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.