A novel method for measuring catheter lock spillage: an in vitro study

Catheters are widely used for blood purification, parenteral nutrition, and for the infusion of drugs. Previous work on catheter lock spillage has focused on the theory and in vitro demonstration of catheter lock spillage caused by the laminar flow profile and by fluid exchange caused by density differences. This work describes an in vitro test with a method that potentially allows measurement of catheter lock spillage in vivo without sampling. The method is based on the change of the electrical resistance of the catheter when the lock solution is injected. This method was tested in vitro with human blood at 36°C using 46.7% trisodium citrate as catheter lock solution. The catheter tip was placed in a beaker filled with whole blood. A stainless steel rod in the beaker served as one electrode and an Arrow-Johans ECG adapter, which was placed on the distal end of the catheter, served as a second electrode. Conductivity was measured with a 5V (rms) 310 Hz sinus voltage and a 10 kOhm resistor in series to the catheter. The driving voltage and the voltage drop at the catheter was continuously measured with a program written under LabView (National Instruments), and the results were converted into mean trisodium citrate concentrations. Within 20 min, the mean trisodium citrate concentration in the catheter decreased to less than 5%. Unlike the previous methods used for catheter lock spillage measurement, this principle can be employed to measure the time course of catheter lock spillage in vivo.