July 2, 1963
`
`w. E. TOLLES
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`3,095,872
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`BLOOD PRESSURE MEASUREMENT
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`'
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`Filed Nov. 27, 1959
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`25$
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`A FIG 2
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`Volts —> Pressure
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`Pressure -—:>
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`FIG. 3
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`INVENTOR
`Walter E. Tolles
`BY
`9‘
`
`ATTORNEYS
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`1
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`United States iiatent
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`1
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`ice
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`3,095,872
`Patented July 2., I963
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`of an individual may be expected not. to change marked
`ly in a short time, measurement of the velocity of
`propagation of the heart pulse wave has been proposed
`as a measurement of blood pressure.
`Direct measurement of the time of travel of heart
`pulse waves between two spaced points along an artery
`has been proposed, but is subject to considerable inac
`curacies. The heart pulse wave is quite complex, and
`it is di?icult to select precisely the same point on the
`Wave as it passes two spaced points so as to get an ac
`curate measurement. Also, the outputs of transducers
`used to pick up the heart pulse wave may be expected
`to vary in amplitude due to changes in the intimacy of
`contact with the artery over which they are placed,
`slight displacements thereof, etc., unless the subject is
`immobilized. This further impairs accuracy of meas
`urement.
`Proposals have also been made to apply single impact
`pulses, or a train of impact pulses, to the artery and
`measure the time of travel thereof between spaced points
`along the artery. However, considerable distortion of
`the pulses occurs as they pass along the artery, so that
`measuring dii?culties arise similar to those present in
`measuring the heart pulse waves. Also, changes in the
`intimacy of contact and location of both the transmitting
`and receiving transducers cause amplitude variations,
`thereby impairing measurement accuracy.
`In accordance with the present invention, alternating,
`substantially symmetrical continuous-wave pressure var
`iations are impressed on the arterial blood stream at a
`frequency substantially higher ‘than the frequency of
`hear-t contraction, and variations in the time of travel
`or phase of the continuous-wave pressure varia ions be
`tween points spaced along the arterial blood stream are
`measured. Due to the changing propagation character
`istics of the artery as a function of the blood pressure
`therein, the continuous-wave pressure variations become
`phase-modulated (or frequency-modulated, since frequen
`cy is the time derivative of phase) as a function of the
`blood pressure. In effect, the applied continuous-wave
`pressure variation is a carrier which is phase-modulated
`by the blood pressure variations. Accordingly a signal
`picked up by a suitable transducer mounted over the
`artery may then be amplitude-limited so as to eliminate
`errors due to varying amplitude, While at the same time
`preserving the necessary information as to blood pres
`sure variations. The phase-modulation may then be
`detected to obtain a signal varying with the blood pres
`sure over a heart beat cycle.
`Advantageously, changes in phase of the applied pres
`sure wave between two spaced points along the arterial
`blood stream are measured. With a ?xed-frequency
`applied pressure variation, changes in phase between two
`?xed points are inversely proportional to changes in ve
`locity of propagation, and hence give ‘an indication of
`changes in blood pressure.
`In a preferred embodiment, a transmitting transducer
`is mounted over an artery and energized with a sinusoidal
`wave having ‘a frequency substantially higher than the
`frequency of heart contraction. The transducer pro
`duces corresponding sinusoidal pressure variations in
`the arterial blood stream. These pressure variations are
`advantageously small compared to the normal arterial
`blood pressures. A pair of receiving transducers are
`mounted at spaced points along the arterial blood stream,
`preferably at a su?icient distance from the transmitting
`transducer so that pickup of pressure waves transmitted
`through the ?esh rather than the artery is negligible.
`Accordingly, the signal outputs of the pair of receiving
`transducers will correspond to the pressure variations
`in the arterial blood stream ‘at the respective spaced
`points. A phase detector is then connected to receive‘
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`1
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`3,695,872
`BLOGD PRESSURE MEASUREMENT
`Walter E. Tolles, Oyster Bay, N.Y., ‘assignor to Cutler
`Hammer, Inc, Milwaukee, Wis, a corporation of Dela
`ware
`
`Filed Nov. 27, 1959, Ser. No. 855,699
`9 Claims. ({1}. 123-2415)
`
`This invention relates to a method of and apparatus
`for measuring arterial blood pressure. The invention
`enables blood pressure to be measured continuously,
`and the apparatus is capable of automatic operation.
