Apparatus and method for non-invasively monitoring a subject's arterial blood pressure

We claim:

1. Apparatus for non-invasively monitoring the pressure within a subject's blood vessel, comprising:

a housing having a substantially planar face configured to compressively engage the tissue overlying the subject's blood vessel, a chamber, and a recess;

a pressure transducer carried on the housing and configured to produce a pressure signal indicative of the pressure applied against it, the pressure transducer further being sized to be received within the chamber of the housing;

a coupling device configured to urge the housing into compressive engagement with the tissue overlying a subject's blood vessel, to compress the vessel and ensure that pressure variations within the vessel are coupled through the tissue to the pressure transducer; and

a blood vessel sensor sized to be received within the recess of the housing and configured to detect the presence of the blood vessel in the space adjacent to the housing, whereby the apparatus can be moved to place the pressure transducer is in a prescribed position overlying the subject's blood vessel.

2. Apparatus as defined in claim 1, wherein:

the pressure transducer has a substantially planar face that is oriented substantially parallel with the planar face of the housing; and

the blood vessel sensor is an ultrasonic transducer having a substantially planar face that is oriented at a prescribed acute angle relative to the planar face of the housing.

3. Apparatus as defined in claim 2, wherein the substantially planar face of the ultrasonic transducer is oriented at an angle of about 35 degrees relative to the planar face of the housing.

4. Apparatus as defined in claim 2, wherein:

the recess of the housing is configured such that the ultrasonic transducer is spaced above the planar face of the housing; and

the apparatus further comprises a lubricant disposed on the planar face of the housing, for enhancing the coupling between the ultrasonic transducer and the subject's blood vessel.

5. Apparatus for non-invasively monitoring the pressure within a subject's blood vessel, comprising:

a housing;

a pressure transducer carried on the housing and configured to produce a pressure signal indicative of the pressure applied against it;

a coupling device configured to urge the housing into compressive engagement with the tissue overlying a subject's blood vessel, to compress the vessel and ensure that pressure variations within the vessel are coupled through the tissue to the pressure transducer; and

a blood vessel sensor carried on the housing and configured to detect the presence of the blood vessel in the space adjacent to the housing, whereby the apparatus can be moved to place the pressure transducer is in a prescribed position overlying the subject's blood vessel;

wherein the housing is configured to carry the pressure transducer and the blood vessel sensor in positions such that the space adjacent to the housing in which the blood vessel sensor detects the blood vessel is directly aligned with the space adjacent to the pressure transducer.

6. Apparatus for non-invasively monitoring the pressure within a subject's blood vessel, comprising:

a housing having at least one inner rigid portion an outer, compliant portion that encircles the at least one inner rigid portion;

a pressure transducer carried on the housing and configured to produce a pressure signal indicative of the pressure applied against it;

a coupling device configured to urge the housing into compressive engagement with the tissue overlying a subject's blood vessel, to compress the vessel and ensure that pressure variations within the vessel are coupled through the tissue to the pressure transducer; and

a blood vessel sensor carried on the housing and configured to detect the presence of the blood vessel in the space adjacent to the housing, whereby the apparatus can be moved to place the pressure transducer is in a prescribed position overlying the subject's blood vessel;

wherein the at least one rigid portion of the housing carries the pressure transducer and the blood vessel sensor, and the inner and outer portions cooperate to define a substantially planar face configured to compressively engage the tissue overlying the subject's blood vessel.

7. Apparatus for non-invasively monitoring the pressure within a subject's blood vessel, comprising:

a housing;

a pressure transducer carried on the housing and configured to produce a pressure signal indicative of the pressure applied against it;

a coupling device configured to urge the housing into compressive engagement with the tissue overlying a subject's blood vessel, to compress the vessel and ensure that pressure variations within the vessel are coupled through the tissue to the pressure transducer; and

a plurality of blood vessel sensors carried on the housing, the plurality of sensors being disposed in an array and configured to detect the presence of the blood vessel in the space adjacent to the housing, the array further being configured to determine a direction to move the apparatus to place the pressure transducer is in a prescribed position overlying the subject's blood vessel.

