Abstract:
A sphygmomanometer with a cuff for use on a patient wrist, upper or lower arm incorporates an inflatable bladder and a support structure. The cuff is subdivided into two sections. The first section holds the bladder against an arterial side of the limb, while the second section abuts a non-arterial side of the limb and is mechanically coupled to the support structure. When the cuff is attached to the patient limb, the bladder is positioned to avoid receiving a gravitational force caused by the weight of the limb. Rather, the gravitational force is absorbed by the support structure in an interior area of the cuff removed from the bladder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 13/505,673, filed May 2, 2012, which was the National Stage of International Application No. PCT/US2009/063972, filed on Nov. 11, 2009. The contents of all of these applications are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to methods and medical apparatuses for non-invasive monitoring of arterial blood pressure, and specifically to the devices and methods that use inflatable cuffs. 
       BACKGROUND OF THE INVENTION 
       [0003]    Blood pressure monitoring has rapidly become an accepted and, in many cases, essential aspect of human and veterinary treatment. Blood pressure monitors are now a conventional part of the patient environment in emergency rooms, intensive and critical care units, in the operating theater, and in homes. 
         [0004]    Several well known techniques have been used to non-invasively monitor a subject&#39;s arterial blood pressure waveform, namely: auscultation, oscillometry, tonometry and flowmetry. The auscultation, oscillometric and flowmetry techniques use a standard inflatable cuff that occludes an artery (for example, the subject&#39;s brachial artery). The auscultatory technique determines the subject&#39;s systolic and diastolic pressures by monitoring certain Korotkoff sounds that occur as the cuff is slowly deflated or inflated. The oscillometric technique, on the other hand, determines these pressures, as well as the subject&#39;s mean pressure, by measuring the small pressure oscillations that occur in the cuff as the cuff is deflated or inflated. The flowmetric technique relies on detecting variations in blood flow downstream from the cuff 
         [0005]    The oscillometric method of measuring blood pressure is currently the most popular method in commercially available automatic systems. This method relies on measuring changes in arterial counter pressure, such as imposed by an inflatable cuff, which is controllably relaxed or inflated. In some cases, the cuff pressure change is continuous, and in others it is incremental. In all oscillometric systems, a transducer (pressure sensor) monitors arterial counter pressure oscillations, and processing electronics convert selected parameters of these oscillations as represented by signals produced by the transducer into blood pressure data. 
         [0006]    In the oscillometric method, the mean blood pressure value is the mean of the cuff pressure values that correspond in time to a peak of the envelope of the pressure oscillations. Systolic blood pressure is generally estimated as the pressure of a decaying pressure slope prior to the peak of the pressure oscillations envelope, corresponding to a point in time where the amplitude of the envelope is equal to a fraction of the peak amplitude. Generally, systolic blood pressure is the pressure on the decaying pressure of the cuff prior to the peak of the envelope where the amplitude of the envelope is 0.57 to 0.45 of the peak amplitude. Similarly, diastolic blood pressure is the pressure on the decaying pressure of the cuff after the peak of the envelope that corresponds to a point in time to where the amplitude of the envelope is equal to a different fraction of the peak amplitude. For example, diastolic blood pressure may be conventionally estimated as the pressure on the decaying pressure of the cuff after the peak where the amplitude of the envelope is equal to 0.82 to 0.74 of the peak amplitude. Other algorithms are also well known in the art. 
         [0007]    The auscultatory method also involves inflation of a cuff placed around a cooperating artery of the patient. Systolic pressure is indicated when the Korotkoff sounds disappear as the cuff is inflated above the highest pressures exerted by the heart onto the arterial walls. Diastolic pressure is indicated when the Korotkoff sounds first appear as the cuff pressure is elevated above the atmospheric pressure. The auscultatory method can only be used to determine systolic and diastolic pressures, and it does not determine mean pressure. 
