For adults with small radial arteries, ultrasound-guided radial artery cannulation remains challenging and the relevant data is currently lacking. The study aimed to test the hypothesis that modified long-axis in-plane ultrasound guidance (M-LAIP) would improve success rates of radial artery cannulation in this population.

This was a prospective, randomised, and controlled clinical study that enrolled 201 adult patients with diameters of the radial artery less than 2.2 mm. Patients were randomised to M-LAIP, short-axis out-of-plane (SAOP), or conventional palpation (C-P) group according to different approaches of radial artery cannulation (M-LAIP, SAOP, and C-P). Outcome measurements included the success rate, cannulation time, and cannulation-related adverse events.

The cannulation success rate was significantly higher in the M-LAIP group than in the SAOP or C-P groups (first success rate: 80.3% vs. 53.8% or 33.8%; P < 0.001; total success rate: 93.9% vs. 78.5% or 50.8%; P < 0.001). Total cannulation time in the M-LAIP group was shorter than that in the SAOP group (P = 0.002) or the C-P group (P < 0.001). The rates of posterior wall puncture and haematoma in the M-LAIP group were lower than that in the SAOP group or C-P group (P < 0.008).

The use of the M-LAIP approach significantly improved the success rate of radial artery cannulation, shortened procedure time, and lowered the rates of posterior wall puncture and haematoma in adults with radial artery diameters less than 2.2 mm, compared with that achieved by the SAOP or C-P approach.