Med. & Biol. Eng. & Comput., 1979, 17, 11-18
Noninvasive automatic determination of mean arterial pressure Maynard
Rarnsey III
Director of Research, Applied Medical Research Corporation, 5041 West Cypress Street, Tampa, Florida 33607, and Clinical Associate Professor of Paediatrics, University of South Florida, College of Medicine, Tampa, Florida 33620
new instrument for the indirect noninvasive measurement of mean arterial pressure (m.a.p.) has been constructed and evaluated in man, The instrument does not require an external microphone or transducer and determines m.a.p, rather than systolic and diastolic pressure. Instead, the method employs the point of maximal oscillations as an indicator of m.a.p. The instrument automatically inflates a standard blood pressure cuff and determines the m.a.p, by measuring the cuff pressure oscillations as the cuff pressure is reduced by discrete increments. Cuff deflation in discrete increments, instead of continuously, allows the oscillation data obtained at each cuff pressure to be tested for artefacts and averaged, greatly enhancing artefact-rejection ability, The m.a.p, is selected as the lowest cuff pressure at which the oscillation amplitude is a maximum. The instrument was tested on the bicep and ankle in a series of 28 studies involving 17 human subjects with intra-arterial catheters. Averaging the mean errors from each of the 28 studies, there was an overall mean error of --O.23 mmHg, with a standard deviation of 4.21 mmHg. The correlation coefficient was 0.98. The instrument was found to give good results in a wide variety of clinical subjects and physiologic states.
Abstraet--A
Keywords--Blood-pressure monitoring, Mean arterial pressure, Noninvasive techniques
1 Introduction THE measurement of blood pressure (b.p.) is an important part of the monitoring procedure for all acute patients and during surgery. In the majority of these patients a noninvasive, indirect method must be used. The auscultatory measurement of systolic (s.p.) and diastolic (d.p.) pressure in the normal range of pressures and in the normal circulatory state presents few difficulties, although its accuracy has been found to be poor in some situations (VAN BERGEN et aL, 1954; COrIN, 1967; SIMPSON et al., COLLINS and MAGORA, 1963). For example, in abnormal pressure ranges, particularly in shock, the accuracy of the auscultatory method is often unacceptable, if indeed a determination can be made (VAN BERGEN et al., 1954; COHN, 1967). The range over which indirect blood pressures can be measured can be extended by the use of ultrasound (KIRBY et al., 1969; BtJGGS et al., 1973). However, this technique requires an ultrasonic transducer, coupling jelly and relatively accurate transducer positioning over the underlying artery in order to obtain satisfactory results. First received22nd July 1977 and in final form 4th February 1978
0140-0118/79/010011 + 08 $01.50/0 ~) IFMBE: 1979 Medical & Biological Engineering & Computing
It is well known that the oscillometric method can be used to estimate the systolic pressure with reasonable accuracy (VAN BERGEN et al., 1954). Not as well known, however, are the studies in animals that have shown that an oscillometric technique can also be used to measure the mean arterial pressure (m.a.p.) (PosEr et al., 1969; GEDDES et al., 1977). These animal studies show that the lowest cuff pressure that gives maximum oscillations in the cuff pressure correlates closely with the mean arterial pressure. It should be remembered that the mean arterial pressure is not the arithmetic average of the s.p. and d.p., but is the average with respect to time of the total arterial pressure waveform. It is sometimes approximated in the brachial artery as d . p . + l / 3 (s.p.-d.p.). This paper describes the operation and evaluation of a new noninvasive automatic instrument for measuring m.a.p.* 2 Materials and methods 2.1 Equipment The design and operation of the instrument can best be understood by reference to Fig. 1. The *DINAMAP, Applied Medical Research Tampa, Florida 33607
January 1979
11
major components of the instrument are a 4040 microprocessor central processing unit (c.p.u.), which controls the entire operation of the device, an air pump to inflate the cuff, a bleed valve to deflate the cuff in discrete increments of pressure, a pressure transducer and an overpressure switch to prevent the pressure in the system from exceeding 300 mmHg at any time.
