Continuous arterial blood pressure monitoring via direct arterial cannulation provides anaesthesiologists with vital information in the perioperative period and easy access for blood sampling.1 But arterial cannulation in children, particularly in infants, can be challenging even for experienced anaesthesiologists, and it often involves repeated unsuccessful attempts, often resulting in haemorrhage and haematoma.2
Different ways of facilitating arterial cannulation in children have been explored, including ultrasound guidance and puncture depth adjustment using ultrasound.3 Ultrasound guidance has been useful for vascular access, demonstrating increased safety and efficiency over conventional methods.4 Many guidelines have recommended the use of ultrasound-guided vascular access both in adults and children.4–6 Recent meta-analyses have reported that ultrasound-guided radial artery catheterisation is associated with an increased success rate at first attempt in adults and children.7,8 The American Society of Echocardiography and Society of Cardiovascular Anesthesiologists have recommended ultrasound as an effective rescue technique for arterial access, as it can identify the location and patency of suitable arteries for cannulation or procedural access.6
Using ultrasound, vascular structures can be viewed in two orientations: short-axis/out-of-plane (SAX) or long-axis/in-plane (LAX). In the SAX view, the image plane is perpendicular to the course of the vessel and needle, whereas in the LAX view, the image plane is parallel to the course of the vessel showing the shaft and tip of the needle as it is advanced.9 The advantages of the SAX view are better visualisation of surrounding structures and easier imaging. The LAX view is better at visualising the whole needle throughout its course and depth of insertion, thereby avoiding insertion of the needle beyond the vessel.6
There is only one study that compares SAX and LAX ultrasound-guided arterial cannulation. It was conducted in adults and favoured the LAX approach because of higher success rate at first insertion, shorter cannulation time and decreased incidence of complications.10 However, there have been no reports comparing the two approaches in children. Given that ultrasound guidance would be more beneficial in children because of the small vessel size, the aim of our study was to compare the SAX approach with the LAX approach for ultrasound-guided arterial cannulation in children in terms of efficacy and safety.
This randomised, controlled, patient-blinded trial was performed at the Seoul National University Hospital, South Korea, between January 2015 and April 2015. The study was approved by the Seoul National University Hospital Institutional Review Board (23 October 2014/No.1409-095-610). After obtaining written informed consent from the parents or legal guardians, children younger than 5 years of age, American Society of Anesthesiologists’ physical status 1 or 2, scheduled for elective surgery (cardiothoracic, general, orthopaedic or neurosurgery) under general anaesthesia and requiring invasive arterial blood pressure monitoring were enrolled prospectively. Children with signs of skin infection, a wound (including haematoma near the puncture site), peripheral vascular diseases, visible scars of recent arterial puncture and prominent differences in blood pressure between the four extremities were excluded from the study. The study was registered at ClinicalTrials.gov (NCT02333786).
The study was conducted in two separate age groups: infants between 1 day and 1 year of age and children between 1 and 5 years of age. After age classification, children were randomised by computer-generated numbers in sealed opaque envelopes into two groups according to the ultrasound-guided arterial cannulation technique: SAX vs LAX. The seal of the envelope was broken by trained study personnel after the induction of general anaesthesia. The study was an intention-to-treat evaluation. Operators started the procedure with the technique based on randomisation; however, if the randomised technique was unsuccessful, they were allowed to cross over to the other technique, although the results were categorised in the randomised group.
Following conventional general anaesthesia, either the radial artery or posterior tibial artery was chosen depending on each child's position and operative field. The wrist or ankle was extended over a roll and dorsiflexed for best visualisation of the artery. After sterile skin preparation, ultrasound-guided arterial cannulation was performed by two paediatric anaesthesiologists who had performed more than 20 arterial cannulations and 200 central venous catheterisations in children under ultrasound guidance. The 4 to 10 MHz hockey stick transducer (i12L-RS; GE Healthcare, Wauwatosa, Wisconsin, USA) of the ultrasound (LOGIQ e; GE Healthcare, Wauwatosa, Wisconsin, USA) was protected with a long sterile cover and placed over the skin in a transverse approach to localise the artery. Palpation was not allowed. Once the radial or posterior tibial artery was identified, the cross-sectional view of the artery was frozen and saved to measure the diameter and depth of the artery.
