An implantable drainage device for draining the cerebro-spinal fluid of a patient, the device including: a drainage valve including a valve member and a valve seat; a control unit suitable for modifying an operating parameter of said valve, at least as a function of a measurement (D) of the fluid flow rate through said valve; an actuator controlled by the control unit, suitable for modifying the operation of said valve as a function of said operating parameter as modified by the control unit; and a mass flowmeter suitable for evaluating said flow rate of fluid through said valve.