EXTERNAL
and delay before the next determination. After the pressure has been determined, the c.p.u, displays the m.a.p, and then checks to see if it is within the high and low alarm limits that have been set by the user. Should it be beyond these limits, the c.p.u. then activates an alarm for 6 s before proceeding with the delay which has been specified by the user. This delay is established by setting the delay switches
INTERNAL
TIME OSCILLATIONS
/ TIME
U ~I I i
,, ,-, ,-,1 DISPLAY
Fig. 1 Schematic of major parts that comprise the device. In the upper right-hand corner of the Figure are shown schematically the waveforms that result during a single determination
The cuff is inflated through one tube from the pump; the pressure is sensed through another tube which leads from the cuff to the pressure transducer within the instrument. The electronic signal from the pressure transducer is used in two ways: (a) after suitable scaling, it is used by the c.p.u, for measuring the actual pressure in the cuff to control inflation and deflation (b) after signal processing, the pulsatile component is used by the central processor for determining the amplitude of the cuff pressure oscillations (Fig. 2). The mean pressure is chosen as the lowest cuff pressure at which the oscillations are a maximum. The instrument repeatedly measures the m.a.p, at user-selected cycle times. Each cycle consists of the cuff inflation, pressure determination, cuff deflation
12
and can range between 1 rain and 8-5 rain in 0-5 rain increments. At the end of the delay, the system automatically measures and compensates for any drift in the transducer zero, before a new pressure determination is made. Motion artefact is rejected by requiring two successive oscillations of almost equal amplitude and by constraining the rise time and maximum slope of the oscillations to be within allowable limits. When all of these criteria are met, the two oscillation amplitudes are averaged and the cuff pressure is reduced in increments of 3 to 6 mmHg. This process is repeated until an oscillation maximum is found and then five more cycles of deflating the cuff an increment of pressure and measuring the oscillation amplitude are executed to be certain that it is the true maximum that has been found and not a premature maximum.
Medical & Biological Engineering & Computing
January 1979
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3 Evaluation: methods and materials The instrument was used to measure m.a.p, in 28 studies involving 17 different patients. The distribution relative to age, sex, location of b.p. cuff and clinical circumstance is given in Table 1. All patients were selected because they already had an indwelling arterial catheter for routine care. 23 studies were conducted on males and five on females with the subjects ranging in age from 2 to 75 years with a mean of age 51. The appropriate standard cuff size for the extremity monitored was chosen by using the index marks on the adult and child-size calibrated V-Lok cuffs.t The child cuff was used in three studies and the adult cuff in 25 studies. There were 14 studies in which the cuff was used on the bicep, 13 on the ankle and one on the calf, the latter being on a patient with an extremely thin ankle and calf. Multiple readings were obtained in each patient with a range of 7 to 73 readings per study, the mean number of readings per study being 21. For each indirect m.a.p, obtained, a direct m.a.p, was measured from the indwelling cannula in either the radial or brachial artery. The intra-arterial reading was generally obtained immediately after completion of the indirect reading. The m.a.p, was measured by hydraulic or electronic damping of the intraarterial waveform. Since the only pressure of interest was the m.a.p., the frequency response of the catheter and transducer system was not of great concern. However, care was taken to assure a proper calibration of the intra-arterial system, and, where necessary, corrections for the hydrostatic pressure
f
Fig. 2 Upper trace shows pressure in cuff as it is deflated in discrete increments during a determination. Middle trace shows oscillations in cuff pressure during determination: the upward deflections on this trace represent the oscillations and the downward deflections represent the artefact caused by the incremental cuff deflation. Notic~ that in the oscillation trace, before a deflation occurs, two consecutive oscillations must be approximately the same amplitude. The mean pressure is chosen as the minimum cuff pressure at which the oscillations are maximum, which is the sixth group of oscillations counting from the right. Bottom trace is time in seconds 1"Manufactured by W. A. Baum Company
Medical & Biological Engineering & Computing
