Patent Publication Number: US-7708694-B2

Title: Bladder-based cuff for measuring physiological parameters and method of measuring physiological parameter using same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present patent application is a continuation of application Ser. No. 10/695,441, filed Oct. 29, 2003 now U.S. Pat. No. 7,247,143, which is incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The invention relates to inflatable cuff devices. More specifically, it relates to inflatable cuff devices for measuring volume and/or the change in volume with respect to some physiological parameter of a body appendage, and a method of measuring physiological parameters using an inflatable cuff device. 
   BACKGROUND OF THE INVENTION 
   Inflatable cuff devices are used in a variety of applications to measure such parameters as volume, change in volume, pressure, response to pressure, occlusion, and response to occlusion. See, for example, U.S. Pat. Nos. 4,747,415 and 5,692,520 to Lavoisier. These measurements are used widely in, but not limited to, the medical community to assay physiological parameters. Examples of such physiological parameters are blood pressure, cardiac cycle (plythesmography), blood flow, and pulse rate, to name a few. In each case, a bladder-based cuff device is placed around or in contact with the object to be measured (usually a body appendage such as an arm or a fingertip) and a gas, generally air, is pumped into the bladder, causing the bladder to inflate and thus reduce the inner diameter of the cuff. It should be noted that media other than air can be used; however, alternative media such as liquids have the disadvantage of being essentially non-compressible. The reduction of the inner diameter of the cuff results in an excess of bladder material being compressed into the area where the cuff engages the object to be measured. This excess of material creates folds, bends, wrinkles, and voids at the cuff/object interface. Depending upon the nature of the measurement, these irregularities generally result in a source of measurement error. 
   Also, there is a need for a device that accurately measures both the volume and the change in volume of a body appendage as blood or other fluids (such as water) pulse in and out or as fluid accumulates to cause swelling. For a device in which measurements of additional physiological parameters are made using optical emitters and detectors, the device preferably also holds the emitters and detectors against the body appendage in a predictable manner, simultaneous with measuring the volume or the volume changes in the body appendage. In holding the components against the body appendage, the bladder must not allow any air gaps or incorrect alignment of the emitters or detectors. The bladder must also be able to conform to the many different profiles of body appendages, while still allowing for accurate volume and optical measurements. 
   The prior art includes many devices in which a bladder inflates against the finger. Examples of such prior art are disclosed in U.S. Pat. Nos. 4,202,347 and 4,331,155 to Sacks, U.S. Pat. No. 5,025,793 to Richley et al., and U.S. Pat. No. 5,218,966 to Yamasawa. In each case, as the bladder inflates against the finger, gaps are caused by the folding of the bladder material. In addition, the bladders do not conform in a predictable way to each of the different shapes of fingers. These variances cause any measurements made by the bladders to be unpredictable and prone to inaccuracies. Through experimentation, we have concluded that it is not possible to build a bladder that simply inflates from a static, un-inflated position to a variety of body appendage shapes and sizes without having gaps, folds, bends, or wrinkles. Thus, the prior art devices inherently introduce errors into the measurements. 
   It is to the solution of these and other problems that the present invention is directed. 
   BRIEF SUMMARY OF THE INVENTION 
   It is therefore a primary object of the present invention to provide a bladder-based cuff device for measuring physiological parameters of an appendage that conforms to any known shape and size of appendage without folds, bends, wrinkles, and voids at the cuff/appendage interface, so as to insure that the measurements are accurate. 
   It is another object of the present invention to provide a bladder-based cuff device that accurately measures both the volume and the change in volume of a body appendage as blood or other fluids (such as water) pulse in and out or as fluid accumulates to cause swelling. 
   It is still another object of the present invention to provide a bladder-based cuff device that conforms to any known body appendage shape and size without gaps and insures accurate and predictable placement of the emitters and detectors. 
