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Slide 1 - Blood Pressure and Flow Measurements S-108.4010Licentiate Course in Measurement Science and Technology
Slide 2 - Contents Invasive Non-Invasive Blood Pressure Blood Flow Palpatory Method (Riva-Rocci Method) Auscultatory Method Ultrasonic Method Oscillometric Method Tonometry Extravascular Sensor Intravascular Sensor Ultrasound Doppler Laser Doppler Flowmetry Dye Dilution Method Thermal Dilution Method Strain Gage Plethysmography Electric-Impedance Plethysmogr. Photoelectric Plethysmography Radioisotopes Thermal Convection Probes 1 2 3 4 General on System Parameters
Slide 3 - Blood Pressure (1) One of the oldest physiological measurements Originates from the heart Value depends on 3 factors: cardiac output diameter of arteries the quantity of blood Commonly refers to arterial blood pressure Values should be lower than 120 / 80 mmHg (systolic pressure (SP) / diastolic pressure (DP)) peripheral resistance Observation of blood pressure allows dynamic tracking of pathology and physiology affecting to the cardiovascular system, which has profound effects to all other organs of the body
Slide 4 - Blood Pressure (2) Pulse pressure (PP) = SP-DP MP = DP+PP/3 Mean pressure (MP) average pressure during one cardiac cycle driving force of the peripheral perfusion. an estimate can be done by using an empirical formula: SP and DP may vary significantly throughout the arterial system but MP is quite uniform (in normal situations) High value increases the risk of heart attack and strokes Low value increases the risk of lower oxygen perfusion e.g. in brains However, the ’normal values’ differ from person to another
Slide 5 - Blood Pressure (3)
Slide 6 - Indirect Methodsin Blood Pressure Measurements
Slide 7 - General Facts Indirect measurement = non-invasive measurement Brachial artery is the most common measurement site Close to heart Convenient measurement Other sites are e.g.: forearm / radial artery wrist (tends to give much higher SP) The most common indirect methods are auscultation and oscillometry
Slide 8 - General Facts (cont.) An occlusive cuff is placed on arm and inflated to P > SP. Then the cuff is deflated gradually and the measurement of blood flow is done cuff The occlusive cuff should be of a correct size in order to transmit the pressure to the artery evenly and thus to obtain accurate results A short cuff requires special attention in placement. Longer cuff reduces this problem. The cuff should be placed at the heart level in order to minimize the hydrostatic effects
Slide 9 - Palpatory Method (Riva-Rocci Method) When the cuff is deflated, there is a palpable pulse in the wrist. P = BP Several measurements should be done as the respiration and vasomotor waves modulate the blood pressure levels +) The blood pressure can be measured in noisy environment too ADVANTAGES -) Only the systolic pressure can be measured (not DP) DISADVANTAGES -) The technique does not give accurate results for infants and hypotensive patients cuff +) Technique does not require much equipment
Slide 10 - Auscultatory Method The Korotkoff sounds are ausculted with a stethoscope or microphone (automatic measurement) Pulse waves that propagate through the brachial artery, generate Korotkoff sounds. There are 5 distinct phases in the Korotkoff sounds, which define SP and DP Also with this method, several measurements should be done. The frequency range is 20-300 Hz and the accuracy is +/- 2mmHg (SP) and +/- 4mmHg (DP)
Slide 11 - Auscultatory Method (cont.) -) The observations differ from observer to another -) A mechanical error might be introduced into the system e.g. mercury leakage, air leakage, obstruction in the cuff etc. -) The observations do not always correspond with intra-arterial pressure -) Auscultatory tecnique cannot be used in noisy environment +) Auscultatory technique is simple and does not require much equipment ADVANTAGES DISADVANTAGES -) The technique does not give accurate results for infants and hypotensive patients
Slide 12 - Ultrasonic Method A transcutaneous (through the skin) Doppler sensor is applied here. The motion of blood-vessel walls in various states of occlusion is measured. The frequency difference between transmitted (8 MHz) and received signal is 40-500 Hz and it is proportional to velocities of the wall motion and the blood. The vessel opens and closes with each heartbeat when DP < P < SP cuff
Slide 13 - Ultrasonic Method (cont.) +) Can be also used in noisy environment ADVANTAGES & DISADVANTAGES +) Can be used with infants and hypotensive individuals -) Subject’s movements change the path from sensor to vessel As the cuff pressure is increased, the time between opening and closing decreases until they coincide Systolic pressure Again as the cuff pressure is decreased, the time between opening and closing increases until they coincide Diastolic pressure
