Fluid delivery device for delivering a substance to a patient at a rate of suministrodecreciente stages, comprising: a system (A, B ... N) reservoir adapted to apply at least a first pressure in a fluid system such contenidoen reservoir for a fluid flow being further adapted such deposition system for applying at least a second pressure in a fluidocontenido in said reservoir system for a fluid flow, being further adapted such deposition system for applying said first pressure, and then, said second pressure when fluid flows from said reservoir system, pressure being applied dichasegunda as decreased stepwise in applied pressure from said first pressure, a device (13) infusion and a fluid connection for communicating said fluid flow from said reservoir system with dichodispositivo infusion, wherein said reservoir system comprising: at least a first and a second reservoirs (29, 31) characterized in that said tank system further comprises at least a first spring (29) constant force placed adjacent to said first tank (29) to apply pressure to said first reservoir, applied in response dichaprimera said first fluid pressure in said reservoir and at least one second spring (37) constant force located adjacent to said second tank (31) to apply pressure in said second reservoir, applied in response said second depósitodicha second pressure in said fluid, wherein the first spring (29) constant force I estádiseñado to apply a substantially constant first pressure in the fluid at least unprimer intermediate range (M) of operation and the second spring (37) constant force estádiseñado for applying a second substantially constant pressure in the fluid at least unsegundo intermediate range (M) of operation, wherein said second pressure is lower adicha first pressure.Dispositivo de suministro de fluido para suministrar una sustancia a un paciente a un caudal de suministrodecreciente por etapas, que comprende: un sistema (A, B ... N) de depósito adaptado para aplicar al menos una pr
