Interstitial fluid monitoring

Details Interstitial fluid monitoring Interstitial fluid monitoring

1999-11-19

2002-05-14

Winakur, Eric F. (Department: 3736)

Surgery

Diagnostic testing

Liquid collection

C606S009000, C606S002000, C128S898000

Reexamination Certificate

active

06387059

ABSTRACT:

FIELD OF THE INVENTION
This invention is in the field of medical procedures, namely laser medical equipment used to perforate or alter tissue for monitoring analyte concentration in bodily fluids.
BACKGROUND
The traditional method of measuring blood glucose, or other analytes, consists of taking a blood sample and then measuring the analyte concentration in the blood or plasma. The blood is typically collected from a patient utilizing mechanical perforation of the skin with a sharp device such as a metal lancet or needle.
This procedure has many drawbacks, including the possible infection of health care workers and the public by the sharp device used to perforate the skin, as well as the cost of handling and disposal of biologically hazardous waste.
When skin is perforated with a sharp device such as a metal lancet or needle, biological waste is created in the form of the “sharp” contaminated by the patient's blood and/or tissue. If the patient is infected with blood-born agents, such as human immunodeficiency virus (HIV), hepatitis virus, or the etiological agent of any other diseases, the contaminated sharp poses a serious threat to others that might come in contact with it. For example, many medical workers have contracted HIV as a result of accidental contact with a contaminated sharp.
Post-use disposal of contaminated sharps imposes both logistical and financial burdens on the end user. These costs are imposed as a result of the social consequences of improper disposal. For example, in the 1980's improperly disposed biological wastes washed up on public beaches on numerous occasions. Improper disposal also permits others, such as intravenous drug users, to obtain contaminated needles and spread disease.
There exists an additional drawback of the traditional method of using a needle for drawing fluids. The pain associated with being stabbed by a the sharp instrument can be a traumatizing procedure, especially in pediatric patients, causing significant stress and anxiety in the patient. Moreover, the stabbing procedure often must be repeated before sufficient fluid is obtained. For analytes that need to be constantly monitored, patients may not comply with the frequency of measurement due to the pain involved. In the case of diabetics, failure to measure glucose levels can result in a life-threatening situation.
In addition to blood withdrawal, concentrations of analytes in interstitial fluid can be measured for accurate representation of analyte concentration in the blood. Because of the strong barrier properties of the stratum corneum, however, collecting interstitial fluid through the stratum corneum poses problems. To reduce the barrier function of the stratum corneum, a number of different techniques are presently used, these include: (1) using a metal lancet to cut the skin, (2) chemical enhancers, (3) ultrasound, (4) tape stripping, and (5) iontophoresis. Chemical enhancers pose the problem of potentially reacting with the analyte to be measured. Moreover, the time lapse after application to propagation of interstitial fluid is great. Tape stripping is unsatisfactory because of the pain to the patient. Iontophoresis and ultrasound, similarly have drawbacks in the collection time and the quantity of fluid removed. As previously discussed, the use of a metal lancet has the drawback of patient discomfort and the possibility of contamination.
Thus, a need exists for a method to easily measure the constituents in the blood or other bodily fluids, without: (1) the use of a sharp object, (2) the slow speed of fluid collection, or (3) the pain currently associated with the elimination or reduction of the barrier function of the stratum corneum. The method would further obviate the need for disposal of contaminated sharps and eliminate the pain associated with sharp instruments. The desired method would also, ideally, increase patient compliance for monitoring the desired analyte. The method and apparatus disclosed herein achieves these and other goals.
Lasers have been used in recent years as a very efficient precise tool in a variety of surgical procedures. Among potentially new sources of laser radiation, the rare-earth elements are of major interest for medicine. One of most promising of these is a YAG (yttrium, aluminum, garnet) crystal doped with erbium (Er) ions. With the use of this crystal, it is possible to build an erbium-YAG (Er:YAG) laser which can be configured to emit electromagnetic energy at a wavelength (2.94 microns), among other things, which is strongly absorbed by water. When tissue, which consists mostly of water, is irradiated with radiation at or near this wavelength, energy is transferred to the tissue. If the intensity of the radiation is sufficient, rapid heating can result followed by vaporization of tissue can result. In addition, or alternatively, deposition of this energy can result in photomechanical disruption of tissue. Some medical uses of Er:YAG lasers have been described in the health-care disciplines of dentistry, gynecology and ophthalmology. See, e.g., Bogdasarov, B. V., et al., “The Effect of Er: YAG Laser Radiation on Solid and Soft Tissues”, Preprint 266, Institute of General Physics, Moscow, 1987; Bol'shakov, E. N. et al., “Experimental Grounds for Er:YAG Laser Application to Dentistry”, SPIE 1353:160-169, Lasers and Medicine (1989) (these and all other references cited herein are expressly incorporated by reference as if fully set forth in their entirety herein). Laser perforators of the type explained in U.S. Pat. No. 5,643,252, said patent being incorporated by reference herein, have generally been designed to perforate or alter the tissue of a patient to reduce the barrier function of the stratum corneum, thus allowing for transport of fluid through the target tissue.
SUMMARY OF THE INVENTION
The present invention employs a laser to perforate or alter the skin of a patient for removal and subsequent analysis of interstitial fluid. These measurements can then be used to approximate analyte concentrations in other bodily fluids, such as blood. Prior to application, the care giver properly selects the wavelength, energy fluence (energy of the pulse divided by the area irradiated), pulse temporal width and irradiation spot size so as to precisely perforate or alter the target tissue to a select depth and eliminate undesired damage to healthy proximal tissue. After perforation or alteration, interstitial fluid is allowed to propagate to the surface of the skin for collection and testing.
According to one embodiment of the present invention, a laser emits a pulsed laser beam, focused to a small spot for the purpose of perforating or altering the target tissue. By adjusting the output of the laser, the laser operator can control the depth, width and length of the perforation or alteration as needed, such as to avoid drawing blood into the interstitial fluid sample.
In another embodiment, continuous-wave or diode lasers may be used to duplicate the effect of a pulsed laser beam. These lasers are modulated by gating their output, or, in the case of a diode laser, by fluctuating the laser excitation current. The overall effect is to achieve brief irradiation, or a series of brief irradiations, that produce the same tissue permeating effect as a pulsed laser.
The term “perforation” is used herein to indicate the ablation of the stratum corneum to reduce or eliminate its barrier function. The term “alteration” of the stratum corneum is used herein to indicate a change in the stratum corneum which reduces or eliminates the barrier function of the stratum corneum and increases permeability without ablating, or by merely partially ablating, the stratum corneum itself. A pulse or pulses of infrared laser radiation at a subablative energy of, e.g., 60 mJ using a TRANSMEDICA™ International, Inc. (“TRANSMEDICA™”) Er:YAG laser (see U.S. Pat. No. 5,643,252, Waner et al., which is incorporated herein by reference) with a beam of radiant energy with a wavelength of 2.94 microns, a 200 &mgr;s (microsecond) pulse, and a 2 mm spot size)

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