Antecubital cephalic or median veins are typically preferred for emergent peripheral venous access owing to their ease of access [3]. However, repeated venous access can cause narrowing or obstruction of these veins. Consequently, catheter placement in these veins is often unsuccessful in patients with difficult venous access. Systematic, stepwise approaches and a broad repertoire of techniques are essential for managing such cases [4]. Ultrasound guidance is instrumental in facilitating peripheral venous catheter placement in patients with difficult venous access [5]. The midline catheter has also gained attention as a safe and long-indwelling ultrasound-guided peripheral venous catheter for patients with difficult venous access [6]. In Case 2, a midline catheter was a feasible alternative. However, its placement requires a sterilized environment, including using sterilized surgical gloves and drapes, and the arm of the patient must be abducted during the procedure, which complicates insertion during surgery under general anesthesia, as evidenced in Case 1.

Witting et al. reported higher success rates of cannulation for venous diameters > 4 mm than for < 4 mm and venous depths of 3–15 mm versus < 3 or > 15 mm [7]. In Case 1, the venous diameter was 1 mm, and the venous depth was 6 mm. In Case 2, the venous diameter was 2 mm, and the venous depth was 7 mm. While the diameters in these two cases were narrow, the depths were within the ranges for ultrasound puncture. If the needle is inserted shallower than 3 mm, ultrasound focusing is insufficient, and lateral resolution is poor, resulting in poor image quality. In addition, the needle should strike the center of the vein as soon as it is advanced subcutaneously. However, if the needle is deflected, the lateral wall of the vein will be damaged and cannulation will not be possible due to hemorrhage. Therefore, ultrasound-guided venous cannulation requires a needle to be inserted to a depth of at least 3 mm. In contrast, if the needle is inserted too deep, advancing the needle into the target vein becomes difficult. Concerning the optimal depth of the vein, no precise reference can be found because there are no other reports on cannulation of the radial vein.

Radial veins are a pair of veins that accompany the radial artery. To our knowledge, this is the first case report of targeting the radial vein for cannulation. Therefore, the optimal site or technique for radial vein cannulation is not conclusive. However, because the radial vein always accompanies the radial artery, the optimal conditions for cannulation of the radial artery to the radial vein are expected to be extrapolated. Wu et al. reported that the success rate of ultrasound-guided radial artery cannulation was higher in the mid-forearm than in the wrist [8]. Yücel et al. also reported higher success rates in the distal quarter of the forearm than in the mid-forearm [9]. The radial nerve to the radial artery and veins in the proximal quarter are closely located. Therefore, the radial vein cannulation in this region carries a risk of nerve injury. In summary, the optimal cannulation site of the radial vein may be the distal quarter of the forearm, whereas the proximal from the mid-forearm and wrist are less likely to be suitable for cannulation. The most important technical aspect of cannulation would be the entry angle of the needle into the vein. If the entry angle is too steep, the posterior wall of the vein is easily damaged during the cannulation. In our experience, the best entry angle for cannulation is a shallow angle of less than 30°. As in ultrasound-guided radial artery cannulation, the needle is gradually advanced into the target vein while confirming its position with ultrasound [10, 11]. Once the needle tip reaches the vein, further decreasing the entry angle is recommended to achieve cannulation. The shallower the needle entry angle, the greater the distance from the sticking site to the target vein. Therefore, we use relatively long needles (usually approximately 5 cm) for deeper veins such as the radial vein.

We demonstrated that the radial vein is a viable target for ultrasound-guided peripheral venous access in patients with difficult peripheral venous access and the cannulation can be performed safely with ultrasound guidance.