Abstract:
A system for generating pressure pulses to calibrate a blood pressure (BP) monitor or other pressure sensing device is disclosed. The oscillometric blood pressure (BP) simulator of the preferred embodiment comprises an elastomeric bladder pneumatically coupled to the BP monitor, and an actuator adapted to reversibly compress the elastomeric bladder and induce one or more pressure pulses of predetermined magnitude, duration, and frequency in the BP monitor. The elastomeric bladder generally comprises an elastomeric tube such as medical grade hose or tubing that is compressed by a piston and bearing driven by a linear actuator, preferably a stepper motor and cam. The simulator further includes a plurality of serially coupled H-bridges for energizing the stepper motor windings with bipolar square pulses.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/529,289 filed Dec. 15, 2003, entitled “COMPACT OSCILLOMETRIC BLOOD PRESSURE SIMULATOR,” which is hereby incorporated by reference herein for all purposes. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention generally relates to a system for generating pulses to simulate the pressure waves transmitted by a blood pressure cuff. In particular, the invention relates to a system for calibrating a blood pressure monitor using a bi-directional actuator and elastomeric coupling to generate user-defined pulses.  
       BACKGROUND  
       [0003]     Oscillometry refers to the measurement of a patient&#39;s blood pressure, typically characterized by the systolic and diastolic blood pressure measurements. Oscillometry is used throughout the medical industry at every stage of patient care to assess cardiovascular function as well as numerous other medical conditions that affect peoples&#39; blood pressure. The most prevalent oscillometers include a blood pressure monitor for pressurizing and controllably deflating a cuff while simultaneously listening for the presence or absence of the pressure pulses caused by the patient&#39;s heart beat. Due to the prevalence of blood pressure monitors and the significance of their role in the health care system, there is a need for blood pressure simulators to test and calibrate the blood pressure monitors. Unfortunately, the blood pressure simulators in the prior art suffer from a number of disadvantages. For example, most simulators are unduly large which prevents them from being conveniently carried by a technician to a hospital or within a hospital where the monitor to be tested in located. Some prior art simulators are also able to generate fixed amplitude pressure pulses and therefore lack the flexibility in settings needed to simulate variable blood pressure conditions that may hamper the blood pressure monitor&#39;s accuracy. There is therefore a need for a compact, and portable blood pressure simulator adapted to emulate any number of cardiovascular conditions.  
       SUMMARY  
       [0004]     The invention in the preferred embodiment features a system for generating pulses having a user-defined waveform to simulate the pressure waves acquired by a blood pressure cuff and transmitted to a blood pressure (BP) monitor, for example, thereby permitting the blood pressure monitor to be calibrated. The oscillometric blood pressure (BP) simulator of the preferred embodiment comprises an elastomeric bladder pneumatically coupled to the BP monitor, and an actuator adapted to reversibly compress the elastomeric bladder and induce one or more pressure pulses of a predetermined magnitude, duration, and frequency in the BP monitor. The elastomeric bladder generally comprises an elastomeric tube such as medical grade hose or tubing. The hose or tubing preferably has a circular cross section, although other configurations may also be suitable. Hose and tubing are particular well suited as an elastomeric bladder because they are single-component devices that are easily sealed, highly portable, light-weight, and inexpensive. Any of various types of elastomeric materials may be used including rubber, latex, vinyl, polyester, polypropylene, polyethylene, polyvinyl chloride (PVC), silicone, and nylon, for example.  
         [0005]     The simulator actuator in the preferred embodiment includes a linear actuator adapted to precisely compress the elastomeric bladder and induce the one or more pressure pulses in the device under test. The simulator may include a cam coupled to the actuator for driving a piston that directly contacts and compresses the elastomeric bladder. The actuator may be a stepper motor, rotary actuator, linear actuator, solenoid, piezoelectric actuator, or speaker coil actuator, for example.  
