Abstract: Of 735 identified citations, 35 studies enrolling 5108 participants fulfilled the inclusion criteria. The quality of evidence was very low for most of the outcomes and was moderate at best for four of the outcomes. Most trials had an unclear risk of bias across the six domains, and heterogeneity among the studies was significant. Use of two-dimensional ultrasound reduced the rate of total complications overall by 71% (14 trials, 2406 participants, risk ratio (RR) 0.29, 95% confidence interval (CI) 0.17 to 0.52; P value < 0.0001, I² = 57%), and the number of participants with an inadvertent arterial puncture by 72% (22 trials, 4388 participants, RR 0.28, 95% CI 0.18 to 0.44; P value < 0.00001, I² = 35%). Overall success rates were modestly increased in all groups combined at 12% (23 trials, 4340 participants, RR 1.12, 95% CI 1.08 to 1.17; P value < 0.00001, I² = 85%), and similar benefit was noted across all subgroups. The number of attempts needed for successful cannulation was decreased overall (16 trials, 3302 participants, mean difference (MD) -1.19 attempts, 95% CI -1.45 to -0.92; P value < 0.00001, I² = 96%) and in all subgroups. Use of two-dimensional ultrasound increased the chance of success at the first attempt by 57% (18 trials, 2681 participants, RR 1.57, 95% CI 1.36 to 1.82; P value < 0.00001, I² = 82%) and reduced the chance of haematoma formation (overall reduction 73%, 13 trials, 3233 participants, RR 0.27, 95% CI 0.13 to 0.55; P value 0.0004, I² = 54%). Use of two-dimensional ultrasound decreased the time to successful cannulation by 30.52 seconds (MD -30.52 seconds, 95% CI -55.21 to -5.82; P value 0.02, I² = 97%). Additional data are available to support use of ultrasound during, not simply before, line insertion. Use of Doppler ultrasound increased the chance of success at the first attempt by 58% (four trials, 199 participants, RR 1.58, 95% CI 1.02 to 2.43; P value 0.04, I² = 57%). No evidence showed a difference for the total numbers of perioperative and postoperative complications/adverse events (three trials, 93 participants, RR 0.52, 95% CI 0.16 to 1.71; P value 0.28), the overall success rate (seven trials, 289 participants, RR 1.09, 95% CI 0.95 to 1.25; P value 0.20), the total number of attempts until success (two trials, 69 participants, MD -0.63, 95% CI -1.92 to 0.66; P value 0.34), the overall number of participants with an arterial puncture (six trials, 213 participants, RR 0.61, 95% CI 0.21 to 1.73; P value 0.35) and time to successful cannulation (five trials, 214 participants, each using a different definition for this outcome; MD 62.04 seconds, 95% CI -13.47 to 137.55; P value 0.11) when Doppler ultrasound was used. It was not possible to perform analyses for the other outcomes because they were reported in only one trial. Based on available data, we conclude that two-dimensional ultrasound offers gains in safety and quality when compared with an anatomical landmark technique. Because of missing data, we did not compare effects with experienced versus inexperienced operators for all outcomes (arterial puncture, haematoma formation, other complications, success with attempt number one), and so the relative utility of ultrasound in these groups remains unclear and no data are available on use of this technique in patients at high risk of complications. The results for Doppler ultrasound techniques versus anatomical landmark techniques are also uncertain.

Plain-language summary: This Cochrane systematic review compared landmark techniques versus ultrasound to guide the insertion of a catheter into the large vein in the neck (the internal jugular vein). In 2013 we included in the review 35 studies enrolling 5108 participants (adults and children). These studies were varied, and their quality was moderate at best. We reran the search in August 2014. We will deal with any studies of interest when we update the review. Nevertheless, ultrasound offered some benefits. Using ultrasound reduced the rate of complications (-71%), including severe bruising (-73%) and accidental puncturing of an artery instead of the vein (72%). It also increased success rates, including success rates at the first attempt (+57%) and reduced the time taken to perform the procedure. None of the included studies reported on death or patient-reported outcomes (patient discomfort). Based on available data, we conclude that two-dimensional ultrasound offers improved safety and quality when compared with an anatomical landmark technique, but these findings do not necessarily hold for all users or for patients at high risk of complications. The relative utility of ultrasound when operators are experienced or inexperienced in central line insertion, however, remains unclear for some outcomes. The results for Doppler ultrasound techniques versus an anatomical landmark technique are also uncertain.