difference in the monitored locations were made. For instance, if the intra-arterial pressure transducer could not be positioned at the same level as the monitored extremity, this difference in level was corrected. Most of the readings were obtained at roughly 1 min intervals, but occasionally it was necessary to suspend the study temporarily while the clinical staff manipulated the patient or when the patient was too active. 4 Results The results of this series of studies are presented in Table 1 and Figs. 3 and 4. Table 1 presents the average results, as well as some descriptive information, for each of the 28 studies. Study number 6 had the highest m.a.p, with a range of 102 to 130. Study number 27 had the lowest m.a.p, with a range of 39 to 54. For each study, the difference between the data obtained directly (d.m.a.p.) and indirectly (i.m.a.p.) was calculated for each reading and then the mean and standard deviation of this difference was found. As can be seen in the Table, study number 8 had the highest mean difference (8-00+ 4.92 mmHg) and study number 9 had the lowest mean difference (0"05 + 3-36 mmHg). The scatter in the difference between the d.m.a.p. and the i.m.a.p, values in each individual study can be deduced from the standard deviation. The largest standard deviation (s.d.) occurred in study number 11, where s.d. = 7.68 mmHg, and the lowest in study number 6, where s.d. = 2.93 mmHg. The mean percentage difference between d.m.a.p. and i.m.a.p, for each of the individual studies is presented as well as the standard deviation of that percentage difference. This standard deviation is a good measure of the percentage error to be expected in a given indirect determination of m.a.p, using this device. Studies marked with a dagger indicate that there was a single direct-indirect measurement pair excluded from analysis. This was done because the difference between the direct and indirect readings was of such magnitude as to indicate unnoticed movement artefact caused by either patient motion or the clinical personnel. At the bottom of Table 1 there are summary statistics. The average mean difference between the direct and indirect measurement was - 0 - 2 3 mmHg and the standard deviation of that mean is 4"21 mmHg. This mean, using Students 't-test', shows no significant difference from zero. Linear regression of d.m.a.p, on i.m.a.p, gives the regression equation d.m.a.p. = 0.979 i.m.a.p. + 1.608 with a standard error of 3"096 and a correlation coefficient of 0.984. This regression equation is obtained by using the mean d.m.a.p, and i.m.a.p. values from each patient and assigning equal weight to them. This regression equation and the line of identity are plotted in Fig. 3. Superimposed on January 1979
15
often fails (COHN, 1967). However, since the instrument described herein does not measure a flow dependent parameter, but rather oscillations in cuff pressure, this method seems better able to make an accurate reading in such difficult circumstances than the auscultatory method. This ability to function reliably in shock is additionally enhanced because the instrument chooses as its end-point the cuff pressure at which the oscillations are maximum, and not the appearance and disappearance of Korotkoff sounds as in the auscultatory method. Thus, in shock, when the amplitude of the oscillations is decreased, the maximum is still usually measurable. In our experience, the instrument rarely fails to function on either the bicep or on the upper leg, just proximal to the knee, even in shock patients. Since the instrument uses the oscillations in cuff pressure in determining the b.p., there is no external microphone or ultrasound transducer to be applied over an artery. Thus, the application of the cuff is somewhat less critical than with other methods measuring blood pressure,and,indeed, the cuff can be applied to any extremity, including the ankle, which
those lines are the mean values for each study, bracketed by plus and minus one standard deviation of the mean difference between the d.m.a.p, and i.m.a.p, values. Fig. 4 presents graphically the d.m.a.p, and i.m.a.p, measurements from three different studies and illustrates the ability of the instrument to track pressure in these different pressure ranges. 5 Discussion The data presented here show that the instrument performs acceptably over a wide range of subject types and over a wide range of m.a.p. Only those measurements for which there were corresponding intra-arterial measurements are presented, but the instrument has been used clinically in many patients that did not have arterial catheters. The lowest readings obtained have been in the low 20s and the highest readings obtained in the 180 to 190 range. The instrument has been used on patients ranging in weight from 4.6 kg to 110 kg. In low blood flow states, such as in cardiovascular shock, the auscultatory method of measuring b.p.