         [0008]    To use either of the oscillometric and ausculatory methods of arterial pressure computation, an oscillatory signal of sufficient quality must be obtained from the artery. The signal quality (for example, as determined by pulse shape distortion and noise level) is greatly influenced by a matching between the inflatable cuff and the patient limb geometry. The cuff size should correspond to the length and circumference of the limb. A fluid bladder positioned inside the cuff should be wrapped around at least a portion of the limb in such a manner as to fully envelop the arterial path, and to effect a gradual and full compression of the artery when pressure in the bladder reaches the systolic pressure inside the artery. The pressure generated by the cuff should not be affected by a gravitational force exerted by the weight of the limb. In other words, the bladder should not compressed by any external forces except the fluid pump and the arterial blood pressure. In addition, when the cuff is positioned on or near the wrist, the wrist should be elevated approximately at the aorta level, otherwise a hydrostatic pressure of blood will cause additional errors. Generally speaking, with consideration of the above-described objectives, prior art pressurizing cuffs have had the following deficiencies: a need for a manual adjustment of the cuff size to match the limb size, and deleterious effects caused by hydrostatic pressure and the limb weight on the accuracy of the pressure measurement. 
         [0009]    To minimize errors that arise from the above deficiencies, numerous cuff designs have been proposed. U.S. Pat. No. 3,527,204 to Lem, which is incorporated by reference herein in its entirety, discloses a dual cuff having a liquid-filled chamber positioned on the top of an air-filled chamber, configured so that the pressure exerted over a patient&#39;s limb is developed by applying pressure to both air and liquid. A dual-cuff design with side-by-side bladders is described in U.S. Pat. No. 3,752,148 to Schmalzbach, which is incorporated by reference herein in its entirety. A dual air chamber cuff design with two chambers positioned in layers is disclosed in U.S. Pat. No. 7,250,030 to Sano et al., which is incorporated by reference herein in its entirety. A cuff designed with a semi-rigid outer layer on an outside surface of the cuff is described in U.S. Pat. No. 6,224,558 to Clemmons, which is incorporated by reference herein in its entirety. U.S. Pat. No. 6,336,901 to Itonaga et al., which is incorporated by reference herein in its entirety, discloses a cuff design including two air bags that are sequentially inflated to provide for a more uniform arterial compression. 
         [0010]    Other cuff designs have been proposed to improved the manner in which the cuff is initially fit over a patient&#39;s limb. See, e.g., U.S. Pat. No. 6,565,524 issued to Itonaga et al. (elastic cuff with elastic plate having a curvature matched to a limb site to be measured), U.S. Pat. No. 7,144,374 to Sano et al. (cuff having adjustable belt applied over a radially changeable elastic member) and U.S. Pat. No. 7,083,573 to Yamakoshi et al. (cuff configured as split ring with pivot), each of which is incorporated by reference herein in its entirety. However, each of the above-referenced cuff designs fails to provide sufficient measurement accuracy. As a result, it would be of benefit to provide a cuff design which can be easily applied to a limb while exhibiting improved measurement accuracy. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention is directed to a cuff for a sphygmomanometer that can be used to measure arterial blood pressure from a patient&#39;s limb (for example, at a patient&#39;s wrist, upper arm or lower arm) while the limb is positioned in a gravitational field. The cuff includes interconnected first and second sections, where the first section is configured to position a pressurizing device (for example, an air bladder) against an arterial side of the patient&#39;s limb, while the second section is mechanically coupled to a support. The pressurizing device is coupled with a pressure sensor for monitoring pressure oscillations in the pressurizing device that are indicative of an arterial blood pressure. 
