Ultrasound vessel sizing before cannulation: a measurement-first approach for anesthesiologists.

The catheter gauge and length are often chosen based on visual estimation. Ultrasound allows for the direct measurement of vessel diameter and depth, which may reduce failed attempts, particularly in patients with difficult access and pediatric patients. This narrative review evaluated trials, meta-analyses, and guidelines on ultrasound-guided vascular access in adults and children to derive practical sizing rules for venous and arterial cannulation in anesthesiology. Key recommendations include targeting a catheter-to-vein ratio (CVR) ≤ 0.45 for peripheral IVs and using long catheters for deeper veins (≥ 8-10 mm); limiting catheter size to ≤ 1/3 of the vessel's luminal area (outer diameter/inner diameter ratio ≤ 0.577) for arterial lines to prevent spasm. A pediatric reference table links typical radial artery diameters to appropriate 22-24G catheter choices. Selecting central venous catheters that occupy approximately ≤ 33% of the vein's diameter may be a reasonable strategy to minimize complications. Evidence from clinical trials supports US-guided peripheral IV and radial artery cannulation over landmark-based techniques. We propose a five-step workflow (scan, measure, select gauge/length, US-guided cannulation, and confirmation) to implement this measurement-first approach. In conclusion, an ultrasound measurement-first strategy aligns catheter selection with vessel dimensions, improving first-pass success and reducing complications in both adult and pediatric practices. The proposed sizing tables and algorithm are intended to serve as practical decision-support tools for bedside applications.