Effects of infusion tubing line lengths and syringe sizes on infusion system compliance: An experimental study using a syringe-type infusion pump at low flow rate

DOI: https://doi.org/10.21203/rs.3.rs-1833191/v1

Abstract

Ideally, the flow delivery of an infusion system is proportional only to the rate of mechanical actuation of the syringe pump plunger. However, in the real world, overall infusion system compliance can vary with components such as tubing lines and syringes. With higher compliance, there may be greater chances of flow irregularity. In this experimental study, we investigated the effects of lengths of infusion lines and syringe sizes on the compliance of syringe pumps with low flow rate (2 ml·hr-1). In the first experiment, infusion system compliance was measured in various settings by occlusion-release. As the infusion tubing length and size of syringe increased, the time to reach each pressure was delayed and the infusion system compliance increased. The contributions to system compliance from syringes were significantly greater compared to those of extended infusion lines. In the second pre-occlusion alarm experiment, the occlusion alarm was delayed by 69.67 ± 3.98 minutes for the 50-ml syringe with a 560 cm infusion line set-up, requiring more attention when using a highly compliant infusion system. Thus, the performance of the syringe pump is affected by loaded syringe size and the length of the infusion line by changing the system compliance.

Introduction

Continuous intravenous drug infusion is one of the principal methods for treating patients. Therefore, it is essential that an accurate dose of drug is delivered to the patient through the infusion system as desired by the attending clinician [1-5]. Compliance, defined as the change in internal volume divided by the change in applied pressure of the system, plays an important role in the accuracy of drug infusion system. In an ideal state, the infusion system can be assumed as a noncompliant rigid system. In a syringe-type infusion pump, rotation of the motor pushes the plunger constantly and delivers the drug at a desired fixed rate. In the real world, when the syringe plunger is pushed by the pump system, the internal volume of the infusion system slightly expands, delaying and decreasing the initial outward flow. In a system with higher compliance, the desired flow is reached at a slower pace, which makes it more difficult to deliver the correct dose of the drug at initiation [6-8]. Likewise, after pushing the stop button on the syringe pump, the plunger movement can stop immediately, but expanded volume by compliance sustains the outflow for a period of time until zero delivery is obtained. Detection of any occlusion of the system can also be extremely delayed [9, 10].

Recent studies have focused on the effects of external factors on the flow rate of low-flow infusion systems. Vertical displacement and vibration conditions with various types of syringe pumps have been explored [11-14]. However, even excluding the effects of external factors in a stable environment, the flow rate of infusion systems can be affected by the internal compliance within the systems. Internal components, like syringe sizes or wall thickness, the material of the plunger head, the length of the extension tubing line, and the viscosity of a drug, can also affect the compliance of the infusion system and, consequently, the performance of the infusion system [15-21]. In some clinical scenarios such as treating a patient infected with a fatal and highly infectious virus, to minimize unnecessary contact and reduce the chance of infection, maintaining a physical distance is necessary along with wearing personal protection equipment to protect the caregivers from infection [22]. An infusion system can be installed in such case outside the patient’s room using a longer extension line so that the empty syringe can be easily replaced. Another option is to keep the infusion system near the patient using a larger syringe to reduce the number of syringe replacements. Compliance will increase in both cases even though it remains difficult to anticipate the exact compliance especially with a low flow rate. The proper length of the tubing line and size of the syringe remain in question.

In this in vitro experimental study, we aimed to investigate the effect of various lengths of infusion line and syringe sizes on the compliance and performance of the infusion system using a syringe-type infusion pump with a low flow rate.

Methods

1. Study Design

This is an in vitro experimental study observing the effect of various lengths of infusion line and syringe sizes on the compliance and performance of the infusion system. The requirement for ethical approval was waived by the Institutional Research Ethics Committee due to the in vitro experimental nature of this study. The experimental design was based on the intravenous drug infusion doses at low flow rate settings. The infusion system for the experiment was composed of a syringe pump (Injectomat MC Agilia, Fresenius Kabi) loaded with a saline-filled syringe (Cong Ty Tnhh mtv Shincahng vina, Vietnam) and an infusion tubing line (polyvinyl chloride extension tubes, JMS Hankook Medical, Republic of Korea). The flow rate of the syringe pump was calibrated with an infusion analyzer (IDA-4 plus, Fluke Biomedical) prior to conducting the experiments. The syringe pump and infusion tubing line were placed at the same height without kinking or coiling to prevent any influence of gravity and the loop effect. To measure the applied pressure, a pressure transducer (TruWave; Edwards Lifesciences, Irvine, CA, USA) was assembled between the infusion tubing line and a 7-Fr central line catheter (3-lumen Arrow Guard Blue, Teleflex Medical) using a three-way connector (Sungwon Medical, Republic of Korea). The tip of the central line catheter was dipped in a half-filled well on an analytical balance (FX-200i; A&D Company) (Fig. 1) [23]. Visible air bubbles were removed by tapping and flushing. In this experimental system, infusion tubing lines and syringes were changeable components that affected the compliance. Various sizes of syringes (10-, 30-, or 50-ml sized) and multiple infusion lines (length for each setting: 140, 280, 420, or 560 cm) were prepared for the experiment. Infusion flow was fixed at 2 ml·h-1. Each set of experiments were repeated five times in a separate room at 22˚C room temperature. 

