Ethics
Ethical approval for this study (Ethical Committee N° #287/2019) was obtained from the Institutional Review Board of Children’s Hospital of Chongqing Medical University, Chongqing City, China (Chairperson Professor Zhongyi-Lu) on 10 January 2020. The registry Uniform Resource Locator of this study is chictr.org.cn (ChiCTR2000029353, registration date: 26 January 2020). Written informed consent was obtained from parents or guardians as part of the standard requirement for the procedure. The study started in May 2020 and ended in December 2020.
Confirming the Scale on the Ultrasonic Probe and Ultrasonic Screen
To confirm the suitability of the ultrasonic probe and ultrasonic screen for infants under the age of 3 months, we measured the inner diameters of 140 radial arteries in infants ranging from 0 to 3 months. The mean inner diameter of the arteries was 1.61 ± 0.34 mm, the mean age was 20 ± 14.5 days, and the mean weight was 3.6 ± 1.39 kg. Based on our statistical results, the inner diameters of the radial and ulnar arteries reported in other literature,14 and our pre-experiment, we confirmed the scale on the ultrasonic probe as follows (Figure 1): the total length of the scale on the ultrasonic probe was 2 cm, each scale was spaced 2 mm apart, the midpoint of the ultrasonic probe was the zero point, and the scale lines were evenly distributed to the left and right sides in the direction of ultrasonic probe length detection. The left side was marked as -5, -4, -3, -2, and -1; the midpoint of the ultrasonic probe was marked 0; and the right side was marked as 1, 2, 3, 4, and 5. The length, style, direction and spacing of the scale on the ultrasonic screen correspond to the ultrasonic probe, and the actual distance between the two scales on the ultrasonic screen was determined by the actual magnification of the ultrasound. Ultrasound with scale on the ultrasonic probe and screen was defined as the scale ultrasound.
Sample Size Calculation
The sample size of this study was calculated by a previous study, in which the first-attempt success rate of radial artery cannulation in the traditional ultrasound group was 60%.14 Based on our preliminary experiment, the first-attempt success of radial artery cannulation in the scale ultrasound group was 90%. Therefore, we wanted to improve the first-attempt success of radial artery cannulation from 60% to 90%, considering the rate of withdrawal (15%), and the final sample size was identified as 38 for each group.
Arterial Catheterization Procedure
Infants scheduled for elective surgical procedures who required continuous arterial pressure monitoring were enrolled in this study between May and December 2020 at the Children’s Hospital of Chongqing, China. The inclusion criteria were as follows: age range of 0 to 3 months and American Society of Anaesthesiologists (ASA) grades I to IV. The exclusion criteria were as follows: an abnormal Allen’s test, malformation in the forearm arteries, erosions near the radial artery puncture in the skin, cardiogenic or haemorrhagic shock, and arterial puncture received within 1 month before the commencement of the trial. Patients were randomly assigned to the scale ultrasound group or the traditional ultrasound group using a computer-generated random number (1:1 ratio). Group allocation was enclosed in sealed envelopes. At the time of enrolment, the research assistant opened the envelopes, and the researchers enrolled the patient. Residents who were blinded to the group allocation recorded the data. A flow diagram shows the patient selection process (Figure 2).
Blood pressure, electrocardiogram, and peripheral oxygen saturation were monitored in the operating room. After general anaesthesia was induced, radial artery cannulation was performed. Radial artery catheterization was performed by residents who had experience with at least 50 cases of arterial puncture. The left hand for radial artery puncture was preferred.
