Flow dynamics of a retrograde i.v. drug infusion system

Drug delivery by the i.v. retrograde method was studied at flow rates and dosage volumes typically used in neonatal intensive-care units. Methylene blue was injected into the administration tubing used for delivery of the maintenance solution. Sets of serial samples delivered by the infusion system were obtained until no color was visible in the ejected solution. Samples were collected at a flow rate of 6 ml/hr for dosage volumes of 1-8 ml and at a dosage volume of 3 ml for flow rates of 2-16 ml/hr. Dye concentrations were measured by spectrophotometric assay, and the cumulative amounts of dye ejected were computed. The percent of total dose delivered was plotted against the cumulative volume of fluid delivered. At all dosage volumes, approximately 1.7 ml of additional maintenance fluid was required for delivery of 95% of the dose. Flow rate did not affect the total volume required for delivery of 95% of a 3-ml dosage volume. Drug delivery by the retrograde system was predictable, and the dilution factor was not dependent on dosage volume or flow rate. A dilution constant of approximately 2 ml can be used in computing appropriate dosage volumes and predicting time required for drug infusion.