         [0006]     The simulator of the preferred embodiment further comprises an actuator control mechanism adapted to drive a stepper motor or comparable actuator. Where the stepper motor includes a plurality of windings, the actuator control mechanism may employ a plurality of H-bridges, each of the plurality of H-bridges adapted to drive one of the plurality of stepper motor windings with a bipolar square pulse. When configured in series, the plurality of H-bridges are able to drop substantially all the voltage from commercially available power supplies without the need for a high-current resistor to dissipate the excess power. The actuator control mechanism, in cooperation with the plurality of H-bridges, may be adapted to drive the stepper motor with a sustaining pulse for maintaining the orientation of the cam in a distended position, thereby holding a peak pulse pressure for a finite period of time. The sustaining pulses are adapted to energize the stepper motor windings sufficient to resist the force exerted by the elastomeric bladder, thereby preventing the cam from inadvertently reversing direction when the driving pulses are discontinued. A sustaining pulse generally has a duty cycle less than half the duty cycle of the one or more bi-polar waveforms used to drive the stepper motor in the forward and reverse directions.  
         [0007]     In some embodiments, the simulator further includes a microprocessor and a pressure feedback mechanism adapted to control the actuator and induce one or more pressure pulses as the blood pressure monitor deflates the cuff in a step-wise manner. The microprocessor is adapted to induce the one or more pulses in accordance with an oscillometric envelope between a simulated systolic pressure and a simulated diastolic pressure provided by the user.  
         [0008]     In a second embodiment, the oscillometric BP simulator includes an actuator, a cam fixedly attached to the actuator, and a piston adapted to induce one or more pressure pulses in the BP monitor. The piston engages the cam via a bearing adapted to operatively couple the piston to the cam with minimal friction and therefore minimal wear at the region of contact. The bearing is preferably fixedly attached to the piston, although it may also be integrally formed with the cam. The bearing preferably includes a ball bearing assembly or a needle bearing assembly, for example. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, and in which:  
         [0010]      FIG. 1  is blood pressure simulator, in accordance with the preferred embodiment of the present invention;  
         [0011]      FIG. 2  is an exemplary pressure pulse waveform to which the blood pressure monitor is subjected by the simulator, in accordance with the preferred embodiment of the present invention;  
         [0012]      FIG. 3  is a graphical depiction of cuff pressure verses time as measured by the blood pressure monitor during an oscillometric measurement cycle, in accordance with the preferred embodiment of the present invention;  
         [0013]      FIG. 4  is a motor drive circuit used with the blood pressure simulator, in accordance with the preferred embodiment of the present invention;  
         [0014]      FIGS. 5A and 5B  are exemplary bi-polar waves used to the excite the first and second windings of the stepper motor, respectively, to drive the cam in the forward direction, in accordance with the preferred embodiment of the present invention; and  
         [0015]      FIGS. 6A and 6B  are exemplary sustaining pulses that drive the first and second windings of the stepper motor, respectively, to hold the cam in a particular position, in accordance with the preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]     Illustrated in  FIG. 1  is the novel pressure simulator adapted to evaluate diagnostic devices ordinarily used to measure pressure or pressure differentials. The preferred embodiment presented herein is an oscillometric blood pressure simulator  100  for testing and calibrating pressure sensing systems including non-invasive blood pressure units  150  used in medical application, for example. The pressure simulator  100  is preferably adapted to generate a pressure waveform including a plurality of pressure pulses to simulate the cardiac rhythm of a patient that would otherwise be acquired by a blood pressure cuff. The pressure simulator  100  is also adapted to simulate signal attenuation attributable to the pressure-dependent impedance mismatch present at the patient-cuff interface. The frequency of the pressure pulses, the diastolic and systolic pressures, and the profile of the pressure pulses may be predetermined by the user to effectively model any of a number of physiological patient conditions for purposes of calibrating blood pressuring monitor equipment regardless of the manufacturer.  