The wrist was padded up with a small roll, and the palm of the hand was taped to keep the hand positioned in dorsiflexion. Aseptic preparation of the skin was performed around the insertion site. The aseptic preparation included disinfecting the insertion site with povidone iodine solution and wrapping the ultrasonic probe with disposable sterile covers, sterile gloves, and operating towels. An ultrasound device (GE venue40; GE, Boston, Massachusetts, USA) with a linear transducer (5 to 13 MHz) and a depth of 2 cm was applied to localize the radial artery. A standard 22-gauge catheter (BRAUN Company, Melsungen, Germany) percutaneously punctured the radial artery using the short-axis, out-of-plane procedure. Patients in the traditional ultrasound group underwent conventional ultrasound-guided radial artery puncture, whereas radial artery puncture was guided by scale ultrasound in the scale ultrasound group. The probe with scale was adjusted such that the radial artery was positioned between any two scales on the ultrasound image (Figure 3 a1, the radial artery is positioned on the ultrasound screen between scale 0 and 1). Subsequently, we chose the position on the ultrasonic probe corresponding to the ultrasonic screen as the arterial puncture point (Figure 3 a2, the arterial puncture point was positioned between scale 0 and 1 on the ultrasonic probe). Between the two scales on the probe, the needle was inserted into the skin at an angle of 30°~45°. Because the scale ultrasound screen and the scale ultrasound probe can provide an accurate arterial location and improved guidance for the operator, we can easily find the location of the needle tip, which was directed towards the arterial lumen when the needle entered the subcutaneous region. Then, the probe was moved proximally 2–3 mm to the radial artery again. According to the scale on the ultrasound screen and the probe, we could easily determine the position and direction of the radial artery (Figure 3 b1 and b2, the arterial was positioned between scale 0 and 1, closer to scale 0). After adjusting the direction of the tip of the needle and inserting the needle into the radial artery (Figure 3 c1 and c2), the needle appeared as a hyperechoic dot on the screen and could be distinguished from the nearby tissue. According to the scale on the ultrasound screen and the probe, adjusting the angle and direction of the incoming needle made the needle come forward to the centre of the radial artery. While keeping the needle immobile, the probe was moved proximally a few millimetres until the image of the needle tip disappeared, the radial artery was repositioned according to the scale on the ultrasound image, and the needle was inserted towards the corresponding scale on the ultrasound probe until the image of its tip reappeared. This procedure was repeated until the needle was inserted fully into the radial artery (Figure 3 d1 and d2). Then, the stylet was removed, and a pressure sensor was connected to monitor blood pressure.
Data Acquisition
The patient’s general condition was recorded by residents who were blinded to the group allocation, including the patient’s age, weight, sex, ASA physical status, heart rate, mean arterial pressure, depth from the skin to the radial artery, and inner diameter of the radial artery. The primary endpoints were the success rate of the first attempt and the total success rate of arterial cannulation. The secondary endpoints were the time of ultrasound location, the time of the needle entering the radial artery, and the time of successful cannulation, times of the arterial puncture and the incidence of vascular complications. Successful puncture was defined as the arterial waveform being verified after arterial cannulation. Puncture failure was defined as more than three attempts to achieve cannulation of the radial artery or any attempts requiring more than 10 min. The time of ultrasound location was defined as the time from the placement of the ultrasound probe on the skin to the insertion of the needle into the skin. The time of the needle entering the radial artery was defined as the time the skin was punctured by the needle to the time the needle entered the radial artery. The time of successful cannulation was defined as the time between when the needle was inserted into the skin and the time the arterial waveform could be verified after arterial cannulation. Complications were recorded, including bleeding and hematoma formation.
Data Analysis
Minitab 18.0 (Minitab Inc., USA) was used for statistical analysis. Descriptive statistics of normally distributed variables were calculated using counts and means ± SD. Descriptive statistics of nonnormally distributed variables were calculated using the median with the interquartile range. Normality of the data was evaluated by the Anderson–Darling test. If the data exhibited a normal distribution, a two-tailed Student’s t-test was used to measure the difference between groups; otherwise, a nonparametric test was used. The chi-square test was used for other categorical data, such as sex and ASA status. Fisher’s exact test was used to determine the successful puncture rate and the incidence of adverse reactions, and the differences in incidence and associated 95% CI were calculated. Statistical significance was defined as a P-value less than 0.05.