8. Apparatus as defined in claim 7, and farther comprising:

an adjustment mechanism configured to adjust the position of the pressure transducer toward a prescribed position overlying the subject's blood vessel; and

a controller configured to control the adjustment mechanism.

9. Apparatus for non-invasively monitoring the pressure within a subject's blood vessel, comprising:

a housing;

a pressure transducer carried on the housing and configured to produce a pressure signal indicative of the pressure applied against it;

an adjustment mechanism configured to adjust the position of the pressure transducer;

a coupling device configured to urge the housing into compressive engagement with the tissue overlying a subject's blood vessel, to compress the vessel and ensure that pressure variations within the vessel are coupled through the tissue to the pressure transducer; and

a blood vessel sensor carried on the housing and configured to detect the presence of the blood vessel in the space adjacent to the housing, whereby the adjustment mechanism and apparatus can be utilized to place the pressure transducer is in a prescribed position overlying the subject's blood vessel.

10. A method for non-invasively monitoring the pressure within a subject's blood vessel, comprising the steps of:

providing a pressure sensing apparatus that includes:

(i) a housing having a substantially planar face configured to compressively engage the tissue overlying the subject's blood vessel, the housing further including a chamber and a recess;

(ii) a pressure transducer carried on the housing and sized to be received within the chamber of the housing, the pressure transducer being configured to produce a pressure signal indicative of the pressure applied against it; and

(iii) a blood vessel sensor carried on the housing and sized to be received within the recess of the housing, the blood vessel sensor being configured to produce a signal indicative of the presence of a vessel in the space adjacent to the housing;

controllably moving the housing across the tissue near a subject's blood vessel until the signal produced by the blood vessel sensor indicates that the housing has reached a prescribed position overlying the subject's blood vessel, the housing being oriented such that an axis connecting its chamber and its recess is substantially parallel with the subject's blood vessel; and

urging the housing into compressive engagement with the tissue overlying a subject's blood vessel, to compress the vessel and ensure that pressure variations within the vessel are coupled through the tissue to the pressure transducer carried on the housing.

11. A method for non-invasively monitoring the pressure within a subject's blood vessel, comprising the steps of:

providing a pressure sensing apparatus that includes a housing having a planar face, a pressure transducer carried on the housing and configured to produce a pressure signal indicative of the pressure applied against it, and an ultrasonic transducer carried on the housing and spaced above the planar face thereof, the ultrasonic transducer being configured to produce a signal indicative of the presence of a vessel in the space adjacent to the housing;

placing a lubricant on at least a portion of the planar face of the housing, thereby enhancing the coupling between the ultrasonic transducer and the subject's blood vessel;

controllably moving the housing across the tissue near a subject's blood vessel until the signal produced by the blood vessel sensor indicates that the housing has reached a prescribed position overlying the subject's blood vessel; and

urging the housing into compressive engagement with the tissue overlying a subject's blood vessel, to compress the vessel and ensure that pressure variations within the vessel are coupled through the tissue to the pressure transducer carried on the housing.

12. A method for non-invasively monitoring the pressure within a subject's blood vessel, comprising the steps of:

providing a pressure sensing apparatus that includes:

(i) a housing forming a compression surface;

(ii) a pressure transducer having a compression surface and being carried on the housing such that the compression surfaces of the pressure transducer and housing are flush, the pressure transducer further being configured to produce a pressure signal indicative of the pressure applied against it; and

(iii) a blood vessel sensor carried on the housing and configured to produce a signal indicative of the presence of a vessel in the space adjacent to the housing;

controllably moving the housing across the tissue near a subject's blood vessel until the signal produced by the blood vessel sensor indicates that the housing has reached a prescribed position overlying the subject's blood vessel; and

urging the compression surface of the housing and the pressure transducer into compressive association with first and second portions of the tissue overlying a subject's blood vessel, respectively, the first portion surrounding the second portion, to compress the vessel and ensure that pressure variations within the vessel are coupled through the tissue to the pressure transducer carried on the housing.

13. A method as defined in claim 12, wherein, in the step of providing, the compression surface of the housing includes an inner section and an outer section, the outer section surrounding the inner section, and the outer section being more compliant that the inner section.

