WO2008036890A1

WO2008036890A1 - Method and device to identify the impedance of tissues and other materials

Info

Publication number: WO2008036890A1
Application number: PCT/US2007/079152
Authority: WO
Inventors: James H. Philip; Thomas Edrich
Original assignee: The Brigham And Women's Hospital, Inc.
Priority date: 2006-09-22
Filing date: 2007-09-21
Publication date: 2008-03-27

Abstract

A method for measuring impedance of tissues or other materials resisting injection of fluid, including the steps of providing a flow sensing device. The flow sensing device includes a fluid flow passage and flow restricting element located within the flow passage. The fluid to be measured is delivered through the flow passage. At least one pressure sensing element is provided in communication with the flow passage upstream of the flow restricting element and at least one pressure sensing element is provided downstream of the flow restricting element. The flow of the fluid though the flow sensing device is measured using the pressure difference generated due to the flow restricting element. The downstream fluid pressure of the fluid is measured and the hydraulic impedance is computed from the relationship between the measured downstream fluid pressure and the measured fluid flow.

Description

METHOD AND DEVICE TO IDENTIFY THE IMPEDANCE OF TISSUES AND OTHER MATERIALS

BACKGROUND OF THE INVENTION:

Field of the Invention:

[0001] The present invention relates to a method and device for measuring impedance that the material to be investigated offers to a fluid that is being injected, more particularly the method utilizes two pressure sensors to measure the flow rate through the device. The impedance of the material that the fluid is being injected into can be calculated using the data from these two pressure sensors.

Description of the Related Art:

[0002] Techniques for direct injection of medication or catheters into the epidural space around the spinal cord are associated with substantial patient risk. It is necessary to identify the epidural space in order to inject the medication and to place the catheter for purpose of anesthesia and analgesia. An epidural needle is used to enter the epidural space. After entering the epidural space, care must be taken not to advance the needle tip further and risk penetrating the dura and possibly damaging the spinal cord.

[0003] One of the usual methods used by anesthesiologists for locating the epidural space involves the injection of saline through needle as the needle is advanced manually past the tissue planes in the back. The anesthesiologist feels a loss of resistance to injection as the needle tip passes into the epidural space. The medication and/or catheter are then inserted through the needle.

[0004] Complications occur when the epidural space is not recognized rapidly and the needle is advanced through the dura into the subarachoid space. This occurs in 1-7% of placements and can carry significant morbidity.

[0005] The standard clinical methods for identifying the epidural space have been the "hanging drop" and the "loss-of-resistance" methods. Several methods have mechanized the indicator for the loss-of-resistance. U.S. Patent No. 6,773,417 includes a bellows that collapses upon entering the epidural space. U.S. Patent No. 5,902,273 performs the same function using a mechanical manometer. U.S. Patent No. 4,175,567 and U.S. Published Patent Application No. 2004/0186430 provide for a membrane attached to the user end of the needle which flexes inward to indicate the loss-of-resistance. U.S. Patent Nos. 5,517,846 and 5,517,846 use the negative pressure to close an electric circuit, thereby providing the user with an alarm. U.S. Patent No. 5,902,273 pressurizes the epidural needle, providing an early warning once the epidural space is entered and also injecting air, thereby pushing the dura away from the needle tip. This may reduce the likelihood of dural puncture.

[0006] A more limited technique of resistance monitoring to differentiate veins from surrounding tissues has been described, studied and commercialized. See U.S. Patent Nos. 6,158,965and 5,087,245.

[0007] However, there still exists a need for a method and device of identifying the epidural space that allows for monitoring of the tissue impedance while allowing the practioner to use the familiar manual loss-of-resistance method. Thus, the user should still be able to "feel" the loss of resistance with his fingers as the device monitors the same property. This will enhance the clinical acceptance and perceived safety of the device.