         [0017]     An exemplary device under test is represented by the pressure sensing system  150  which includes an inflatable cuff  152  adapted to engage the patient&#39;s arm or wrist, for example; and a blood pressure (BP) monitor  153  including a pressure regulator  154  and a pressure display unit  155 . The cuff  152  and BP monitor  153  are pneumatically coupled via an elastic hose  156  and T-connector  142 . The pressure regulator  154  is adapted to pressurize the cuff  152  above a patient&#39;s systolic pressure, slowly dissipate the pressure, and determine the patient&#39;s systolic and diastolic pressures which are registered on the pressure display  155 .  
         [0018]     The pressure simulator  100  preferably includes a bi-directional actuator operatively coupled to the pressure sensing system  150 , an actuator control mechanism, and a pressure feedback mechanism. The actuator control mechanism subjects the BP monitor to a plurality of pulses having a determined frequency, magnitude, and profile by controlling the bi-directional actuator under the guidance of the feedback mechanism. In the preferred embodiment of the blood pressure simulator  100 , the bi-directional actuator is incorporated into an actuator housing  102  while the actuator control mechanism and a pressure feedback mechanism are incorporated into a simulator control housing  130 . Both the actuator housing  102  and control housing  130  are pneumatically coupled to the cuff  152  and BP monitor  153  by means of a pneumatic conduit  126  and extension hose  140 . The collective pressure induced by the pressure regulator  154  and the pressure pulses induced by the bi-directional actuator are therefore sensed by the BP monitor  153  as well as the pressure feedback mechanism.  
         [0019]     The bi-directional actuator in the preferred embodiment comprises a cam  106  driven by a stepper motor  108 . The stepper motor  108  is fixedly attached to the actuator housing  102  and adapted to rotate the motor shaft  110  at least 180 degrees and as much as a full 360 degrees in a clock-wise (CW) or counter clockwise (CCW) manner under the direction of the motor drive control circuit  134 . The cam  106  is fixedly attached to the motor shaft  110  and drives a cam follower  112  via a bearing  114  including a ball bearing assembly or needle bearing assembly, for example. The cam follower  112  is fixedly attached to a slider  118  which is concealed within the actuator housing  102  where it is constrained to move solely in the vertical direction. In the preferred embodiment, the displacement of the follower  112  drives the slider  118  up and down within the housing  102 , thereby adapting the slider to serve as a piston  120  for exerting a force against the pneumatic chamber  104 .  
         [0020]     In the preferred embodiment, the pneumatic chamber  104  is formed of an elastomeric material, e.g., rubber or latex hose, that is inserted into a cavity  128  where it is captured between the upper section of the actuator housing  102  and a backing plate  116 . The pneumatic chamber  104  in some embodiments is an section of the same hose used for the pneumatic conduit  126 . When the cam  106  is turned, the piston  120  compresses the chamber  104  and the increased pressure observed by the pressure monitor  153  as if transmitted from the cuff  152 . After the peak pulse pressure is reached, the direction of the cam  106  is reversed and the pressure in the cuff  152  and pneumatic hose  156  restored to the test level controlled by the BP monitor  153 . The pressure induced in the cuff  152  is concurrently measured by the pressure feedback mechanism, i.e., the pressure sensor  136 , and the sensed pressure signal  160  transmitted to the actuator control mechanism in real-time via the amplifier  138 .  
         [0021]     In the preferred embodiment, the cam  106  is a linear cam adapted to displace the follower  112  linearly in proportion to the angular displacement of the cam  106 . One skilled in the art will appreciate that a non-linear cam  106  may also be employed in alternative embodiments provided a bi-directional actuator is adapted to rotate the cam CW and CCW. In addition to the bi-direction rotary actuator of the preferred embodiment, various other actuators may be employed including linear actuators, solenoids, and piezoelectric devices, for example.  