14. A method as defined in claim 12, wherein, in the step of providing, the shape of the compression surface of the housing is longer than it is wide, and in the step of urging, the shape is oriented relative to the blood vessel such that the vessel runs in the compression surface's long direction.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

This invention relates generally to apparatus and methods for monitoring a subject's arterial blood pressure and, more particularly, to such apparatus and methods that monitor arterial blood pressure non-invasively by applying a pressure sensor against tissue overlying an arterial blood vessel, to partially applanate or compress the vessel.

Two well-known techniques have been used to non-invasively monitor a subject's arterial blood pressure waveform, namely, auscultation and oscillometry. Both techniques use a standard inflatable arm cuff that occludes the subject's brachial artery. The auscultatory technique determines the subject's systolic and diastolic pressures by monitoring certain Korotkoff sounds that occur as the cuff is slowly deflated. The oscillometric technique, on the other hand, determines these pressures, as well as the subject's mean pressure, by measuring actual pressure changes that occur in the cuff as the cuff is deflated. Both techniques determine pressure values only intermittently, because of the need to alternately inflate and deflate the cuff, and they cannot replicate the subject's actual blood pressure waveform. Thus, true continuous, beat-to-beat blood pressure monitoring cannot be achieved using these techniques.

Occlusive cuff instruments of the kind described briefly above generally have been effective in sensing long-term trends in a subject's blood pressure. However, such instruments generally have been ineffective in sensing short-term blood pressure variations, which are of critical importance in many medical applications, including surgery.

One technique that has been used to provide information about short-term blood pressure variations is called arterial tonometry. One device for implementing this technique includes a rigid array of miniature pressure transducers that is applied against the tissue overlying a peripheral artery, e.g., the radial artery. The transducers each directly sense the mechanical forces in the underlying subject tissue, and each is sized to cover only a fraction of the underlying artery. The array is urged against the tissue, to applanate the underlying artery and thereby cause beat-to-beat pressure variations within the artery to be coupled through the tissue to the transducers.

The rigid arterial tonometer described briefly above is subject to several drawbacks. First, its discrete transducers are relatively expensive and, because they are exposed, they are easily damaged. In addition, the array of discrete transducers generally is not anatomically compatible with the continuous contours of the subject's tissue overlying the artery being sensed. This has led to inaccuracies in the resulting transducer signals. In addition, in some cases, this incompatibility can cause tissue injury and patient discomfort. Another drawback is that such rigid arterial tonometers have failed to correct for signal artifacts that arise when the subject's arm is moved. This is a particular problem when the subject is exercising or otherwise ambulating.

Yet another drawback to the arterial tonometer described briefly above is its inability to continuously monitor and adjust the level of arterial wall compression to an optimum level of zero transmural pressure. Generally, optimization of arterial wall compression has been achieved only by periodic recalibration. This has required an interruption of the patient monitoring function, which sometimes can occur during critical periods. This drawback is perhaps the most severe factor limiting acceptance of tonometers in the clinical environment.

Another device functioning similarly to the arterial tonometer includes a housing having a closed, liquid-filled chamber with one wall of the chamber defined by a flexible diaphragm. The device is applied against a subject's skin, with the flexible diaphragm pressed against the tissue overlying a peripheral artery, e.g., the radial artery, and several electrodes located in separate compartments of the chamber sense volume changes in the compartments that result from the beat-to-beat pressure variations in the underlying artery. Although the device seeks to replicate the arterial pressure waveform, it is considered to have a relatively low gain, making it unduly susceptible to noise. Further, the device must be calibrated periodically, during which time its continuous monitoring of the subject's blood pressure waveform necessarily is interrupted.

It should, therefore, be appreciated that there is a continuing need for an apparatus, and related method, for non-invasively and continuously monitoring a subject's blood pressure, with reduced susceptibility to noise and without the need to intermittently interrupt the device's normal operation for calibration. Various embodiments of the present invention can fulfill some or all of these requirements.

