The trial, published as a study protocol in Frontiers in Medicine on October 19, 2026, and registered at ChiCTR under identifier ChiCTR2600124338, is a single-center, parallel-group, randomized noninferiority trial that will enroll 198 surgical patients aged 65 years or older who require invasive arterial pressure monitoring. Participants will be allocated 1:1 to distal or conventional radial artery catheterization. The design is engineered to isolate the effect of the access site itself: both groups will receive the identical real-time ultrasound-guided dynamic needle-tip positioning technique, the same ultrasound platform, the same 20-gauge, 45-millimeter polyurethane over-the-needle arterial catheter, the same sterile preparation, and the same pressurized flush system. Only the puncture site and the limb position will differ. The primary outcome is first-attempt success, defined as catheter placement with free aspiration of blood and an undamped arterial waveform after a single skin puncture. Noninferiority will be concluded if the lower limit of the two-sided 95 percent confidence interval for the risk difference, distal minus conventional, is greater than minus 15 percentage points in both the intention-to-treat and per-protocol populations.

The technical heart of the trial is the dynamic needle-tip positioning method, a real-time ultrasound guidance approach that has become the gold standard for difficult arterial access. In both groups, the operator will visualize the target artery in a short-axis out-of-plane view using a high-frequency linear-array transducer, then advance the needle in small increments while alternately sliding the transducer to keep the needle tip visible within the center of the arterial lumen. This alternating probe movement is what distinguishes the technique from a static out-of-plane puncture, where the needle tip can drift out of the imaging plane and pierce the posterior wall unseen. Once sufficient intraluminal advancement is achieved, the catheter is threaded over the needle, connected to a zeroed transducer, and secured with a standardized transparent dressing. Correct placement requires free aspiration and an undamped arterial waveform, a strict objective criterion that leaves little room for ambiguity about whether the catheter is truly delivering usable pressure data.

What makes the distal site anatomically interesting is its position over a firm osseous base. The distal radial artery, in the anatomical snuffbox or dorsoradial hand near the Hegu acupuncture point region, lies close to bone, which may facilitate compression after catheter removal, reduce catheter kinking or waveform damping during wrist movement, and preserve the conventional forearm segment for future access. These advantages could matter enormously during prolonged surgery or when operative positioning requires wrist movement, situations in which a conventional wrist catheter can kink, damp, or lose its waveform entirely. The trade-off is caliber: the distal artery is generally smaller and may be deeper, potentially reducing first-attempt success and prolonging catheterization time. The clinical value of distal access therefore depends on whether its gains in catheter stability and access-site safety offset any loss of procedural efficiency, and the existing comparative evidence is frustratingly inconsistent.

Previous trials have produced conflicting estimates. Xiong and colleagues reported noninferior first-attempt success with distal versus conventional access, 87.7 percent versus 91.3 percent, together with fewer dampened waveforms but a longer procedure. Maitra and colleagues, however, found substantially lower first-attempt success with distal access, 57 percent versus 77 percent, and longer catheterization times. More recent trials in critically ill and anesthetized adults found comparable insertion success but fewer waveform-loss events with distal placement, particularly during wrist movement. A 2026 meta-analysis of 12 randomized trials involving 1,790 participants found no overall difference in first-attempt success and suggested fewer hematomas, bleeding events, catheter blockages, and unstable waveforms with distal access, although insertion took longer. Critically, the age-stratified analysis suggested lower first-attempt success in patients older than 65 years, but certainty was low, techniques were heterogeneous, and ultrasound was not consistently used from the initial attempt. That gap, an underrepresented older population with inconsistent imaging, is precisely what the Ningbo team hopes to close.

The trial’s methodology reflects a meticulous effort to control every variable that could confound the comparison. Before allocation disclosure, both candidate sites in the selected limb will be examined in short and long axes with a high-frequency linear-array transducer, and anteroposterior internal diameter, depth from skin to anterior arterial wall, visible calcification or plaque, and color-Doppler flow will be recorded at end diastole and averaged across three images. If the distal arterial diameter is less than 1.5 millimeters, the procedure is deemed unsafe, no puncture will be performed, and the case will be directly classified as a failure. Randomization will use randomly varying block sizes generated by an independent statistician, stratified by planned operative position, supine versus non-supine, because positioning may influence catheter stability and wrist movement. The allocation sequence will be concealed in sequentially numbered, opaque, tamper-evident sealed envelopes opened immediately before catheterization. Operators cannot be masked, but postoperative outcome assessors and the statistician will remain masked where feasible, with groups coded as A and B until the primary analyses are finalized.