[0008] Not only is it necessary to identify the point of injection of the fluid into the patient, it is also important to be able to monitor the flow of the medicament as it enters the patient. As is disclosed in U.S. Patent No. 4,898,576, the entirety of which is hereby incorporated by reference, it is known to provide an intravenous flow monitor to provide an indication of resistance to flow at the point of insertion of the catheter. An intravenous flow monitor measures the resistance to flow of fluid in the line connecting the fluid source and catheter or cannula. The measurement can be used to provide an indication of the conditions at the end of the catheter, for example, whether the vein is healthy, phlebitic, or occluded. Thus, by measuring infusion pressure at a known rate, the resistance (impedance) of the vein can be assessed for possible occlusion, infiltration, etc.

[0009] In contrast to U.S. Patent No. 4,898,576, the present invention does not rely on a set flow rate, but instead lets the user determine the flow rate as the syringe plunger on the device is depressed manually by the user. Thus, the user retains the manual "feel" associated with injecting fluid into the tissue, while the device simply measures pressures at two points within the device, thereby calculating the instantaneous flow rate and impedance of the tissue.

[0010] U.S. Patent No. 6,964,204 describes a device used for measurement of flow rate. It includes pressure sensors locally integrated into the device. This device is not described for the measurement of tissue impedance. In contrast to this device, the present invention does not integrate pressure sensors, but rather provides for connection ports to attach clinically approved pressure transducers. Thus, the present invention is less complex to manufacture since it does not require integrated electronics with the inherent risk of exposing the patient to toxins. It can be manufactured from one single material (e.g. Plexiglass™). Another advantage is that the present invention makes use of already available and FDA- approved pressure transducers. Thus, there are less patient- safety concerns.

[0011] Also, in the hospital setting, patients frequently require intravenous (IV) administration of liquids and medications dissolved in liquids. Most often, an IV catheter is inserted into a suitable vein and joined to an injection port. Caregivers can then either inject liquid into the injection port using a syringe (called an "IV bolus"), or attach an IV infusion system consisting of a liquid-filled bag, connection tubing, and an infusion pump that pushes fluid through the connection tubing and into the vein. Administration of the correct fluid volume is critical: In the case of the IV bolus, the caregiver visually checks the gradations on the syringe to verify the volume administered. In the case of an IV infusion, the infusion pump is set to a rate and/or total volume to be infused.

[0012] In the setting of medical simulation it is important to establish the flow rates and volumes administered by the team of trainees while they attempt to resuscitate the "patient." A common method is to require the team member to announce the dosing loudly to the observers as he or she pretends to administer the fluid. The appropriate reaction of the simulated patient is then determined by the observer using a set of rules or by using mathematical modeling techniques.

[0013] In the above described setting of medical simulation modern manikins can include "veins" in the forearms that can be accessed in the same manner as in patients using IV catheters. Here, the trainee actually injects fluid as he announces the dose. One previous known system automatically identifies syringes using bar codes and then continuously weighs the cumulative fluid administered into one simulated "vein" using a digital scale. Disadvantages of this system are that when two or more medications are given simultaneously or in close succession (this is often the case during resuscitation) the individual doses cannot be distinguished in real time. For instance, if two IV infusions are attached, the digital scale will only recognize the total volume given -not the volumes from each pump. Likewise, individual boluses given through the same IV tubing as a simultaneously running IV infusion would not be easily quantified in real-time. Also, the moving syringes are require specific implementation which is likely more cumbersome than the current invention. SUMMARY OF THE INVENTION

[0014] One aspect of the present invention is to facilitate the placement of epidural catheters for anesthesia and analgesia. The present invention provides for a easier, and safer epidural placement.

[0015] While allowing for the conventional "loss-of-resistance" technique, the present invention provides for real-time monitoring of the tissue impedance at the end of the needle and gives early warning of correct needle placement. This may be beneficial in the setting of resident training because an attending anesthesiologist can now supervise as the resident handles the epidural needle. But the attending still has an independent monitor to provide reassurance of the correct placement. Thus, the attending physician is less likely to have to take control of the needle. Safety may also be enhanced because dural punctures and the above-described sequel may be less likely. Likewise, risk management may benefit because the care team is likely to encounter fewer complications. Also, scheduling may be faster due to possibly more efficient epidural placement.

