A method of estimating the blood flow rate of a heart ventricle assist device whichis positioned externally of, or implanted in, a patient. The assist device comprisesa blood pump having a rapidly rotating, electrically powered impeller, and comprisesbriefly interrupting power to the impeller to cause its rotation to slow. Fromthis, blood viscosity can be estimated, which viscosity is used to obtain realtime, estimated blood flow rates and pressure heads. Apparatus for accomplishingthis is disclosed.