`In many ?elds of medicine there is a need for appara
`tus capable of continuously measuring arterial blood
`pressure in a simple and convenient manner. The need
`is particularly keen in hospital operating rooms and the
`like, and in many experimental ?elds of medicine, for
`example in space ‘medicine. In the latter ?eld partic
`ularly, there is great need for continuous blood pressure
`measuring apparatus which will allow moderate activity
`by the subject and will not produce any general disco-In
`fort.
`In conventional sphygmomanometry, the subject must
`remain relatively quiet for many seconds and withstand
`periodic circulatory occlusion. Furthermore, the meas
`urement gives only two pressure values (i.e. systolic and
`diastolic) over a considerable number of pulse cycles,
`which results in the discarding of a substantial amount
`of information.
`Automatic methods of measuring blood pressure are
`known, based on the sphygmomanometer and digital
`plethysmograph principles. In addition to the limited
`information given, these methods are unsatisfactory for
`monitoring active subjects and the periodic occlusion of
`the blood circulation causes discomfort. The weight
`and bulk of the required auxiliary air supply, and equip—
`ment to calibrate for environmental pressure, are also
`disadvantages.
`Continuous measurement of blood pressure is possible
`by arterial cannulation, but serious drawbacks are pres
`ent. In addition to the trauma of initial arterial punc
`ture, movement of an active subject may cause sec
`ondary traumata and there is danger that the cannula
`may be dislodged from the artery.
`Accordingly, it is an object of the present invention
`to provide a method and apparatus for continuously
`measuring arterial blood pressure without interfering
`with the blood circulation or unduly impairing the physi
`cal activity and general comfort of the subject.
`It is a further object of this invention to provide ap
`paratus which ‘continuously and automatically records
`arterial blood pressure without the need for constant
`supervision by an attending physician.
`The invention is based on the fact that the velocity of
`propagation of pressure waves in the blood stream varies
`with changes in arterial blood pressure. Although the
`relationship is not linear, it is a single-valued function
`and hence changes in velocity of propagation can be
`used to indicate changes in blood pressure.
`As is well known, the beating of the heart causes heart
`pulse pressure waves to travel down the arteries. Since
`the arterial walls are elastic, expansion and contraction
`of the walls take place as the pressure changes during
`a heart beat cycle. This results in transverse waves
`which propagate down the arterial blood stream with a
`velocity of propagation which varies with the blood pres-,
`sure and the elastic condition of the arterial walls.
`Commonly, velocities of propagation of tens of feet per
`second are encountered in humans, as contrasted with
`velocities of propagation of longitudinal compression
`waves of thousands of feet per second common in liquids.
`Inasmuch as the elastic condition ‘of the arterial walls
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`3,095,872
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`F of the impressed wave and the wavelength A by the
`following equation:
`
`(1)
`c=hF
`Thus, for a constant frequency F, a measurement of
`wavelength will give the velocity of propagation.
`The velocity of propagatilon can also be expressed
`as:
`
`(2)
`CZD/f
`wher D is the distance between the two points of meausre
`ment and t is the time required for the wave to travel
`from one point to the other. It is also possible to ex
`press the velocity of propagation as:
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`the outputs and respond to changes in the phase angle
`therebetween.
`‘By employing continuous-Wave excitation and measur
`ing changes in phase angle of the pressure variations at
`spaced points, important advantages may be obtained.
`Some well-known phase detectors are inherently insensi
`tive to amplitude variations, and where necessary limit
`ers may be employed in the output circuits of the re
`ceiving transducers, so that the phase measurement is
`substantially independent of amplitude variations. Thus,
`variations in input signal amplitude, different amounts
`of attenuation between the transducers and the artery,
`different amounts of attenuation along the arterial blood
`stream, etc. will not markedly affect the accuracy of
`the measurement. Furthermore, since only a single
`frequency sinusoidal wave is required for measuring pur
`poses, ?ltering may be employed to eliminate the effect
`of ambient environmental pressures which might give
`rise to extraneous outputs from the receiving transducers.
`Also adverse effects due to frequency distortion in the
`artery are eliminated.
`The ‘invention will be more fully understood by ref
`erence to the following description of speci?c embodi
`ments thereof, taken in conjunction with the drawings,
`in which:
`FIG. 1 is a block diagram of a system embodying the
`principles of the present invention;
`FIGS. 2A through 2B are waveforms explanatory of
`the system of FIG. 1; and
`~
`'
`FIG. 3 shows a modi?cation of the receiver portion
`of FIG. 1.