         [0022]     The actuator control mechanism in the preferred embodiment comprises a microprocessor  132  and a motor drive circuit  134 . The microprocessor  132  monitors the pneumatic instantaneous pressure reading  160 , calculates the size and shape of one or more pressure pulses, and induces the appropriate pulses by exciting the stepper motor  108  via the drive circuit  134 . In the preferred embodiment, the size and shape of the induced pressure changes conform to a predetermined pressure waveform retained in local memory, preferably random access memory (RAM)  133 . Illustrated in  FIG. 2  is an exemplary pressure pulse waveform  200 . The waveform  200  includes a plurality of pressure pulses  202  modulated by an oscillometric envelope  204 . The oscillometric envelope  204 , in turn, is characterized by a mean pressure point  210  and is bounded on the left and right sides by points coinciding with a simulated systolic pressure and a simulated diastolic pressure, respectively. The first pressure pulse  206  of the waveform  200  is introduced at or below the simulated systolic pressure point  212 , while the last pulse  208  is introduce prior to or at the simulated diastolic pressure point  214 . The train of pressure pulses  202  are generally characterized by a nominal pulse frequency of approximately one Hertz, although one skilled in the art will readily appreciate that the device under test may also be subjected to various other periodic and non-periodic pulse waveforms. The simulated mean pressure point  210  coincides with the point at which a patient&#39;s blood pressure optimally matches the cuff pressure and the maximum pulse energy transmitted through the patient-cuff interface. The lower pulse magnitudes on either side of the mean  210  simulate sonic attenuation due to pressure mismatch at the patient-cuff interface that would be observed if an actual patient were being tested.  
         [0023]     Illustrated in  FIG. 3  is a graphical depiction of cuff pressure verses time as measured by the blood pressure unit  150  during an oscillometric measurement cycle. The cuff  152  is initially inflated above the simulated systolic pressure  312  and the pressure released from the cuff  152  in a step-wise manner. When the simulator  100  detects that the cuff pressure  302  has dropped to the simulated systolic pressure  312 , the microprocessor  132  causes the drive circuit  134  to turn the cam  106  in a CCW direction wherein the piston  120  deforms the pneumatic chamber  104 , thus introducing the first pressure pulse  206 . The direction of the cam  106  is reversed after the cuff pressure  302  has been increased by the magnitude of the first pressure pulse  206  given in  FIG. 2 . As the cuff pressure continues to decrease under the control of the pressure regulator  154 , the magnitude of the pressure pulses injected by the actuator control mechanism increases in accordance with the oscillometric envelope  204  until the ambient cuff pressure equals the simulated mean pressure  310  which coincides with the mean pressure point  210 . As the cuff pressure further decreases, the actuator control mechanism further reduces the magnitude of the pressure pulses until the last pulse  208  is injected and the ambient cuff pressure falls below the simulated diastolic pressure  314 . The simulated systolic pressure and simulated diastolic pressure may then be compared to the measured pressures determined by the BP monitor  153  for purposes of calibrating the monitor, for example.  
         [0024]     Referring to  FIG. 4 , the bi-directional actuator  108  in the preferred embodiment is a stepper motor including two sets of winding schematically represented by the first winding  402 , i.e., WINDING — 1, and a second winding  404 , i.e., WINDING — 2. The windings  402 ,  404  may be excited in different sequences and with different polarities to obtain various possible motor speed in both the forward and reverse directions. In the preferred embodiment, the two windings  402 ,  404  are adapted to receive simultaneous bi-polar square waves from the motor drive circuit  134 , the first square wave  162 A being 90 degrees out of phase with respect to the second square wave  162 B. The number of cycles of the bi-polar square pulses determines the angular displacement of the cam  106  while the polarity of the phase difference between the square waves  162 A,  162 B determines the direction of rotation.  
         [0025]     In the preferred embodiment the motor drive circuit  134  includes two H-bridges for toggling the polarity of the bi-polar square waves used to control the stepper motor  108 . The first H-bridge  414  controls or drives the first winding  402  while the second H-bridge  416  controls or drives the second winding  404 . In the novel implementation of the preferred embodiment, the H-bridges  414 ,  416  are coupled in series instead of parallel. Each of the H-bridges includes four switches  410 - 413  made to open or close under the direction of the motor drive circuit  134 . In steady state, the switches are toggled every half cycle, T/2, where the full period, T, is determined by the temporal resolution of the selected stepper motor  108 .  