         [0012]    The second section and the support are mutually arranged within the gravitational field to direct a vector of the gravitational field away from the arterial side and toward a rear side of the patient&#39;s limb, such that substantially no gravitational force is applied to the pressurizing device. In this arrangement, the force generated by the limb within the gravitational field is instead absorbed by the second section and the support. The cuff has a variable geometry that allows the patient&#39;s limb to be easily inserted and then fixedly gripped so that it may be supported by the second section. By diverting the effects of gravitational force away from the pressurizing device, a signal-to-noise ratio of the signals provide by the pressure sensor is improved for more accurate blood pressure measurement 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of illustrative embodiments of the invention, in which: 
           [0014]      FIG. 1  provides a perspective view of a sphygmomanometer including a measurement cuff according to an embodiment of the present invention; 
           [0015]      FIG. 2  provides a cross-sectional view of the sphygmomanometer of  FIG. 1 ; 
           [0016]      FIG. 3  provides a front view of another sphygmomanometer including a measurement cuff according to an embodiment of the present invention; 
           [0017]      FIG. 4  provides a side view of the sphygmomanometer of  FIG. 3 ; 
           [0018]      FIG. 5  provides a side view of another sphygmomanometer including a measurement cuff according to an embodiment of the present invention; 
           [0019]      FIG. 6  provides a perspective view of a variant to the sphygmomanometer of  FIG. 4  including a measurement cuff according to an embodiment of the present invention; 
           [0020]      FIG. 7  provides a perspective view of another sphygmomanometer including a measurement cuff according to an embodiment of the present invention; 
           [0021]      FIG. 8  is a cross-sectional view of the sphygmomanometer of  FIG. 7 ; 
           [0022]      FIG. 9  provides a perspective view of another sphygmomanometer including a measurement cuff according to an embodiment of the present invention; and 
           [0023]      FIG. 10  a cross-sectional view sphygmomanometer of  FIG. 9 . 
       
    
    
       [0024]    Like reference numerals are used in the drawing Figures to connote like components of the sphygmomanometer and measurement cuff. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    The present invention relates to non-invasive arterial blood pressure measurement methods using pressurizing cuffs with suitable pressurizing devices (for example, inflatable bladders). Pressure inside the bladder may be generated by a compressed fluid. For example, the compressed fluid may be selected to be air that is compressed and provided to the bladder by a conventional air pump and released from the bladder by a conventional decompression valve). The pressure generated by the bladder is preferably monitored using a pressure sensor coupled to the bladder. 
         [0026]    The oscillometric method described above may be performed by analyzing oscillations in cuff pressure measurements caused by blood surges passing through a pliant artery that transmit pressure pulses to the bladder. The auscultatory method described above may be performed by analyzing the characteristics of acoustic waves (Korotkoff sounds) produced inside the compressed artery. In each case, embodiments of the method rely on accurate detection of the mechanical oscillations or vibrations of the artery that are of arteries that are transmitted to the bladder. 
         [0027]    These oscillations and vibrations may be detected by a corresponding sensor coupled to the bladder. One source of error operating when a conventional cuff is wrapped around a patient&#39;s wrist and positioned on a tabletop is the weight of the arm and hand. Even small variations in the gravitational force can result in spurious oscillations and vibrations inside the cuff, and thereby contaminate the signals indicating oscillations and vibrations from the arteries. For example, such pressure variations may be caused by patient motions or external vibrations (generated, for example, when the patient is being transported). To minimize such spurious signals, embodiments of the present invention rely on a combination of two design features: a decoupling of the inflatable cuff from the support structure, and a cuff geometry that is adjusted for the size and shape of the patient limb. A key idea behind preferred embodiments of the invention is decoupling the gravitational force from the arterial side of the limb, and directing it toward a back side of the cuff that is adjacent to the rear side of the limb. 
         [0028]      FIG. 1  illustrates a sphygmomanometer according to an embodiment of the present invention that includes a cuff  16  divided into two sections. A first section  101  contains a bladder  11 , and a second section  102  comprises a back support  10  supported by a stem  4 . The cuff  16  is wrapped around a patient&#39;s limb  1 , and locked in place with a suitable locking device such as a locking tape  13 ,  14  (for example, a hook and loop fastener such as a VELCRO® fastener). An inflatable bladder  11  is positioned on the inner side of the cuff  16  (within the boundaries of the first section  101 ) to face an inner side of a wrist of the patient&#39;s limb  1 . In other words, the first section  101  is configured to face the arterial side of the patient&#39;s limb. The bladder is preferably formed from an elastic material, such as latex, synthetic or natural, or any elastomeric material, such as polyurethane. 