   These and other objects of the present invention are achieved by the provision of a cuff for measuring physiological parameters of an appendage wherein the cuff includes a hollow, rigid tube having an inner surface and opposed ends; and a bladder having an inner surface, an outer surface, and opposed ends. The ends of the bladder are sealed to the ends of the tube to create an enclosed internal volume between the inner surface of the bladder and the inner surface of the tube and an external volume defined by the outer surface of the bladder and surrounded by the internal volume. The bladder has a normal, relaxed state, in which the internal volume is filled with a fluid and a retracted state in which the fluid is evacuated from the internal volume. Two stiffener ribs are placed on the inner surface of the bladder, parallel to each other and to the lengthwise axis of the tube at diametrically opposite positions. A plurality of emitters and detectors are arranged in a linear array are embedded in one of the ribs, so as to emit and detect light through the bladder. A fluid port extends through the tube and communicates with the internal volume, through which the internal volume can be filled with or emptied of the fluid. 
   In one aspect of the invention, the bladder is tubular in shape, with the ends of the bladder overlapping the ends of the tube, and wherein the thickness of the bladder is greater where the ends of the bladder overlap the ends of the tube. 
   In another aspect of the invention, the bladder, in its normal, relaxed state, has an inside diameter smaller than the diameter of the smallest of the type of appendage to be measured, and is made of a material, such as a thin wall rubber or a thin wall silicone rubber, that allows the inside diameter to stretch at least to the diameter of the largest of the type of appendage to be measured, and return back to its original size without deforming. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a bladder-based cuff in accordance with the present invention. 
       FIG. 2  is a cross-sectional view taken along line  2 - 2  of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIGS. 1 and 2 , a bladder-based cuff  10  is shown in accordance with the present invention, comprising a hollow, rigid tube  20  having an inner surface  20   a  and opposed ends  20   b , and a bladder  30  having an inner surface  30   a , outer surface  30   b , opposed ends  30   c , and a center portion  30   d  between the opposed ends  30   c , the ends  30   c  of the bladder  30  being sealed to the ends  20   b  of the tube  20  to create an enclosed internal volume  40   a  between the inner surface  30   a  of the bladder  30  and the inner surface  20   a  of the tube  20  and an open external volume  40   b  defined by the outer surface  30   a  of the bladder  30  and surrounded by the enclosed internal volume  40   a . Preferably, the bladder  30  and the enclosed internal volume  40   a  are both tubular in shape, while the open external volume  40   b  is cylindrical, with the bladder  30 , the enclosed internal volume  40   a , and the open external volume  40   b  all having a common longitudinal axis that is collinear with the longitudinal axis of the tube  20 . 
   The bladder  30  has a normal, relaxed state, in which the enclosed internal volume  40   a  is filled with a fluid, preferably air or another gas, and a retracted state, in which the fluid is evacuated from the internal volume  40   a . In its relaxed state, the bladder  30  does not provide sufficient room for an appendage to be inserted into the external volume  40   b ; while in its retracted state, the bladder  30  does provide sufficient room for the appendage to be inserted into the external volume  40   b . The appendage can be a finger (more specifically, a fingertip), an arm, or any other appendage having one or more physiological parameters normally measured by an inflatable cuff device. 
   A plurality of emitters  50   a  and detectors  50   b  are positioned within the interior of the tube  20 , preferably inside the enclosed internal volume  40   a , as discussed in greater detail hereinafter. Preferably, the emitters  50   a  and detectors  50   b  are positioned in a linear fashion parallel to the longitudinal axis of the tube  20 , with the emitters  50   a  being side-by-side and the detectors  50   b  being side-by-side. In the embodiment shown in  FIGS. 1 and 2 , there are two emitters  50   a  and four detectors  50   b . The emitters  50   a  may generate and emit either monochromatic light or multi-wavelength light, and each emitter  50   a  can include a variety of different types of light sources, including multiple discrete LEDs, multiple discrete laser diodes, and multiple emitting sources combined and coupled into multi-mode fiber optics, as examples. The number of emitters  50   a , or more specifically, the number of wavelengths employed by the cuff  10 , is governed by the number of parameters being measured other than volume or change in volume, as a different wavelength is required for each parameter. The number of detectors  50   b  is determined by the need for spatial differentiation. The factors governing the number of emitters  50   a  and detectors  50   b  are described in U.S. Pat. No. 6,181,958, which is incorporated herein by reference in its entirety. 
