,Heine Sphygmomanometer,This top model of the HEINE GAMMA Sphygmomanometer range features one tube system and a new screw-type valve for precise selection of air release rate. The HEINE GAMMA 5.5 is an elegant precision instrument which is designed to last a lifetime. Screw-type valve system. All-metal case with matte chrome finish. Oversized inflator speeds up the inflation process. Big 60 mm scale is clear and easy to read. Detachable rubber ring protects instrument and glass. Patented “2 in 1” tube design. Microfilter protects valve and pressure proof manometer. Gamma 5.5 with adult cuff (14 cm) and pouch.,B L O O D P R E S S U R E,Sphygmomanometer,Tycos ProCheck TR2 - Sphygmomanometer,Lightweight and compact, the Tycos TR-2 hand aneroid provides portability in an affordably priced, hand-held design. The trigger-release air valve is positioned for optional left or right-handed use and allows for easy fingertip control of cuff deflation. Large easy-to-read dial face with bright graphics. Balanced palm design allows for comfortable use over repeated measurements. Meets AAMI accuracy standard of 3mmHg. Comes in a zippered case. Five year warranty. All latex free,B L O O D P R E S S U R E,ABPM(Ambulatory Blood Pressure Monitoring),Ambulatory blood pressure measurement (ABPM) is becoming more and more popular as a standard method in clinical practice.,Due to various inaccuracies and phenomenons associated with conventional Riva-Rocci/Korotkoff technique of blood pressure measurement (e.g. white-coat hypertension), the method of ambulatory blood pressure measurement is becoming increasingly popular as a standard in clinical practice.,INTRODUCTION,Advantages of ABPM,Series of measurements,More accurate documentation of blood pressure,Blood pressure profile independent of medical environment, therefore:,Documentation of blood pressure behaviour over 24-h period during daily life activities,Identification of blood pressure characteristics during the night,Consistent efficacy control of antihypertensive medication,Consistent efficacy control of antihypertensive medication,Adequate treatment with antihypertensive agents based on multiple measurements,B L O O D P R E S S U R E,ABPM(Ambulatory Blood Pressure Monitoring),Indications for ABPM,Suspected white-coat hypertension Suspected nocturnal hypertension Differentiation of dipper/non-dipper/super-dipper status Therapy-resistant hypertension Elderly patients/isolated systolic hypertension Type 1 diabetes Hypertension during pregnancy Evaluation of hypotension,B L O O D P R E S S U R E,ABPM(Ambulatory Blood Pressure Monitoring),B L O O D P R E S S U R E,History Measurement of blood pressure was first attempted by Hales in 1733. He apparently inserted pipes into the arteries of animals. The blood pressure cuff was developed by Riva-Rocci in 1896. Cushing introduced the measurement of blood pressure into Anaesthetic practice in 1901. Stephen Hales (1677-1761).English naturalist. Scipione Riva-Rocci (1863-1937). Italian Physician. Harvey Williams Cushing (1869-1939). American neurosurgeon. Pioneered diathermy, advocated record keeping and monitoring in Anaesthesia. Described Cushings disease, syndrome, reflex and ulcer.,Mechanism of action A microprocessor controls the sequence of inflation and deflation of the cuff. The cuff is inflated to a pressure above the previous systolic pressure, it is then deflated incrementally. A transducer senses the pressure changes which are processed by the microprocessor. This has an accuracy of +/- 2%. The mean arterial pressure (MAP) corresponds to the maximum oscillation at the lowest cuff pressure. The systolic pressure corresponds to the onset of rapidly increasing oscillations. Diastolic pressure corresponds to the onset of rapidly decreasing oscillations. It is also calculated from the systolic and MAP (MAP= diastolic + 1/3rd pulse pressure),NIBP(non invasive blood pressure),The Cuff The cuff should cover at least 2/3rds of the upper arm. The width of the cuffs bladder should be 40% of the mid-circumference of the limb. The middle of the cuff should overlay the brachial artery. The device has a fast rate of inflation and a slow cuff deflation. This avoids venous congestion and allows time to detect arterial pulsation. Sources of error If the cuff is too small the blood pressure over-reads. Similarly, if too large then the blood pressure under-reads, (greatest error is seen with an undersized cuff). Systolic pressure over-reads at low pressures (60mmHg) and under reads at high systolic pressures. Arrhythmias such as