Operator standardization is another pillar of the design. All catheterizations will be performed by a prespecified group of senior anesthesiologists with more than 10 years of clinical experience in radial artery catheterization, each of whom must demonstrate protocol adherence in simulation and complete at least 20 successful supervised distal and 20 successful conventional ultrasound-guided radial catheterizations before enrolling participants, or provide documented equivalent recent experience. Operator identity and cumulative study case number will be recorded to assess clustering and learning effects. In the distal group, the forearm will be pronated with the thumb neutral or slightly abducted, and the puncture point will be selected where ultrasound shows the most favorable combination of lumen diameter, depth, and straight course. In the conventional group, the forearm will be supinated with the wrist extended approximately 30 degrees over a soft roll, avoiding excessive extension that may flatten the vessel. The conventional radial artery will be identified 1 to 2 centimeters proximal to the wrist crease, lateral to the flexor carpi radialis tendon.

Secondary outcomes extend the evaluation well beyond immediate cannulation success. The trial will measure overall assigned-site success within three skin punctures and five minutes, procedural efficiency including the number of skin punctures, needle redirections, and needle-to-waveform time, intraoperative catheter performance defined as at least one episode of damping or waveform loss requiring manipulation, flushing, limb repositioning, or catheter replacement, access-site safety and hemostasis assessed at 24 hours, and Doppler-confirmed arterial patency of both the distal and conventional radial arteries. Patency will be assessed by sliding the high-frequency probe along the long axis of the punctured arterial segment to confirm continuity of blood flow signals, with occlusion defined as the absence of both color-flow and pulsed-wave Doppler signals. Complete hemostasis is defined as the first time no visible bleeding recurs for 30 seconds after release of manual compression. A structured telephone interview seven days after the procedure will explore late complications including local redness, swelling, persistent pain, abnormal masses, and paresthesia.

The statistical framework is equally rigorous. The sample size calculation assumes an 85 percent first-attempt success rate in the conventional group, informed by prior studies in older or high-risk patients, and an equal 85 percent rate in the distal group. With a one-sided alpha of 0.025 and 80 percent power, 89 evaluable participants per group are required, and allowing for 10 percent attrition yields 99 per group, or 198 total. The primary analysis will use the absolute risk difference in first-attempt success with a two-sided 95 percent Miettinen-Nurminen score confidence interval, and distal catheterization will be declared noninferior only if the lower confidence limit is greater than minus 0.15 in both the intention-to-treat and per-protocol populations. Missing primary endpoint data will be handled conservatively as a failure in the distal group and a success in the conventional group. A supportive adjusted analysis will use logistic regression with prespecified prognostic variables including age, pre-cannulation mean arterial pressure, assigned-site arterial diameter, visible calcification or plaque, and operator, with adjusted risk differences obtained by marginal standardization.

The investigators are candid about the trial’s limitations. The 15-percentage-point noninferiority margin, although used in previous perioperative trials, is clinically generous, and noninferiority alone will not establish distal access as the default approach. The single-center setting and experienced operators may limit generalizability, follow-up of 24 hours may miss delayed thrombosis, infection, pseudoaneurysm, or neurologic symptoms, and the trial is not powered for rare events or superiority on secondary outcomes. Ethics approval was granted by the Ethics Committee of Ningbo No. 6 Hospital on March 12, 2026, and the first subject was enrolled on July 26, 2026, with recruitment expected to complete by June 2027. The most useful result may be a conditional one: if distal access is noninferior and improves waveform continuity or hemostasis, it may be favored when wrist movement, operative positioning, or preservation of the conventional radial segment matters. Either outcome should inform operator training and a subsequent multicenter effectiveness trial, and the team plans to share the protocol, statistical analysis plan, data dictionary, analysis code, and a de-identified participant-level dataset after publication.

Subject of Research: Ultrasound-guided distal versus conventional radial artery catheterization in older surgical patients: protocol for a randomized controlled noninferiority trial

Article Title: Ultrasound-guided distal versus conventional radial artery catheterization in older surgical patients: protocol for a randomized controlled noninferiority trial

Article References: Zhang, L., Diao, S., Zhang, L., & Li, X. (2026). Ultrasound-guided distal versus conventional radial artery catheterization in older surgical patients: protocol for a randomized controlled noninferiority trial. Frontiers in Medicine, 13, Article 1985261. https://doi.org/10.3389/fmed.2026.1985261

Image Credits: AI Generated

DOI: 10.3389/fmed.2026.1985261

Keywords: Ultrasound-guided, distal, versus, conventional, radial, artery, catheterization, older, surgical, patients, protocol, randomized