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`Referring to FIG. 1, a transmitting transducer 10 is
`strapped or otherwise a?ixed to an arm 11 of a subject
`over the brachial artery '12. The transducer may be of
`any suitable type, for example a crystal or magnetic
`transducer. The brachial artery is particularly suitable
`since it is sufficiently close to the surface to permit pres
`sure waves to be impressed thereon and responses to
`be obtained therefrom. However, other arteries such
`as those in the leg or neck may be used if desired.
`Transducer 10 is energized from oscillator 13, the
`output of which is advantageously sinusoidal in wave
`form and substantially constant in frequency. The me
`chanical vibrations of the transduceryapply an alternating
`force to the artery wall which in turn produces alternat
`ing pressure variations in the arterial blood stream.
`These constant-frequency pressure variations are super
`posed on the normal blood pressure variations and are
`advantageously maintained at a low amplitude level com
`pared to the blood pressures expected to be encountered,
`for example an amplitude variation corresponding to
`a few millimeters of mercury.
`A pair of pressure-sensitive receiving transducers 14
`and 15 are strapped or otherwise af?xed to the arm over
`the brachial artery downstream from transducer 10, in
`order to respond to the incremental pressure waves pro
`duced by the transmitting transducer and yield corre~
`spending outputs. The receiving transducers may be
`small pressure-sensitive microphones of the crystal, mag
`netic or capacity types, or the like. They are advanta
`geously mounted in a unit with ?xed separation there
`between.
`The transmitting transducer, in impressing pressure
`variations on the artery, may also produce pressure
`waves in and near the skin. Consequently it is ad
`vantageous to place the receiving transducers su?iciently
`far away from the input transducer to avoid responses
`to such pressure waves.
`The pressure waves impressed on the arterial blood
`stream by transducer 10 will travel along the artery at
`a velocity of propagation which varies with the blood
`pressure, and accordingly will become phase-modulated
`'thereby. It is possible to measure changes in the veloc
`ity of propagation in several ways. As is well known,
`the velocity of propagation c is related to the frequency
`
`where (p is the phase angle between the sinusoidal waves
`at the two points.
`It is preferred to obtain an indication of the blood
`pressure by measuring the phase angle between the out
`puts of the two receiving transducers ‘14, 15. vTo this
`end an ampli?er and phase detector 16 is supplied with
`the outputs of the receiving transducers and yields an
`output which varies with the phase angle therebetween.
`The ‘output of the phase detector is supplied to a suitable
`indicator 17, which maybe a meter, recorder, etc, as
`desired.
`The operation of the apparatus of FIG. 1 is illustrated
`by the waveforms shown in FIG. 2. Here, FIG. 2A
`shows a continuous plot of the arterial blood pressure as a
`function of time covering approximately one heart pulse
`cycle. The maximum (systolic) pressure developed
`during the heart contraction is shown at 21, and the
`minimum (diastolic) pressure present during heart ex—
`pansion is shown at 22. This waveform will repeat at
`the frequency of heart contraction, normally of the order
`of 72 cycles per minute.
`FIG. 2B illustrates the constant-frequency sinusoidal
`pressure variation produced in the arterial blood stream
`by the transmitting transducer 10. The frequency is
`substantially higher than the frequency of heart con
`traction, for example, of the order of 50 to 300 cycles
`per second. This sinusoidal pressure variation is super
`posed on the normal arterial pressure variation, giving a
`resultant pressure variation illustrated in FIG. 2C. FIGS.
`2A and 2B are drawn to different scales for clarity.
`As indicated in FIG. 2C, the impressed sinusoidal pres
`sure variation has an amplitude which is small compared
`to the normal arterial blood pressure. Thus, the instan
`taneous blood pressure measured at any time is substan
`tially unchanged by the presence of the superimposed
`continuous-wave pressure variation.
`FIG. 2D illustrates the output of the phase detector
`16. As the blood pressure rises to the maximum or
`systolic pressure, the velocity of propagation increases
`and accordingly the phase angle between the outputs of
`receiver transducers 14 and =15 decreases. A number of
`types of phase detectors are known in the art and may
`be employed as meets the requirements of a given ap
`plication. It is here assumed that the phase detector
`yields a maximum output at zero phase angle, and lower
`outputs as the phase angle increases. Consequently, the
`output varies in a manner similar to the blood pressure
`variation.
