**9. Areas of research**

With an increase in clinical studies showing the advantages of radial access also came insight into complications including radial loops, high take off of radial artery, spasm, dissection [5]. The ulnar artery became another option, initially avoided due to location, as the ulnar artery is often deeper beneath the skin and concern for ulnar nerve damage or hand ischemia but reports for both coronary and peripheral angiography and interventions rasie doubts regarding those concerns [33–35]. A meta analysis of five trials found similar complications between radial vs. ulnar approach [36].*,* crossover was higher with ulnar versus radial approach but this was driven by one trial [37]. This trial was to enroll 2286 patients but was stopped early with 902 enrolled after finding cross over to another site was 26% more likely with ulnar approach compared to radial, with the caveat that ultrasound was not used for access. Further studies are warranted in comparing radial versus ulnar using ultrasound.

Radial artery is being reinvestigated as a favored coronary artery bypass graft (CABG) over veins with recent meta analysis of 1036 patients having lower mortality with arterial grafts over venous grafts [38]. This has prompted the ROMA prospective randomized trial comparing vein to arterial grafts for CABG. One study from 2003 found radial grafts that were previously cannulated had a lower patency rate [39]. Several other studies have found changes in the radial artery including arterial tears, radial intimal hyperplasia and loss of reactivity after sheath insertion [40–42]**.** This has prompted some surgeons to request interventional cardiologists not to use nondominant radial artery for angiography. Further studies investigating radial or ulnar access prior to CABG are recommended.

Radiation exposure is a constant worry in the catheterization laboratory [43]. Advances in technology have lowered radiation exposure including improved shielding. Clinical data have shown radial, particularly right radial, to have more radiation exposure compared to femoral approach [6]. Comparison of left radial to femoral approach in one randomized trial [44] found higher radiation compared in radial approach, although this was done prior to newer technology to assist in left radial such as sheath extension (eg. Stand Tall, Radux Devices) and left arm support systems. Multiple randomized trials found less radiation with left versus right radial [45–49] although one trial found more radiation with left radial [50]. Avoiding steep angles, particularly LAO -Caudal, lower magnification, lower frame rate with fluoroscopy, and distance is recommended [51, 52]. Further research comparing access sites is warranted to better understand with current technology the risks of radiation exposure.

Elderly have higher risk for CV procedures [53, 54] and benefits of radial approach for reduction in bleeding complications is a valid concern. Age appears to be a predictor of failure or cross over to another site [5, 19]. Yet studies in the elderly including a retrospective analysis have not shown increased time to treat ST elevation myocardial infarction [55]. A review of patients enrolled in randomized Rival trial found less complications but higher cross over rates in the elderly [56]. Further studies are warranted in those 75 years and older to compare radial (left versus right) and femoral access points in examining cross over rates, radiation, bleeding and success.

Radial access has dramatically changed over the past twenty years with advances in both technology and technique to bring this approach to the forefront in both the acute setting as well as for complex procedures.