[0016] Yet another aspect of the present invention is to identify tissues for the purpose of obtaining needle biopsies or fluid samples. Currently, interventional radiology procedures are guided by imaging such as ultrasound or CT. The needle is advanced under visualization. However, both ultrasound and CT cast artifacts around a metal needle, obscuring the exact positioning of the tip. In addition, the resolution in space of these imaging techniques may not adequately identify the boundary from one tissue to another. For instance, one may wish to obtain a needle biopsy of a liver tissue, but inadvertently advance the needle into a bile duct that is within the liver. Current imaging may not have the necessary spatial resolution to detect this and therefore lead to a useless biopsy. The present invention can more accurately identify the tissue because, for example, the impedance obtained from within the bile duct is quite different from the impedance in the liver parenchyma.

[0017] Still another aspect of the present invention is to provide a method of identifying a specific structure in interventional radiology, i.e., drainage of cystic or encapsulated fluid collections. The current state of the art is to insert a needle with or without an imaging device or blindly while aspirating in the hope of entering the collection and identifying it by withdrawing the expected fluid through the needle. In orthopedic, internal medicine or rheumatology, the method of the present invention facilitates entry into joint capsules for steroid or other medication injection or for diagnostic fluid withdrawal. [0018] Another aspect of the present invention provides a method for quantifying tissue impedance as medications, such as local anesthetics, are injected into tissues. Injection of local anesthetic into tissue for anesthesia is usually performed manually with a syringe and needle and is often painful. Commercially available devices, for example, The Wand™ manufactured by Milestone Scientific Inc., have the ability to limit pressure applied to the tissues during injection, thus limiting pain. However, this is done without explicit calculation of the tissue impedance. The injection occurs at low flow rates that are adjusted to avoid exceeding preset pressure limits regardless of the tissue impedance. The method of the present invention may be able to further reduce the likelihood of pain by measuring the tissue impedance at a low flow rate which lies well below the pain threshold. Thus, the maximum allowable flow rate could be calculated based on this impedance model. This would avoid simple empirical testing of the pain threshold and would avoid sensitizing the patient during the beginning of the injection.

[0019] Another aspect of the present invention is to provide a method of quantifying the impedance of tissue to more objectively help delineate the borders of a tumor for operative planning in oncology and surgery. Typically, surgeons must anticipate the extent of the tumor tissue by palpitating the tissue intra-operatively.

[0020] During injection of fluid, any possible dislodging of the catheter or needle from the desired tissue space may be detected because the impedance may change, thus providing early warning of incorrect fluid delivery.

[0021] According to these and other aspects there is provided a method for measuring impedance of tissues or other materials resisting injection of fluid, comprising the steps of providing a flow sensing device, the flow sensing device including a fluid flow passage and flow restricting element located within the flow passage. Next the fluid to be measured is delivered through the flow passage. At least one pressure sensing element is provided in communication with the flow passage upstream of the flow restricting element and at least one pressure sensing element is provided downstream of the flow restricting element. The flow of the fluid though the flow sensing device is measured using the pressure difference generated due to the flow restricting element. The downstream fluid pressure of the fluid is measured and the hydraulic impedance is computed from the relationship between the measured downstream fluid pressure and the measured fluid flow.

[0022] The method of the present invention can measure the impedance of living tissue or other material as fluid is delivered via injection needle or catheter. The device includes a proximal chamber that has a port for measurement of hydraulic pressure, followed by a flow restricting element located within the flow passage, and a distal chamber which has another port for measurement of hydraulic pressure. The flow of the fluid though the flow sensing device is calculated using the pressure difference between the distal and proximal ports. The hydraulic impedance is computed from the relationship between the measured downstream fluid pressure and the computed fluid flow.