2. 100mmHg Experiment

Each experiment started after the flow reached a steady state to exclude the effect of start-up delay. The internal pressure at the stopcock was same as the pressure on the monitor since the space was filled with homogeneous fluid. To initiate the experiment, the three-way stopcock was rotated and the infusion system was occluded. Time was manually recorded with a stopwatch for every increment of 20 mmHg to a pressure of 100 mmHg. When the pressure reached 100 mmHg, the three-way stopcock was rotated to release occlusion while the syringe pump continued infusion. After 30 seconds of window period flushing, the accumulated flushed volume was weighed. The final volume was calculated as the weighed volume reduced by the volume administered for the window period.

3. Pre-occlusion Alarm Experiment

The occlusion pressure alarm was set to 900 mmHg by default factory setting with the pre-occlusion alarm turning on 50mmHg below the occlusion setting. In this experiment, the time to the pre-occlusion pressure alarm and the accumulated fluid volume during system occlusion were measured for each syringe size and infusion tubing line length.

4. Statistical Analysis

The mean and standard deviation were used for descriptive analysis. Normality of variables was evaluated using the Shapiro-Wilk test. Continuous variables were analyzed using the Kruskal-Wallis test followed by a multiple comparison using Dunn’s test with Bonferroni correction. Statistical analyses were performed with R version 4.0.2 (R Foundation for Statistical Computing, Vienna, Austria). Graphs were presented using Graphpad Prism 8 (GraphPad Software Inc., CA, USA). The error range for flow regularity was 2% for the pump on the syringe. The error ranges for the length, diameter, and thickness of the syringe were 0.3% to 5% for the infusion tubing line according to the manufacturer’s manual. 

Results

Each experiment consisted of a total of 12 sets of three sizes of syringes and four lengths of infusion lines. Each set of experiments was repeated 5 times. Therefore, a total of 60 runs for 12 sets of compliance experiments and 60 measurements of occlusion experiments were performed and included in the analyses.

1. Compliance of the Infusion System

The changes of the internal compliance for the infusion system according to the length of the infusion line and the loaded syringe size are presented in Figure 2. The overall compliance of the system, which was directly calculated from the accumulated flushed volume after occlusion release when the system pressure reached 100 mmHg, exhibited a consistent tendency to increase as the loaded syringe size increased and the length of the infusion line increased (Table 1). There was a statistically significant difference in system compliance even after post hoc analysis when using a large 50-ml syringe compared to using a small 10-ml syringe (p-value < 0.001). In addition, use of a long infusion line of 560 cm significantly increased the system compliance compared to the setting with the infusion line of 140 cm or 280 cm (p-value < 0.001). In the infusion system loaded with 10-ml, 30-ml, and 50-ml syringes, the increase in system compliance per 140 cm increase in infusion tubing length was 0.04 μL·mmHg-1, 0.07 μL·mmHg-1, and 0.10 μL·mmHg-1, respectively. The difference in overall system compliance was 0.46 μL·mmHg-1 between the 10-ml and 30-ml syringe loaded systems, and the compliance difference between 30-ml and 50-ml syringe loaded systems averaged 0.375 μL·mmHg-1. Both factors (increase of infusion tubing length and loaded syringe size) affected system compliance, but as shown in Figure 2, the syringe size contributed significantly to the increase in infusion system compliance compared to the length of the infusion line. 

2. Pre-occlusion Alarm Experiment

Consistent with the compliance experiment, the pre-occlusion alarm experiment demonstrated that both the time to activation of the occlusion alarm and the accumulated bolus dose at the release of the occlusion increased as the length of the infusion line and the syringe size both increased (Fig. 3). Activation of the pre-occlusion alarm was delayed from 5.20 ± 0.05 minutes with the 10-ml syringe and 140 cm infusion line to 69.76 ± 3.98 minutes with the 50-ml syringe and 560 cm infusion line setup, and the flushed fluid volume at the release of the occlusion increased in weight from 201.40 ± 2.15 mg to 2101.20 ± 110.48 mg. There were statistically significant differences according to infusion line length or syringe size (Table 2).

Discussion

In this experimental study, we evaluated the effects of the length of the infusion tubing line and the size of the loaded syringe on the overall compliance of the syringe-type infusion pump system. We observed that increases in the loaded syringe size and the length of the infusion line both increased the overall compliance of the infusion system. When the loaded syringe size was constant, the system compliance increased slightly as the line length increased by 140 cm. However, the increase in syringe size had a greater effect on system compliance change compared to the increase in infusion tubing line length. 

