During prolonged major surgical procedures in paediatric, significant fluctuations in vital signs occur. To ensure real-time, precise monitoring of haemodynamic parameters such as blood pressure and heart rate for maintaining intraoperative safety, radial artery catheterisation remains the most conventional monitoring method for such operations. Ultrasound-guided radial artery puncture has gained increasing favour among clinicians over traditional palpation and blind techniques due to its visualisation and real-time advantages, with its benefits becoming increasingly evident. Nevertheless, this technique still carries a relatively high failure rate. This study proposes the 'laser-assisted T-shaped three-dimensional ultrasound localisation technique' for paediatric radial artery puncture and catheterisation. This aims to enhance puncture success rates, reduce the need for repeated attempts, lower complication incidence, safeguard the patient's life during surgery, and improve the quality of medical services.

Following induction of anaesthesia in the paediatric patient, position one arm in slight abduction with a thin rolled bandage placed beneath the wrist to maintain mild dorsal extension. Disinfect the radial artery puncture site with povidone-iodine solution and drape the area. Select the appropriate ultrasound system and probe according to the assigned group (T-shape ultrasound guidance group or out-of-plane ultrasound guidance group).

T-shaped probe ultrasound guidance Group:

Mount the 3D-printed housing with a laser emitter onto the T-shaped ultrasound probe. The laser emitter is positioned directly above the Mark point on the short-axis plane of the T-shaped probe. Given the probe's hardware structure-where the long and short-axis probes are vertically and tightly fused, with the long-axis probe's centreline perpendicular to and passing through the short-axis probe's centreline-the emitted laser lies on the skin surface extension of the short-axis plane's centreline. Instruct the assistant to apply an appropriate amount of coupling agent evenly across the ultrasound probe. The operator and assistant shall collaboratively cover the entire 3D-printed three-axis adjustable support arm and ultrasound probe with a sterile sleeve. Remove any air between the probe and the sterile sleeve. Apply coupling agent evenly to the outer layer of the sterile sleeve where it contacts the patient's skin to complete device preparation. To commence positioning, the operator gently places the T-shaped probe 0.5 cm below the wrist crease using the arm, orienting the short axis distally and the long axis proximally. Adjust the arm and rotation axis so the midline of the short-axis ultrasound image traverses the centre of the radial artery cross-section. Stabilise the short axis, Slightly adjust the longitudinal axis angle left or right to achieve optimal visualisation of the radial artery in the longitudinal ultrasound image. The laser projection direction now indicates the radial artery's subcutaneous course. Once the arm is positioned to ensure stable probe contact with the skin surface, hands-free, stable, and continuously monitored ultrasound guidance is established. The operator's left hand secures the skin near the puncture site, while the right hand holds a 24G arterial cannula needle. Insert the needle along the laser-projected course of the radial artery at approximately a 30° angle to the skin (with the insertion point 0.5cm from the ultrasound probe). Once the needle tip enters the subcutaneous tissue, if deviation is visible in the short-axis ultrasound image, adjust the insertion angle to pierce the anterior wall directly above the radial artery. Upon visualising blood return, displace the ultrasound probe 0.5 cm distally. Confirm needle entry into the vessel on the longitudinal ultrasound image. Adjust the needle tip position to align with the centre of the radial artery's longitudinal section as displayed on the longitudinal view. Subsequently, while maintaining the needle position with the right hand, slide the probe proximally with the left hand. Utilise the short-axis image to determine the pre-cannulation needle tip position using the 'dynamic needle tip method'. Gradually reduce the needle insertion angle while slowly withdrawing the needle core. Once unobstructed blood return is achieved, advance the cannula. Should resistance persist, withdraw the needle from the radial artery and apply pressure to the puncture site with sterile gauze. Attempt the procedure again after approximately 3 minutes, once no significant bleeding is observed at the puncture site.

Out-of-plane ultrasound guidance Group:

The assistant applies coupling agent to the surface of the ultrasound probe. The operator and assistant then collaboratively cover the probe with a sterile sleeve, evacuating any air between the probe and sleeve. Coupling agent is evenly applied to the outer layer of the sterile sleeve where it contacts the child's skin, completing device preparation. To commence positioning, the operator gently places the ultrasound probe 1 cm below the wrist crease, aligning its short-axis centreline parallel to the radial artery's course. Once the radial artery is centred within the ultrasound image, mark the midpoint of the probe with a sterile marker pen. Slide the probe 1 cm distally along the short-axis midline to ensure the needle tip approaches the maximum transverse plane level upon subcutaneous entry into the radial artery. Stabilise the probe with the left hand to complete positioning. Adjust the ultrasound plane to form approximately a 60° angle with the skin. Align the midpoint of the probe's short axis plane with the marked point. Using the right hand, advance a 24G arterial cannula needle vertically into the skin at approximately a 30° angle relative to the probe. Upon observing blood return, gradually reduce the needle insertion angle while slowly withdrawing the needle core. Insert the cannula when blood return is unobstructed and resistance ceases. Should resistance persist, withdraw the needle from the radial artery and apply pressure to the puncture site with sterile gauze. Attempt again after approximately 3 minutes when no significant bleeding is observed at the puncture site.

Inclusion Criteria:

• The paediatric patient's family fully understands and voluntarily participates in this study, having signed the informed consent form.
• Paediatric patients undergoing critical surgery requiring radial artery puncture and catheterisation for continuous blood pressure monitoring are selected.
• ASA physical status classification of I-IV.
• Age ≤ 24 months, gender not restricted.

Exclusion Criteria:

• Paediatric patients who have undergone radial artery puncture within the past month.
• Paediatric patients with abnormal development of the radial or ulnar arteries, peripheral vascular disease at the radial puncture site, congenital heart disease, or haemorrhagic or cardiogenic shock.
• Paediatric patients with wounds, infections, or haematomas near the radial puncture site.
• Ultrasound imaging demonstrating inadequate collateral circulation or a negative Allen test.
• Cases where the paediatric patient's guardian refuses to sign the informed consent form for study participation.
• Other circumstances deemed unsuitable for trial participation by the investigator.