   A fluid port  60  extends through the tube  20  and communicates with the enclosed internal volume  40   a , by which the internal volume  40   a  can be filled with or emptied of the fluid. The port  60  is attachable to means (not shown) for emptying fluid from the internal volume  40   a  and filling it with the fluid. This means can comprise a manifold connected to the fluid port  60  via a valve in the manifold and a pump to move the fluid into and out of the manifold, a syringe and a tube connecting the syringe to the fluid port  60 , or any other mechanism designed to move fluids of any type. To allow the appendage to be inserted into the external volume  40   b , the mechanism draws the fluid from the enclosed internal volume  40   a , causing the bladder  30  to assume its retracted state (thus minimizing the enclosed internal  40   a  and maximizing the external volume  40   b ). Once the appendage has been placed into the external volume  40   b , the mechanism releases the fluid back into the enclosed internal volume  40   a  so that the bladder  30  assumes a state in which it conforms to the contours of the appendage. 
   In order for the bladder  30  to conform to any shape of appendage, without gaps, it is necessary for the bladder  30 , in its normal, relaxed state, to have an inside diameter smaller than the smallest of the type of appendage to be measured. To achieve this goal, the bladder  30  is made of a material that allows the inside diameter to stretch at least to the diameter of the largest of the type of appendage to be measured. When the fluid is evacuated from the enclosed internal volume  40   a , a vacuum is created and the bladder  30  stretches, causing the diameters of the bladder  30  and the external volume  40   b  to increase until the bladder  30  conforms to the inner surface  20   a  of the tube  20 , thus becoming larger than the largest of the type of appendage to be tested. When the fluid is returned to the enclosed internal volume  40   a  and the vacuum is released with an appendage inside the external volume  40   b , the bladder  30  attempts to retract to its natural state and conforms to the appendage without any wrinkles or gaps. 
   The selection of the bladder material is very important. The material must be very soft and elastic, and must be able to stretch approximately two to three times its original diameter and return back to the starting size without deforming. In addition, the bladder material must be sufficiently impermeable to the fluid so as to minimize leakage, regardless of the amount the bladder  30  is stretched. The bladder material also must be of a type that can achieve a stable state with respect to its elasticity after it has been stretched. Due to these considerations, materials that are suitable for the bladder  30  include, but are not limited to, a rubber or a silicone rubber. 
   In addition, for practical purposes, the vacuum necessary to stretch the bladder  30  must be kept low to minimize the amount of force necessary to expand the bladder  30  so as to conform to the inner surface  20   a . If the bladder  30  is made of a rubber or a silicone rubber material, then to meet the requirement that the bladder  30  be able to stretch to approximately two to three times its original diameter using a low vacuum, the bladder  30  must have a thin wall. However, the requirement of a thin wall raises another consideration. Thin wall rubbers and silicone rubbers are permeable. Because in some appendages, particularly a finger, the volume change is very, very small as blood or other fluids pulse in and out or as fluid accumulates to cause swelling, the change in pressure is also very small. As a result, when the change in pressure due to the pulsing or swelling of the body appendage is measured, it is also possible to measure the pressure loss in the bladder  30  due to the loss of fluid due to leakage through the wall of the bladder  30 . A thin wall, between 0.012 inch and 0.016 inch thick, allows for the required amount of stretch while minimizing both fluid leakage and the force required to pull the bladder  30  against the surrounding inner surface  20   a  of the tube  20 . 
   With a thin material of the specified thickness, it is necessary to provide a material having a high tear strength (for silicone rubber, for example, about 125 lb/in.), so that the bladder  30  will not be damaged in use as a result of being snagged by a fingernail or rough, course skin. 
