Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Utility Patent
1998-09-25
2001-01-02
Noori, Max (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
C600S454000
Utility Patent
active
06167765
ABSTRACT:
TECHNICAL FIELD
This invention relates to the field of hemodynamics, and more particularly to a system and method for measuring blood flow rate in a vessel, such as a hemodialysis access.
BACKGROUND ART
Hemodialysis is a process by which blood is passed through an external dialysis circuit to replace the function of a patient's kidney. Blood is removed from the patient's vascular system via an arterial line, is passed through a dialysis filter, and is returned to the patient via a venous line. In order to simplify the withdrawal and return of blood, many dialysis patients have an arteriovenous shunt, or access, surgically created between an artery and vein in a location in the body, such as the upper or lower arm. The access provides a permanent site where the arterial line and venous line can be connected to the patient. A vascular access may be constructed from a native arteriovenous fistula, which is a direct connection of a patient's artery to one of his/her veins, or alternatively may be constructed from a synthetic material, typically polytetrafluoroethylene (PTFE).
While a permanent vascular access provides a convenient connection site for arterial and venous lines, malfunction of such an access is a frequent occurrence in patients receiving chronic hemodialysis. Specifically, unpredictable thrombosis and stenosis in an access causes a reduction in blood flow which necessitates correction through angioplasty or other surgical means. If untreated, low blood flow can cause undesired recirculation in the access, where some part of the freshly dialyzed blood from the venous line flows upstream to the arterial line where it is again filtered. Studies have shown that decreased hemodialysis access flow is associated with an increased risk of access thrombosis and stenosis, such that early detection of an access with a low flow rate is essential in order to prevent more serious complications (see May et al.,
Kidney Int.
52: 1656-1662, 1997).
Therefore, the importance of sufficient access flood flow has resulted in the emergence of access surveillance as a necessary component in the care of patients on hemodialysis. Surveillance techniques have been developed to detect low blood flow predictive of future thrombosis and stenosis.
An early method of calculating the access flow rate involves occluding the access, placing a needle into the access to monitor the pressure therein, and pumping blood around the occlusion to determine the relationship between blood flow rate and pressure within the access. This intra-access pressure monitoring may be performed either upstream (see Langescheid et al.,
Dialysis and Transplantation
June: 54-55, 1977) or downstream (see Brosman et al.,
J. Am. Soc. Nephrol.
7: 966-969, 1996) from the occlusion. Unfortunately, occlusion of the access may lead to thrombosis, and placement of the needle or pressure sensor within the access is invasive. Static and dynamic venous pressure monitoring, whereby the pressure within the access is measured with the dialysis blood pump off (static) or on (dynamic), have also been used for surveillance (see Besarab et al.,
ASAIO J.
Jan-Feb: 35-37, 1998; Schwab et al.,
Kidney Int.
36: 707-711, 1989). However, these methods do not correlate well enough with blood flow rate and lack the sensitivity and specificity needed for accurate access surveillance.
At present, the most reliable methods for surveillance of access blood flow utilize conventional Doppler ultrasound (see Stauch et al.,
Am. J. Kidney Dis.
19: 554-557, 1992; Kirshbaum and Compton,
Am. J. Kidney Dis.
25: 22-25, 1995; Findley et al.,
Radiographics
13: 983-999, 1993; Sands,
ASAIO J.
Jan-Feb: 41-43, 1998; Oates et al.,
Ultrasound Med. Biol.
16: 571-579, 1990; Sands et al.,
ASAIO J.
38: M524-M527, 1992) or indicator dilution techniques (see Depner,
ASAIO
Jan-Feb: 38-39, 1998; Krivitski,
Kidney Int.
48: 244-250, 1995; Lindsay et al.,
ASAIO J.
Jan-Feb: 62-67, 1998).
To evaluate a vascular access using Doppler ultrasound, an ultrasound unit with both imaging and spectral flow Doppler capabilities, termed duplex ultrasonography, is typically utilized. Access blood flow is calculated using the time-velocity integral of a spectrum obtained from a representative area of the access. The cross-sectional area of the access is measured via imaging, and from these measurements volume blood flow is calculated. However, Doppler ultrasound techniques are fraught with sources of operator error, most often associated with the determination of cross-sectional area. In addition, conventional Doppler ultrasound is labor intensive and expensive, such that measurements are not usually made with high enough frequency to effectively monitor the onset of reduced access flow.
Indicator dilution methods have also been utilized to measure access blood flow. U.S. Pat. No. 5,685,989 issued to Kravitski et al. discloses a dilution technique which uses ultrasonic sensors on the arterial and venous lines. For the measurement of access blood flow, the blood lines are reversed and a temporary recirculation is created. Then, a known quantity of an indicator, such as saline, is injected into the venous line. This dilutes the flow of blood in the access, resulting in Doppler velocity changes measured by the ultrasonic sensor on the arterial line. Because this change is proportional to the concentration of injected saline in the blood, access flow can be calculated. The use of other indicator dilution methods to determine blood flow can be found in U.S. Pat. No. 5,312,550 issued to Hester, U.S. Pat. No. 5,510,716 issued to Buffaloe, IV et al., and U.S. Pat. No. 5,644,240 issued to Brugger. Unfortunately, conditions affecting indicator mixing and recirculation of the indicator through the cardiovascular system can affect the accuracy of results using this method. Furthermore, due to the necessity for the reversal of blood lines during dialysis, dilution techniques are cumbersome and time-consuming.
DISCLOSURE OF THE INVENTION
Therefore, a principal object of the present invention is to provide a system and method for determining the blood flow rate in a vessel.
It is a further object of the present invention to provide a system and method for accurately measuring blood flow rate in a vessel without relying on a measurement of vessel cross-sectional area.
It is another object of the present invention to provide a system and method for determining blood flow rate in a hemodialysis access in a simple, safe, and efficient manner.
Accordingly, a system is provided for determining the blood flow rate in a vessel of a patient without requiring a measurement of the cross-sectional area of the vessel. A conduit is provided in communication with the vessel, and has an inlet for diverting blood from the vessel into the conduit. Disposed within the conduit is a pump for diverging blood out of the vessel using at least one pump flow rate. An ultrasonic sensor in communication with the vessel generates at least one Doppler frequency signal correlated with a blood flow rate in the vessel downstream from the inlet, wherein the downstream flow rate depends on the known pump flow rate and an unknown blood flow rate in the vessel upstream from the inlet. A processor is provided in communication with the ultrasonic sensor and the pump for determining the upstream flow rate from the signal and the pump flow rate.
Correspondingly, a method is provided for determining the blood flow rate in a vessel. The method includes diverting blood from the vessel at a diversion point to obtain a flow of diverted blood in a conduit, then pumping the diverted blood through the conduit using at least one pump flow rate. The method further comprises generating at least one Doppler frequency signal correlated with a blood flow rate in the vessel downstream from the diversion point, wherein the downstream flow rate depends on the known pump flow rate and an unknown blood flow rate in the vessel upstream from the diversion point. Lastly, the method includes processing the signal and the pump flow rat
Brooks & Kushman P.C.
Noori Max
Patel Jagdish
The Regents of the University of Michigan
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