Table 1.

Characteristics of the included randomized controlled trials

Study	Number of patients (ultrasound/control)	Population	Age	Setting	Control	Operator
Levin et al., 2003 [2]	69 (34/35)	Adult	Ultrasound/Palpation: 59.9 yr/66.4 yr (mean)	Elective abdominal, cardiothoracic, vascular surgery and neurosurgery	Palpation	Anesthetists with experience of ultrasound-guided central venous catheterization but no experience of ultrasound-guided arterial catheterization
Schwemmer et al., 2006 [3]	30 (15/15)	Small children and infants	28 months (median)	Elective neurosurgery	Palpation	Anesthetists with experience of >20 ultrasound-guided arterial catheterization
Shiver et al., 2006 [4]	60 (30/30)	Adult	≥18 yr	Emergency department	Palpation	Anesthetists with experience of ultrasound-guided peripheral and central venous catheterization but no experience of ultrasound-guided arterial catheterization
Ganesh et al., 2009 [5]	152 (72/80)	Children	Ultrasound/Palpation: 99.1 months/99.6 months (mean)	Elective abdominal, craniofacial, orthopedic, thoracic surgery and neurosurgery	Palpation	Anesthetists with experience of <10 ultrasound-guided arterial catheterization
Bobbia et al., 2013 [6]	72 (37/35)	Adult	Ultrasound/Palpation: 69 yr/71 yr (mean)	Emergency department	Palpation	Physicians with absence of an observational training period
Ueda et al., 2013 [7]	104 (52/52)	Small children and infants	Ultrasound/Doppler: 6 months/5 months (median)	Elective major surgery (mainly cardiac surgery)	Doppler	Anesthetists with experience of <5 ultrasound-guided arterial catheterization
Ishii et al., 2013 [8]	59 (59/59)*	Small children and infants	18.4 months (median)	Elective cardiac surgery for congenital heart disease	Palpation	Anesthetists with experience of ultrasound-guided central venous catheterization but no experience of ultrasound-guided arterial catheterization

*Matched data (the radial arteries are matched as they belong to the same patient).