SUMMARY OF THE INVENTION

The present invention resides in an improved apparatus, and related method, for non-invasively monitoring a subject's arterial blood pressure, with reduced susceptibility to noise and without the need to intermittently interrupt the pressure monitoring for calibration. The apparatus includes a pressure transducer that produces a pressure signal indicative of the pressure applied against it and further includes a coupling device that urges the pressure transducer into compressive association and engagement with tissue overlying the subject's blood vessel, to compress the vessel and ensure that pressure variations within the vessel are coupled through the tissue to the pressure transducer.

A controller controls the coupling device to controllably modulate the location of the pressure transducer relative to a nominal location that is static with respect to the subject's blood vessel. The controller also monitors the resulting effect of the modulation on the pressure signal, producing an error signal (i.e., a control signal) that is indicative of the deviation of the nominal location from a preferred target location. Preferably, the coupling device is configured to respond to the error signal by controllably adjusting the nominal location toward the target location. When the nominal location is at or substantially close to the target location, the blood vessel is compressed according to a prescribed requirement, which is preferably a prescribed mean amount providing a transmural pressure of substantially zero. This requirement optimizes the coupling between the blood vessel and the sensor assembly. The pressure sensor assembly thereby senses the subject's blood pressure in an optimal manner.

More particularly, in one form of the invention, the pressure sensor assembly includes a base configured to provide reaction forces for urging the pressure transducer into compressive association with the tissue overlying the subject's blood vessel. The coupling device includes a first variable positioning device, such as a first motor with an eccentric cam, configured to controllably adjust the nominal location toward the target location. The coupling device also includes a second variable positioning device, such as a second motor with an eccentric cam, configured to vary the pressure transducer through a range of locations relative to the nominal location, i.e., modulating the pressure transducer about the nominal location.

Preferably, the first and second variable positioning devices actuate a first and second end of a lever arm, respectively, to actuate the pressure transducer, which is connected to the lever arm between the two ends. By monitoring the effect on the pressure signal of the second variable positioning device, the controller can direct the first variable positioning device to adjust the lever arm such that the nominal location moves, and thus the subject's blood vessel becomes compressed by the prescribed mean amount.

In a separate aspect of the invention, the pressure transducer is in compressive association with the tissue and blood vessel through a substantially incompressible, and preferably compliant plug, whereby the pressure signal is a reasonably accurate representation of the pressure being applied to the plug by the tissue. The plug preferably fills a chamber that connects the pressure transducer to the tissue. For the purposes of this application, a plug is broadly defined to be any intermediary device that puts the pressure transducer in compressive association with the tissue. For example, the plug might comprise a liquid sealed into a chamber by a flexible diaphragm, where the diaphragm contacts the tissue and the liquid contacts the pressure transducer. Likewise, the plug might be a rigid, solid device in contact with both the pressure transducer and the tissue.

In another separate aspect of the invention, the pressure transducer is configured to be in compressive association with a first portion of the tissue overlying the subject's blood vessel. A compression surface of the device is configured to compress a second portion of the tissue overlying the subject'blood vessel, the second portion preferably being a section of tissue surrounding the first portion of the tissue. The compression surface preferably compresses the second portion of tissue in concert with the pressure transducer's compressing of the first portion of tissue. This will preferably cause the pressure transducer to only sense the pressure immediately beneath it, minimizing edge effects and the pounding of artery pressure against the edge of the occluded section of the vessel.

It is also preferable that the compression surface includes an inner section and an outer section, the outer section being more compliant that the inner section. This may, in some embodiments, further minimize edge effects and pounding, as well as potentially adding to the subject's comfort due to the compliant portion forming a radius to transition between the compressed and non-compressed portions of the arm.

Another aspect of the invention resides in a blood vessel sensor carried on a housing that contains the pressure transducer. The blood vessel sensor is configured to detect the presence, and preferably the location, of a blood vessel such as an artery, thus allowing the pressure transducer to be placed in a prescribed position overlying the subject's blood vessel. The blood vessel is preferably a blood flow sensor that senses the flow (although, not necessarily the flux) of blood in the space adjacent to the housing. The blood flow sensor is preferably an ultrasonic transducer having a substantially planar face that is oriented at a prescribed acute angle relative to a planar face of the housing that adjoins the tissue overlying the blood vessel (e.g., the compression surface). The ultrasonic transducer preferably senses the velocity of the blood flow, providing an indication of blood flow when the velocity is sensed. Preferably there is a coupling medium disposed on the planar face, which in various embodiments can enhance the coupling between the ultrasonic transducer and the subject's blood vessel, and can aid in the positioning of the pressure transducer.

