Measurement of vital signs blood pressure

Measurement of vital signs blood pressure

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  MARCH/APRIL 2011 ▼IEEE PULSE 39 TUTORIAL B lood pressure is the last of the four physiologic variables known as clini- cal vital signs that we will consider in this tutorial series. In essence, pressure and heart rate are the two vital signs that evaluate the cardiovascular system. Al- though pressure is important throughout the cardiovascular system, the vital sign of blood pressure refers to the arterial blood pressure, and this is a time-varying quantity because of the cyclic pump- ing nature of the heart. An example of a blood-pressure waveform as a function of time is shown in Figure 1. The wave- form shows a peak in pressure following ventricular contraction, and this pressure is known as the systolic blood pressure. The waveform also shows a minimum just before ventricular contraction, with this pressure value known as the diastolic blood pressure. Clinically, blood pres- sure is described by what appears to be a fraction: systolic blood pressure is the numerator, and diastolic blood pressure is the denominator. The pressure difference between the systolic and diastolic blood pressures is known as the pulse pressure. The mean arterial pressure is the average of the pressure waveform taken over one or several cardiac cycles. Before we specifically consider the measurement of blood pressure, let us look at how pressures are measured in general. A schematic view of a pressure sensor is shown in Figure 2. The basic idea is of a fluid-filled chamber that is coupled to the fluid for which the pres- sure is to be measured. The chamber is rigid, with the exception of one wall or a portion of a wall that consists of a thin, distendable diaphragm. When the pres- sure within the chamber is equal to that outside it [Figure 2(a)], the diaphragm is in its neutral position. If the pressure within the chamber is greater than that outside it, this will cause the diaphragm to deflect away from the body of the chamber, and depending on the elastic properties of the diaphragm, this deflec- tion will be proportional to the pressure difference between the fluid within the chamber and the pressure external to it. This distention of the diaphragm can be measured by a displacement sensor that will, in turn, provide an electrical signal proportional to the pressure difference. When the external pressure is equal to atmospheric pressure, which it usually is, the pressure measured by the sensor is known as the gauge pressure. If the pres- sure sensor is located within a vacuum so that the external pressure is zero, the pressure measured is known as the ab- solute pressure. The displacement sensor can be any type of device capable of mea- suring small displacements. Most often, it is a strain gauge or a set of strain gauges, but linear variable differential transform- ers have been used as well. Two common types of pressure sen- sors used to measure blood pressure are illustrated in Figure 3. Much of the early work quantitatively describing the physi- ology of the cardiovascular system was done using the unbonded fine-wire strain gaugetypeofsensorshowninFigure3(a). In this case, pressure differences between the fluid in the dome and that external to the sensor cause the diaphragm to de- flect, which, in turn, stretch two of the fine-wire strain gauges, while the other two are allowed to contract. This causes MeasurementofBloodPressure MichaelR.Neuman Digital Object Identifier 10.1109/MPUL.2011.940568 Date of publication: 28 April 2011 2154-2287/11/$26.00©2011 IEEE Systolic Blood Pressure Diastolic Blood Pressure BloodPressure Time Mean Arterial Pressure Pulse Pressure FIGURE 1 An example of an arterial blood-pressure waveform showing systolic, diastolic, pulse, and mean pressures. © INGRAM PUBLISHING, PHOTODISC, IMAGESOURCE, & TECHPOOL STUDIOS
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  40 IEEE PULSE▼ MARCH/APRIL 2011 the former strain gauges to experience a small increase in their electrical resis- tance, while the latter causes a decrease in this quantity. These strain gauges are connected in a bridge circuit such that the bridge is close to balance when there is no pressure difference across the diaphragm, and it becomes unbalanced producing an output voltage as the pressure in the dome increases with respect to the exter- nal pressure. Although this type of pressure sensor was extensively used in biomedical re- search and clinical care to measure blood pressure several years ago, it has been re- placed today by disposable semiconductor strain-gauge-based sensors, as illustrated in Figure 3(b). Pressure differences