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Medical
& Biological Engineering
& Computing
January 1979
patients find very comfortable. Also, since there are no wires or Sensors in the cuff, the instrument is less susceptible to damage and provides no electrical hazard. The instrument is housed in a rugged aluminium cabinet measuring 10.8 x 28.6 x 28.6 mm and is easily integrated into critical care and surgical areas. N o cross interference with other electronic devices has been noted.
measuring cardiac output, it also permits the computation of cardiovascular parameters such as peripheral resistance, without invading the systemic arterial system.
6 Summary An instrument for the automatic and noninvasive determination of m.a.p, has been designed, con-
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Despite the advantages enumerated above, the strategy that this instrument uses to determine b.p. has two disadvantages. First, because it pauses at a given pressure until a 'match' is found, it takes longer to make a reading should there be any motion artefact. This implies that the time necessary for a determination is not constant. However, in practice, most readings are completed within 40 s. Secondly, since before the development of this instrument m.a.p, could only be measured directly, many are not accustomed to using m.a.p, clinically. The use of this instrument may, therefore, require a period of familiarisation for those not accustomed to using m.a.p. The ability to noninvasively and automatically determine the m.a.p, permits the use and accurate dosage titration of antihypertensive agents such as sodium nitro-prusside and inotropic agents such as dopamine, without the use of an intra-arterial catheter in many patients. When coupled with the use of Swan-Ganz type thermodilution catheters for Medical & Biological Engineering & Computing
structed and evaluated clinically. The results show that the instrument measures m.a.p, with acceptable accuracy in a variety of clinical subjects with normal, high and low blood pressures.
References BUGGS, H., JOHNSON, P. E., GORDON, L. S., BALGUMA, F. B. and WET-rACrI, G. E. (1973) Comparison of systolic arterial blood pressure by transcutaneous doppler probe and conventional methods in hypotensive patients. Anesthesia & Analgesia, 52, 776-778. COliN, J. N. (1967) Blood pressure measurement in shock. J. A M , I , 199, 118-122. COLLINS, V. J. and MA6ORA, F. (1963) Sphygmomanometry: the indirect measurement of blood pressure. Anesthesia & Analgesia, 42, 443-452. GEDDES,L. A , MOORE,A. G., GARNER,H., ROSaOROUGH, J., Ross, J. and AMEND,J. (1970) The indirect measurement of mean blood pressure in the horse. The Southwestern Veterinarian, 23, 289-294. J a n u a r y 1979
17
GEDDES, L. A., CHAFFEE,V., WHISTLER,S. J., BOUKLAND, J. D. and TACKER, W. A. (I977) Indirect mean blood pressure on the anesthetized pony. Amer. J. Vet. Res., 38, 2055-2057. KIRBY, R. R., KEMMERER, W. T. and MORGAN, J. L. (1969) Transcutaneous doppler measurement of blood pressure. Anesthesiology, 31, 86-89. POSEr, J. A., GEDD~S, L. A., WILLIAMS,H. and MOORE, A. G. (1969) The meaning of the point of maximum oscillations in cuff pressure in the indirect measurement
18
of blood pressure. Cardiovascular 2~esearch Center Bull., 8, 15-25. SIMPSON, J. A., JAMtESON, G., DICKHAUS, D. W. and GROVER, R. F. (1965) Effect of size of cuff bladder on accuracy of measurement of indirect blood pressure. Amer. Heart J., 70, 208-215. VAN BERGEN, F. H., WEATHERHEAD, D. S., TRELOAR, A. E., DOBKTN, A. B. and BVCKLEY, J. J. (1954) Comparison of indirect and direct methods of measuring arterial blood pressure. Circulation, 10, 481-490.