   The exact shape of the bladder  30  is also very important. In early testing we found that when a vacuum is created behind a simple tubular bladder, the ends of the bladder always retracted first. This prevented the bladder from retracting with a smooth surface at the center section where the appendage is placed. Instead, there would be a fold in the center section that would prevent the bladder from conforming to the body appendage. The present invention employs a combination of features to overcome this problem. 
   First, two stiffener ribs  70  are placed on the inner surface  30   a  of the bladder  30 , parallel to each other and to the longitudinal axis of the tube  20  at diametrically opposite positions, to reduce buckling. The ribs  70  must be wide enough and thick enough to provide the stability needed, but they must be narrow enough so that the stretch area of the bladder  30  is not reduced to the point that the force necessary to cause the bladder  30  to retract against the inner surface  20   a  of the tube  20  is too great. In addition, the height of the stiffener ribs  70  must be minimized in order to maximize the retracted diameter of the bladder  30  for a given diameter of the inner surface  20   a , and to allow for the insertion of the largest appendage of the type being targeted. The exact dimensions of the ribs  70  will therefore depend on the relative size of the bladder  30 , the desired maximum retraction diameter of the inner surface  20   a , and the desired force required to retract the bladder  30 . 
   Second, we have discovered that the shape and thickness of the bladder  30  at the transition region  30   e  between the ends  30   c  and the center portion  30   d  is critical for proper bladder retraction. There must be a relatively gradual transition from the ends  30   c  of the bladder  30  to the center portion  30   d . Furthermore, the bladder thickness must be about 30% greater within the transition region  30   e  as compared to the thickness of the center portion  30   d , to increase the stiffness of the bladder  30  and decrease transition buckling upon bladder retraction. We have also found that the ends  30   c  of the bladder  30  must overlap the ends  20   b  of the tube  20  by about 30% to achieve the desired stiffness, that is, to decrease buckling at the transition region  30   e  between the ends  30   c  and the center portion  30   d  upon retraction of the bladder  30 . 
   The thickness of the bladder  30  also is greater (about 70% thicker) where the ends  30   c  of the bladder  30  overlap the ends  20   b  of the tube  20 . End caps  80  are provided at the ends  20   b  of the tube  20  to cover the ends  30   c  of the bladder  30  where they overlap the ends  20   b  of the tube  20 . 
   In an embodiment in which the cuff  10  is used to make one or more optical measurements in addition to the volume measurement, the emitters  50   a  and detectors  50   b  must be attached to the cuff  10  such that they are held firmly against the appendage without allowing an air gap or creating a ridge line that pushes against the appendage. The manner in which the emitters  50   a  and detectors  50   b  are attached must be such that they are secure and cannot separate from the bladder  30 . In order to make a smooth surface against the appendage with no ridge lines, the emitters  50   a  and detectors  50   b  are placed on the inner surface  30   a  of the bladder  30 , and the emitters  50   a  and detectors  50   b  respectively emit and detect light through the bladder  30 . In particular, one of the stiffener ribs  70  includes a receiving cavity  72  dimensioned to accommodate the emitters  50   a  and detectors  50   b  so that they are always placed in a consistent manner. Once the emitters  50   a  and detectors  50   b  have been placed within the receiving cavity  72 , an adhesive specifically made for the bladder material is applied as an overlay to attach the emitters  50   a  and detectors  50   b  to the bladder  30 . The application and adhesion methods are critical to ensure that no air or excess glue remain between the surface of the emitters  50   a  or detectors  50   b  and the inner bladder surface  30   a.    
   To ensure consistent optical and pressure measurements, the placement of the appendage within the external volume  40   b  must be consistent with respect to the emitters  50   a  and detectors  50   b . Furthermore, the placement of the appendage must be in a manner to minimize any anatomical or physiological disturbances of the appendage. In the embodiment in which the cuff  10  is used, a sensitive capacitive touch sensor is used as a finger-stop  82  to ensure proper finger positioning within the external volume  40   b  and to minimize blanching. In addition, a gap  84  between the upper surface of the finger sensor and the corresponding end cap  80  accommodates a large range of finger nail sizes. 
