Downward movement of syringe pumps reduces syringe output

H. Kern1, A. Kuring2, U. Redlich1, U. R. Döpfmer1, N. M. Sims3, C. D. Spies1 and W. J. Kox1

1Department of Anaesthesiology and Intensive Care Medicine, University Hospital Charite, Campus Mitte, Humboldt University of Berlin, Schumanstrasse 20/21, D-10117 Berlin, Germany. 2Department of Biomedical Engineering, University of Jena, Germany. 3Department of Anesthesia and Partners Healthcare Biomedical Engineering, Massachusetts General Hospital, Harvard Medical School, Boston, USA*Corresponding author

Accepted for publication: February 1, 2001


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
We studied how lowering a syringe pump and changing the outflow pressure could affect syringe pump output. We experimentally reduced the height of three different syringe pump systems by 80 cm (adult setting) or 130 cm (neonatal setting), as can happen clinically, using five flow rates. We measured the time of backward flow, no flow and the total time without flow. An exponential negative correlation was present between infusion rate and time without flow (r2=0.809 to 0.972, P<0.01). Minimum flow rates of 4.4 and 2.6 ml h–1 respectively were calculated to give 60 and 120 s without infusion. The compliance of the different syringe pumps and their infusion systems was linearly correlated with the effective time without infusion (r2=0.863, P<0.05). We conclude that the height of the syringe pumps should not be changed during transportation. If vertical movement of the syringe pump is necessary, the drugs should be diluted so that the flow rate is at least 5 ml h–1.

Br J Anaesth 2001; 86: 828–31

Keywords: equipment, syringe pumps


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Anecdotal reports have noted changes in haemodynamics and oxygenation of neonates and older children due to flow-rate variability of dopamine or epinephrine infusions, in some cases requiring cardiopulmonary resuscitation.1 2 Variation in flow rate affects nitroprusside pharmacodynamics in adults and children receiving continuous nitroprusside infusions.3 Moving patients after surgery to the intensive care unit can destabilize them if they are being treated with catecholamines.4 Lifting an infusion pump containing epinephrine by 80–100 cm while moving a patient from the operating table into bed after surgery led to critical increases in heart rate, blood pressure and left atrial pressure.5 Experimentally, raising or lowering a syringe pump by 100 cm caused bolus administration or a critical delay in infusion, depending on the infusion rate. We found similar effects especially in newborns after correction of congenital heart disease. Lowering the syringe pump by a maximum of 80 cm in adults and by 130 cm in neonates could occur when preparing patients for transport from the theatre to the intensive care unit (Fig. 1). We set out to measure (i) the delay of different syringe pumps after lowering the height of the pump by the maximum distance found in clinical practice; (ii) the effect of infusion rate on the output of the syringe pump when the outflow pressure was altered by lowering the syringe pump; (iii) the minimum infusion rate for each syringe pump system that gave a predetermined delay of infusion.



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Fig 1 The drawing demonstrates the clinical practice of lowering the syringe pump from the position used during the operation to the bottom of the incubator for transport.

 

    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The infusion volume during the change in outflow pressure and the time without efficient infusion for the patient were measured in a closed system using glass capillaries of 0.5 mm diameter and a total length of 1 m. At the end of the capillaries, a manometer was used to simulate a central venous pressure of 8 mm Hg (Fig. 2).



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Fig 2 The experimental study simulating lowering the syringe pumps by 80 cm (adult setting) or 130 cm (neonatal setting). CVP, central venous pressure.

 
One hour before changing outflow pressure, syringes were connected with the closed system and pumps were started with a minimum flow rate of 1 ml h–1. They were kept running throughout the experiment. When a coloured fluid reached a marked area of the capillaries, the syringes were lowered by 80 or 130 cm for adult and neonatal settings respectively, and the time until the marked area was reached again was measured. The following values were determined: (i) the time of the reflux back into the infusion system of the liquid column; (ii) the distance of the reflux of the liquid column; (iii) the time without movement of the liquid column; and (iv) 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 (Fig. 3). This experiment was done using flow rates of 1, 2, 3, 5 and 10 ml h–1 and repeating each measurement eight to 10 times. Afterwards, the syringes and the infusion systems were connected to a commercially available transducer (Ohmeda, Murray Hill, NJ, USA) and a patient monitor (Solar 8000; Marquette Medical Systems, Milwaukee, WI, USA) to determine the compliance of the different systems. The pressure increase from 0 to 300 mm Hg was timed at 25 mmHg intervals and the compliance of the different systems was calculated according to c={Delta}v/{Delta}p using two flow rates.



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Fig 3 Changes in infusion volume–time relationship during movement of the syringe pump. The drawing shows the experimental procedure and the measurement of time with no flow and the missing volume.

 
Materials
Three syringe pumps with their infusion systems were tested: (i) Injectomat-C (Frisenius, Oberursel, Germany) with a 150 cm infusion system (9000911); (ii) Ivac 770 (Ivac, San Diego, CA, USA) with a 200 cm infusion system (G30402); and (iii) Perfusor fm (B. Braun Melsungen Medical, Melsungen, Germany) with a 150 cm infusion system (N872 296/0).

A 50 ml Injectomat syringe was used for all experiments. The tested syringe pumps were used in clinical routine and were therefore checked regularly according to German law (Medizinproduktegesetz).

Statistical analysis
Results are given as mean (standard deviation) after normal distribution had been confirmed with the Kolmogorov– Smirnov test. The exponential correlation between pairs of variables was tested by r2 and linear correlation was determined with the Pearson correlation coefficient. An exponential regression model was tested and the flow rate for a predetermined time without infusion was calculated for each system. The level of significance was P<0.05. We used the software package SPSS 8.0 (SPSS, Chicago, IL, USA).


