An implantable infusion apparatus, comprising: an infusate reservoir; a metering assembly comprising an accumulator having an inlet, an outlet, a first mechanical stop and a second mechanical stop; an inlet valve in communication with said infusate reservoir and with said accumulator inlet, and an outlet valve in communication with said accumulator outlet; a diaphragm disposed in said accumulator such that said diaphragm has a resting state position, deflects in a first direction in response to the opening of said inlet valve while said outlet valve is closed to cause infusate to flow into said accumulator from said reservoir, and deflects in a second direction in response to the opening of said outlet valve while said inlet valve is closed to cause infusate to flow out of said accumulator outlet, wherein said first direction and said second direction are different: and a gas chamber containing a pressurized gas, wherein said diaphragm provides a barrier between said pressurized gas and any infusate in said accumulator, and said pressurized gas biases said diaphragm in said second direction, where the bias of said diaphragm by said pressurized gas is overcome when said inlet valve is open and said outlet valve is closed; and a valve accumulator pump programmed to selectively actuate said inlet valve to cause infusate to flow into said accumulator and deflect the diaphragm in said first direction and against said first mechanical stop and selectively actuate said outlet valve to cause infusate to flow out of said accumulator and deflect the diaphragm in said second direction and against said second mechanical stop; characterized in that the infusion apparatus further comprises: a spacer having a concave surface defining said second mechanical stop, said concave surface having an annular groove and a trough in fluid communication with said annular groove and with said inlet and said outlet of said accumulator, a diameter of said annular groove being smaller than a length of said trough and smaller than a distance from said inlet to said outlet, said annular groove is the only annular groove in said spacer, and said annular groove and said trough defining a fluid flow path, wherein the volume of said fluid flow path is equal to or less than about 5% of a total volume of infusate discharged by said accumulator. an infusate reservoir; a metering assembly comprising an accumulator having an inlet, an outlet, a first mechanical stop and a second mechanical stop; an inlet valve in communication with said infusate reservoir and with said accumulator inlet, and an outlet valve in communication with said accumulator outlet; a diaphragm disposed in said accumulator such that said diaphragm has a resting state position, deflects in a first direction in response to the opening of said inlet valve while said outlet valve is closed to cause infusate to flow into said accumulator from said reservoir, and deflects in a second direction in response to the opening of said outlet valve while said inlet valve is closed to cause infusate to flow out of said accumulator outlet, wherein said first direction and said second direction are different: and a gas chamber containing a pressurized gas, wherein said diaphragm provides a barrier between said pressurized gas and any infusate in said accumulator, and said pressurized gas biases said diaphragm in said second direction, where the bias of said diaphragm by said pressurized gas is overcome when said inlet valve is open and said outlet valve is closed; and a valve accumulator pump programmed to selectively actuate said inlet valve to cause infusate to flow into said accumulator and deflect the diaphragm in said first direction and against said first mechanical stop and selectively actuate said outlet valve to cause infusate to flow out of said accumulator and deflect the diaphragm in said second direction and against said second mechanical stop; characterized in that the infusion apparatus further comprises: a spacer having a concave surface defining said second mechanical stop, said concave surface having an annular groove and a trough in fluid communication with said annular groove and with said inlet and said outlet of said accumulator, a diameter of said annular groove being smaller than a length of said trough and smaller than a distance from said inlet to said outlet, said annular groove is the only annular groove in said spacer, and said annular groove and said trough defining a fluid flow path, wherein the volume of said fluid flow path is equal to or less than about 5% of a total volume of infusate discharged by said accumulator. a spacer having a concave surface defining said second mechanical stop, said concave surface having an annular groove and a trough in fluid communication with said annular groove and with said inlet and said outlet of said accumulator, a diameter of said annular groove being smaller than a length of said trough and smaller than a distance from said inlet to said outlet, said annular groove is the only annular groove in said spacer, and said annular groove and said trough defining a fluid flow path, wherein the volume of said fluid flow path is equal to or less than about 5% of a total volume of infusate discharged by said accumulator.