`It should be understood that the output of the phase
`detector will not necessarily be linearly proportional
`to the blood pressure. However, variations in blood
`pressure will be readily apparent, and the magnitude
`may be determined by suitable calibration of the indica
`tor.
`It is advantageous to supply the output of the
`phase detector to a continuous recorder so that a com
`plete record of the subject’s blood pressure may be
`obtained. The recorder output may be calibrated in
`units of pressure as indicated in FIG. 2E, where the
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`3,095,872
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`5
`systolic pressure 23 and diastolic pressure 24- are shown
`for one heart cycle.
`Calibration may be accomplished in any convenient
`manner. For example, the subject’s blood pressure may
`be determined by conventional techniques at the same
`time it is obtained by the apparatus of the invention, and
`the indicator calibrated accordingly.
`It will be noted that by making the impressed fre
`quency high compared to the heart beat frequency, the
`manner in which the blood pressure varies during a
`single heart beat cycle may be determined. The fre
`quency components in the blood pressure variation will
`also be considerably lower than the impressed frequency,
`and may readily be eliminated in the ampli?er so as not
`to interfere with the measurement.
`It may often be desired to measure a subject’s blood
`pressure in an environment where ambient pressure
`variations are present. There may also be amplitude
`variations in the outputs of the receiver transducers due
`to many factors. For example, even though the output
`of oscillator 13 is maintained constant, the pressure
`variation impressed on the arterial blood stream by trans
`ducer iii may vary due to variations in the attenuation
`from the skin to the artery. There may also be a
`varying attenuation along the arterial blood stream, and
`between the artery and transducers l4 and 15. It is ‘ac
`cordingly highly desirable that the indicated blood pres
`sure be made independent of environmental conditions
`and insensitive to amplitude variations. ‘In such cases
`the embodiment shown in FIG. 3 may be employed.
`Inasmuch as a substantially ?xed applied sinusoidal
`freqeuency is employed for measuring purposes, it is
`possible to amplify and sharply ?lter the outputs of the
`receiving transducers so as to prevent response to other
`than the desired frequency, thereby eliminating the
`e?ects of environmental pressure changes at other fre
`quencies. Accordingly, the outputs of transducers 14
`and 15 are supplied to respective ‘ampli?ers and ?lters
`31, '31’. The ?lters are tuned to the frequency of
`oscillator 13 (FIG. 1) so that frequencies substantially
`di?erent therefrom will be discriminated against.
`The outputs of ampli?er-?lters 31, 31’ are then sup
`plied to respective limiters 32, 32' so as to substantially
`eliminate amplitude variations. With suf?cient ampli?
`cation and limiting, the outputs of limiters 32, 32’ may be
`substantially square waves, whose relative phase is the
`same as that of the input sinusoidal waves. These square
`waves are then supplied to a phase detector 33 and thence
`to indicator 17. Suitable circuits for measuring the phase
`between square waves are well known in the art. In the
`event that a phase detector is employed which is in
`herently insensitive to amplitude variations, limiters 32,
`32’ may be omitted.
`As illustrated in the drawings, it is preferred to em—
`ploy a pair of receiving transducers spaced from the
`transmitting transducer, and utilize Variations in the phase
`angle between the receiver transducer outputs in order
`to obtain an indication of blood pressure. However,
`it is possible within the broad scope of the invention to
`employ only a single receiving transducer and measure
`variations in the time of travel of the continuous~wave
`pressure variations between transmitting and receiver
`transducers. Here also, it is preferred to measure changes
`in phase between the impressed pressure variation and
`the received pressure variation.
`The invention has been described in connection with
`preferred embodiments thereof. It will be understood,
`however, that variations and modi?cations of the ar
`rangements described may be made within the spirit and
`scope of the invention.
`I claim:
`1. Apparatus for continuously measuring arterial blood
`pressure which comprises transmitting transducer means
`and driving ‘source for externally impressing an alternating
`substantially symmetrical continuous-wave pressure varia
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`tion on the arterial blood stream at a frequency substan
`tially higher than the frequency of heart contraction and
`with an amplitude which is substantially smaller than
`the normal arterial blood pressures, whereby the im
`pressed continuous-wave pressure variation is phase-modu
`lated in accordance with the arterial blood pressure over
`the heart beat cycle as the continuous-wave pressure varia
`tion travels along the artery, receiver transducer means
`externally positioned over the arterial blood stream and
`spaced from the transmitting transducer means and re
`sponsive to said phase-modulated continuous-wave pres
`sure variation to yield an output corresponding thereto,
`and detecting means insensitive to amplitude variations
`for detecting the phase-modulation of the output of the
`receiver transducer means.