In the SAX group, after viewing the artery in cross section, a 24-G needle (Jelco, Smith Medical, Dublin, Ohio, USA) was inserted with bevel-up orientation at approximately 30 to 45° to the skin and perpendicular to the transducer, adhering close to the middle of the transducer's long axis. The artery appeared as a circular anechoic structure and the needle appeared as a hyperechoic dot. The needle was advanced until the artery collapsed and re-expanded or flashback of blood occurred.
In the LAX group, after viewing the artery in cross section, the transducer was rotated 90°, keeping the image in the centre of the ultrasound screen to identify the artery in its long axis. When the artery appeared as a tubular anechoic structure, a 24-G needle was inserted with bevel-up orientation at about 30 to 45° to the skin and parallel to the transducer directly under the centre of the transducer. The needle and artery were both visualised entirely within the plane of ultrasound imaging. After confirming the needle tip within the lumen and the arterial blood flashback in the needle hub, the angle of the needle was lowered and gently advanced slightly more distally under ultrasound guidance.
In both groups, after arterial blood flashback, the stylet was removed and with pulsatile flow through the needle, the latter was advanced further for complete cannulation. When there was no backflow of blood after removing the stylet, especially in the SAX group, the catheter was withdrawn slowly until the backflow reoccurred and advanced in the same manner. A guide wire was not used in either group. Successful arterial cannulation was confirmed by the arterial waveform on the monitor.
The following data were collected for every child: age, height, weight, sex and cannulated artery. The primary outcome was the total time to successful cannulation. The secondary outcomes included diameter and depth of the artery, time variables (imaging time, time to first successful puncture and time between first successful puncture and cannulation), number of puncture attempts, success rates (first puncture and final cannulation), posterior wall puncture rate and complications (thrombosis, oedema or infection). Imaging time was defined as the interval between skin contact with the transducer and skin penetration of the needle. Time to first successful puncture was defined as the interval between skin penetration of the needle and flashback of blood. Time between first successful puncture and cannulation was defined as the interval between the first flashback of blood and confirmation of the arterial waveform on the monitor. Total time to successful cannulation was defined as the interval between skin contact of the transducer and confirmation of the arterial waveform on the monitor. The posterior wall was considered to be punctured when there was no backflow of blood after removing the stylet.
The sample size was calculated from the previous study based on data obtained in adults.10 Mean arterial cannulation times using the SAX and LAX approaches were 46.8 ± 34 and 23.7 ± 17 s, respectively. The required sample size was 23 for each group, for an α-error of 0.05 and a power of 80%. Considering a dropout rate of 20%, a total of 108 patients were planned for enrolment.
All data were expressed as mean ± SD unless otherwise specified. The Student's t test was used for normally distributed continuous variables. The Mann–Whitney U test was used for nonnormally distributed continuous variables. The χ2 test or Fisher's exact test was used for categorical variables. A P value less than 0.05 was considered statistically significant. Statistical analysis was performed using IBM SPSS Statistics 21 (SPSS Inc., IBM Corporation, Armonk, New York, USA).
Of the 108 enrolled, 101 children were randomised and seven were excluded. Two were excluded because of a faulty invasive blood pressure transducer set, four because of a change in the operation plan making arterial cannulation unnecessary and one because of withdrawal of consent. Fifty infants were randomised into the SAX (n = 25) and LAX (n = 25) groups. Fifty-one children were randomised into the SAX (n = 25) and LAX (n = 26) groups (Fig. 1). Patient characteristics are summarised in Table 1. No significant differences were observed between the groups. Procedural data comparing the SAX and LAX groups according to patient age are summarised in Table 2. Ultrasound-guided arterial cannulation was successful in 97 patients (96.0%). All arterial lines functioned well during the intraoperative period. No serious complications, such as thrombosis, oedema or infection, were reported postoperatively. Crossover to the other group did not occur.