In the first experiment, as the infusion tubing length increased, the time to reach the target pressure was delayed and the infusion system compliance increased. However, the effect of tubing line length extension on the increase of the overall infusion system compliance was smaller than anticipated. Four times increase in infusion tubing line increased the overall infusion system compliance by approximately one-third from baseline when the loaded syringe size was constant: from 0.17 μL·mmHg-1 to 0.28 μL·mmHg-1 (with a 10-ml loaded syringe), from 0.60 μL·mmHg-1 to 0.80 μL·mmHg-1 (30-ml syringe), and from 0.94 μL·mmHg-1 to 1.24 μL·mmHg-1 (50-ml syringe) (Table 1). However, since small-diameter tubing was used in the experiments, the effect of tubing line extension might have been enlarged when using different tubing lines with higher compliance or larger diameter. 

Consistent with previous studies regarding syringe-type infusion systems, we confirmed that the increase in the loaded syringe size significantly increased the overall infusion system compliance [7, 16, 19]. With a 30-ml syringe, which is three times larger than a 10-ml syringe, compliance increased more than three times, and with a 50-ml syringe, it increased more than four times. Numerous previous studies have reported the increased risk of flow irregularities or the occurrence of inadvertent bolus doses when using syringe-pumps loaded with large sized syringes [13, 14]. In the syringe-type infusion pump system, the use of a smaller syringe would be ideal to minimize system compliance and flow irregularity. However, in real-world clinical practice, the use of small-volume syringes presents issues that inevitably accompany frequent syringe exchanges causing discontinuation of drugs with subsequent start-up delays, increasing the workload of the caregivers as well as the risks of contamination or human errors [24-26]. For this reason, syringe-type infusion pumps are frequently used with large-sized syringes. Large compliance by large-sized syringes and long infusion lines is associated with start-up delays and late recognition of any unexpected occlusion, as seen in our results [6, 7, 9, 16, 27]. Therefore, comprehensive understanding and caution about possible inadvertent drug delivery situations are highly recommended for caregivers when using syringe-type infusion pumps loaded with a large sized syringe. 

The second experiment revealed quantification of the accumulated fluid bolus to the patient after accidental release of system occlusion. If the infusion light is on without any warning alarm activation, an increasing pressure gauge displayed on the corner of the screen should not be of concern. The occlusion pressure pre-alarm is triggered when the pressure reaches 50 mmHg below occlusion while the pump continues infusing drugs. In clinical practice, clinicians are notified first by a pre-occlusion alarm and respond accordingly to resolve the issue. If the pressure is not released and increases, the occlusion alarm turns on, and the syringe pump stops to prevent the accidental bolus injection. A previous simulation study showed different behaviors on decompressing the occluded infusion system [28]. Even with education and simulation to prevent accidents, human error cannot be completely eliminated. Although the anti-bolus system affects up to a 0.2 ml bolus after occlusion release, a mechanical improvement is required for abrupt pressure release or sustained pressure increase below the occlusion pressure. 

It is well-known that flow irregularity increases at a low flow rate [19, 29, 30]. In all experiments in this study, the flow rate of the infusion system was fixed at 2 ml·h-1, which is an acceptable range in clinical settings. Highly potent vasoactive drugs are often started or maintained with a low flow rate. The infusion of norepinephrine or epinephrine at a concentration of 40 μg/ml and rate of 0.02 μg/kg/min for a 60-kg adult is converted to a flow rate of 1.8 ml·h-1. Infusion of remifentanil at a concentration of 50 μg/ml at a rate of 0.03 μg/kg/min for a 60-kg adult is at a flow rate of 2.16 ml·h-1. A diluted drug can be a solution to a high-compliance system by increasing the absolute flow rate and lowering fluid viscosity, especially for patients requiring meticulous management. 

Previously, many studies have investigated the effects of external factors on the performance of infusion pumps with a main focus on the effect of syringe size among internal components. The present study has the strengths of actual quantification of the increased compliance of the infusion system according to length of the infusion line, which was considered a natural concept. However, when using infusion tubing lines with a small lumen diameter, extending the infusion line length does not appear to significantly increase the overall system compliance compared to an increase in syringe volume capacity. With respect to the potential occlusion of the infusion system, the occlusion alarm might not be useful in highly compliant systems, and special caution should be taken to ensure that the accumulated bolus is not flushed into the patient.

