This Week in PBS&DBlood PressureBP In and Around the HeartBlood Pressure ClassificationDirect MeasurementExtravascular SensorsExtravascular SensorsBlood Pressure TransducersStrain gage w/ Wheatstone BridgeArterial/Venous BP transducersIntravascular SensorsDynamic Properties of the Measurement System(a) Simplified analogous circuit. Compliance of the sensor diaphragm is larger than compliance of catheter or sensor cavity foBandwidth RequirementsDistortionHeart SoundsHeart SoundsHeart SoundsIndirect MeasurementsAuscultatory MethodIndirect MeasurementsOscillometric MethodOscillometric Automated BP MonitorsIndirect MeasurementsUltrasonic (Doppler) MethodPrinciples of Biomedical Systems & DevicesPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Principles of Biomedical Systems & DevicesLecture 10Measuring Blood PressurePBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404This Week in PBS&D HemoDynamicsª Blood vesselsª Pressure gradientª Factors affecting blood pressure and homeostasis of blood pressureª Shockª Blood and its components Measuring Blood Pressure and Soundsª Direct measurement of BPª Dynamic properties of the direct measurement catheter systemsª Heart soundsª Indirect measurements of the BP• Auscultation and osscilometric methodsPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Blood Pressure Blood transports O2and nutrients to tissues, and carry metabolic waste away from the cells The transportation is made possible by a “pressurized vessel”system, the arteries, veins, arterioles, venuoles and capillaries, 100,000 km in all… The pressure is provided by a mechanical pump, the heart. Measuring this pressure at various locations of this transportation network carries significant clinical information. These measurements can be made directly or indirectlyPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404BP In and Around the HeartPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Blood Pressure ClassificationClassification of blood pressure levels for adults 18 years and older. Systolic pressure, the higher number of a blood pressure reading, is the pressure as the heart pumps; diastolic pressure is the pressure when the heart relaxes between beatsCATEGORY SYSTOLIC (mm Hg†) DIASTOLIC (mm Hg†)Normal* < 130 < 85High normal 130-139 85-89HypertensionSTAGE 1 (Mild) 140-159 90-99STAGE 2 (Moderate) 160-179 100-109STAGE 3 (Severe) 180-209 110-119STAGE 4 (Very Severe) >209 >119Optimal blood pressure is less than 120/80 mm Hg. Unusually low readings should be evaluated by a physician.PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Direct Measurement Invasive measurements – two major techniques:ª Couple vascular pressure to an external sensor via a liquid-filled catheter (extravascular pressure sensor)ª Place the sensor on the catheter tip which is directly inserted into the vessel of interest (intravascular pressure sensor).PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404ExtravascularSensorsSensing portSample and transducerzero stopcockElectrical connectorDisposable pressure transducer with an integral flush deviceFlush solution under pressureRoller clampPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Catheter connected to a pressure sensor through 3-way stopcock System is filled with saline-heparin solution (anticoagulant agent), must be flushed every few minutes Catheter inserted through surgical cut down or percutaneous insertion BP info is transmitted via the catheter fluid to the sensor diaphragm (why use water, but not air?)ExtravascularSensorsPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Blood Pressure TransducersA thin flexible metal diaphragm is stretched across the opening of the transducer top. The diaphragm is connected to an inductive bridge (or resistive Wheatstone bridge) strain gauge which flexes the strain gauge an amount proportional to the applied pressure.A clear plastic dome, filled with fluid sits atop the diaphragm and provides the hydraulic coupling / connection to the catheter. Electrical connector typically houses the bridge circuitPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Strain gage w/ Wheatstone BridgeR4R3R1R2ArmatureStrain-gage wiresADCBDiaphragmcbdaR4=C∆υoυiR3=DR2=ARyRiRxR1=B342100RRRRV =⇔=∆Let all R initially be equal to R0« Ri. If R1and R3increase and R2and R4 decrease by ∆R, then ivRRv00∆=∆PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Arterial/Venous BP transducersPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Intravascular Sensors Disadvantage of the previous system: The frequency response of the system is limited by the hydraulic properties, in particular the low pass filter effect of the tubing system Using an intravascular system eliminates the entire plummingsystem, by making the measurement at the site!ª Eliminates the time delay introduced by the tubing systemª Allows high fidelity measurement of the high frequency components of the BP signal Typical sensors used:ª Strain gages bonded onto a flexible diaphragm at the catheter tipª Fiber optic systems where the displacement measurement of the diaphragm is made opticallyPBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Dynamic Properties of the Measurement System Errors in measurement of dynamic pressure can lead to significant consequences, underdamped systemÆ Overestimated BP. The liquid filled catheter-sensor is a hydraulic system that can be best modeled by distributed parameters.DiaphragmSensorLsLcRsRcLcRcLcPRcCcCcCcCd =Cs∆V∆V∆PLiquidCatheterIncrementallengthEach segment of the catheter has its own resistance Rc, inertance Lc, and compliance Cc. In addition, the sensor has resistor Rs, inertance, Ls, and compliance Cs. The compliance of the diaphragm is Cd.PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404Dynamic Properties of the Measurement System(a) Simplified analogous circuit. Compliance of the sensor diaphragm is larger than compliance of catheter or sensor cavity for a bubble-free, noncompliant catheter. The