**4.2 Determination of the appropriate catheter and vein entry angle in difficult venous access**

The size of intravenous catheters [14, 15, 18, 21, 23, 25] is referred to as the gauge. The diameter and the length of the cannula vary with the size of the catheter. As the number/size of the catheter increases, the cannula diameter decreases. In addition, different lengths are available for the catheters of the same size [29, 34]. Generally, a 20-gauge to a 24-gauge catheter is preferred for peripheral catheterization. Peripheral catheters larger than 20 gauge in size are more likely to cause phlebitis. A 22 to 24 gauge catheter for neonates, pediatric patients, and older adults generally minimizes the insertion-related trauma. A 20-gauge to 24-gauge catheter should be used based on the vein size for blood transfusion; when rapid transfusion is required, a larger-size catheter gauge is recommended [29].

The catheter should be placed at an angle of 10–30 degrees to the skin, and after entering the vein at the PIVC, the angle should be reduced. However, the veins that are superficial, thin, slippery, and present a risk of difficult venous access should be entered at an angle of 30–45 degrees to the skin, from the lower side of the entry point and parallel to the vein, and immediately after entering the vein, the angle should be reduced to 15 degrees [34]. In the process of catheter placement with the assistance of the ultrasound technique, if a vessel is 16 mm deep and is entered at a 45° angle to the vein, then a catheter with a minimum length of 23 mm is required to reach the vessel. At a 30° angle, the catheter would have to be 32 mm to reach the anterior wall of the vessel [36].