be- tween the fluid in the sensor and external pressure result in a deflection of the dia- phragm portion of the silicon chip. In this case, however, the strain gauges are inte- grated into the diaphragm, and they have a much higher sensitivity to diaphragm deflection than the wires of the un- bounded strain gauge sen- sor. Thus, the diaphragm can be much smaller than it was for the unbonded strain gauge sensor, and it needs to deflect only a very small amount; so, the sensor itself is also much smaller. The overall sensitivities of both the semiconductor and unbounded strain gauge pressure sensors are essentially the same at 5 mV/mmHg per volt of excitation. There are two general categories of methods to measure blood pressure: di- rect and indirect methods. Each of these will be described in the following sections. Direct Measurement of Blood Pressure The direct method of measuring blood pressure involves having the pressure sensor coupled directly to the vessel or chamber in which the blood pressure is to be measured. This coupling is done by either a fluid-filled catheter or minia- ture pressure sensor located at the mea- surement site itself. The arrangement for direct blood-pressure measurement is illustrated in Figure 4. The fluid in the catheter conducts the pressure seen at its distal tip to the pressure sensor at its proximal end according to Pascal’s law. When measuring pressure in this way, it is important to maintain the pressure sensor at the same vertical level as the distal tip of the catheter so that there is no hydrostatic pressure component added to or subtracted from the pressure being measured. There are several con- cerns that must be consid- eredwhenmakingadirect blood-pressure measure- ment with a fluid-filled catheter. Since the cath- eter is made of a foreign material in contact with blood, it can cause clot- ting of the blood at contact points. This can create emboli that can break away from the catheter and block blood ves- sels downstream. The clots can also block the communication of the blood with the fluid in the catheter if the clot cov- ers the openings at the catheter tip. This obstruction can lead to incorrect pressure readings, especially when the Port Dome Unbonded Wire Strain Gauges Unbonded Wire Strain Gauges Vent Vent Housing Lead Wires Diaphragm Diaphragm Fluid-Filled Chamber Catheter Port Flush Port Silicon Chip Port ome Unb Wir Ga nt Housing L Diaph Fluid-F Chamb Vent Diaphragm Catheter Port Flush Port Silicon Chip Strain Gauge (a) (b) FIGURE 3 Pressure sensors used to measure blood pressure: (a) unbonded fine-wire strain gauge sensor and (b) semiconductor strain-gauge-based sensor. Brachial Artery Fluid-Filled Catheter in Artery Flush Line Pressure Sensor FIGURE 4 The direct measurement of blood pressure. FIGURE 2 The fundamental idea of a pressure sensor. (a) When there is no pressure difference between the interior of the pressure sensor and the exterior, there is no displacement of the diaphragm, (b) but when the internal pressure is higher than the external pressure, the diaphragm is displaced proportional to the pressure difference. This can be sensed by the displacement sensor. P0 P > P0 Diaphragm Displacement Sensor Displacement Sensor (a) (b) External Pressure = P0 Thepressure differencebetween thesystolicand diastolicblood pressuresisknownas thepulsepressure.
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  MARCH/APRIL 2011 ▼IEEE PULSE 41 pressure varies over time, as is gener- ally the case while measuring the arte- rial blood pressure. When one considers measuring the pressure change, the dia- phragmofthepressuresensorisdisplaced; thus, the fluid must flow either into or out of the pressure sensor as the pressure changes due to diaphragm deflection. In some sensors, such as the semiconductor pressure sensor, the amount of displace- ment of the diaphragm and its size mean that very little fluid flow will be seen, but the much larger unbonded strain-gauge pressure sensor will experience a greater flow into or out of the sensor during pres- sure changes because of its greater size and larger diaphragm. The fluid flowing into the pressure sensor flows along the catheter, but the catheter, being a rela- tively small-diameter tube, offers some resistance to this flow. Furthermore, the fluid in the catheter has mass and, there- fore, will have inertia. If the catheter is made of a flexible, elastic material, it will have some compliance and, certainly, the pressure sensor itself with the mov- able diaphragm will have compliance. By compliance, we mean that a change in pressure will result in a change in volume. If we recall electrical analogs to fluid mechanical variables, fluid resistance, such as