Fig. 2

Schematic, illustrating the “push-out” effect, i.e.: the temporary increase in the outflow of the “red” fluid into the patient due to an increase in speed of the “blue” pump, as is explained below. a Two infusion pumps, filled with a “blue” and a “red” solution, respectively, produce the same flow rate, which is constant in time. The two flows are merged at the “mixing point” M, and subsequently travel towards point P, which represents the distal tip of the catheter where the fluid enters the blood stream of the patient. Since the flow rates of both pumps are equal, the catheter from point M to point P contains equal amounts of red and blue fluid, in the form of a mixture. This mixture is travelling with a flow rate that is twice the flow rate of each pump. A constant amount r of red fluid is entering the patient during each unit time interval (illustrated by a canister at point P for each unit time interval). b The same set-up, but now, at \(\hbox {t} = 0\), the flow rate of the “blue” pump is suddenly increased by a factor 5. As a result, after \(t=0\), the amount of red fluid entering the patient at point P now temporarily equals 3r per unit time. Therefore, in b, the “memory effect” is visible: the distal part of the catheter (near P) still contains the “old” mixing ratio corresponding to the situation before \(t=0\). This “old” mixture is now being pushed out at the new speed; hence we dubbed this temporary increase in output of red fluid the “push-out” effect. In the literature, this effect is sometimes referred to as “dead volume” effect, or “catheter memory effect”. c After \(\hbox {t} = \hbox {t}_{delay}\), the amount of red fluid entering the blood stream per unit time equals r again, which equals the intended dose, just like the situation before \(t=0\) in a