[0023] A method of identifying the epidural space around the spinal cord in order to inject fluid medications and/or to place a catheter for the purpose of anesthesia and analgesia, comprising the steps of providing a flow sensing device, the flow sensing device including a fluid flow passage and flow restricting element located within the flow passage, and connecting a source of the fluid to an inlet of the flow sensing device. An outlet of the flow sensing device is connected to a medication patient delivery device such as a needle or catheter. The fluid to be measured is delivered through the flow passage. A least one pressure sensing element is provided in communication with the flow passage upstream of the flow restricting element and at least on pressure sensing element is located downstream of the flow restricting element. The flow of the fluid though the flow sensing device is calculated using the pressure differences. The downstream fluid pressure of the fluid is also measured. The hydraulic impedance is computed from the relationship between the measured downstream fluid pressure and the measured fluid flow. The impedance level is monitored to determine the location of medicament patient delivery device, wherein as the epidural space is being entered the impedance will be lowered and an alarm triggered notifying the caregiver to stop advancement.

[0024] These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment relative to the accompanied drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Fig. 1 is a side view of the device of the present invention.

[0026] Fig. 2 is a perspective view of the system of the present invention.

[0027] Fig. 3 is a cross-sectional view of the impedance measuring device of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] Referring to Figs. 1-3, the method of the present invention will be described in conjunction with a detailed description of the system and device. The present invention contemplates a device and method for identifying the impedance of tissues and other substances. The present invention is applicable in numerous settings. For example, an anesthesia care provider can use the system of the present invention to more confidently and quickly identify the epidural space. However, it should be appreciated that although the present invention is described in relation to locating the epidural space of a patient, other uses are contemplated by the present invention.

[0029] During anesthesia delivery an epidural needle is inserted into the epidural space of the patient's spine. However, according to the present invention, as shown in Fig. 1, instead of inserting a syringe 50 directly into the hub of an epidural needle 60, a short rigid flow and/or impedance sensing device 10 is placed in series between the needle and the syringe. Sensing device 10 has a large diameter with a short resistance piece or stenosis 20. As will be described further herein, sensing device 10 has attachments 30, 40 to measure the pressure on both sides of stenosis 20.

[0030] The user injects fluid with the syringe through the impedance sensor 10 and through the needle 60 into the tissue of the patient. Meanwhile, the instantaneous flow rate of the fluid can be calculated from the pressure difference across the resistance 20 according to Poiseuille's effect and the injection pressure can be measured distally to the resistance piece at the port 40.

[0031] As shown in Figs. 1 and 3, sensing device 10 obtains the flow characteristics as will be described further herein. Sensing device 10 includes a body 12 that has an inlet 14 and an outlet 16. Fluid from the syringe 50 or any other patient medication delivery device, such as an IV 52 (Fig. 2), flows into sensing device 10 via inlet 14 and into a first flow passage 22. From flow passage 22 the fluid flows through flow restricting element/stenosis 20 and into a second flow passage 24. Flow passageways 22, 24 form a flow path through the device. From flow passage 24 the fluid exits sensing device 10 via outlet 16 and into the patient through needle 60.

[0032] As illustrated in Fig. 3, an upstream pressure port 26 is provided to attach the tubing 32 (Fig. 2) leading to a hydraulic pressure sensor 30 (Figs. 2 and 3). Likewise, the downstream chamber 24 can communicate with a hydraulic pressure sensor 40 (Fig. 2 and 3) via a tubing connection 34 (Fig. 2) to port 28. The pressures can be measured by two single pressure transducers or via a single differential pressure transducer added to on single pressure transducer attached to the distal port 28. Moreover although being shown located outside of the flow passages, it should be appreciated that pressure sensors can be located within flow passages 22, 24. Also, it should also be appreciated that numerous alternative locations and sensor structures or types can be used and are contemplated by the present invention.

