Overspill of catheter locking solution: safety and efficacy aspects

Avoidance of clotting in catheter lumens between treatments usually entails locking with an anticoagulant solution such as heparin. Catheter manufacturers specify internal volume of the catheters, and it is commonly assumed that injection of this precise filling volume is safe and efficient. However, it has been shown that activated partial thromboplastin time (aPTT) increases after instillation of the heparin lock volume specified by the manufacturer. We have investigated this phenomenon in vitro using dye and saline dilution and found that injection of the catheter volume results in spillage of up to 15% of the catheter volume into the patient's blood. Depending on fluid dynamics of the instillation process and catheter compliance, the concentration of the locking solution at the catheter tip is much reduced. As a consequence of this overshoot, the mean concentration at the tip is reduced to approximately 90%. In conclusion, the injection volume must exceed 120% of the catheter lumen for achieving the full strength of the locking solution at the tip. The overshoot may have clinical consequences, (e.g., bleeding if highly concentrated heparin is used).