that seen in the catheter, can be represented by an electrical resistor, the inertia of the fluid in the catheter can be represented by an electrical inductor, and the compliance of the catheter and pres- sure sensor can be represented by an elec- trical capacitor. Thus, in very simplified terms, the catheter–pressure sensor com- bination can be represented by the elec- trical circuit shown in Figure 5. The fig- ure shows that the catheter and pressure sensor behave as a second-order system and, in this case, will act like a low-pass filter with a resonant peak near the cutoff frequency. One can also see that the catheter and sensor factors that af- fect the components of the circuit will change the frequency response of the system. A smaller diameter catheter will lower the cutoff frequency when com- pared with a larger diameter catheter. Similarly, a longer catheter will have a lower cutoff frequency than a shorter one, and the larger and more compliant the pressure sensor, the observed cutoff frequency will be lower. In practice, with the unbonded strain-gauge pressure sen- sor, typical cutoff frequencies seen with cardiac catheters range from 20 to 100 Hz. One can see that it is possible to in- troduce frequency distortion into the blood-pressure waveform with this type of instrumentation. On the other hand, if the semicon- ductor pressure sensor is used, its compli- ance is much lower than that of the un- bonded strain-gauge pressure sensor, and the capacitor in the circuit, as shown Fig- ure 5, will be much smaller. This means that the resonant and low-pass cutoff frequency will be much higher and well out of the frequency range of blood- pressure signals. The problems associated with the fluid-filled catheter and the external pressure sensor can be greatly reduced by moving the pressure sensor to the distal tip of the catheter as opposed to having it at the proximal end. This could not be done with the unbonded strain- gauge sensor, but semiconductor pres- sure sensors can be made sufficiently small that they can be easily placed at the tip of a catheter measuring 1–2 mm in diameter. In this case, there is either no fluid or a very short fluid column, and the semiconductor pressure sensor has a very low compliance. Thus, the low-pass cutoff frequency of such a sys- tem is generally of the order of 10–100 kHz, well beyond the frequencies con- tained in the blood-pressure signal. To- day, semiconductor pressure sensors can be batch produced at a relatively low per-unit cost, and catheters contain- ing such sensors can be relatively inex- pensive compared with what they were several years ago. This means that these devices can be used as single-use, dis- posable medical devices and can help to avoid cross-patient contamination. Indirect Measurement of Blood Pressure The primary limitation of the direct method of blood-pressure measurement is that one must invade the body for the measurement. This can put the individ- ual on whom the pressure is measured at risk of hemorrhage due to perforation secondary to catheter placement, it in- troduces a foreign material into the body, and there is always a risk of infection or blood clotting and emboli production. Furthermore, discomfort and anxiety of the patient or study subject associ- ated with the placement of the catheter or sensor could raise the blood pressure, giving readings that are not truly rep- resentative of the patient’s or subject’s condition before placement. Although these risks are outweighed by the ben- efits of having a continuous measure of blood pressure for patients undergoing intensive care, and in some research situations, this is clearly not an accept- able approach for the routine assessment of blood pressure as a screening proce- dure, as a part of a clinic visit, or even at home. Indirect methods of measuring blood pressure were developed to get rid of these problems. Today, the principal method of indi- rect blood-pressure measurement in use is the Riva-Rocci method using an exter- nal cuff to apply a known pressure to a limb, usually an arm. The cuff contains a compliant rubber bladder that is con- nected to a source of air pressure, most often a rubber bulb, and a pressure gauge that measures the pressure in the rubber bladder within the cuff. The cuff is placed around an arm, as shown in Figure 6, such that its outer surface is tight and Catheter Flow Resistance Catheter Fluid Inertia Catheter and Sensor Compliance Catheter and Sensor Compliance FIGURE 5 Simplified equivalent circuit for the fluid-filled catheter and pressure sensor used for direct blood-pressure measurement. FIGURE 6 The indirect measurement of blood pressure in the arm. (Photo courtesy of Shutterstock.)
