A fluid delivery system includes a first fluid container having infusion fluid for a patient. The first fluid container has an outlet and a conduit attached to the outlet. A second fluid container applies pressure to the first fluid container to force fluid out of the first container and into the conduit at a desired flow rate. A pressure sensor is coupled to the second fluid container and a flow sensor is coupled to the conduit. The processor monitors the flow rate and controls the duty cycle of a downstream flow control actuator to maintain the desired flow rate. The processor determines the downstream resistance of the system by varying the flow rate about the selected flow rate, receiving pressure signals from the pressure sensor and flow signals from the flow sensor, and calculating the change in pressure over the change in flow rate to produce the fluid resistance measurement.