         [0029]    The sphygmomanometer of  FIG. 1  further includes an electronic module that is incorporated inside a base  3 , a display  19  and at least one control button  17 . The back support  10  preferably includes a cushion  12  to comfortably support the patient&#39;s limb  1  against the back support  10 . The bladder  11  is inflatable to compress arteries inside the limb  1 , causing a restriction of the blood flow inside the arteries. The restriction generates arterial oscillations which can be detected by a conventional pressure sensor or accelerometer coupled to the bladder  11 . 
         [0030]    As illustrated for example in  FIG. 2 , the back support  10  can be attached to a base  3  by a stem  4 . During operation, the base  3  is preferably placed on a platform such as a tabletop. Referring again to  FIG. 1 , two armrests  7 ,  8  are coupled to the base  3  by corresponding stand-offs  5  and  6 . The armrests  7 ,  8  support the patient limb  1  at positions away from the first section  101 . By supporting the weight of the limb  1  during blood pressure monitoring, the armrests  7 ,  8  relieve the cuff  16  from supporting the full weight of the limb  1  and assist in reducing the effect of the weight of the limb  1  on signal noised generated at the pressurizing device. 
         [0031]    Besides the pressure sensor, base  3  may contain other components, such as a power supply, other sensors, electronic circuitry, an internal pump, valves, and the like. A hose assembly for connecting the bladder  11  to the internal pump, pressure sensors and valves may preferably be hidden inside the base  3  and stem  4 . A liquid-filled bag  31  as shown in  FIG. 2  may preferably be provided at a position between the bladder  11  and limb  1  to improve pressure compliance with the arterial blood flow. 
         [0032]    The sphygmomanometer of  FIGS. 1 and 2  may be operated as follows. Initially, as locking tape  13 ,  14  is unlocked, the first section  101  of the cuff  16  is released from the back plate  10 , and the limb  1  (a patient&#39;s arm, as illustrated in  FIG. 1 ) is placed on the cushion  12  in a manner such that a wrist  30  faces outwardly to position an inner surface (arterial side) of the limb  1  outwardly such that arteries  22  are positioned away from the cushion  12 . The cuff  16  is then wrapped around the limb  1 , and the locking tape  13 ,  14  is secured. In this configuration, the bladder  11  and liquid-filled bag  31  (if provided) face the arteries  22 . 
         [0033]    An operator proceeds to press a switch  17 , which initiates a measurement cycle of the sphygmomanometer. The internal pump pressurizes the bladder  11  to compress the arteries  22  against supporting bones  23  inside the limb  1 . As illustrated in  FIG. 2 , an axis  21  of the back plate  10  is tilted by an angle α with respect to a vertical direction  20  of the sphygmomanometer. The base  3  of the sphygmomanometer is preferably positioned so that the vertical direction  20  is parallel to a gravity vector  24 . Because the limb  1  in this configuration is primarily supported by the cushion  12  and back  10 , the gravitational force vector  24  is directed toward the support  5 , and away from bladder  11  and the arterial side of the limb  1 . The angle α should preferably be set between 20° and 60° (see also  FIG. 3 ). 
         [0034]    The bladder  11  receives arterial oscillations from the arterial side of the limb  1 , and transmits the oscillations to the internal pressure sensor. In response, the internal pressure sensor transmits a signal to the electronic circuit, and the electronic circuit translates the signal to determine a pressure inside the bladder  11 , to compute systolic and diastolic pressure values, and to transmit signals to the display  19  for displaying the systolic and diastolic pressure values. Since the gravity vector  24  is directed away from the bladder  11 , distortions in the arterial pressure arising from variations in the weight vector  24  (for example, as would arise from movements by the patient of the limb  1 ) are reduced. As illustrated in  FIG. 2 , the stem  4  may preferably incorporate a pivot and/or spring  18  configured to further absorb variations in the gravity vector  24  due to patient movement of the limb  1  while it supported by armrests  7 ,  8 . 