    Results
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Changing the outflow pressure by lowering the syringe pump by 80 and 130 cm reduced the delivered infusion by between 633 and 23 s depending on the flow (Table 1). There was a statistically significant (P<0.01), exponentially negative correlation between flow rate and time without effective infusion for all three systems tested. Using this exponential regression, we estimated the flow rates when there would be an effective time without infusion of 60 and 120 s (Table 2). These rates were a minimum of 4.4 ml h–1 for 60 s without infusion and a minimum of 2.6 ml h–1 for 120 s without infusion for the adult setting. Lowering the syringe pump by 130 cm (neonatal setting), a minimum infusion rate of 6.0 ml h–1 gave a time of 60 s and 4.3 ml h–1 gave a time of 120 s without infusion. Testing the compliance of the different systems resulted in similar values for the Injectomat-C and the Perfusor fm (81 and 85 µl mm Hg–1 respectively). The compliance of the Ivac 770 system was greater, at 103 µl mm Hg–1. There was a statistically significant positive correlation (r2=0.863, P<0.05) between time without infusion and the compliance of the system.


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Table 1 Time without effective infusion after changing hydrostatic pressure using different flow rates. Results are shown as mean (standard deviation) after confirming normal distribution with the Kolmogorov–Smirnov test
 

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Table 2 Flow rates for predicted times without infusion, calculated for each system according to a logistic linear regression model
 

    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
We found that moving syringe pumps downwards, as we have been doing during transport after cardiac surgery, reduced the flow from the syringe pumps. The time without infusion was related to the infusion rate. Differences between the systems tested were probably caused by the different compliances of the connected infusion systems, and were correlated linearly with the observed time without infusion. The data presented do not endorse any particular manufacturer or model as an infusion device.

Variation in flow rate from syringe pumps has been shown in experimental and clinical settings.2 3 6 Differences in relative syringe pump height during transport are frequent during paediatric cardiac surgery. When transport incubators are used the syringe pumps may be lowered suddenly. Changes in flow of vasoactive drugs may contribute to the haemodynamic instability often seen when moving intensive care patients.7 Relevant time without infusion was found up to infusion rates of >5 ml h–1. The greater compliance of the Ivac 770 system influenced the marked effect of changes in outflow pressure on this apparatus. The greater length of the system (200 cm compared with 150 cm for the Injectomat-C and Perfusor fm) and the specific pressure transducer membrane could cause an enlarged filling volume, despite the favourably low relative piston compliance.8 Because we wished to simulate clinical conditions, we did not change the infusion systems and we compared the Ivac 770 system as it is used clinically.

Variation in drug delivery at different flow rates was simulated in previous studies using a computerized gravimetric technique.9 We wanted to go further and demonstrate the individual flow rates using a fixed pressure of 8 mm Hg working against the infusion, comparable to the central venous pressure in a clinical setting. With this technique we could show the time of retrograde flow, the time with no flow and the time taken to return to the original infusion volume (i.e. the volume before the syringe pump’s position was changed). A clinical study was not appropriate for ethical reasons, because a direct relationship between haemodynamic response and drug administration is known.3

In conclusion, the height of a syringe pump, used to give vasoactive agents, relative to the patient should not be changed for transport. If a change in the syringe pump’s position is inevitable, the concentration of the drugs administered should be adjusted so that flow rates of at least 5 ml h–1 are required. Infusion systems with a small filling volume and small compliance are advantageous.


    Acknowledgements
 
We would like to thank Dr H. Redlich (Centre for Information and Communication, University of Potsdam, Germany) for statistical advice, Mr H.-J. Piper (Department of Medical Engineering, University Hospital Charité, Campus Mitte, Humboldt University of Berlin) for technical help and Mrs A. Todd for linguistic advice.


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
1 Schulze KF, Graff M, Schimmel MS, et al. Physiologic oscillations produced by an infusion pump. J Pediatr 1983; 103: 796–8[ISI][Medline]

2 Klem SA, Farrington JM, Leff RD. Influence of infusion pump operation and flow rate on hemodynamic stability during epinephrine infusion. Crit Care Med 1993; 21: 1213–7[ISI][Medline]

3 Hurlbut JC, Thompson S, Reed MD, et al. Influence of infusion pumps on the pharmacologic response to nitroprusside. Crit Care Med 1991; 19: 98–101[ISI][Medline]

4 Russel GB, Myers JL, Kofke WA. The first 24 h postoperatively. In: Hensley FA, Martin DE, eds. The Practice of Cardiac Anesthesia. Boston: Little, Brown, 1990; 289–301

5 Krauskopf KH, Rauscher J, Brandt L. Influence of hydrostatic pressure on continuous application of cardiovascular drugs with syringe pumps. Anaesthesist 1996; 45: 449–52[ISI][Medline]

6 Stull JC, Erenberg A, Leff RD. Flow rate variability from electronic infusion devices. Crit Care Med 1988; 16: 888–91[ISI][Medline]

7 Evans A, Winslow EH. Oxygen saturation and hemodynamic response in critically ill, mechanically ventilated adults during intrahospital transport. Am J Crit Care 1995; 4: 106–11[Medline]

8 Crisp CB, Slate J, Lovy D. Infusion pump operation, flow rate, and hemodynamic stability during epinephrine infusion. Crit Care Med 1994; 22: 1339–40[Medline]

9 Leff RD, True WR, Roberts RJ. A gravimetric technique for evaluating flow continuity from two infusion devices. Am J Hosp Pharm 1987; 44: 1388–91[ISI][Medline]