   Placing the emitters  50   a  and detectors  50   b  on the inside surface of the bladder  30  raises another consideration that must be addressed. Light passing though the bladder  30  from the emitters  50   a  to the detectors  50   b  without passing though the appendage is called light piping, and is an error source that must be eliminated. To eliminate light piping, the bladder material is tinted with appropriate pigments selected to absorb the specific wavelengths of light employed, effectively damping the light piping signal. The nature of the tint must be such that at the wavelength of light of interest is heavily attenuated for the minimum separation distance of the source and detector. In a preferred embodiment of the bladder-based cuff  10 , the minimum distance is the distance between the illuminating source (the emitters  50   a ) and the closest of the detectors  50   b . This distance is generally no less than 4-5 mm. The attenuation should be such to decrease the intensity of piped light to below that of the governing signal-to-noise ratio (“SNR”) of the detecting channels. That is, the voltage induced within the detectors  50   b  due to light piping should be less than the inherit noise of the electronics and other sources. 
   In addition to damping the light piping signal, it is desirable to allow as much light as possible to pass from the emitters  50   a  through the bladder  30  and into the appendage. As a result, the amount and combination of pigment in the bladder  30  is critical. Acceptable tinting is therefore a function of the initial electromagnetic intensity, wavelength, bladder thickness, and distance to the nearest detector  50   b . An acceptable tinting has been accomplished for wavelengths between 600 and 900 nm at a concentration of 100 parts adhesive to 22 and 20 parts black and blue silicone pigments, respectively. 
   For the most accurate optical measurements, illumination and detection surfaces should not produce inhomogeneities in the finger or tissue beds. The soft and pliable surface of the bladder  30  offers an ideal tissue-to-optical interface by supplying a continuous engagement surface for the finger and measuring system. To maintain the best engagement surface possible, the placement of the electro-optical components (that is, the emitters  50   a  and detectors  50   b ) is therefore on the side of the bladder  30  opposite the finger placement. However, this opposite placement of the emitters  50   a  and detectors  50   b  relative to the finger necessitates the transmission of the light through the bladder  30  twice, once for illumination and once for detection. Thus, in addition to the optimization of tinting substance(s) and concentration(s), it is also necessary to optimize the thickness of the bladder  30  between the finger and the illumination source (that is, the emitters  50   a ). In other words, the bladder material must be optimized to allow sufficient light to pass therethrough into and out of the tissue to: (1) properly illuminate the tissue and (2) properly detect the light back-scattered from the tissue. 
   Building on the need both to attenuate unintentional traverse piping of light between the illuminating source (emitters  50   a ) and the detectors  50   b , specifically selected tinting substances employed within the bladder material can provide wavelength selectivity. For example, any ambient or stray light that couples through finger or tissue bed into the detectors  50   b  that does not originate from the designated illumination source(s) (emitters  50   a ), results in measurement errors or uncertainties. Accordingly, the selection and concentration of tinting substances should provide wide-spectrum attenuation to minimize non-specific light detection. Conversely, the tinting selection and concentration may be such that a specific wavelength is selected or allowed to pass through the bladder  30  without significant attenuation, while all others are inhibited. Carefully considering the characteristics of the bladder material, the geometric spacing of the optical components, and the wavelengths of the emitters  50   a , the tinting and design of the bladder  30  can be optimized to meet a wide variety of light filtering needs. These filtering needs can include, but are not limited to, high-pass, low-pass, and notch-filter configurations. 
   To accommodate appendages other than fingers and finger tips, it is only necessary to modify the size of the above-described cuff  10 , thus maintaining its functionality. 
   Modifications and variations of the above-described embodiments of the present invention are possible, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described.