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  42 IEEE PULSE▼ MARCH/APRIL 2011 cannot be stretched, but the inner surface and, hence, the bladder lie against the arm without any tension in the cuff material in contact with the skin or, for that mat- ter, the skin itself. The cuff and bladder should be designed in such a way that the portion of the cuff containing the bladder covers most of the circumference of the arm so that it will apply a uniform pres- sure against the limb. As air is introduced into the bladder, it will expand, but since it cannot expand outward due to the limi- tation of the outer cuff fabric, the bladder and cuff must expand against the arm. This exerts a pressure on the arm that is transmitted through the soft tissue of the arm, and under ideal circumstances, the pressure in the tissue of the arm under the central portion of the cuff should be the same as the air pressure in the cuff. Everything within the soft tissue of the arm under the central portion of the cuff should experience this pressure; thus, if the cuff is placed around the brachial (up- per) part of the arm, the brachial artery (the major artery of the upper arm) will also see this pressure. If this pressure is greater than the systolic blood pressure, it will cause the brachial artery to collapse under the cuff, and no blood will flow distal to the cuff. If the pressure in the cuff and, hence, the arm under the cuff is lower than the diastolic pressure, then there should be no interruption of blood flow in the brachial artery since the blood pressure will always be greater than the pressure in the tissue under the cuff. When the pressure in the cuff is between systolic and diastolic blood pressure, there will be a portion of the cardiac cycle when the cuff pressure is greater than the blood pressure, and this will cause the brachial artery to collapse and stop the blood flow. When the arterial blood pressure increas- es above the cuff pressure, the blood pres- sure will open the brachial artery, and a bolus of blood will flow through it until, following systole, the ar- terial blood pressure again drops below the cuff pres- sure and the artery col- lapses. Thus, it can be seen that if one has some way of detecting flow of blood distal to the cuff, it should be possible to identify cuff pressures corresponding to systolic and diastolic blood pressures. There are several ways for doing this. The simplest way is to listen to the sounds in the brachial artery as it opens and collapses, and a bolus of blood passes down the arm (Figure 7). This can be done quite easily with a stethoscope, and some electronic blood- pressure instruments use a microphone in the cuff positioned over the brachial artery to detect these sounds electroni- cally. These sounds are known as Korot- koff sounds, and they actually change in character as the cuff pressure slowly drops from systolic to diastolic pressure. You can hear an example of Korotkoff sounds at the Web site: http://www.thin- klabsmedical.com/stethoscope_community/ Sound_Library at the bottom of the Web page. For the purpose of this tutorial, we will only consider whether the Ko- rotkoff sounds are present or not rather than examining the changing charac- ters of these sounds as the cuff-pressure changes. If we manually raise the pres- sure in the cuff to a level that we know will be greater than the systolic blood pressure and slowly reduce the pressure in the cuff while listening over the bra- chial artery with a stethoscope, when the cuff pressure goes below systolic blood pressure, we will begin to hear Korotkoff sounds, one thump per heart- beat. As the cuff pressure continues to reduce, the intensity of the sounds will increase, and then, as the pressure con- tinues to drop, the sounds will become more muffled and ultimately disappear altogether once the cuff pressure de- scends through diastolic blood pressure. Thus, by listening to the sounds and ob- serving the pressure in the cuff, one can determine systolic and diastolic blood pressure without actually contacting the blood. There can be a number of sources of errors when this tech- nique is used. First, the cuff-arm system is not a closed entity. The cuff is open at each end, and the arm extends well beyond the edges of the cuff. Thus, to assume that the pres- sure in the soft tissue of the arm is the same as that in the cuff violates some physical principles. Nev- ertheless, if the cuff is wide enough, the portion of the arm under the central por- tion of the cuff can have the same pres- sure as in the cuff because the soft tissue in the arm behaves more like a gel than as a liquid. The American Heart Associa- tion has established a set of standard cuff sizes based upon the circumference of the arm or thigh to give pressures within the central part of the arm under the cuff that are equivalent to cuff pressures (Table 1). Another source of error can arise when the stethoscope or microphone is not properly placed and only the loudest of the Korotkoff sounds gets picked up. A Systolic Pressure Diastolic Pressure Time 140 120 100 80 60 40 20 CuffPressure(mmHg)Korotkoff Sounds FIGURE 7 A schematic view of the production of Korotkoff sounds as the pressure in the cuff is reduced through systolic and diastolic pressures. TheAmericanHeart Associationhas establishedasetof standardcuffsizes baseduponthe circumferenceofthe armorthigh