         [0026]     To drive the cam  106  in a CCW direction, for example, switches  410 ,  413  of the first H-bridge  414  are closed while switches  411 ,  412  of the first H-bridge  414  are opened. Every half cycle, T/2, the switches are 410-413 are toggled, thus giving rise to the bi-polar square pulse  500 A illustrated in  FIG. 5A . Concurrently, the switches  410 ,  413  of the second H-bridge  416  are opened while switches  411 ,  412  of the second H-bridge  416  are closed. The switches  410 - 413  of the second H-bridge  416  are toggled after a quarter cycle, T/4, and toggled every half cycle thereafter, thus giving rise to the bi-polar square wave  500 B illustrated in  FIG. 5B . To drive the cam  106  in the CW direction, relative phase of the bi-polar drive signals is reversed, i.e., the first wave  500 A inputted to the first winding  402  is made to lag the second wave  500 B inputted to the second winding  404  by a ninety degrees. Whether the cam  106  is driven CC or CCW, each of the windings  402 ,  404  drops one half the system voltage, V, provided by the power supply  418 . Coupling the H-bridges  414 ,  416  in series instead of parallel obviates the need for high-current resistors used in the prior art to drop the voltage difference between the power supply and a single winding.  
         [0027]     Referring to  FIGS. 6A and 6B , the actuator control mechanism of the preferred embodiment is also adapted to produce one or more sustaining pulses used to hold the cam in a desired position. In particular, sustaining pulses are employed to maintain the cam in position while resisting the torque created by the piston  120  when it is engaged against the pneumatic chamber  104 . As such, the cam  106  may be locked in position to extend the width of one or more pressure pulses  202 . In the preferred embodiment, the sustaining pulses  602 ,  604  are represented by intermittent square pulses that are enabled when the cam has reached the position to be maintained. In the preferred embodiment, the sustaining pulses  602 ,  604  are square pulses having the same amplitude as their respective bi-polar square pulses immediately prior to the initiation of the sustaining pulses. The sustaining pulses are preferably uni-polar, have a pulse width of approximately T/12, and have a repetition period of approximately T/6. After the sustaining pulses, the cam  106  may be driven in the reverse direction to its home position, for example.  
         [0028]     The implementation of the novel sustaining pulses presented herein allows the pressure simulator  100  extend its pulse duration while appreciably reducing its power consumption. As such, the voltage source  410  may employ disposable batteries, thus increasing its portability while reducing its cost.  
         [0029]     Referring to  FIG. 1  again, the pressure simulator  100  in some embodiments further includes a photo interrupt detector that insures the cam  106  returns to the same position—termed the home-position—at the onset of each pressure pulse. The sensor in the preferred embodiment includes a light source, e.g., a light emitting diode (LED)  122 , a light detector, e.g., a photovoltaic cell  124 , and an opaque blade  123  fixedly attached to the slider  118 . When the cam  106  is away from it home position and engaged against the pneumatic chamber  104 , for example, the position of the blade  123  permits the cell  124  to receive the light emitted by the LED  122 . When the slider  118  is biased towards the upper side of the housing  102 , however, the opaque blade  123  interrupts the light beam between the LED  122  and cell  124 , thus confirming that the cam  106  is located in the home position. Once the home position is confirmed, the cam  106  position is initialized before it is advanced to one of a plurality of possible staging positions where it will await the initiation of the next pressure pulse. In the preferred embodiment, the slider  118  may be staged: (a) in immediate proximity to the pneumatic chamber  104  or (b) in a position biased slightly against the pneumatic chamber. In the latter case, the piston  120  induces minor deformation of the pneumatic chamber  104  which enables the simulator  100  to induce the peak pulse pressure more rapidly compared to the alternate staging position or the home position. The simulator  100  may be designed such that the angular offset between the home position and either of the two staging positions is given by a predetermined number of stepper motor increments to which the cam  106  may be automatically advanced preceding the initiation of a pressure pulse.  
         [0030]     Although the description above contains many specifications, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention.  
         [0031]     Therefore, the invention has been disclosed by way of example and not limitation, and reference should be made to the following claims to determine the scope of the present invention.