Central venous catheters can lead to the formation of fibrous sheaths. The origin of the fibrous sheath is the contact point between the catheter and the vein wall, and it extends all the way to the tip of the catheter. This can affect the normal function of the venous catheter and is also prone to causing other complications [6] . The main components of the fibrous sheath include cellular and non-cellular elements, and it is a membrane-like substance that envelops the surface of central venous catheters [7] . Within 1 to 2 weeks after the insertion of a central venous catheter, the fibrous sheath rarely causes catheter dysfunction. However, as the dwell time increases, catheter-related complications may occur around 15 weeks post-insertion. These complications include catheter blockage, extravasation, and catheter-related thrombosis [8] .

The formation of a fibrous sheath is a dynamic process. As the dwell time of the catheter increases, the composition of the fibrous sheath varies. It mainly includes fibrin, red blood cells, platelets, fibroblasts, endothelial cells, smooth muscle cells, collagen fibers, and elastic fibers [9] . After the insertion of a PICC catheter, injury to the vascular endothelium occurs, and continuous pressure is exerted on the vein wall. Additionally, with the patient’s breathing, heartbeat, and physical movements, friction between the catheter and vein wall can cause endothelial damage. Endothelial injury, reduced blood flow velocity, and hypercoagulability of blood are crucial factors contributing to thrombus formation, and they also play a significant role in the development of fibrous sheath. Currently, there is no definitive conclusion regarding the formation of fibrous sheaths both domestically and internationally. However, existing research has indicated a correlation with catheter-related thrombus formation. The three key factors in thrombus formation are endothelial injury, slowed blood flow, and a hypercoagulable state. The development of fibrous sheath is a protective response of the human body to the invasion of foreign objects, such as a PICC catheter, and it is also a process of self-regulation and self-repair.

A fibrous sheath is a catheter-related thrombus, distinct from the commonly referred-to mural thrombus. Unexpected displacement of the catheter can lead to treatment delays and thrombus formation, among other complications. PICC catheters are particularly prone to displacement [10] . Despite the high feasibility of PICC catheterization, patients undergoing PICC placement are susceptible to a variety of complications, such as infection, allergic reactions to transparent dressings, catheter blockage, phlebitis, and catheter insertion injuries. Therefore, during clinical treatment, it is essential to analyze the causes of these complications and provide targeted nursing care and interventions [11] [12] . Studies have identified key risk factors for fibrous sheath formation after PICC placement, including the type of infused fluid, infusion filter use, infusion rate, techniques for catheter flushing and locking, frequency of flushing and locking, and the specific tools employed for these procedures [13] . During the PICC insertion process, the vascular endothelium is injured, which activates the coagulation system and leads to endothelial cell damage. This, in turn, causes the adhesion of platelets and white blood cells and activates coagulation factors, resulting in the production of thrombin that converts fibrinogen into fibrin. However, because the catheter is a foreign body that is suspended within the blood vessel, it alters the original blood flow direction to varying degrees. This results in the accumulation of a large amount of fibrin around the catheter, leading to the formation of a fibrous sheath [14] .

Currently, the primary diagnostic methods for fibrous sheaths both domestically and internationally include venography, CT scans, intravascular ultrasound, echocardiography, and histopathological staining [15] . Given the high detection rate and non-invasive nature of vascular ultrasound, it has become the most commonly used diagnostic tool for fibrous sheath in clinical practice. However, this method cannot accurately differentiate between fibrous sheath and thrombus, resulting in low specificity. Moreover, the accuracy of diagnosing fibrous sheaths using color Doppler ultrasound is not high [16] .

Given that the early symptoms of fibrous sheath are difficult to detect, by the time they are identified, the sheath has often transformed into fibrous connective tissue, necessitating surgical removal, which can cause secondary injury. Therefore, it is suggested that Doppler ultrasound be combined with probe compression techniques for the clinical examination of fibrous sheaths to increase the positive detection rate [7] . Additionally, the integration of ultrasound with laboratory tests can be applied for the prediction of fibrous sheath formation [17] .

Therefore, in the future, clinical nursing staff can adopt a multidisciplinary approach to the diagnosis of fibrous sheath. This can improve the diagnostic accuracy and prevent missed diagnoses.

