1.

Characteristics of the included RCTs

Study	No. of Patients	Vein	Clinical setting	BMI(LAX/SAX, kg/m2)	Age(LAX/SAX, year)	Male/Female(LAX/SAX)	Operator	Ultrasound equipment	Outcome
LAX: Long- axis; SAX: Short- axis; SCV: Subclavian vein; IJV: Internal jugular vein; AV: Axillary vein; OR: Operating room; SICU: Surgical intensive care unit; PACU: Post-anesthesia care unit; ED: Emergency department; HC: Hemodialysis center. Outcomes: ① First-attempt success rate; ② Overall puncture success rate; ③ Incidence of accidental arterial puncture; ④ Incidence of hematoma; ⑤ Insertion time; ⑥ Number of needle redirection; ⑦ Number of skin punctures.
Vezzani et al 2017[6]	190(95/95)	SCV	SICU	26±4/27±5	70±12/70±12	74±68/21±27	2 anesthesiologists with 3-6 years of experience	Philips CX50	①②③④⑤⑥⑦
Shrestha et al 2016[7]	82(41/41)	IJV	OR/SICU	unknown	59.68±22.01/49.59±18.58	23±30/18±11	An anesthesiologist with a 3 years of experience	SonoSiteⓇ, Micromaxx, Bothwell, WA, USA	①②③⑤⑥⑦
Batllori et al 2016[8]	148(73/75)	IJV	OR/PACU	27.7±4.8/27.3±4.1	mean:64.9/64	45±28/49±26	3 anesthetists with appropriate proficiency	SonoSite, Bothell, WA	①②④⑤⑦
Maddali et al 2017[9]	86(43/43)	AV	OR	27.4±5.1/26±4.3	59.7±13/61.8±13.7	unknown	3 proficient anesthesiologists	GE Healthcare, Wauwatosa, WI	①②③④⑥
Ozkan et al 2015[10]	64(32/32)	IJV	OR	unknown	unknown	unknown	unknown	unknown	①
Chittoodan et al 2011[11]	99(49/50)	IJV	OR	Weight (kg):85.2±13.5/84.1±15.7	62.9±13.2/62.9±13.1	37±37/12±13	2 experienced anesthetists	SonoSiteⓇ, Micromaxx, Bothwell, WA, USA	①②③⑤⑥
Tammam et al 2013[12]	60(30/30)	IJV	SICU/HC	27.41±7.17/28.15±8.2	56.03±14.52/51.33±15.29	13±16/17±14	Well-trained attending anesthetists	GE LogiqBook XP Portable	①④⑤⑦