Slide 14 - Oscillometric Method The intra-arterial pulsation is transmitted via cuff to transducer (e.g. piezo-electric) The arterial pressure oscillations (which can be detected throughout the measurement i.e. when P > SP and P < DP) are superimposed on the cuff pressure SP and DP are estimated from the amplitudes of the oscillation by using a (proprietary) empirical algorithm. cuff cuff The cuff pressure is deflated either linearly or stepwise
Slide 15 - Oscillometric Method (cont.) DISADVANTAGE -) Many devices use fixed algorithms leading to large variance in blood pressures +) In the recent years, oscillometric methods have become popular for their simplicity of use and reliability. ADVANTAGES +) MP can be measured reliably even in the case of hypotension
Slide 16 - Tonometry A sensor array is used here, because at least one of the pressure sensors must lay directly above the artery Linear array of pressure sensors is pressed against a superficial artery, which is supported from below by a bone (radial artery). The pressure is increased continuously and the measurements are made when the artery is half collapsed When the blood vessel is partly collapsed, the surrounding pressure equals the artery pressure. The hold-down pressure varies between individuals and therefore a ’calibration’ must be done
Slide 17 - Tonometry (cont.) ADVANTAGES +) Can be used for non-invasive, non-painful, continuous measurement DISADVANTAGES -) Relatively high cost -) The wrist movement and tendons result in measurement inaccuracies
Slide 18 - Direct Methodsin Blood Pressure Measurements
Slide 19 - General Facts Direct measurement = Invasive measurement Used only when essential to determine the blood pressure continuously and accurately in dynamic circumstances A vessel is punctured and a catheter (a flexible tube) is guided in The most common sites are brachial and radial arteries but also other sites can be used e.g. femoral artery A division is made into extravascular and intravascular sensor systems This method is precise but it is also a complex procedure involving many risks….
Slide 20 - Extravascular Sensor The sensor is located behind the catheter and the vascular pressure is transmitted via this liquid-filled catheter. The ’normal’ measuring system The actual pressure sensor can be e.g. strain gage variable inductance variable capacitance optoelectronic piezoelectric, etc…
Slide 21 - Extravascular Sensor (cont.) The hydraylic link is the major source of errors. The system’s natural frequency may be damped and degraded due (e.g.): too narrow catheter too long tubing various narrow connections air bubbles in the catheter The catheter-sensor system must be flushed with saline-heparine solution every few minutes in order to prevent blood from clotting at the tip. .
Slide 22 - Extravascular Sensor (cont.) Normally the interesting frequency range is 0 – 100 Hz. If only MP is measured the bandwidth is 20 Hz (harmonics > 10 are ignored)
Slide 23 - Intravascular Sensor The sensor is located in the tip of the catheter. This way the hydraulic connection is replaced with an electrical or optical connection +) The frequency response is not limited by the hydraulic properties of the system. No time delay. -) Breaks easily -) More expensive +) Electrical safety and isolation when using fiber optics The dispacement of the diaphragm is measured
Slide 24 - Disposable Sensors Disposable sensors decrease the risk of patient cross-contamination and reduce the amount of handling by hospital personnel Cheaper and more reliable than reusable pressure sensors General on System Parameters Even minute air bubbles in catheter have a dramatic effect on frequency response The natural frequency and the length of the catheter have a following relationship: The catheter diameter has a linear relationship to natural frequency Stiffer catheters have a higher frequency response Teflon Polyethylene Silicon rubber BETTER WORSE
Slide 25 - Indirect Methodsin Blood Flow Measurements
Slide 26 - Blood Flow O 2 Blood Flow Blood Pressure ECG O and other nutrition concentration in the cells are one of the primary measurements. 2 Blood flow helps to understand basic physiological processes and e.g. the dissolution of a medicine into the body. Usually the blood flow measurements are more invasive than blood pressure measurements / ECG It also helps to understand many pathological conditions, since many diseases alter the blood flow. Also the blood clots in the arterial system can be detected.