This study has several limitations. First, single types of syringe pump, syringe, and infusion tubing line were used for the experiments. However, other syringe pumps might exhibit similar phenomenon. Although the syringe type did not previously affect the time to occlusion alarm activation [27], the syringe and infusion line can make a slight difference depending on the material, thickness, and diameter, which directly affect the resistance and compliance of the system. The anti-reflux valves can contribute to limiting the changes of compliance by increasing the resistance of the system. The density and viscosity of different drugs also have an influence on compliance. Second, stopcock rotation and time measurement could not be automated and executed manually. All the experiments were performed by a single investigator to reduce measurement bias. Third, in the second experiment, the weight increased rapidly with 30 ml and 50 ml syringes. Thirty seconds might not be enough for the accumulated volume to flow out, and the measured weight might have been underestimated. 

In conclusion, the compliance of the syringe-type infusion pump system is increased when the loaded syringe size or the length of the infusion tubing line increases. Ideally, using small syringes and short lines will minimize compliance and reduce flow rate variability. In real-world clinical practice, when treating patients using syringe-type infusion pumps loaded with large syringes with long infusion lines, clinicians should consider the increased compliance of the infusion system and subsequent start-up delays, delayed drug dose controls, and extremely delayed alarms for any occlusion within the system.

Abbreviations

none

Declarations

Funding: The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Conflicts of interest/Competing interests” 

The authors declare that they have no conflict of interest.

Research involving human participants and/or animals: This work did not to include research involving human participants and/or animals.

Ethics approval: Ethical approval was waived by the Institutional Research Ethics Committee due to the in vitro experimental nature of this study.

Informed consent: No human subjects were included in this study, waiving the need for informed consent.

Author contributions

EKL, KYH, and JJM contributed to the study conception and design. EKL and KYH performed experiments. EKL, KYH, YYK, DCC and JJM analyzed data and interpreted results. EKL, KYH and JJM drafted manuscript. KYH and JJM edited and revised the manuscript. All authors read and approved the final version of manuscript. JJM supervised the project.

Acknowledgements: none

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Tables

Table 1. Released weight (mg) at 100 mmHg and calculated compliance (μL·mmHg-1) for each size of syringe and length of line.

Syringe

140cm

280cm

420cm

560cm

p-value

10ml

Weight

33.60±2.94

39.00±0.89

42.20±1.17

45.20±1.17bc

<0.001

Compliance

0.17±0.03

0.22±0.01

0.25±0.01

0.28±0.01bc

<0.001

30ml

Weight

76.60±1.02

80.40±3.38

90.00±3.03

97.00±1.67bc

<0.001

Compliance

0.60±0.01

0.64±0.03

0.73±0.03

0.80±0.02bc

<0.001

50ml

Weight

111.00±2.68a

115.60±2.33a

128.00±1.10a

140.80±2.14abc

<0.001

 

Compliance

0.94±0.03a

0.98±0.03a

1.11±0.01a

1.24±0.02abc

<0.001

Values are mean ± standard deviation and p-values calculated by Kruskal-Wallis test of released weight and compliance of line length of 140, 280, 420, and 560 cm are all 0.002.

a Statistical significance (p<0.025) compared to released weight or compliance with 10-ml syringe and same line length by Dunn’s test with Bonferroni correction.

b Statistical significance (p<0.025) compared to released weight or compliance with 140 cm line and same size of syringe by Dunn’s test with Bonferroni correction

c Significance (p<0.025) compared to released weight or compliance with 280 cm line and same size of syringe by Dunn’s test with Bonferroni correction

Table 2. Time to alarm (second) and released weight after the pre-occlusion alarm (mg) for each size of syringe and length of line.

 

140cm

280cm

420cm

560cm

p-value

10ml 

Time

312.01±3.07

356.85±8.85

436.91±23.65b

484.35±7.21bc

<0.001

 

Weight

201.40±2.15

224.80±2.32

230.00±10.10

249.20±25.51b

0.017

30ml

Time

1840.98±33.26

1973.22±39.53

2047.77±70.30

2478.03±138.84b

<0.001

Weight

1000.00±18.25

1026.80±16.22

1071.80±24.36

1098.80±42.48b

0.002

50ml

Time

3762.74±163.95a

3682.35±163.75a

3993.33±102.59a

4185.84±238.87ac

0.017

 

Weight

2062.60±39.60a

1922.20±56.80a

2073.40±29.39a

2101.20±110.48a

0.027

Values are mean ± standard deviation and p-values calculated by Kruskal-Wallis test of pre-occlusion time and measured weight of line length of 140, 280, 420m, and 560cm are all 0.002.

a Statistical significance (p<0.025) compared to pre-occlusion time or measured weight with 10-ml syringe and same line length by Dunn’s test with Bonferroni correction.

b Statistical significance (p<0.025) compared to pre-occlusion time or measured weight with 140 cm line and same size of syringe by Dunn’s test with Bonferroni correction

c Statistical significance (p<0.025) compared to pre-occlusion time or measured weight with 280 cm line and same size of syringe by Dunn’s test with Bonferroni correction