[0033] Referring again to Fig. 2, the data from pressure sensors 30, 40 can be collected by a microprocessor 70 and used to compute the impedance of the tissue in realtime. The principle of the impedance calculation is that the flow (F) due to the pressure gradient (P₁-P₂) across the impedance Z_channei is the same as the flow through the needle and into the tissue having the impedances Z_needi_e and Z_tlssue. This can be expressed as follows:

F = (P₁-P₂)/ Zchannei (equation 1) and F = P₂ / (Z_needie + Z_tlssue) (equation 2) so

Z_tlsSue= Zchannei X P₂/ (Pl "P₂) " Z_needle (equation 3)

where F denotes the direction of fluid flow; and Z_channei, Z_needie and Z_tlSsue denote the impedances of presented by the channel, needle and tissue. Pi and P₂ are ports meant for a fluid communication with a hydraulic pressure sensor. As can be seen from the above equations, four quantities must be known to calculate the impedance of the tissue in question: Zchannei, Z_needie, Pi, and P₂. The impedances Z_cnannei and Z_needie are constants, and the pressures Piand P₂ are measured in real-time by pressure transducers 30 and 40 and processed in realtime by microprocessor 70.

[0034] When using a differential pressure sensor to find the difference between Pi and P₂, a second sensor must be used to measure the downstream pressure P₂ separately. This can be appreciated from equation 3 above. The differential pressure sensor can supply the term Pi- P₂ in equation 3 as highlighted below:

■^tissue ⁼ ^channel X *^* 2' v"l^~ "i> ^" ^needle

The second sensor must provide the absolute pressure P₂ which is alone in the numerator as highlighted in equation 3 below:

Ztissue = Zchannei X P₂/ (Pl "P₂) " Z_needle-

Only then can the equation be solved for Z_tlSsue- Thus, the downstream resistance or impedance is subtracted from the total measured impedance. [0035] In order to determine the location of the epidural space, the method of the present invention operates in the following manner. While advancing through the interspinous ligament the flow rate will be very low and the pressure high, corresponding to a high impedance. When the epidural space is entered there will be a sudden change to low pressure, possibly with high flow as the user's fingers respond to the change in tissue (low impedance), triggering an alert. This alert will prompt the user to stop advancing the needle. In a teaching hospital where trainees learn to perform epidural analgesia, the physician supervisor will not only be able to witness the loss of resistance by watching the trainee perform the injection, but he will also have a more objective measure by watching the display. The impedance can be converted into an audible signal that would not require visualization, thus allowing the supervisor to watch the trainee continuously.

[0036] In the hospital setting, patients frequently require intravenous (IV) administration of liquids and medications dissolved in liquids. Most often, an IV catheter is inserted into a suitable vein and joined to an injection port. Caregivers can then either inject liquid into the injection port using a syringe (called an "IV bolus"), or attach an IV infusion system consisting of a liquid-filled bag, connection tubing, and an infusion pump that pushes fluid through the connection tubing and into the vein. Administration of the correct fluid volume is critical. In the case of the IV bolus, the caregiver visually checks the gradations on the syringe to verify the volume administered. In the case of an IV infusion, the infusion pump is set to a rate and/or total volume to be infused.

[0037] Fluid delivery to the patient can also be continuously monitored whether it is delivered by simple IV infusion or by IV pump (Fig. 2). Moreover, the device may be connected to the patient with tubing 62 instead of a needle 60 as shown in Fig. 1. The device of the present invention may also be used in conjunction with other features, such as a barcode based syringe-identification system to enhance patient safety and an early alarm system.

[0038] The method of the present invention can also serve to measure the flow of liquid administered intravenously to a patient. This may be useful in the clinical setting because errors in IV fluid or IV medication administration could be detected. The present invention may also have application in the setting of medical simulation training where a team of medical personnel practice mastering critical events using a life-size manikin. Another environment the system and method of the present invention has application is in the automatic recording of drugs administered in OR, ICU, or other care settings. The present invention can independently quantify the fluid rate and volumes administered by the team to the manikin, and thus lend the simulation a further dimension of realism. [0039] Any center for medical simulation could use the present device and method to lend further realism to the simulated scenario. The trainee would no longer have to be instructed to call out the doses so that the observers can react. In the operating room, this device could be used to provide surveillance over fluid administration and used by nursing or MD personnel. This device could be incorporated into all Anesthesia Information Management Systems (AIMS) as a direct input of time and quantity of liquid injected. This would serve as a time and drug quantity input to the recording system.

