Surgery – Means for introducing or removing material from body for... – Material introduced into and removed from body through...
Reexamination Certificate
2000-10-12
2003-03-25
Huson, Gregory (Department: 3751)
Surgery
Means for introducing or removing material from body for...
Material introduced into and removed from body through...
C600S581000, C604S048000, C604S093010, C604S175000, C604S180000, C604S264000, C604S290000, C604S500000, C604S507000
Reexamination Certificate
active
06537243
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a device and a method for obtaining interstitial fluid from the body of a patient for use in a diagnostic test. More particularly, this invention relates to a device and a method for obtaining interstitial fluid from the body of a patient at a rapid rate of collection.
2. Discussion of the Art
Interstitial fluid is the substantially clear, substantially colorless fluid found in the human body that occupies the space between the cells of the human body. Several methods have been used to obtain interstitial fluid from the body of a patient for diagnostic tests. Diagnostic tests that can be run with samples of interstitial fluid include, but are not limited to, glucose, creatinine, BUN, uric acid, magnesium, chloride, potassium, lactate, sodium, oxygen, carbon dioxide, triglyceride, and cholesterol.
It is much more difficult to obtain a sample of interstitial fluid from the body of a patient than it is to obtain a sample of blood from the body of a patient. Blood is pumped under pressure through blood vessels by the heart. Consequently, a cut in a blood vessel will naturally lead to blood flowing out of the cut because the blood is flowing under pressure. Interstitial fluid, which is not pumped through vessels in the body, is under a slight negative pressure. Moreover, the amount of interstitial fluid that can be obtained from a patient is small because this fluid only occupies the space between the cells of the human body. Currently available methods for obtaining large amounts of interstitial fluid are unsatisfactory, because these methods are accompanied by undesirable side effects.
Several methods have been employed to obtain access to interstitial fluid for glucose monitoring. These methods include, but are not limited to, microdialysis, heat poration, open flow microperfusion, ultrafiltration, subcutaneous implantation of a sensor, needle extraction, reverse iontophoresis, suction effusion, and ultrasound.
Microdialysis involves placing microdialysis tubes in the body, introducing a fluid into the tubes, allowing the fluid to traverse the length of tubing in the body, and withdrawing the fluid to a location outside the body. As the fluid passes through the microdialysis tubing in the body, glucose from the body is exchanged with the fluid inside the tubing, resulting in a change in glucose concentration primarily in the fluid inside the tubing. The change in the concentration of glucose in the tubing can be measured with a sensor that is external to the body.
There are several drawbacks in the use of microdialysis equipment for measuring the concentration of glucose. Microdialysis tubes have walls. The walls of the microdialysis tubes are formed from a material called dialysis membrane. This membrane allows molecules below a certain size to pass but restricts the movement of larger molecules. The amount of glucose that is exchanged may be small, leading to small changes in the concentration of glucose in the fluid inside the microdialysis tubing. These small changes in the concentration of glucose can be difficult to detect accurately. Moreover, the amount of time required for the fluid to circulate through the microdialysis tubes can be great. Accordingly, the concentration of glucose being measured by a sensor that is external to the body can lag behind the actual concentration of glucose inside the body by several minutes. Reducing the length of the tubing and increasing the rate of the pumping of the fluid can decrease the duration of this lag, but such actions also decrease the amount of glucose being transferred to the tubing. In addition, obtaining accurate measurements of the concentration of glucose from solutions having a low concentration of glucose is difficult. Furthermore, the microdialysis tubing can break off during use or upon withdrawal from the body, thereby presenting a hazard to the user. Finally, the exchange of glucose across the membrane of the tubing can vary over time, resulting in erroneous determinations of the concentration of glucose.
Heat generated by a light from a laser that acts upon a dye or heat generated by a heated wire can be used to form openings in the outermost layer of the skin, the stratum corneum. The formation of openings in the skin by means of heat is described in WO 97/07734. Interstitial fluid can be extracted from the openings in the skin by means of a vacuum or by application of pressure around the periphery of the openings. The use of a laser to form openings in the skin is expensive, because the laser must not only be powerful enough to cause the formation of the openings in the skin, but must also be properly focused to create small openings in the skin. A plurality of openings must be formed in the skin order to obtain a sufficient quantity of interstitial fluid. If one laser is used, the mechanism housing the laser will be complex and costly, on account of the necessity of additional components for moving the laser to a plurality of locations on the stratum corneum. Alternatively, a plurality of lasers could be incorporated into an instrument to form a plurality of openings in the stratum corneum. This approach would be costly because of the additional cost of extra lasers. Because of the limitations of the laser and because of unsightly discoloration caused by the formation of openings in the skin, the number of openings per each interstitial fluid extraction operation is typically limited to three to six. The amount of interstitial fluid extracted will be limited to the amount that can be drawn through these openings. A greater number of openings could provide an increased rate of collection of interstitial fluid, but a greater number of openings would be impractical. The openings would have to be distributed over a wide area of skin, thereby making the harvesting of the interstitial fluid difficult.
Open flow microperfusion is similar to microdialysis. A fluid flows through a tube placed in the body, and the fluid is exchanged between the body and the tube. The concentration of glucose in the fluid exiting the body is proportional to the concentration of glucose in the body. Typically, if the concentration of glucose in the fluid inside the tube is initially zero, by the time the fluid leaves the body, the concentration of glucose in the exiting fluid will be one-third that of the concentration of glucose in the body. The difference between open flow microperfusion and microdialysis resides in the type of tube used. Microdialysis tubes have very small pores that are designed to allow only small molecules to diffuse through the walls of the tube. Pore sizes in microdialysis tubing are typically on the order of 1 to 10 nm. Open flow microperfusion systems have pores typically on the order of 200 micrometers. In the case of open flow microperfusion, the pores should not restrict the movement of any molecules in the interstitial space. Neither the microdialysis method nor the open flow microperfusion method extracts a pure sample of interstitial fluid; accordingly, these methods require a calibration factor.
Ultrafiltration involves placing microdialysis tubing inside the body and extracting interstitial fluid from the body through the tubing by means of vacuum. A steady stream of fluid cannot be obtained because the application of vacuum leads to the formation of bubbles in the fluid. A lower level of vacuum would reduce bubble formation but would increase the amount of time required to remove the sample of interstitial fluid from the body and transfer it to a glucose detector. The pores of the microdialysis tubing become clogged over time, thereby leading to lower flow rates or the need to increase levels of vacuum. The interstitial fluid that is obtained does have concentrations of glucose similar to that found in blood, making the determination of the concentration of glucose more accurate than that of microdialysis. However, the length of tubing that must be inserted under the skin is typically on the order of centimeters in length. A
Henning Timothy P.
Lowery Michael G.
Abbott Laboratories
Huson Gregory
Prunner Kathleen J.
Weinstein David L.
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