646
D. Schuster
J. B. Cazalaà D. J. Baker
Reply
Design flaw in continuous syringe pumps
Received: 20 March 1998 Accepted: 22 March 1998 Sir: I appreciate Dr. Lengas' comments. I was not aware of these previous reports in the pathology literature. I agree that it would be quite interesting if a correlation could be established between abnormal cytology (of the kind referred to in his letter) and the physiologic derangements in permeability or lung water that I discussed in my paper. Such a combined approach might indeed improve the specificity of making the diagnosis of ªARDS.º
)
D. Schuster ( ) Washington University School of Medicine, University Box 8225, 660 S. Euclid Ave St. Louis MO 63110, USA Tel.: + 1 31 43 62 37 76 Fax: + 1 31 47 47 82 00 email:
[email protected]
Table 1 Effects of vertical position changes of different syringe pumps and different brands of syringe. Device flow rate 1 ml ´ h1 (BI volume of bolus injection following elevation (1 m) of the device, BA volume of bolus aspiration into the system following the return of the device to its position, ZI time of zero infusion following BA)
Received: 9 December 1997 Accepted: 22 January 1998 Sir: It was with considerable interest that we read the article by Lönnqvist and Löfqvist [1] concerning their observation of flow rate discontinuity from syringe pump infusions related to the height of the infusion device above the patient. Since we have never noticed this phenomenon in the intensive care unit at our hospital, we have examined possible reasons why our experience should be so different. We have considered the following hypotheses: (i) that we may have a better protocol for setting up a syringe pump infusion device, (ii) that we use a relatively large syringe (20 ml) during low rate infusions, (iii) that the brand of syringe used may, in itself, affect the continuity of infusion. We consider that the first hypothesis is untenable since we follow the standard user advice for syringe pump infusers published throughout Europe [2, 3]. The second and third hypotheses could, however, be tested by examination of the linearity of infusion against back pressure variations. Using the same test protocol as described in the paper, we have established that the flow rate discontinuity phenomenon depends on the brand of syringe used. The results shown in the Table 1 indicate
Syringe
that the phenomenon is not evident when using BD Plastipak (50 ml), BD Plastipak (20 ml) or the Monoject (50 ml) syringes. Since these are the types of syringes used exclusively in our unit, this would explain why the irregular flow phenomenon has not been observed at Hôpital Necker. The differences between syringe performance are due to the resultant force (F) acting on the syringe plunger. Three factors influence this force: (i) the siphon force (P) related to the height of the syringe pump, (ii) the cross-section of the syringe (S), (iii) the peak frictional force (Fr) present in the syringe itself during movement of the plunger. When the syringe pump is mounted above the patient, P is negative and the force required to move the syringe plunger is given by the following relationship: F = Fr ½ P ½.S If the term ½ P ½.S < Fr, there will be no movement of the plunger until the pump is active. Many syringe pump manufacturers include the frictional force anticipated in the driving force calculation. This parameter varies between values of 1 and 6, depending on the type of syringe used. The cross-section factor (S) varies approximately between 1 and 2.5 between 20 ml and 50 ml syringes (manufacturers' information provided in confidence). From our preliminary study, we conclude that any specific syringe pump should only be used with the syringes for which it is designed to operate. The characteristics of syringes play an important role in the linearity of infusion arte. To reduce the possibility of errors arising as a result of heightrelated infusion discontinuity, the following
Braun perfusor FM
Braun compact
Graseby 3300
Ivac P 4000
Vial Module type IEC
Vial Pilote type IEC
0.12 0.05 4
0.14 0.03 5
0.11 0.05 4
0.11 0.04 2
0.15 0.01 2
0.15 0.04 4
BD Plastipak 20 ml BI (ml) 0.05 BA (ml) 0 ZI (min) 1
0.05 0.05 2
0.06 0.06 1
0.08 0.04 1
0.05 0.05 0
0.08 0.02 0
0.19 0.06 6.5
1.4 0.1 75
0.35 0.13 15
0.7 0.1 25
1 0.1 45
0.05 0.02 0
0.07 0.02 2
0.1 0.03 1
0.07 0.02 1
0.06 0.02 1
BD Plastipak 50 ml BI (ml) BA (ml) ZI (min)
Braun perfusor 50 ml BI (ml) 0.3 BA (ml) 0.1 ZI (min) 10 Sherwood monoject 50 ml BI (ml) 0.05 BA (ml) 0.03 ZI (min) 1
