**Background:** Midline catheters (MCs) are increasingly used for mid- to long-term intravenous therapy, particularly in elderly patients who often have fragile veins and chronic diseases requiring infusion. However, there is no consensus on the optimal position of the MC tip, with guidelines differing between the US (tip should not exceed the axillary vein) and China (tip can be located in the subclavian vein). This study aimed to investigate the relationship between MC tip position and catheter-related complications and indwelling duration in elderly patients.
**Methods:** This cohort study included patients aged ≥60 years admitted to a single institution from May 2018 to December 2021 who required MC insertion for estimated infusion time >1 week. Exclusion criteria included contraindications for catheterization, malignant tumour with concurrent chemotherapy, or refusal of MC treatment. MCs were inserted using the modified Seldinger technique. Following insertion, bedside ultrasound was used to confirm the MC tip position. Patients were divided into two groups: superficial implementation (distance between catheter tip and clavicle >3 cm, n=42) and deep implementation (distance ≤3 cm, n=53). Clinical data including catheter-related complications, day of occurrence, and indwelling duration were recorded. Catheter-related complications included phlebitis, infiltration, occlusion, dislodgement, catheter-related thrombosis, and catheter-related bloodstream infection.
**Key Results:** A total of 95 patients were included (mean age approximately 86 years in both groups; no significant differences in baseline characteristics). The median catheter indwelling duration was significantly longer in the deep implementation group (29 days, range 14-77) compared to the superficial implementation group (18.5 days, range 11-31; p=0.014). Catheter-related complications occurred in 14 patients overall (14.74%). The incidence was significantly higher in the superficial implementation group (10/42, 23.81%) than in the deep implementation group (4/53, 7.55%; p=0.026). The complication rate per 1,000 catheter days was 9.03/1,000 in the superficial group versus 1.23/1,000 in the deep group (p=0.001). Complications in the deep implementation group occurred significantly later (median day 96, IQR 46.5-140) compared to the superficial group (median day 14, IQR 9-18; p=0.003). In the superficial group, complications included phlebitis (n=1), infiltration (n=5), catheter dislodgement (n=2), and catheter occlusion (n=2). In the deep group, complications included infiltration (n=1), catheter occlusion (n=1), and catheter-related venous thrombosis (n=2). No catheter-related bloodstream infections occurred in either group. The proportion of patients completing treatment was higher in the deep implementation group (92.45% vs. 76.19%, p=0.04). Kaplan-Meier survival analysis showed significantly higher complication-free survival in the deep catheterization group (log-rank test, p<0.001).
**Clinical Implications:** This study suggests that ultrasound-guided placement of the MC tip in the distal segment of the axillary vein may reduce catheter-related complications, delay their occurrence, and prolong indwelling duration in elderly patients. The authors propose that the larger diameter and increased blood flow in the distal axillary vein may allow for greater haemodilution of administered medications, potentially reducing chemical phlebitis and infiltration. However, two cases of catheter-related venous thrombosis occurred in the deep implementation group, both in patients with multiple high-risk factors (advanced age, malignant tumours, prolonged bed rest, malnutrition, high Padua scores). The authors caution that for patients at high risk of thrombosis, positioning the catheter tip in the distal axillary vein may require careful consideration. Study limitations include its non-randomized design, single-centre setting, relatively small sample size, and inability to account for all confounding variables such as illness severity and medication history.