[0040] As noted above, this would update the current practice of calling out administered dosages in the simulation setting. In the perioperative setting, this would update the current practice of visual feedback (syringe injections) or reliance on the settings of an infusion pump, or reliance on the rough manual setting of an IV infusion system. In an AIMS, this would replace the caregiver trying to remember how much and when a drug was administered to the patient.

[0041] The present invention also provides additional objective quantitative information about fluid administration. As such, the present invention provides the first practical input to simulated patients and to an AIMS.

[0042] Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method for measuring impedance of tissues or other materials resisting injection of fluid, comprising the steps of: providing a flow sensing device, the flow sensing device including a fluid flow passage and flow restricting element located within the flow passage; delivering the fluid to be measured through the flow passage; providing at least one pressure sensing element in communication with the flow passage upstream of the flow restricting element and at least one pressure sensing element downstream of the flow restricting element; measuring the flow of the fluid though the flow sensing device using the pressure difference generated due to the flow restricting element; measuring the downstream fluid pressure of the fluid; and computing the hydraulic impedance from the relationship between the measured downstream fluid pressure and the measured fluid flow.

2. The method of claim 1, wherein at least one pressure sensing element comprises a differential pressure sensor and the further comprising the step of measuring the difference between the downstream fluid pressure and the pressure of the fluid upstream of the flow restricting element.

3. The method of claim 1, wherein at least one pressure sensing element comprises a first pressure sensor communicating with the fluid flow upstream of the flow restricting element and a second pressure sensor communicating with the fluid flow downstream of the flow restricting element.

4. A method of identifying the epidural space around the spinal cord in order to inject fluid medications and/or to place a catheter for the purpose of anesthesia and analgesia, comprising the steps of: providing a flow sensing device, the flow sensing device including a fluid flow passage and flow restricting element located within the flow passage; connecting a source of the fluid to an inlet of the flow sensing device; connecting an outlet of the flow sensing device to a medication patient delivery device; delivering the fluid to be measured through the flow passage; providing at least one pressure sensing element in communication with the flow passage downstream of the flow restricting element; calculating the flow of the fluid though the flow sensing device by measuring the pressures generated upstream and downstream of the flow restricting element; measuring the downstream fluid pressure of the fluid; computing the hydraulic impedance from the relationship between the measured downstream fluid pressure and the calculated fluid flow; and monitoring the impedance level to determine the location of medication patient delivery device, wherein as the epidural space is being entered the impedance will be lowered and an alarm triggered notifying the caregiver to stop advancement.

5. A method for measuring impedance of a substance or other material resisting injection of fluid, comprising the steps of: providing a flow sensing device, the flow sensing device including a fluid flow passage and flow restricting element located within the flow passage; delivering the fluid to be measured through the flow passage; providing at least one pressure sensing element in communication with the flow passage upstream of the flow restricting element and at least one pressure sensing element downstream of the flow restricting element; measuring the flow of the fluid though the flow sensing device using the pressure difference generated due to the flow restricting element; measuring the downstream fluid pressure of the fluid; and computing the hydraulic impedance from the relationship between the measured downstream fluid pressure and the measured fluid flow.

6. A method for measuring impedance of tissues or other materials resisting an injection of fluid, comprising the steps of: providing a flow sensing device, the flow sensing device including a fluid flow passage and flow restricting element located within the flow passage; connecting a source of the fluid to an inlet of the flow sensing device; connecting an outlet of the flow sensing device to a medication delivery device; delivering the fluid to be measured through the flow passage; providing at least one pressure sensing element in communication with the flow passage downstream of the flow restricting element; calculating the flow of the fluid though the flow sensing device by measuring the pressures generated upstream and downstream of the flow restricting element; measuring the downstream fluid pressure of the fluid; computing the hydraulic impedance from the relationship between the measured downstream fluid pressure and the calculated fluid flow; and monitoring the impedance level to determine the location of the medication delivery device.