Medical & Biological Engineering & Computing
January 1979
Noninvasive automatic determination of mean arterial pressure Maynard
Rarnsey III
Director of Research, Applied Medical Research Corporation, 5041 West Cypress Street, Tampa, Florida 33607, and Clinical Associate Professor of Paediatrics, University of South Florida, College of Medicine, Tampa, Florida 33620
new instrument for the indirect noninvasive measurement of mean arterial pressure (m.a.p.) has been constructed and evaluated in man, The instrument does not require an external microphone or transducer and determines m.a.p, rather than systolic and diastolic pressure. Instead, the method employs the point of maximal oscillations as an indicator of m.a.p. The instrument automatically inflates a standard blood pressure cuff and determines the m.a.p, by measuring the cuff pressure oscillations as the cuff pressure is reduced by discrete increments. Cuff deflation in discrete increments, instead of continuously, allows the oscillation data obtained at each cuff pressure to be tested for artefacts and averaged, greatly enhancing artefact-rejection ability, The m.a.p, is selected as the lowest cuff pressure at which the oscillation amplitude is a maximum. The instrument was tested on the bicep and ankle in a series of 28 studies involving 17 human subjects with intra-arterial catheters. Averaging the mean errors from each of the 28 studies, there was an overall mean error of --O.23 mmHg, with a standard deviation of 4.21 mmHg. The correlation coefficient was 0.98. The instrument was found to give good results in a wide variety of clinical subjects and physiologic states.
Abstraet--A
Keywords--Blood-pressure monitoring, Mean arterial pressure, Noninvasive techniques
1 Introduction THE measurement of blood pressure (b.p.) is an important part of the monitoring procedure for all acute patients and during surgery. In the majority of these patients a noninvasive, indirect method must be used. The auscultatory measurement of systolic (s.p.) and diastolic (d.p.) pressure in the normal range of pressures and in the normal circulatory state presents few difficulties, although its accuracy has been found to be poor in some situations (VAN BERGEN et aL, 1954; COrIN, 1967; SIMPSON et al., COLLINS and MAGORA, 1963). For example, in abnormal pressure ranges, particularly in shock, the accuracy of the auscultatory method is often unacceptable, if indeed a determination can be made (VAN BERGEN et al., 1954; COHN, 1967). The range over which indirect blood pressures can be measured can be extended by the use of ultrasound (KIRBY et al., 1969; BtJGGS et al., 1973). However, this technique requires an ultrasonic transducer, coupling jelly and relatively accurate transducer positioning over the underlying artery in order to obtain satisfactory results. First received22nd July 1977 and in final form 4th February 1978
0140-0118/79/010011 + 08 $01.50/0 ~) IFMBE: 1979 Medical & Biological Engineering & Computing
It is well known that the oscillometric method can be used to estimate the systolic pressure with reasonable accuracy (VAN BERGEN et al., 1954). Not as well known, however, are the studies in animals that have shown that an oscillometric technique can also be used to measure the mean arterial pressure (m.a.p.) (PosEr et al., 1969; GEDDES et al., 1977). These animal studies show that the lowest cuff pressure that gives maximum oscillations in the cuff pressure correlates closely with the mean arterial pressure. It should be remembered that the mean arterial pressure is not the arithmetic average of the s.p. and d.p., but is the average with respect to time of the total arterial pressure waveform. It is sometimes approximated in the brachial artery as d . p . + l / 3 (s.p.-d.p.). This paper describes the operation and evaluation of a new noninvasive automatic instrument for measuring m.a.p.* 2 Materials and methods 2.1 Equipment The design and operation of the instrument can best be understood by reference to Fig. 1. The *DINAMAP, Applied Medical Research Tampa, Florida 33607
January 1979
11
major components of the instrument are a 4040 microprocessor central processing unit (c.p.u.), which controls the entire operation of the device, an air pump to inflate the cuff, a bleed valve to deflate the cuff in discrete increments of pressure, a pressure transducer and an overpressure switch to prevent the pressure in the system from exceeding 300 mmHg at any time.
