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Downward movement of syringe pumps reduces syringe output.

H Kern, A Kuring, U Redlich, UR Döpfmer, NM Sims, CD Spies, WJ Kox. Br J Anaesth 2001: 86: 828-31

The aim of this study was to measure the function of different syringe pumps after lowering the height of the pumps by the maximum distance found in clinical practice. In particular, the effect of infusion rate on the output of the syringe pump, and the minimum infusion rate that gave a predetermined infusion delay were measured. Three syringe pumps with their respective infusion lines were tested. Injectomat C (German) with its 150 cm long infusion line, Ivac 770 (USA) with its 200 cm infusion line, and Perfusor fm (German) with its 150 cm long infusion line. A 50 ml Injectomat syringe was used for all experiments. In-vivo closed system simulation was achieved by using glass capillaries, 0.5 mm diameter and 1 meter long. At the end of the capillaries, a manometer was used to simulate a central venous pressure of 8 mm Hg. The syringes were connected to the closed system, and pumps were started with a minimum flow rate of 1 ml/hr and kept running throughout the experiment using a colored fluid. When the fluid reached a marked area on the capillaries, the syringes were lowered by 80 or 130 cm for adult and neonatal settings, respectively. The time until the marked area was reached again was recorded. The following variables were determined: the distance of the reflux back into the infusion system of the liquid column; the distance of the reflux of the liquid column; the time without movement of the liquid column; and the time for the liquid column to reach the starting position again. The infusion volume that was not administered was calculated according to the stopped time and flow rate. The experiment was repeated using the flow rates of 1, 2, 3, 5, and 10 ml/hr, and repeating each measurement 8 to 10 times. The compliance of each system was caculated.

Comments: There was a significant exponential negative correlation between flow rate and time without effective infusion for all three systems tested. Using an exponential regression, it was estimated that at the adult setting, where the pumps were lowered by 80 cm, that at a minimum flow rates of 4.4 ml/hr, there would be an effective time without infusion of 60 sec; and at a minimum of 2.6 ml/hr the duration of time without effective infusion would be 120 sec. Likewise, for the neonatal settings, when lowering the infusion pump by 130 cm, a minimal infusion rate of 6 ml/hr resulted in a time of 60 sec, and 4.3 ml/hr gave a time of 120 sec without infusion. Relevant time without infusion was found up to infusion rates of >5ml/hr. The compliance of the Injectomat-C and Perfusor fm pump systems were similar, 81 and 85µl/mm Hg, respectively, but that of the Ivac 770 was significantly greater, at 103 µl/mm Hg. This compliance was positively correlated with the time without infusion. This demonstrated that the height at which the pump is placed, the compliance of the delivery system, and the dialed flow rate, all play a significant role in the amount of infusion that is actually delivered.

When we transport a precariously labile infant after surgery to the intensive care unit, there are usually several infusion pumps attached to the patient. It is not uncommon that there is insufficient space near the patient to secure all the pumps. Often, these pumps are placed on a shelf below the patient. We are all very familiar with the changes in flow rate associated with the diameter and length of an intravenous cannula and the height of the infusion bag. Nevertheless, when an infusion pump is used, many of us think that the pump will deliver whatever we set, regardless of where we place the pump. If it is not possible to maintain the height of the infusion pump at the same level during transport as in the operating room, the flow rate should be adjusted to <5ml/hr. An infusion system with minimum filling volume and compliance is always preferable. With advances in technology, we tend to put too much faith in machines and their arrogant digital readouts. This paper jogs us back to reality.

Reviewed by: Hoshi J. Khambatta, MD
Babies & Children's Hospital
Columbia Presbyterian Medical Center, NY
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