         [0035]    To minimize effects of hydrostatic pressure generated by the weight of the blood fluid, it is desirable to elevate the cuff approximately to a vertical level  36  substantially equal to the vertical level of an aorta of the patient. In an embodiment of the present invention as illustrated in  FIG. 4 , the stem  4  is configured to tilt the cuff  16  with respect to a horizon  34  to form an angle β between the horizon  34  and a cuff axis  35 . A horizontal plane defined by the horizon  34  is perpendicular to the direction of the gravity vector. By tilting the cuff  16  in this manner, it can be positioned in proximity to the level  36 . 
         [0036]    Further, to set the cuff  16  at a predetermined position in relation to the wrist  3  of the limb  1 , a guide  33  is preferably provided. When the limb  1  is held by the cuff  16 , the guide  33  is configured to rest at the base  32  of the patient&#39;s thumb, thereby setting a longitudinal position of the cuff  16  relative to the patient&#39;s wrist  30 . In this manner, the guide  33  positions the cuff  16  consistently, thereby improving repeatability of successive blood pressure measurements. A pillow  85  is preferably provided on the base  3  for supporting an elbow  53  of the limb  1  in a comfortable and stable manner. 
         [0037]    Alternate configurations for tilting and supporting the limb  1  to relieve the bladder  11  from the gravity vector  24  are illustrated in  FIGS. 5 and 6 . Both configurations employ one or more legs  52  that may be positioned to rest on and against a tabletop  50  to support the sphygmomanometer and the limb. 
         [0038]    As illustrated in  FIG. 6 , the effect of the gravity vector  24  can be further isolated from the bladder  11  by providing links  54  and a hand rest  55  for further stabilizing the position of the wrist  30  of the limb  1  in relation to the cuff  16 . The links  54  preferably comprise a flexible material (for example, nylon or another suitable plastic) to further absorb variations in the gravity vector  24  due to patient movement of the limb  1 . As shown in  FIG. 5 , an axis  51  of the limb  1  is tilted with respect to a horizon  34  by an angle β that is preferably set between 20 and 45°. As previously described, this positioning helps to keep the level of the cuff  16  approximately at the level of the aorta, and away from tabletop  50  by a distance  56  to safely ensure that the cuff  16  makes no contact with the tabletop  50  during use to negatively affect measurement accuracy. In this configuration, an inner part  58  of the limb  1  (artery side) and the bladder  11  are accordingly not affected by the weight of the limb  1 . 
         [0039]    In addition to relieving the bladder  11  from effects of the gravity vector  24 , the cuff  16  may be sized to provide good compliance in gripping the limb  1 . In other words, the limb  1  is preferably well-supported by the cuff  16 , while at the same time decoupling the weight of the cuff  16  from the bladder  11 . Thus, a rear side of the limb  1  (away from the arteries) is preferably not mechanically coupled to the bladder  11 , but instead is coupled to a weight-supporting part of the cuff. This is illustrated for example in  FIG. 7 , which illustrates a sphygmomanometer according to another embodiment of the present invention. 
         [0040]    The sphygmomanometer of  FIG. 7  contains a base  65  that supports the bladder  11 , and is coupled with a retractable belt  60  that is soft and pliant (for example, a rubberized woven fabric). The belt  60  may preferably be retractably rolled onto a spool  63  rotatably provided within a holding cylinder  62 . The spool  63  is preferably spring-loaded to retract the belt  60  within the spool  63  under the control of a grip  67  positioned inside a handle  66 . The handle  66  serves as a support for the sphygmomanometer, and is in effect a functional equivalent to the support  4  of  FIGS. 1 and 2 . During operation, it is held by an operator in order to support the weight of the limb  1  against the belt  60 . 