EXTERNAL
and delay before the next determination. After the pressure has been determined, the c.p.u, displays the m.a.p, and then checks to see if it is within the high and low alarm limits that have been set by the user. Should it be beyond these limits, the c.p.u. then activates an alarm for 6 s before proceeding with the delay which has been specified by the user. This delay is established by setting the delay switches
INTERNAL
TIME OSCILLATIONS
/ TIME
U ~I I i
,, ,-, ,-,1 DISPLAY
Fig. 1 Schematic of major parts that comprise the device. In the upper right-hand corner of the Figure are shown schematically the waveforms that result during a single determination
The cuff is inflated through one tube from the pump; the pressure is sensed through another tube which leads from the cuff to the pressure transducer within the instrument. The electronic signal from the pressure transducer is used in two ways: (a) after suitable scaling, it is used by the c.p.u, for measuring the actual pressure in the cuff to control inflation and deflation (b) after signal processing, the pulsatile component is used by the central processor for determining the amplitude of the cuff pressure oscillations (Fig. 2). The mean pressure is chosen as the lowest cuff pressure at which the oscillations are a maximum. The instrument repeatedly measures the m.a.p, at user-selected cycle times. Each cycle consists of the cuff inflation, pressure determination, cuff deflation
12
and can range between 1 rain and 8-5 rain in 0-5 rain increments. At the end of the delay, the system automatically measures and compensates for any drift in the transducer zero, before a new pressure determination is made. Motion artefact is rejected by requiring two successive oscillations of almost equal amplitude and by constraining the rise time and maximum slope of the oscillations to be within allowable limits. When all of these criteria are met, the two oscillation amplitudes are averaged and the cuff pressure is reduced in increments of 3 to 6 mmHg. This process is repeated until an oscillation maximum is found and then five more cycles of deflating the cuff an increment of pressure and measuring the oscillation amplitude are executed to be certain that it is the true maximum that has been found and not a premature maximum.
Medical & Biological Engineering & Computing
January 1979
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3 Evaluation: methods and materials The instrument was used to measure m.a.p, in 28 studies involving 17 different patients. The distribution relative to age, sex, location of b.p. cuff and clinical circumstance is given in Table 1. All patients were selected because they already had an indwelling arterial catheter for routine care. 23 studies were conducted on males and five on females with the subjects ranging in age from 2 to 75 years with a mean of age 51. The appropriate standard cuff size for the extremity monitored was chosen by using the index marks on the adult and child-size calibrated V-Lok cuffs.t The child cuff was used in three studies and the adult cuff in 25 studies. There were 14 studies in which the cuff was used on the bicep, 13 on the ankle and one on the calf, the latter being on a patient with an extremely thin ankle and calf. Multiple readings were obtained in each patient with a range of 7 to 73 readings per study, the mean number of readings per study being 21. For each indirect m.a.p, obtained, a direct m.a.p, was measured from the indwelling cannula in either the radial or brachial artery. The intra-arterial reading was generally obtained immediately after completion of the indirect reading. The m.a.p, was measured by hydraulic or electronic damping of the intraarterial waveform. Since the only pressure of interest was the m.a.p., the frequency response of the catheter and transducer system was not of great concern. However, care was taken to assure a proper calibration of the intra-arterial system, and, where necessary, corrections for the hydrostatic pressure
f
Fig. 2 Upper trace shows pressure in cuff as it is deflated in discrete increments during a determination. Middle trace shows oscillations in cuff pressure during determination: the upward deflections on this trace represent the oscillations and the downward deflections represent the artefact caused by the incremental cuff deflation. Notic~ that in the oscillation trace, before a deflation occurs, two consecutive oscillations must be approximately the same amplitude. The mean pressure is chosen as the minimum cuff pressure at which the oscillations are maximum, which is the sixth group of oscillations counting from the right. Bottom trace is time in seconds 1"Manufactured by W. A. Baum Company
Medical & Biological Engineering & Computing
