Midline IV Catheters: The Battle of AST vs MST
Edward Korycka, MSN, RN
The use of midline catheters has grown dramatically over the last 5 years - seemingly related to various hospital initiatives and clinical studies. Midline catheters often solve a nice “middle ground” between the need for a central line and the need for a PIVC that will not quickly clot and fail – though we know not all medications and therapies should use a midline (see blog post written by Dr. Nancy Moureau that outlines the use of midlines vs PICCs).
There are two main types of midline catheters: AST midlines and MST midlines. AST midlines are inserted using an “all-in-one” technique, while MST midlines are inserted using a two-step technique. Both MST and AST midline catheters share commonalities in their purpose and benefits. They are both minimally invasive techniques for venous access that offer a less invasive alternative to central venous catheters. Midline catheters are typically inserted into the upper arm, providing a longer dwell time than traditional peripheral catheters. They can accommodate the administration of various medications and reduce the need for frequent venipuncture. Additionally, both techniques rely on the Seldinger method, which involves the use of a guidewire to facilitate catheter placement.
The Accelerated Seldinger Technique (AST) is a more direct method for midline catheter insertion. It involves placing the catheter directly into central veins, typically the basilic or brachial veins, using a modified over-the-needle catheter. AST midline catheters are longer in length and reach deeper into the upper arm, approaching the axillary region. This technique bypasses some potential issues associated with MST midlines but may cause discomfort for patients due to the proximity of the catheter to the antecubital fossa vessels. On the surface, AST devices may seem like an easy step towards progress and “vascular access for all” – with similarities to PIVC devices, however, there are several factors that should be considered with regard to midline device selection.
To have a “simple” catheter – there are trade-offs in the functionality of the device – and similarly to PIVCs, AST-MCs are a challenge to keep in through the completion of therapy. In a 2019 study involving AST-MCs, 60 out of 98 catheters were removed before the completion of therapy and the overall incidence of premature catheter removal was 71.8/1000 (Madsen et al., 2019). In another prospective study, the premature removal incidence was 62%. The AST-MCs in this study did outperform PIVC dwell times, however, two AST-MCs were needed on average to complete therapy (Nielsen et al., 2020). If the number of catheters being used and the desire to keep the same catheter in for the duration of therapy is important – then it may be worth reconsidering an AST-MC device.
Another challenge to AST-MCs is their preset length. With a non-trimmable, one-size-for-all length, the catheter may not be long enough and thus may not reach the desired location in the vein. This can make it difficult to administer medications or fluids, and it can also increase the risk of complications, such as phlebitis. Conversely, the catheter may be too long. If the catheter is too long, it can kink or coil, which can also contribute to the risk of complications. A too-long catheter can likewise be more difficult to secure in place, which can increase the risk of dislodgement. ASTs are generally perceived as “easier” on the surface, but the easier technique comes with the trade-off of catheter longevity and flexibility of catheter length.
The Modified Seldinger Technique (MST) uses a more gradual catheter advancement, allowing for preferred positioning in the midline veins. It requires ultrasound guidance for accurate placement and verification. MST midline catheters are trimmable in length and designed to terminate near the shoulder. The MST approach offers improved patient comfort, as it minimizes catheter contact with the vessels in the antecubital fossa.
MST midline catheters also present certain challenges that need to be addressed, with the main challenge being the need for a skilled healthcare professional who is proficient in ultrasound guidance. Accurate positioning of the MST midline is critical to the success of the line. This skill necessity may limit the accessibility of MST midlines in some healthcare settings or for healthcare providers who are not accustomed to placing PICC devices using a similar technique.
Additionally, MST midline catheters can have some significant complications that can negatively impact patients. In one recent study, MST catheters had a 14% phlebitis rate and a 7% rate of DVT – this led to a 24% overall failure rate (Bunch, 2022). While this failure rate isn’t as high as seen in the AST data, it’s still significant. Another common complication with midlines is losing blood return which, according to the INS guidelines, would mean the device needs to be replaced as there could be a more serious complication (Gorski, L., 2021).
The good news is that MST midlines are evolving. The HydroMID® catheter is made from an innovative hydrophilic biomaterial with MIMIX™ technology. This HBM material is thromboresistant, high strength and lubricious (Mannarino et al., 2020). In a recent study the HBM reported a reduction in complications six times less than the standard polyurethane midline catheters (Bunch, 2022) and in another study demonstrated a 90% reduction in tracking force required to insert the device in a bench study (LeRoy 2023). Technological improvements like this show promise in reducing complications, saving time, and improving overall patient outcomes and satisfaction.
AST-MCs may perfectly fit specific patient situations leading to faster insertions for those patients and clinicians. However, we know there is no one-size-for-all patients, and these devices cannot replace MST-MCs. Furthermore, recent advances in MST-MC technology may make them a better choice for complication reduction and decreasing failure rates. Assessment of the patient remains key before choosing any vascular device. Make sure your training and toolbox gives you choices.
References:
Bunch J. A retrospective assessment of peripheral midline failures focusing on catheter composition. J Infus Nurs. 2022;45(5):1-9. DOI: 10.1097/NAN.0000000000000484
Gorski, L.A. MS, RN, et al (2021). Infusion Therapy Standards of Practice, 8th Edition. Journal of Infusion Nursing 44(1S):p S1-S224, January/February 2021.
LeRoy, K. J., & Donahue, D. T. (2023). Trackability of a high strength thromboresistant hydrogel catheter: An In vitro analysis comparing venous catheter forces in a simulated use pathway. Journal of the Mechanical Behavior of Biomedical Materials, 1056070.
Madsen, E. Bundgaard, Sloth, E., Illum, B. Skov, & Juhl-Olsen, P. (2020). The clinical performance of midline catheters-An observational study. Acta anaesthesiologica Scandinavica, 64(3), 394–399. https://doi.org/10.1111/aas.13516
Mannarino MM, Bassett M, Donahue DT, Biggins JF. Novel high-strength thromboresistant poly(vinyl alcohol)-based hydrogel for vascular access applications. J Biomater SCI Polym ED. 2020;31(5):601-21. PubMed PMID: rayyan-788013993.
Nielsen, E. B., Antonsen, L., Mensel, C., Milandt, N., Dalgaard, L. S., Illum, B. S., Arildsen, H., & Juhl-Olsen, P. (2021). The efficacy of midline catheters-a prospective, randomized, active-controlled study. International journal of infectious diseases : IJID : official publication of the International Society for Infectious Diseases, 102, 220–225. https://doi.org/10.1016/j.ijid.2020.10.053
