Radial artery cannulation is a routine procedure for invasive hemodynamic monitoring for patients undergoing cardiovascular surgery patients.

Although ultrasound (USG) guidance is known to improve success rates, there is limited data comparing the efficacy of the 'Direct' versus 'Seldinger' techniques specifically in patients with anticipated difficult radial access.

This study aims to compare the effectiveness of USG-guided Direct cannulation and USG-guided Seldinger techniques in adult patients with anticipated difficult radial arterial cannulation.

Study Protocol and Patient Assessment: Data collection and the procedural intervention will be performed in the preoperative period, prior to the induction of anesthesia. All patients will undergo a modified Allen test. The radial artery site will be selected based on the following hierarchy: preferably the non-dominant hand (unless surgical plan dictates otherwise), an arm with no history of coronary angiography, and the side determined to be easiest for cannulation via ultrasound (USG) assessment.

Ultrasound Assessment and Exclusion: A 12 MHz linear ultrasound probe will be used for all assessments and procedures. Before the intervention, a detailed sonographic evaluation of the radial artery will be recorded, including:

• Artery diameter and wall thickness
• Lumen continuity and flow velocity
• Presence of calcification, vessel irregularity, or tortuosity
• Stenosis (percentage) and plaque characterization (stable/unstable; fibrous/fatty)
• Presence of hematoma, posterior wall injury, or dissection
• Skin- mid radial artery distance Patients with significant stenosis where an appropriate arterial segment matching the cannula diameter and length cannot be identified will be excluded from the study, and cannulation will be performed at an alternative site.

Randomization and Positioning: Patients will be randomized into two groups using a computer-generated random number table:

• Group D (Direct): Cannulation performed using the catheter-over-needle technique.
• Group S (Seldinger): Cannulation performed using the catheter-over-guide-wire technique.

The patient's arm will be abducted, and the wrist will be extended and fixed over a small support. Standard aseptic preparation and draping will be performed.

Procedural Definitions and Outcome Measures: The procedure will be performed under real-time ultrasound guidance. The following time intervals and metrics will be recorded:

• Radial artery cannulation time: Defined as the time elapsed from the initial skin contact of the needle/cannula to the appearance of the radial artery pressure waveform
• Radial artery puncture time: Defined as the time elapsed from the initial skin contact of the needle/cannula to the appearance of blood in the hub Number of puncture: The total number of skin punctures required to achieve successful cannulation.

Number of needle maneuvers: The total number of needle advancements towards the artery

Safety and Follow-up: If arterial cannulation fails, manual compression will be applied to the artery for 5 minutes to prevent hematoma formation. All patients will be monitored for complications (such as hematoma, ischemia, or nerve injury) for 24 hours post-procedure.

Inclusion Criteria:

• Patients aged 18 years or older
• American Society of Anesthesiologists (ASA) physical status I-IV
• Patients scheduled for cardiovascular surgery requiring invasive arterial monitoring

Exclusion Criteria:

• Patients undergoing emergency surgery
• Coagulation disorders or known coagulopathy
• Presence of an arteriovenous fistula
• Patients in shock or hemodynamically instable
• Patients requiring mechanical circulatory support
• History of Raynaud's phenomenon
• Refusal to participate in the study