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  MARCH/APRIL 2011 ▼IEEE PULSE 43 similar error can occur when the individ- ual listening for the sounds has a hearing impairment. In either case, this may yield a systolic blood pressure that is lower than the true value and a diastolic blood pres- sure that is higher. An error that the author has observed several times as a patient is that the person taking the blood pressure is in a hurry and reduces the pressure in the cuff too quickly. According to the American Heart Association’s recommendations, the cuff pressure should not be reduced at a rate greater than 3 mmHg/heartbeat. Reduc- ing the cuff pressure quickly can result in underdeterming systolic and diastolic blood pressures. A second method for the indi- rect measurement of blood pressure is known as the oscillometric method. The setup is the same as for the previous method, but the detection of systolic and diastolic pressures is different. In- stead of using a microphone, pressure oscillations in the cuff itself are mea- sured. In Figure 7, we can view the cuff pressure as it drops from its initial value above systolic pressure to well below the diastolic pressure. Figure 7 shows the cuff pressure plotted as a function of time. If one observes closely, one can see slight variations in the cuff pressure when it lies between the systolic and diastolic pressures. This is due to the slight change in the volume of the limb as a bolus of blood moves through the artery under the cuff. It is known that when the cuff pressure is greater than the arterial pressure the artery under the cuff collapses; however, when the arterial pressure is greater than the cuff pressure, the blood forces the artery to open and increase its volume. Thus, the limb under the cuff will increase in volume by this amount, so the volume of the cuff will have to decrease by the same amount. This slight decrease in the volume of the cuff causes the pres- sure in the cuff to increase by a small amount, since there is no change in the quantity of air in the cuff. Once the arterial pressure drops below the cuff pressure, blood is squeezed out of the artery, causing it to collapse and thereby reducing the volume of the limb under the cuff to its original value. This results in an increase in volume of the air in the cuff and a corresponding decrease in pressure. Unlike the Korotkoff sounds, the pressure pulsations in the cuff do not start exactly at the point where the cuff pressure falls below the systolic pressure nor do they disappear when the cuff pressure drops below the diastolic pres- sure. In the former case, because the pressure in the limb is not uniform under the cuff, being lower at the edges of the cuff than it is under the cuff’s center, the pressure under the center of the cuff can be above the systolic pressure and block arterial blood flow when the pressure under the cuff near its edges is lower due to the limb being an open system. Thus, arterial blood can open that portion of the artery under the edge regions and in- crease limb volume, resulting in a small decrease in cuff volume, both to a lesser extent than when the cuff pressure is less than the systolic pressure. As shown in Figure 8, the pulsations in cuff pressure begin at higher pressures than systolic pressure, but they increase at a slower rate as the cuff pressure drops until sys- tolic pressure is reached, at which point their amplitude increases more rapidly with each heart beat. Similarly, as the cuff pressure drops below the diastolic pressure, the pressure pulsations in the cuff do not completely disappear. Since the artery remains open throughout the cardiac cycle, its diam- eter will change as the instantaneous pressure goes from diastolic to systolic and back to diastolic and so on. This is the basis of the pulse that one can feel over a superficial artery. This also means that as the limb volume changes by a small amount over the cardiac cycle, the cuff volume will also change, yield- ing pressure variations in the cuff. These pressure variations will have constant amplitude as the cuff pressure contin- ues to drop below the diastolic pres- sure, whereas at pressures in the region above diastolic pressure, the amplitude of these pulsations in cuff pressure drops FIGURE 8 A schematic view of cuff-pressure pulsations during the oscillometric measurement of blood pressure. Systolic Pressure Diastolic Pressure Mean Arterial Pressure Time 140 120 100 80 60 40 20 CuffPressure(mmHg)CuffPressure Pulsations Systolic Pressure Diastolic Pressure Mean Arterial Pressure TABLE 1. STANDARD CUFF SIZES (AMERICAN HEART ASSOCIATION). Cuff Cuff Width (cm) Bladder Length (cm) Arm Circumference (cm) Newborn 3 6 < 6 Infant 5 15 6 – 15 Child 8 21 16 – 21 Small adult 10 24 22 – 26 Adult 13 30 27 – 34 Large adult 16 38 35 – 44 Adult thigh 20 42 45 – 52
