This prospective, single blind, randomized, controlled study was conducted at Seoul National University Bundang Hospital after approval had been obtained from the hospital’s ethics committee (IRB protocol number: B-1201–070–002). Written informed consent include images relating to individual participants was obtained from each participant during a preoperative visit. We explained this is a single-blind study (i.e., subject unaware of group allocation until completion of the study). The study protocol is retrospectively registered with the Korean Clinical Trials Registry (KCT0001711).
Between April 2014 and September 2014, patients between 18 and 80 years of age with an American Society of Anesthesiologists physical status score between I–III and who required radial arterial catheterization for continuous blood pressure monitoring under general anesthesia, were enrolled. Patients with a history of atherosclerosis, hemorrhagic shock, morbid obesity, Raynaud’s disease, or peripheral vascular disease were excluded. Subjects were allocated to either the bevel-up (bevel facing upwards) or bevel-down (bevel facing downwards) group by block randomization. Random numbers generated using a computer-generated randomization table were sealed in an opaque envelope. An operator with experience of more than 100 US-guided radial artery catheterization procedures performed the cannulation.
Following induction of general anesthesia, radial arterial catheterization was performed. With the arm in supination, the wrist was extended over a roll with the hand dorsiflexed at 45°. An observer recorded the time taken (using a stopwatch) by the anesthesiologists to achieve successful US-guided radial artery catheterization. Following antiseptic preparation of the insertion site, an US probe connected to a US system (S-nerve™; Sonosite, Bothell, WA, USA) was used to identify the radial artery in the short-axis view. Using a short-axis approach, a 20-gauge intravenous cannula (BD Angiocath Plus™; Becton Dickinson Medical [S] Pte Ltd, Tuas, Singapore) was placed beneath the center of the transducer at an angle of 30–45°; no local anesthetic was administered prior to catheterization. The puncture needle was inserted with the bevel facing upward in the bevel-up group, and downward in the bevel-down group. When the needle was inserted into a radial artery, an image of its tip appeared on the US screen as a dot within the lumen of the vessel (Fig. 1). Entry of the needle into the artery was indicated by adequate return of free-flowing arterial blood. The diameter (Fig. 2) and depth of the artery (Fig. 3) (the latter defined as the distance from the skin to the outer wall of the artery) were measured from the US image.
Insertion time, defined as the time between the US probe contacting the skin and the placing of the catheter into the radial artery and the number of attempts, were recorded. The ease of insertion was indexed by the success rate on the first attempt. Total success rate, first attempt success rate, second attempt success rate, third attempt success rate, and failure rate in both groups were recorded. Failure of cannulation was defined as more than three attempts at cannulation, because of the possibility for numerous attempts. New attempt was defined as a new penetration of the skin with the needle, followed by an unlimited number of redirections under the skin, as needed. A new catheter was required for new attempt.
Complications including thrombosis, hematoma, vasospasm and posterior wall damage were recorded by an observer. Complications was monitored and detected on US image clinically during and after radial arterial cannulation by an observer (Fig. 4).
Statistical analysis
SPSS for Windows software package ver. 19.0 (SPSS, Inc., Chicago, IL, USA) was used for statistical analysis. Data are presented as mean ± SD or median (range) as appropriate. The categorical end point variables were analyzed using the χ2 test, or Fisher exact test if the subject count in any contingency table cell was expected to be <5. Differences with P value <0.05 and 95 % CIs excluding 0 were considered statistically significant. Student’s t-test was used for group comparison of age, height, weight, and radial artery diameter and depth from the skin. A p-value of 0.05 was taken to indicate statistical significance.
Sample size analysis
The first success rate for US-guided radial artery catheterization using the short-axis approach is reportedly 51 % [6]. Based on this report, we estimated that 92 patients would provide 80 % power for detecting a 50 % improvement in the success rate, from 60 % to 90 % at a 2-tailed α risk of 0.05. Allowing for dropouts and technical problems, 204 patients were enrolled into the study.