         [0041]    As illustrated in  FIGS. 7 and 8 , one end of the retractable belt  60  is fixed to a pin  64 , while the opposite end is attached to the spool  63  so that the belt  60  is movable in a direction  61  into the cylinder  62  until the retractable belt  60  fully embraces the limb  1 . In operation, the operator squeezes the grip  67  which, via links  68 , operates the spool  63  to release and allow the retractable belt  60  to expand outwardly from the cylinder  62 . The limb  1  (for example, beginning with the patient&#39;s hand as illustrated in  FIG. 7 ) can be inserted through the expanded retractable belt  60 . At this time, the bladder  11  is deflated and the pressure sensor coupled to the bladder  11  is “zeroed” with respect to atmospheric pressure. Next, the operator releases the grip  67 , and the spool  63  rotates under spring force to pull the retractable belt  60  in the direction  61  until it tightly encircles the limb  1 . The spool  63  preferably includes a ratchet or other conventional “one-way” mechanism, causing the tightened belt  60  to become locked such that it can no longer be tightened or expanded without further squeezing the grip  67 . An air pump preferably provided within the base  65  inflates the bladder  11 , and arterial pressure is measured by one of the previously-described, known methods known in art. The weight of the limb  1  is supported by the back side  2  of the tightened belt  60  and, subsequently, by handle  66 , while the arterial side of the arm is exposed only to pressure exerted by the bladder  11  and not exerted by the weight of the limb  1 . The weight of the limb  1  may be further supported by resting the limb  1  on a side of the tabletop  50 , or by using one of the supporting structures shown in  FIGS. 1-6 . 
         [0042]    An alternative embodiment of the cuff  16  of  FIGS. 7 and 8  is shown in  FIGS. 9 and 10 . In the embodiment of  FIGS. 9 and 10 , the retractable belt is replaced by an articulated, three-part jaw including a base  78  and clamps  73 ,  77  which are rotatably coupled to the base  78  by pivots  75  and  57 , respectively. The bladder  11  is configured so that it does not protrude beyond open ends of the clamps  73  and  77 . When the clamps  73  and  77  close, they support the limb  1  at lips  74  and  76 , respectively, so that the bladder  11  is relieved from supporting the arm&#39;s weight once the cuff  16  is rotated counter-clockwise to its position as shown in  FIGS. 9 and 10 . 
         [0043]    The clamps  73  and  77  can be opened by squeezing the grip  67  to move in a direction  80 . When the grip  67  is squeezed, the clamps  73  and  77  open so that the bladder  11  may be positioned against the arterial side of the limb  1  in proximity to an interior surface  86  of the base  78 . In this position, the artery  22  can be compressed by the bladder  11  against the bone  23 . Once the bladder  11  is so positioned, the grip  67  is released, the clamps  73 ,  77  are rotated to close and tightly encircle the limb  1 . To facilitate closure, the clamps  73 ,  77  are preferably provided with conventional spring-return mechanisms. 
         [0044]    As illustrated in  FIG. 10 , an internal pump  81  controlled by an electronic control circuit may be housed within an internal cavity  84  of the base  78  of the sphygmomanometer, and inflates the bladder via an inflation tube  83 . A pressure sensor  82  in communication with the bladder  11  via the inflation tube  83  can sense a bladder pressure, and transmits a signal indicating the bladder pressure to the electronic control circuit for processing. The electronic control circuit is preferably housed within a control panel  72  of the sphygmomanometer. 
         [0045]    As illustrated in  FIG. 10 , the control panel  72  may include one or more control buttons  17  for activating the electronic circuit, pump  81 , pressure sensor  82  and electronic control circuit, and is preferably equipped with a reset button  79  to clear the display and reset the sphygmomanometer for a subsequent measurement. The control panel  72  is also preferably equipped with indicator lamps  9  for providing an indication of a current status of the arterial blood pressure measurement. 
         [0046]    While the invention has been particularly shown and described with reference to a number of preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. Accordingly, the invention is to be limited only by the scope of the claims and their equivalents.