Schwemmer 2006.

Study characteristics
Methods	RCT
Participants	Number: 30 radial arteries Number per intervention Ultrasound: 15 radial arteries Palpation: 15 radial arteries Inclusion criteria Small children Exclusion criteria Not reported Surgery/setting: major neurosurgery Baseline characteristics Age: 6 months–9 years; median age 28 months; mean age 40 (SD 33) months Ultrasound group: mean 40.3 (SD 34.9) months Palpation group: mean 39.6 (SD 32.5) months Mean weight: not reported Sex ratio: not reported
Interventions	Randomisation: coin toss Intervention: the radial artery was first localised by ultrasound in its short cross‐section. The cannula was advanced toward the vessel at an angle of 45 degrees. When the cannula appeared to be within the vessel, the transducer was removed and catheterisation was accomplished. Control Palpation technique: the position and course of the artery were identified, the skin was repeatedly disinfected, and the cannula was inserted distally to the fingertip and was directed according to continued palpation. Cross‐over to the other technique: after 3 failed cannulation attempts, the initial approach was changed to the alternative method Co‐Intervention: a normovolaemic status was achieved using crystalloids given the night before the procedure. A linear transducer connected to an ultrasound system (Sonos 5000; Hewlett‐Packard, Andover, MA, USA) was used with a focal length positioned 1.8 cm to identify the radial artery. The cross‐sectional area of the artery was measured at the head of the radius with and without dorsiflexion of the hand by about 45 degrees. The transducer or the physician's fingertip was applied to the skin, and the radial artery was identified as the pulsating vessel. Following further local disinfection, the vessel was approached with standard 24‐G cannulas (Becton Dickinson, Helsinborg, Sweden) via 1 of the 2 techniques. Expertise of operator: experienced personnel (> 20 paediatric arterial catheterisations)
Outcomes	Cross‐sectional area of the radial artery with or without dorsiflexion Cannulation success rates with palpation and ultrasound techniques Cannulation success rate on first attempt Time for successful insertion of the catheter between palpation and ultrasound techniques (interval between skin puncture and successful intra‐arterial advancement of the catheter) Total number of attempts at arterial cannulation with palpation and ultrasound techniques Total number of technique switches Rate of complications for palpation and ultrasound techniques
Notes	No information was provided regarding funding, and no conflicts of interest were declared.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: "The technique to be used for radial artery puncture and insertion of the catheter was selected by tossing a coin: heads for ultrasound guidance and tails for palpation."
Allocation concealment (selection bias)	Unclear risk	No information.
Blinding of participants and personnel (performance bias) All outcomes	High risk	All participants underwent induction of general anaesthesia prior to arterial catheterisation (low risk of bias). The anaesthesiologist was aware of the allocated intervention before performing arterial catheterisation (high risk of bias).
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Quote: "When the cannula appeared to be within the vessel, the transducer was removed and catheterization was accomplished."
Incomplete outcome data (attrition bias) All outcomes	Low risk	No participants withdrawn.
Selective reporting (reporting bias)	Low risk	All outcomes were addressed.
Other bias	Low risk	We identified no other sources of bias.