At present, the most common method for managing fibrous sheath is pharmacological treatment, with the primary medications including urokinase, tissue-type plasminogen activator (t-PA), and streptokinase. Compared to streptokinase and t-PA, urokinase has fewer side effects and is more cost-effective, making it the conventional drug of choice for fibrous sheath [18] . Urokinase is a serine protease that directly activates plasminogen to promote fibrinolysis [19] . Some studies have shown that low doses of urokinase (1.5 mL, 15,000 IU) significantly reduce protein adsorption [20] , while Ariyachaipan-ich et al. [21] demonstrated that adding 1 mg of tPA to 50 mL of saline for catheter recanalization can achieve a success rate of 95.3%. However, these medications do not address the root cause of the problem. Research by Tang Wenshuai has shown that within traditional Chinese medicine, Danshen (Salvia miltiorrhiza) possesses properties of promoting blood circulation to remove blood stasis, anticoagulation, inhibition of platelet aggregation, and vasodilation, which can inhibit the formation of the sheath from the source [22] .

In clinical practice, the decision to use urokinase for thrombolysis or Danshen solution should be based on the individual patient’s condition. It is important to note that during thrombolytic therapy, comprehensive monitoring of the patient’s bleeding, coagulation, and platelet status is essential. Additionally, dissolving agents are not recommended, as they are ineffective against mature fibrous sheaths [23] . If a patient has normal platelet counts and coagulation function, and Doppler ultrasound confirms the presence of a fibrous sheath while ruling out mural thrombus, successful catheter removal can be achieved after the administration of urokinase.

It should not be overlooked that excessive patient anxiety and tension can enhance sympathetic nerve excitability and repeatedly stimulate the vagus nerve, which can easily lead to vasospasm and contraction, thereby increasing the difficulty of catheter removal [24] . Therefore, nursing staff should provide detailed explanations to patients and their families about the causes of difficult PICC removal, coping strategies, and the purpose of the procedure before catheter removal. During the process, they should communicate with the patient to distract their attention and enhance the patient’s trust in the nursing staff. This helps to alleviate anxiety and fear, preventing excessive nerve excitability from causing vasospasm or contraction, which could affect catheter removal. Additionally, nursing staff can use local warm compresses and massage to reduce the difficulty of catheter removal.

In the early stages, fibrous sheath formation does not present with obvious clinical symptoms and is difficult to detect. However, as it progresses to the late stages, it develops into fibrous connective tissue, at which point it cannot be managed solely with medication. In most cases, surgical removal of the catheter is required, causing secondary trauma to the patient. This highlights the importance of early prevention. Studies have shown that standardized catheter maintenance can reduce the incidence of fibrous sheath [25] . This is because the pressure difference generated during pulsatile flushing creates a vortex, and the swirling action can effectively remove adherent material from the catheter wall, reducing the risk of catheter blockage.Moreover, the swirling action is also detrimental to the formation of fibrous sheath. The water-injected surface protection technology plays a positive role in reducing protein adsorption. Water passes through the membrane wall into the bloodstream to form a blood-free boundary layer, preventing protein adsorption on the vessel wall and the formation of fibrous sheath [26] . Studies have shown that after infusing viscous fluids or blood products, nursing staff can effectively prevent the occurrence of fibrous sheath by using more than 10 ml of normal saline for pulsatile flushing and locking of the catheter, regularly observing the status of the indwelling catheter, strengthening health education, avoiding lifting heavy objects and prolonged pressure on the limb with the catheter, and regularly performing exercises with an electronic hand grip combined with fist clenching [27] .

In practice, under the premise of meeting therapeutic requirements, selecting catheters with superior material and high biocompatibility for puncture can ensure adequate blood flow between the catheter and the vessel wall, thereby preventing the formation of fibrous sheath. Meanwhile, in the routine maintenance of the catheter, nursing staff should flush and lock the catheter more than three times a day (every 4 - 6 hours during infusion and every 8 hours without infusion) to avoid the formation of fibrous sheath. Additionally, it is essential to enhance relevant training and management of nursing staff to ensure the safety and effectiveness of PICC catheter maintenance, prolong the lifespan of the catheter, and fully realize the application value of PICC placement [28] .