an infusion pump for infusing solutions from iv bags through tubing to patients . the pump comprises a housing having a compartment which receives an iv bag in a solution - dispensing position . the bag is placed over a bladder within the compartment , and the bladder is expanded by a pressurized fluid to apply a pushing force against the bag which collapses to infuse solution through the tubing . a pump in the housing pumps a fluid such as air into the bladder under influence of a control circuit . a pressure sensor indirectly senses pressure of fluid in the bladder through a pressure of pad which contacts the bladder wall . a control circuit generates a pressure signal responsive to movement of the pressure pad for operating a valve which directs fluid between the pump and bladder . a dispensing spike is provided for interconnecting a dispensing port of the bag in fluid communication with the tubing . the spike is formed with an annulus which cooperates with a wall of the housing to prevent unintended withdrawal of the spike from a dispensing port while infusion takes place . the annulus also operates a switch which is connected in the control circuit to ensure that infusion proceeds only when the bag is in its proper solution - dispensing position in the housing .