In another embodiment, the invention may feature a controller configured to controllably modulate the position of a pressure sensor assembly relative to a nominal location (and the subject's blood vessel) with a periodic signal having a frequency substantially greater than the frequency of the subject's expected heartbeat, and the controller monitors the resulting pressure signal to produce a plurality of pressure waveforms. Each such pressure waveform corresponds to a different phase of the periodic signal, which in turn corresponds to a different nominal amount of vessel compression.

The controller is configured to select the particular one of the pressure waveforms that derives from a selected transmural pressure, preferably being a transmural pressure of substantially zero. For example, the waveform may be selected by choosing the particular waveform for which the pressure signal at systole differs from the pressure signal at diastole by a maximum amount. The controller then directs the coupling device to adjust the nominal location such that the selected waveform generally exhibits a lower mean pressure than half of the waveforms and exhibits a higher mean pressure than the other half of the waveforms.

Other features and advantages of the present invention should become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial, perspective view of a blood pressure monitoring apparatus embodying features of the present invention. Depicted are a base plate, a coupling device and a pressure sensor assembly of the blood pressure monitoring apparatus.

FIG. 2 is a schematic cross-sectional view of the blood pressure monitoring apparatus of FIG. 1, in its prescribed position secured to a subject's wrist, with the pressure sensor assembly disposed adjacently to the tissue overlying the subject's radial artery.

FIG. 3 is a view of the base plate, coupling device and pressure sensor assembly of FIG. 1, from a different perspective than that of FIG. 1.

FIG. 4 is a perspective view of the base plate, coupling device and pressure sensor assembly of FIG. 1, wherein the coupling apparatus has retracted the pressure sensor assembly.

FIG. 5 is a perspective view of the base plate, coupling device and pressure sensor assembly of FIG. 1, wherein the coupling apparatus has advanced the pressure sensor assembly.

FIG. 6 is an exploded perspective view of a sensor housing that is part of the pressure sensor assembly of FIG. 1.

FIG. 7 is a cross-sectional view of the sensor housing of FIG. 6.

FIG. 8 is a block diagram of a first embodiment of a blood pressure monitoring apparatus in accordance with the invention, incorporating the coupling device and pressure sensor assembly of FIG. 1.

FIG. 9 is a block diagram of a second embodiment of a blood pressure monitoring apparatus in accordance with the invention, which incorporates the coupling device and pressure sensor assembly of FIG. 1 and provides an accurate depiction of a subject's arterial pressure waveform within as little as a single heartbeat.

FIG. 10A is a graph of an exemplary pressure signal produced by the blood pressure monitoring apparatus of FIG. 9, incorporating both a sinusoidal modulation component and a subject heartbeat component.

FIG. 10B is a graph of ten reconstructed pressure waveforms reconstructed from the exemplary pressure signal of FIG. 10A by the blood pressure monitoring apparatus of FIG. 9.

FIG. 11 is a block diagram of the signal processing portion of an ultrasound system, used to controllably position pressure sensor assembly of FIG. 1 on a subject's wrist.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention include variations of the apparatus and methods described below. The embodiments incorporate aspects of the invention described in U.S. patent application entitled "Apparatus and Method for Non-invasively Monitoring a Subject's Arterial Blood Pressure," Ser. No. 766,810, filed Dec. 13, 1996, ("the '810 application"), which is incorporated herein by reference.

With reference now to the drawings, and particularly to FIGS. 1 and 2, there is shown a blood pressure monitoring apparatus 10 configured for attachment to a subject's wrist 12, with a pressure sensor assembly 14 compressively engaging the tissue 16 overlying the subject's radial artery 18. Blood pressure variations within the artery are coupled through the tissue to the pressure sensor assembly, to produce a pressure signal output that represents the artery's pressure waveform. The pressure sensor assembly, by engaging the tissue over the radial artery, compre