1. Field of Invention
The present invention generally relates to cerebrospinal fluid shunts and, more particularly, to a method and device for testing for the presence, absence and/or rate of flow in the shunt tubing implanted under the skin.
2. Description of Related Art
A cerebrospinal fluid (CSF) shunt includes a system of tubing that allows CSF to flow from a patient's brain to another part of the body (e.g., abdomen to relieve pressure in the brain). As a result, it is desirable to know, periodically, that the pathway of the CSF shunt remains unobstructed to permit CSF flow and what the flow rate is. It is also desirable to make these determinations non-invasively when quantifying the CSF flow.
The following describe different apparatus and methodologies that have been used to monitor, determine or treat body fluid flow, including CSF flow through a shunt.
“A Thermosensitive Device for the Evaluation of the Patency of Ventriculo-atrial Shunts in Hydrocephalus”, by Go et al. (Acta Neurochirurgica, Vol. 19, pages 209-216, Fasc. 4) discloses the detection of the existence of flow in a shunt by placement of a thermistor and detecting means proximate the location of the shunt and the placement of cooling means downstream of the thermistor. The downstream thermistor detects the cooled portion of the CSF fluid as it passes from the region of the cooling means to the vicinity of the thermistor, thereby verifying CSF flow. However, among other things, the apparatus and method disclosed therein fails to teach or suggest an apparatus/method for quantifying the flow of the fluid through the shunt.
In “A Noninvasive Approach to Quantitative Measurement of Flow through CSF Shunts” by Stein et al., Journal of Neurosurgery, 1981, April; 54(4):556-558, a method for quantifying the CSF flow rate is disclosed. In particular, a pair of series-arranged thermistors is positioned on the skin over the CSF shunt, whereby the thermistors independently detect the passage of a cooled portion of the CSF fluid. The time required for this cooled portion to travel between the thermistors is used, along with the shunt diameter, to calculate the CSF flow rate. See also “Noninvasive Test of Cerebrospinal Shunt Function,” by Stein et al., Surgical Forum 30:442-442, 1979; and “Testing Cerebropspinal Fluid Shunt Function: A Noninvasive Technique,” by S. Stein, Neurosurgery, 1980 Jun. 6(6): 649-651. However, the apparatus/method disclosed therein suffers from, among other things, variations in thermistor signal due to environmental changes.
U.S. Pat. No. 4,548,516 (Helenowski) discloses an apparatus for indicating fluid flow through implanted shunts by means of temperature sensing. In particular, the apparatus taught by Helenowski comprises a plurality of thermistors mounted on a flexible substrate coupled to a rigid base. The assembly is placed on the skin over the implanted shunt and a portion of the fluid in the shunt is cooled upstream of the assembly. The thermistors detect the cooled portion of the fluid as it passes the thermistor assembly and the output of the thermistor is applied to an analog-to-digital converter for processing by a computer to determine the flow rate of the shunt fluid.
U.S. Pat. No. 6,413,233 (Sites et al.) discloses several embodiments that utilize a plurality of temperature sensors on a patient wherein a body fluid (blood, saline, etc.) flow is removed from the patient and treated, e.g., heated or cooled, and then returned to the patient. See also U.S. Pat. No. 5,494,822 (Sadri). U.S. Pat. No. 6,527,798 (Ginsburg et al.) discloses an apparatus/method for controlling body fluid temperature and utilizing temperature sensors located inside the patient's body.
U.S. Pat. No. 5,692,514 (Bowman) discloses a method and apparatus for measuring continuous blood flow by inserting a catheter into the heart carrying a pair of temperature sensors and a thermal energy source. See also U.S. Pat. No. 4,576,182 (Normann).
U.S. Pat. No. 4,684,367 (Schaffer et al.) discloses an ambulatory intravenous delivery system that includes a control portion of an intravenous fluid that detects a heat pulse using a thermistor to determine flow rate.
U.S. Pat. No. 4,255,968 (Harpster) discloses a fluid flow indicator which includes a plurality of sensors placed directly upon a thermally-conductive tube through which the flow passes. In Harpster a heater is located adjacent to a first temperature sensor so that the sensor is directly within the sphere of thermal influence of the heater.
U.S. Pat. No. 3,933,045 (Fox et al.) discloses an apparatus for detecting body core temperature utilizing a pair of temperature sensors, one located at the skin surface and another located above the first sensor wherein the output of the two temperature sensors are applied to a differential amplifier heater control circuit. The control circuit activates a heat source in order to drive the temperature gradient between these two sensors to zero and thereby detect the body core temperature.
U.S. Pat. No. 3,623,473 (Andersen) discloses a method for determining the adequacy of blood circulation by measuring the difference in temperature between at least two distinct points and comparing the sum of the detected temperatures to a reference value.
U.S. Pat. No. 3,762,221 (Coulthard) discloses an apparatus and method for measuring the flow rate of a fluid utilizing ultrasonic transmitters and receivers.
U.S. Pat. No. 4,354,504 (Bro) discloses a blood-flow probe that utilizes a pair of thermocouples that respectively detect the temperature of a hot plate and a cold plate (whose temperatures are controlled by a heat pump. The temperature readings are applied to a differential amplifier. Energization of the heat pump is controlled by a comparator that compares a references signal to the differential amplifier output that ensures that the hot plate does not exceed a safety level during use.
U.S. Patent Publication No. 2005/0171452 (Neff), which is owned by the same assignee as the present application, namely, Neuro Diagnostic Devices, Inc., and which is incorporated by reference herein, discloses a cerebral spinal fluid (CSF) shunt evaluation system that utilizes pairs of temperature sensors, each pair having an upstream and a downstream temperature sensor and whose outputs are analyzed for providing CSF flow rates when an upstream temperature source is applied to the patient.
U.S. Patent Publication No. 2005/0204811 (Neff), which is owned by the same assignee as the present application, namely, Neuro Diagnostic Devices, Inc., discloses a CSF shunt flow measuring system contains upstream and downstream temperature sensors embedded within the wall of a shunt with a temperature source located between the sensors and whose outputs are analyzed for providing CSF flow.
However, there remains a need to quickly and non-invasively determine the flow status or flow rate of a fluid in a subcutaneous tube.
All references cited herein are incorporated herein by reference in their entireties.
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