difference in the monitored locations were made. For instance, if the intra-arterial pressure transducer could not be positioned at the same level as the monitored extremity, this difference in level was corrected. Most of the readings were obtained at roughly 1 min intervals, but occasionally it was necessary to suspend the study temporarily while the clinical staff manipulated the patient or when the patient was too active. 4 Results The results of this series of studies are presented in Table 1 and Figs. 3 and 4. Table 1 presents the average results, as well as some descriptive information, for each of the 28 studies. Study number 6 had the highest m.a.p, with a range of 102 to 130. Study number 27 had the lowest m.a.p, with a range of 39 to 54. For each study, the difference between the data obtained directly (d.m.a.p.) and indirectly (i.m.a.p.) was calculated for each reading and then the mean and standard deviation of this difference was found. As can be seen in the Table, study number 8 had the highest mean difference (8-00+ 4.92 mmHg) and study number 9 had the lowest mean difference (0"05 + 3-36 mmHg). The scatter in the difference between the d.m.a.p. and the i.m.a.p, values in each individual study can be deduced from the standard deviation. The largest standard deviation (s.d.) occurred in study number 11, where s.d. = 7.68 mmHg, and the lowest in study number 6, where s.d. = 2.93 mmHg. The mean percentage difference between d.m.a.p. and i.m.a.p, for each of the individual studies is presented as well as the standard deviation of that percentage difference. This standard deviation is a good measure of the percentage error to be expected in a given indirect determination of m.a.p, using this device. Studies marked with a dagger indicate that there was a single direct-indirect measurement pair excluded from analysis. This was done because the difference between the direct and indirect readings was of such magnitude as to indicate unnoticed movement artefact caused by either patient motion or the clinical personnel. At the bottom of Table 1 there are summary statistics. The average mean difference between the direct and indirect measurement was - 0 - 2 3 mmHg and the standard deviation of that mean is 4"21 mmHg. This mean, using Students 't-test', shows no significant difference from zero. Linear regression of d.m.a.p, on i.m.a.p, gives the regression equation d.m.a.p. = 0.979 i.m.a.p. + 1.608 with a standard error of 3"096 and a correlation coefficient of 0.984. This regression equation is obtained by using the mean d.m.a.p, and i.m.a.p. values from each patient and assigning equal weight to them. This regression equation and the line of identity are plotted in Fig. 3. Superimposed on January 1979
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often fails (COHN, 1967). However, since the instrument described herein does not measure a flow dependent parameter, but rather oscillations in cuff pressure, this method seems better able to make an accurate reading in such difficult circumstances than the auscultatory method. This ability to function reliably in shock is additionally enhanced because the instrument chooses as its end-point the cuff pressure at which the oscillations are maximum, and not the appearance and disappearance of Korotkoff sounds as in the auscultatory method. Thus, in shock, when the amplitude of the oscillations is decreased, the maximum is still usually measurable. In our experience, the instrument rarely fails to function on either the bicep or on the upper leg, just proximal to the knee, even in shock patients. Since the instrument uses the oscillations in cuff pressure in determining the b.p., there is no external microphone or ultrasound transducer to be applied over an artery. Thus, the application of the cuff is somewhat less critical than with other methods measuring blood pressure,and,indeed, the cuff can be applied to any extremity, including the ankle, which
those lines are the mean values for each study, bracketed by plus and minus one standard deviation of the mean difference between the d.m.a.p, and i.m.a.p, values. Fig. 4 presents graphically the d.m.a.p, and i.m.a.p, measurements from three different studies and illustrates the ability of the instrument to track pressure in these different pressure ranges. 5 Discussion The data presented here show that the instrument performs acceptably over a wide range of subject types and over a wide range of m.a.p. Only those measurements for which there were corresponding intra-arterial measurements are presented, but the instrument has been used clinically in many patients that did not have arterial catheters. The lowest readings obtained have been in the low 20s and the highest readings obtained in the 180 to 190 range. The instrument has been used on patients ranging in weight from 4.6 kg to 110 kg. In low blood flow states, such as in cardiovascular shock, the auscultatory method of measuring b.p.