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  44 IEEE PULSE▼ MARCH/APRIL 2011 as the cuff pressure approaches diastolic pressure. Oscillometric blood-pressure instruments use these features of cuff pulse amplitude, as shown in Figure 8, to identify systolic and diastolic pressures. Various manufacturers use different features of the pulse amplitude changes near the systolic and diastolic pressures to locate these values. There is another characteristic of the blood pressure that the oscillometric method can measure, i.e., the mean arteri- al pressure. It turns out that when the limb pressure is equal to the mean pressure in the artery under the cuff, the artery will undergo the greatest change in volume between systole and diastole. This will re- sult in the greatest variation in cuff pres- sure and, hence, the greatest amplitude in cuff-pressure pulsations, which can be easily detected electronically. Thus, oscil- lometric blood-pressure instruments give mean arterial pressure as well as systolic and diastolic pressures. Other Methods of Indirect Blood-Pressure Measurement There are other indirect methods of measuring blood pressure, most of which are still under in- vestigation. It is known that the velocity of prop- agation of the systolic pressure pulse in an artery is affected by the elasticity of the arterial walls and the systolic blood pressure. Some investigators have used this fact to con- tinuously monitor changes in systolic blood pressure over short intervals on a beat-to-beat basis. Since the relation- ship between pulse propagation time and systolic blood pressure varies from one individual and one location on that individual to the next, it is neces- sary to first calibrate the measurement for the individual patient and the loca- tion where the propagation time will be measured on that individual. Blood ves- sel elasticity is unlikely to change over the short intervals, so, once calibrated, the pulse propagation velocity should be related to blood pressure. A second indirect method used to continuously monitor blood pressure in the finger of patients or study subjects is the vascular unloading technique. In this case, a cufflike structure in a solid container covers an entire finger such that, unlike the Riva-Rocci cuff, the sys- tem is open only on one end. The cuff is connected to a rapidly responding pres- sure source controlled by a servosystem that is designed to maintain the cuff and, hence, the finger at a constant volume throughout the cardiac cycle. As dis- cussed earlier for the indirect measure of blood pressure, during systole, the blood pressure increases, and normally, more blood enters the tissue of the fin- ger. Without the cuff, this would mean that the finger volume increases slightly during systole and then decreases dur- ing diastole. In this case, the servosystem prevents the finger volume to increase by increasing the pressure in the cuff. Since the volume of the finger neither increases nor decreases when this servosystem operates, the pressure in the cuff must equal the pressure in the finger, which will be de- termined by the blood pressure. A pressure sen- sor measures the pressure in the cuff and provides a continuous waveform of the finger blood pres- sure. It must be noted that physiologists and engineers have demonstrated that the blood-pressure waveform changes as one moves further out toward the periphery of the body. Thus, the blood- pressure waveform of the finger is differ- ent from that of the upper arm. Vascular tonometry is another method that is used to experimentally show the blood-pressure waveform over time. In this case, the pressure waveform needs to be measured from a peripheral artery that is located near the skin surface. Often, the temporal artery on the side of the head has been used because of the solid structure of the skull behind it. A miniature pres- sure sensor or a linear array of small pressure sensors are placed against the skin over the artery. A force is applied by pressing these sensors into the skin. The sensor that is over the temporal artery will partially flatten it, and so the circumferential tension in the arte- rial wall will be normal to the pressure against this arterial wall segment that is trying to return the arterial wall to a circular cross section. The force that the blood pressure applies to the arte- rial wall is opposed by the force ap- plied by the pressure sensor, keeping this segment of arterial wall flattened. Thus, the pressure seen by this pres- sure sensor over the artery should be the same as the blood pressure within the artery. Many assumptions were made for this to be the case, but several years ago, investigators at NASA were able to demonstrate this technique of noninvasively obtaining the blood- pressure waveform. With this tutorial, we have complet- ed our series of examining methods for measuring the four clinical vital signs: temperature, heart rate, breathing rate, and blood pressure. There are many ad- ditional measurements that are made in clinical medicine and the research labo- ratory today, and the interested reader is referred to biomedical engineering handbooks and textbooks on biomedical instrumentation for more details. A few of these are listed below. For Further Reading [1] L. A. Geddes, The Direct and Indirect Measurement of Blood Pressure. Chica- go: Year Book, 1970. [2] J. G. Webster, Ed., Encyclopedia of Medi- cal Devices and Instrumentation, 2nd ed. New York: Wiley, 2006. [3] R. B. Northrup, Noninvasive Instrumen- tation and Measurement in Medical Diag- nosis. Boca Raton, FL: CRC Press, 2002. [4] T. Togawa, T. Tamura, and P. A. Oberg, Biomedical Transducers and Instru- ments. Boca Raton, FL: CRC Press, 1997. [5] J. G. Webster, Ed., Medical Instrumen- tation: Application and Design, 4th ed. New York: Wiley, 2010. [6] J. D. Bronzino, Ed., The Biomedical En- gineering Handbook, 3rd ed., Boca Ra- ton, FL: CRC Press, 2006. Asecondindirect methodusedto continuouslymonitor bloodpressureinthe fingerofpatientsor studysubjectsisthe vascularunloading technique.