Arterial cannulation is indicated in several clinical settings: high-risk surgery, respiratory failure, circulatory failure. The possible anatomical sites of cannulation are: a.radialis, a.brachialis, a. femoralis. However, the most reported complications in preschool-aged children are associated with femoral and brachial arteries. Therefore, alternative cannulation sites are being searched as ulnar artery, tibial posterior artery.

In pediatric patients, artery cannulation may be associated with a higher risk of complications. The preferred puncture site is the radial artery or the femoral artery. However, puncture of the femoral artery in comparison with the radial artery is based on observational data as a significant risk side of post-puncture complications. The radial artery is the dominant artery of the hand, and the collateral supply (via the ulnar artery) is often unable to provide adequate peripheral perfusion. Therefore, other potential sites suitable for artery cannulation in paediatric patients are currently being investigated with the primary goal of increasing safety and reducing the risk of post-puncture complications. According to published studies, the potential alternatives are: posterior tibial artery and ulnar artery. Both arteries have comparable dimensions with radial artery and can be used for USG-guided cannulation. In this prospective randomized trial, patients with clinical indication for arterial cannulation (high-risk surgery, respiratory failure, circulatory failure) undergoing surgery under general anesthesia and pediatric patients admitted into the pediatric intensive care unit (PICU) will be randomized in form of sealed envelopes into a.radialis and a.ulnaris group as the initial site of arterial catheterization. Ultrasound (USG) measurement of both arteries will be performed before the cannulation. The catheterization success rate (number of percutaneous punctures) and overall success rate will be evaluated. The artery will be cannulated under USG control, initially in in-plane view, only in case of insufficient image, or after 2 unsuccessful attempts, the out-of-plane approach could be used. The maximum 5 percutaneous attempts and 10 minutes could be used for the initial (randomized) site of cannulation). In case of cannulation failure, the other artery could be used for cannulation. The rate of complications in 24 hours after cannulation will be evaluated. The arterial catheter will stay in place until the clinical indication remained.

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Inclusion Criteria:

• Paediatric patients (1 - 6 years)
• high risk surgery (significant blood loss or hemodynamic instability)
• critically ill patients at PICU (respiratory failure, circulatory failure)
• informed consent

Exclusion Criteria:

• refused to participate
• arterial catheterization not clinically relevant

randomization after general anesthesia induction, or deep sedation in PICU