In both infants and children, there were no statistically significant differences in the total time to successful cannulation between the two groups. However, imaging time was significantly longer in the LAX group compared with the SAX group (46.5 ± 39.2 vs 16.0 ± 17.6 s; 95% confidence interval, CI of mean difference, −42.7 to −18.3; P = 0.000). Posterior wall puncture rate was significantly higher in the SAX group compared with the LAX group [95.7% vs 18.0%; P = 0.000; odds ratio (OR) 0.01; 95% CI, 0.002 to 0.048]. There were no statistically significant differences in the other time variables, number of puncture attempts and success rates between the two groups.
Examining the results according to age and ultrasound-guided arterial cannulation technique, imaging time was significantly longer in the LAX group compared with the SAX group both in infants [61.8 ± 43.4 vs 13.4 ± 6.5 s; 95% CI of mean difference: −67.0 to −29.9; P = 0.000) and children (32.3 ± 28.9 vs 18.3 ± 23.4 s; 95% CI of mean difference, −28.8 to 0.8; P = 0.001). In infants, the total time to successful cannulation was significantly longer in the LAX group (176.8 ± 88.5 vs 134.0 ± 109.7 s; 95% CI of mean difference, −101.8 to 16.2; P = 0.03). However in children, the time between first successful puncture and cannulation was significantly longer in the SAX group compared with the LAX group (56.5 ± 68.0 vs 44.9 ± 68.6 s; 95% CI of mean difference, −26.9 to 50.0; P = 0.03). There were no statistically significant differences in the number of puncture attempts and success rates between the two techniques in infants or children.
Dividing the results according to patient age, regardless of the ultrasound-guided arterial cannulation technique, the total time to successful cannulation was significantly longer in infants compared with children (156.3 ± 100.4 vs 115.2 ± 79.9 s; 95% CI of mean difference, 4.7 to 77.6; P = 0.03). Arterial cannulation was 100% successful in children, whereas in infants, the final success rate was 92% (P = 0.06). There were no statistically significant differences in the posterior wall puncture rates between infants and children.
In this prospective randomised study, we compared the SAX and LAX approach for ultrasound-guided arterial cannulation in children. Imaging time was significantly longer in the LAX group because the transducer was rotated, starting from the SAX view to the LAX view, to capture the whole artery in its long axis. Nevertheless, there were no statistically significant differences in the total time to cannulation between the two groups, but despite faster imaging, time between first successful puncture and cannulation was longer in the SAX group than in the LAX group.
Arterial cannulation is composed of two steps: puncture of the artery and insertion of the catheter. Even when arterial puncture is successful, cannulation does not succeed if the catheter fails to stay in the artery. In the SAX group, following posterior wall puncture, withdrawal of the needle tip back into the lumen and sliding it successfully inside the artery could have been difficult and time consuming. Given that posterior wall puncture may cause problems such as loss of pulse, vasospasm and haematoma,11 and that such complications are more severe in children, the LAX approach would be a viable option, if the two techniques have similar total time. From a statistical viewpoint, differences in imaging time between the SAX and LAX group exist: however, the differences are in seconds, which may not have clinical significance. In addition, although it was not statistically significant, the final success rate was higher in the LAX group compared with the SAX group (98 vs 94%), which also favours the LAX approach.
Although it is possible to trace the tip of the needle as a dot using the SAX approach, tracing the tip and cannulating the small artery at the same time is demanding. In contrast to the SAX approach, it is relatively easy to view the whole needle including the tip and the artery using the LAX approach. Furthermore, the bevelled tip of the needle extends about 1 mm from the tip of the catheter, which may cause failure to insert the catheter after successful puncture using the LAX approach. Thus, we advanced the needle at least the length of the bevel and then inserted the catheter.