`2. Apparatus in accordance with claim 1 including
`?lter means between the receiving transducer means and
`the detecting means for passing outputs having the fre
`quency of the continuous-wave pressure variation and dis
`criminating against frequencies substantially different
`therefrom.
`3. Apparatus for continuously measuring arterial blood
`pressure which comprises transmitting transducer means
`and driving source for externally impressing an alternating
`substantially symmetrical continuous-wave pressure varia
`tion on the arterial blood stream at a frequency substan
`tially higher than the frequency of heart contraction and
`with an amplitude which is substantially smaller than
`the normal arterial blood pressures, whereby the im
`pressed continuous-wave pressure variation is phase
`modulated in accordance with the arterial blood pres
`sure over the heart beat cycle as the continuous~wave
`pressure variation travels along the artery, 1a pair of
`receiving transducers externally positioned over the arte
`rial blood stream at spaced points therealong and re
`sponsive to said continuous-wave pressure variation to
`yield respective outputs, and detecting means insensitive
`to amplitude variations for detecting changes in the rela
`tive phase of the outputs of the receiving transducers.
`'4. Apparatus in accordance with claim 3 including
`means for mounting said pair of receiving transducers
`with substantially ?xed separation therebetween.
`5. Apparatus in accordance with claim 3 including
`?lter means between the receiving transducers ‘and the de
`tecting means for passing outputs having the frequency
`of the continuous-wave pressure Variation and discrimi
`iiating against frequencies substantially different there
`rom.
`6. Apparatus for continuously measuring arterial blood
`pressure which comp-rises transmitting transducer means
`and driving source for external-1y impressing an ‘alternat
`ing substantially symmetrical continuous-wave pressure
`variation on the arterial blood stream at a frequency sub
`stantially higher than the frequency of heart contraction
`and with an amplitude which is substantially smaller
`than the normal arterial [blood pressure, whereby the
`impressed continuous-wave pressure variataion is phase
`modulated in accordance with the arterial blood pressure
`over the heart best cycle as the continuous-wave pressure
`variation travels along the artery, 1a pair of receiving
`transducers externally positioned over the arterial blood
`stream at spaced points therealong and responsive to said
`continuous-wave pressure variation to yield respective
`outputs, means for limiting the outputs of the receiving
`transducers to respective predetermined amplitude levels,
`and means ‘for detecting changes in the relative phase of
`said amplitude-limited outputs. '
`7. Apparatus in accordance with claim 6 including
`?lter means between the receiving transducers and the
`means for limiting the outputs thereof for passing out
`puts having the frequency of the continuous-wave pres
`sure variation and discriminating against frequencies
`substantially different therefrom.
`8. The method of measuring arterial blood pressure
`which comprises externally impressing an alternating sub
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`3,095,872
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`7
`stantially symmetrical continuous-wave pressure varia
`tion on the ‘arterial blood stream having a frequency sub
`stantially higher than the frequency of heart contraction
`and an amplitude which is substantially smaller than the
`normal arterial blood pressures, whereby the impressed
`continuous-wave pressure variataion is phase-modulated
`in accordance ‘with the arterial blood pressure over the
`heart beat cycle as the continuous-Wave pressure variation
`travels along the ‘artery, externally deriving a signal corre—
`sponding to said phase-modulated continuous-Wave pres
`sure variation at a point over the arterial ‘blood stream
`spaced from ‘the point of impressing thereof, ‘and detect
`ing the phase-modulation of said signal substantially in
`dependent of the amplitude thereof.
`9. A method in accordance with claim 8 including ex
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`5.
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`ternally deriving signals corresponding to the eonti11u—
`ous-wave pressure variataion at a pair of spaced points
`along the arterial blood stream, limiting ‘the amplitudes
`of the signals, and measuring the relative phase of the
`amplimde-limited signals.
`
`References Cited in the ?le of this patent
`UNITED STATES PATENTS
`Grabau _____________ __ Aug. 30, 1949
`Henning _____________ __ July 18, 1950
`Sheer ______________ __ Nov. 10, 1953
`Uemura et val. ________ __ May 27, 1958
`Brown ______________ __ July 14, 1959
`Barnett et a1 ___________ __ July 12, 1960
`
`2,480,646
`2,515,221
`2,658,505
`2,836,173
`2,894,595
`2,944,542
`
`5
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`