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Fig. 3 Mean error p/us or minus one standard deviation for each of the 28 studies. The so/id fine is the /ine of identity and the dashed line is the /east squares regression line (d,m.a.p. = 0"979, i.m.a.p.-F 1.608). This regression equation is computed by assigning equal weight to the mean error from each of the individual studies. The correlation coefficient is O. 98 16
Medical
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patients find very comfortable. Also, since there are no wires or Sensors in the cuff, the instrument is less susceptible to damage and provides no electrical hazard. The instrument is housed in a rugged aluminium cabinet measuring 10.8 x 28.6 x 28.6 mm and is easily integrated into critical care and surgical areas. N o cross interference with other electronic devices has been noted.
measuring cardiac output, it also permits the computation of cardiovascular parameters such as peripheral resistance, without invading the systemic arterial system.
6 Summary An instrument for the automatic and noninvasive determination of m.a.p, has been designed, con-
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Direct and indirect pressures are p l o t t e d for three different studies to s h o w the ability o f the device to track mean arterial pressure in different pressure ranges
Despite the advantages enumerated above, the strategy that this instrument uses to determine b.p. has two disadvantages. First, because it pauses at a given pressure until a 'match' is found, it takes longer to make a reading should there be any motion artefact. This implies that the time necessary for a determination is not constant. However, in practice, most readings are completed within 40 s. Secondly, since before the development of this instrument m.a.p, could only be measured directly, many are not accustomed to using m.a.p, clinically. The use of this instrument may, therefore, require a period of familiarisation for those not accustomed to using m.a.p. The ability to noninvasively and automatically determine the m.a.p, permits the use and accurate dosage titration of antihypertensive agents such as sodium nitro-prusside and inotropic agents such as dopamine, without the use of an intra-arterial catheter in many patients. When coupled with the use of Swan-Ganz type thermodilution catheters for Medical & Biological Engineering & Computing
structed and evaluated clinically. The results show that the instrument measures m.a.p, with acceptable accuracy in a variety of clinical subjects with normal, high and low blood pressures.
References BUGGS, H., JOHNSON, P. E., GORDON, L. S., BALGUMA, F. B. and WET-rACrI, G. E. (1973) Comparison of systolic arterial blood pressure by transcutaneous doppler probe and conventional methods in hypotensive patients. Anesthesia & Analgesia, 52, 776-778. COliN, J. N. (1967) Blood pressure measurement in shock. J. A M , I , 199, 118-122. COLLINS, V. J. and MA6ORA, F. (1963) Sphygmomanometry: the indirect measurement of blood pressure. Anesthesia & Analgesia, 42, 443-452. GEDDES,L. A , MOORE,A. G., GARNER,H., ROSaOROUGH, J., Ross, J. and AMEND,J. (1970) The indirect measurement of mean blood pressure in the horse. The Southwestern Veterinarian, 23, 289-294. J a n u a r y 1979
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GEDDES, L. A., CHAFFEE,V., WHISTLER,S. J., BOUKLAND, J. D. and TACKER, W. A. (I977) Indirect mean blood pressure on the anesthetized pony. Amer. J. Vet. Res., 38, 2055-2057. KIRBY, R. R., KEMMERER, W. T. and MORGAN, J. L. (1969) Transcutaneous doppler measurement of blood pressure. Anesthesiology, 31, 86-89. POSEr, J. A., GEDD~S, L. A., WILLIAMS,H. and MOORE, A. G. (1969) The meaning of the point of maximum oscillations in cuff pressure in the indirect measurement
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of blood pressure. Cardiovascular 2~esearch Center Bull., 8, 15-25. SIMPSON, J. A., JAMtESON, G., DICKHAUS, D. W. and GROVER, R. F. (1965) Effect of size of cuff bladder on accuracy of measurement of indirect blood pressure. Amer. Heart J., 70, 208-215. VAN BERGEN, F. H., WEATHERHEAD, D. S., TRELOAR, A. E., DOBKTN, A. B. and BVCKLEY, J. J. (1954) Comparison of indirect and direct methods of measuring arterial blood pressure. Circulation, 10, 481-490.
Medical & Biological Engineering & Computing
January 1979
