In this trial, we found that the M-LAIP approach could increase the first-pass success rate and total cannulation and reduce the number of attempts in adults with radial artery diameters less than 2.2 mm compared with the SAOP or C-P approach. We also found that it may take longer to complete the first location with the M-LAIP approach, but it required a shorter cannulation time, and the incidence of hematoma complications was lower.
Currently, the conventional LAX is superior to C-P in radial artery cannulation, however, there are still some insufficiencies or deficiencies in the inaccuracy of locating the needle tip and the lower first-pass success rate [12, 21]. There are two key issues to be well solved: one is how to keep the center axis of the artery in the ultrasonic plane, the other is how to avoid the needle tip deviation from the center axis of the radial artery or insertion into the artery wall. The ultrasound devices assume that all received echoes come from structures located precisely on the central line of the US beam [22]. Therefore, we define the plane that passes through the center of the ultrasonic section as the ultrasonic plane(abcd), as depicted in Fig. 4A. We designed two special centerlines on the sides of the probe, which were formed by the junction point between two pieces of the transducer. (Fig. 4A). The two centerlines are parallel and determine a plane (mnba) that coincides with the ultrasonic plane. The centerlines are contributable to the assessment of the position of the ultrasound plane, and thereby offer a precise localization and orientation for puncture.
The elevation beamwidth of ultrasound varies with depth and has a measurable thickness [23]. The artifacts caused by ultrasound section thickness introduce errors in locating the needle tip during the process of ultrasonic guidance [13]. Only when the ultrasound plane deviates from the radial artery central axis, the needle tip is inserted into the artery wall (Fig. 4B, ②, ④) or close to the outside of the artery wall (Fig. 4B, ①, ⑤), and the arterial lumen and the needle tip are both still in the ultrasound section, can the above-mentioned situation of no backflow blood in the hub of the needle occur. (Fig. 4B, (1), (2), (4), (5)). In practice, we determine the optimal location of the ultrasound probe at the wrist by evaluating the diameter of the radial artery in the long-axis ultrasound view (Fig. 4B). Only when the center axis of the artery is held in the ultrasonic plane, can the maximum diameter of the artery in the ultrasound image be obtained (Fig. 4B, 2, (3), ③), and this approach can significantly reduce the incidence of the error in locating caused by the elevation beamwidth of ultrasound.
The M-LAIP ultrasound technique is significantly different from the conventional LAIP ultrasound technique [12, 14, 2527, Supplemental-Appendix] concerning the centerlines of the probe, the obtaining of the maximum diameter of the artery in the ultrasound view, the operator's seated position, and the manners of holding the needle and probe with hands (Fig. 1, Fig. 4B ). In the M-LAIP approach, the operator's special seated position and needle-holding mode were adopted, which have the advantages of stabilizing the hand-held probe, optimizing the visual field, keeping the needle perfectly aligned with ultrasonic plane, and more precise needle tip control. These factors may be the main reason for the higher success rate of the M-LAIP approach.
Compared with the SAX approach [28], the LAX approach is relatively easy to view the whole needle and the vessel [16] continuously and the relative position of the needle tip to arteries posterior wall [14], which reduces the incidence of posterior wall penetrations and hematoma in our study. The higher rates of posterior wall puncture may be due to the difficulty in tracking a high echo dot image of the needle tip. The small radial artery (diameter < 2.2 mm) and the 2–4 mm 2-point discrimination limit of fingertip palpation [20] may be the cause of the higher cannulation failure rate and higher incidence of complications in the C-P group. Compared with the other study [4], the lower success rate of the C-P in our study may be linked to the small caliber radial artery (1.8 mm), the high proportion of female patients (75%-80%), etc. Besides, the differences in the first location time in the study groups are in seconds, which may not have much clinical benefit for a procedure.
Berk, et al [14] demonstrated that the LAX approach had advantages over the SAX approach. The first-pass success rate was similar to our study(76% VS 80.3%), but, the mean radial artery diameter showed to be larger, the proportion of female patients was lower (50% VS 78%), which might reduce the difficulty of cannulation [29].
A study [16] on radial artery cannulation in children with artery diameters of 2-2.3 mm showed that the LAX approach was not superior to the SAX approach in the first-pass success rate, which was not consistent with our findings. The reason may be attributable to the different ultrasound approaches, etc.
Research showed that the rate of vasospasm was low (3–4%), and there was a correlation between vasospasm and the number of attempts or cannulation time [20], which was consistent with our findings. Therefore, fewer attempts and shorter cannulation times may reduce the occurrence of spasms.
It has been reported [12] that the incidence of hematoma ranges from 15–18% for ultrasound-guided radial artery cannulation, which was similar to the rate found in the SAOP group in our study. The increased incidence of hematoma may be associated with more attempts.
Minimal experience with US guidance may have prevented operators from realizing their full benefit [30]. For cases with difficult cannulation, operator experience in using ultrasound-guided techniques plays a key role in successful arterial cannulation, especially for novices who need a procedure with a learning curve [16, 31]. Therefore, the results of our study were obtained by only four experimental anesthesiologists.
The current study has several limitations. General anesthesia (GA) could increase the diameter of the radial artery [32], therefore, all the procedures in this study were performed before the induction of GA. Vasospasm is difficult to standardize that definition, which may be a limitation in the study. A bias in judgment, recording of procedures, measurement, and the undoubled-blind trial design may be the potential limitation of this study. In the future, we will compare this M-LAIP approach with conventional LAX, the modified dynamic needle tip positioning ultrasound technique [33] to verify whether it has some advantages, especially for infants.