Berk et al.10 supported the use of LAX ultrasound-guided radial arterial cannulation in adults because of faster total cannulation time, fewer attempts and less posterior wall damage. Contrary to Berk et al., we found no statistically significant differences in the total time to successful cannulation and the number of puncture attempts between the SAX and LAX groups, which may be attributable to different study groups. Compared with posterior wall damage rates of 55.6% for the SAX group and 20.4% for the LAX group in adults,10 our data demonstrated posterior puncture rates of 95.7% and 18% for the SAX and LAX groups, respectively. Regarding the small vessel sizes in children, a higher posterior puncture rate is to be expected, which agrees with our result in the SAX group. However, in contrast to our expectations, the LAX approach showed an even lower posterior puncture rate in children than in adults.
Several studies have compared ultrasound-guided arterial cannulation in children with other techniques such as traditional palpation or Doppler-assisted techniques. These studies reported final success rates of 65 to 100% for ultrasound-guided techniques.3,12–14 In this study, the final success rate ranged between 88 and 100%, in agreement with the results of earlier studies and demonstrating a relatively high performance in infants, regardless of the ultrasound-guided arterial cannulation technique used.
Nakayama et al.3 suggested that ultrasound-guided radial artery catheterisation in children was fastest and most reliable when the artery was 2 to 4 mm below the skin surface. In this study, both the radial artery and posterior tibial artery were included with a mean depth of 3.1 ± 1.1 mm (range 1.4 to 6.3 mm), which was within their recommended range. Nonetheless, the depth of the artery did not show a statistically significant association with the success rate.
Ueda et al.14 found that a diastolic arterial diameter of greater than 1 mm was the most statistically significant covariate associated with first-attempt success. In this study, the mean arterial diameter was significantly smaller in infants compared with children (1.3 ± 0.3 vs 2.1 ± 0.7 mm; 95% CI of mean difference, −1.0 to −0.6; P = 0.000), which could have affected the difference in total time to successful cannulation. In the same context, owing to the small size of the artery, imaging time could have been longer in the LAX group, especially in infants, resulting in longer total time to successful cannulation.
The findings of Ganesh et al.15 differed from those of all the other studies. They reported that ultrasound guidance did not facilitate radial artery cannulation in children. They explained the negative result by operator inexperience. Another study also mentioned the importance of operator proficiency when using ultrasound-guided techniques.14 Previous studies on ultrasound-guided arterial cannulation have shown a diverse range of operators’ skill: some restricted operators of ultrasound-guided arterial cannulation to those experienced in its use,3,10,12 and others did not.13–15 The guidelines published by The Royal College of Radiologists recommended that more than 20 ultrasound-guided vascular access attempts in adults are needed for an operator to be competent.16 Regarding the small vessel sizes in children, operator experience may play a critical role in successful cannulation. Therefore, we limited the operators to two experienced anaesthesiologists. Nevertheless, considering that ultrasound-guided arterial cannulation in children is a procedure with a learning curve, future studies on the learning curve and its relationship with the outcomes are needed.
There are several limitations in our study. First, because two experienced anaesthesiologists performed ultrasound-guided arterial cannulation, one should be cautious about applying our data to other practitioners. Second, the mean number of puncture attempts was 1.6 ± 0.8, demonstrating that most of the cannulations were relatively simple and fast. As ultrasound guidance is of more value in difficult cases, our data could have been of greater value if we included complicated cases.
In conclusion, despite longer imaging time with the LAX approach, there were no statistically significant differences in the total time to cannulation between the two groups. The posterior wall puncture rate was lower in the LAX group than in the SAX group in children studied.
Acknowledgements relating to this article
Assistance with the study: none.
Financial support and sponsorship: none.
Conflict of interest: none.
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