System and method for determining the flow rate of blood in a vessel using doppler frequency signals

DETAILED DESCRIPTION What is claimed is: 1.
A system for determining flow rate of blood in a vessel which communicates the blood between two locations of a patient, the system comprising: a conduit in communication with the blood in the vessel, the conduit having a diversion point for diverting the blood from the vessel into the conduit, wherein the vessel has a flow rate (Q.
sub.
A) upstream from the diversion point and a flow rate (Q.
sub.
D) downstream from the diversion point; a pump connected to the conduit for pumping the diverted blood in the conduit using a pump flow rate; an ultrasonic sensor in communication with the blood in the vessel downstream from the diversion point, the ultrasonic sensor sensing the downstream flow rate (Q.
sub.
D) and generating at least two downstream Doppler frequency signals including a first downstream Doppler frequency signal when the pump has a first pump flow rate and a second downstream Doppler frequency signal when the pump has a second pump flow rate; and a processor connected to the ultrasonic sensor and the pump, the processor determining the upstream flow rate (Q.
sub.
A) when the pump has a selected pump flow rate from the first and second downstream Doppler frequency signals and the selected pump flow rate.
2.
The system of claim 1, wherein the vessel is a hemodialysis access.
3.
The system of claim 1, wherein the conduit further comprises means for returning the diverted blood from the conduit into the vessel.
4.
The system of claim 3, wherein the conduit comprises an external dialysis circuit.
5.
The system of claim 1, wherein the conduit comprises an intravascular catheter.
6.
The system of claim 1, wherein the processor determines a time-averaged mean Doppler velocity from at least one of the first and second downstream Doppler frequency signals.
7.
The system of claim 1, wherein the vessel comprises native tissue.
8.
The system of claim 1, wherein the vessel comprises synthetic material.
9.
The system of claim 1, further comprising a second ultrasonic sensor connected to the processor and in communication with the blood in the vessel upstream from the diversion point, the second ultrasonic sensor sensing the upstream flow rate (Q