Slide 27 - Blood Flow (2) Normal blood flow velocity 0,5 m/s – 1 m/s (Systolic, large vessel)
Slide 28 - Doppler Measurements (1) Ultrasound Doppler The blood cells in the fluid scatter the Doppler signal diffusively. In the recent years ultrasound contrast agents have been used in order to increase the echoes. f = 2 – 10 MHz c c = 1500 - 1600 m/s (1540 m/s) f = 1,3 – 13 kHz d The ultrasound beam is focused by a suitable transducer geometry and a lens
Slide 29 - Doppler Measurements (2) Ultrasound Doppler The flow velocity is obtained from the spectral estimation of the received Doppler signal In order to know where along the beam the blood flow data is colledted, a pulsed Doppler must be used
Slide 30 - Doppler Measurements (3) Ultrasound Doppler The ultrasound Doppler device can be either a continuous wave or a pulsed Doppler CW DOPPLER PULSED DOPPLER Range ambiguity Low flow cannot be detected No minimum range Simpler hardware Minimum range Accuracy No minimum flow (Maximum flow) x (range) = limited
Slide 31 - Doppler Measurements (4) Ultrasound Doppler GENERAL PARAMETERS the power decays exponentially because of the heating of the tissue. The absorption coefficient ~ proportional to frequency the far field operation should be avoided due to beam divergence. D = Transducer diameter (e.g. 1 – 5 mm) the backscattered power is proportional to f 4 the resolution and SNR are related to the pulse duration. Improving either one of the parameters always affects inversely to the other
Slide 32 - Doppler Measurements (5)Laser Doppler Flowmetry The principle of measurement is the same as with ultrasound Doppler The laser parameter may have e.g. the following properties: 5 mW He-Ne-laser 632,8 nm wavelength The moving red blood cells cause Doppler frequency 30 – 12 000 Hz. The method is used for capillary (microvascular) blood flow measurements
Slide 33 - Direct Methodsin Blood Flow Measurements
Slide 34 - Indicator Dilution Methods (1)Dye Dilution Method A bolus of indicator, a colored dye (indocyanine green), is rapidly injected in to the vessel. The concentration is measured in the downstream The blood is drawn through a colorimetric cuvette and the concentration is measured using the principle of absorption photometry amount of dye 1% peak C Avg. flow
Slide 35 - Indicator Dilution Methods (2)Thermal Dilution Method A bolus of chilled saline solution is injected into the blood circulation system (right atrium). This causes decrease in the pulmonary artery temperature. A standard technique for measuring cardiac output in critically ill patients An artery puncture is not needed in this technique heat content of injectate Several measurements can be done in relatively short time density of blood (e.g. 1060 kg/m3) specific heat of blood (e.g. 3640 J/(kg*K)
Slide 36 - Plethysmographyin Blood Flow Measurements
Slide 37 - Plethysmography (1)Strain Gage Method Plethysmography means the methods for recording volume changes of an organ or a body part (e.g. a leg) Strain gage is made of silicone rubber tubes, which are filled with conductive liquid (e.g. mercury) whose impedance changes with volume. Venous occlusion cuff is inflated to 40 – 50 mmHg. In this way there will be the arterial inflow into the limb but no venous outflow. If only a segment of limb is measured, there is a need for arterial occlusion cuff also.
Slide 38 - Plethysmography (2)Chamber Method Chamber plethysmograph is the only accurate non-invasive way to measure changes in the blood volume As the volume of the leg increases, the leg squeezes some kind of bladder and decreases its volume Volume transducer can be e.g. water filled tube (level) or air (pressure) The speed of the return of the venous blood is measured
Slide 39 - Plethysmography (3)Electric-Impedance Method Different tissues in a body have a different resistivity. Blood is one of the best conductors in a body ( = 1,5 Ωm) A constant current is applied via skin electrodes The change in the impedance is measured The accuracy is often poor or unknown I = 0,5 – 4 mA rms (SNR) f = 50 – 100 kHz (Zskin-electrode+shock)
Slide 40 - Plethysmography (4)Photoelectric Method A beam of IR-light is directed to the part of the tissue which is to be measured for blood flow (e.g. a finger or ear lobe) The light that is transmitted / reflected is collected with a photodetector The blood flow modulates the attenuated / reflected light which is recorded. Poor measure for changes in volume Very sensitive to motion artefacts Method is simple Heart rate is clearly seen
Slide 41 - Other Methodsin Blood Flow Measurements
Slide 42 - Radioisotopes A rapidly diffusing, inert radioisotope of lipid-soluble gas ( Xe or Kr) is injected into the tissue or passively diffused 133 85 The elimination of the radioisotope from microcirculatory bed is related to the blood flow:
Slide 43 - Thermal Convection Probe The rate of heat removal from the tissue under probe is measured The concentric rings are isolated thermally & electrically from each other The central disk is heated 1 – 2 C over the temperature of tissue A temperature difference of 2- 3 C is established between the disks o o The method is not very common due extreme nonlinear properties and difficulties in practical use (e.g. variable thermal characteristics of skin) This is one of the earliest techniques for blood flow measurements
Slide 44 - Summary (1) BLOOD PRESSURE Describes the physiology and pathology of cardiocvascular system ”Normal” values are 120 / 80 mmHg High values may lead to heart attack and strokes Low values may lead to low oxygen perfusion All direct methods require skin punctuation and a use of catheter. Methods are used only when continuous and accurate measurements are needed. Almost all indirect methods rely on an occlusive cuff which is placed on the bracial artery. The actual measurement is done when the cuff is deflated
Slide 45 - Summary (2) BLOOD FLOW Used for understanding physiological processes (e.g. medicine dissolution). Also used for locating clots in arteries Usually more invasive methods are used than with blood pressure measurements Normal velocity is 0,5 - 1 m/s Indirect measurements are done by using ultrasound or plethysmographic method Direct measurements are done by dilution methods (dye / thermal)