This is not a very new article, but it still appears to be on-line only. I wrote the draft of this post a while ago then forgot about it, but I think it is a valuable publication. Lean WL, et al. Accuracy of 11 formulae to guide umbilical arterial catheter tip placement in newborn infants. Archives of disease in childhood Fetal and neonatal edition. 2017.
The authors, from the Royal Women’s Hospital in Melbourne, compared the accuracy of 11 formulae for deciding how far to insert an umbilical arterial catheter. There have been 11 formulae published and they worked out, based on 103 babies of various sizes who had catheters placed, how well the different formulae worked.
They found that many formulae in common use, especially those based on body weight rather than a body measurement, led to misplaced catheters very frequently. The formulae which worked best were the umbilicus to shoulder tip measurement which is ‘a’ in the figure, and one that I had never heard of which was 0.33 multiplied by the total length of the baby.
Measuring the total body length of a baby is rather inaccurate, and can be difficult when the baby is getting lots of attention or is unstable. I have always used the shoulder tip to umbilicus, since I was a fellow (and add 1 cm for the babies over 2 kg, but I don’t know where any of that comes from). This study confirms that that is one of the best methods, the difference between that method, and the more difficult body length method was minimal. I think it should clearly be the default for UAC insertion depth; I just have to remember to take the measurement before I gown up.
The second article is from the same group of people, and is also on-line first, since March this year : Lean WL, et al. Accuracy of five formulae to determine the insertion length of umbilical venous catheters. Archives of disease in childhood Fetal and neonatal edition. 2018. In this study they had 118 umbilical venous catheters, but only 70 in a position that they could use; the others being either displaced in the portal venous system or too low to be useful. None of the formulas they evaluated were much good, with at best 55% being well-placed. Shukla’s formula, based on body weight is as follows (((3xBW)+9)/2) +1. That is a bit unwieldy for daily use, and prone to errors. The next best had slightly more well-placed UV catheters but many more that were too low (37% vs 4%), that was umbilicus to xiphisternum plus 1cm.
My own practice has been umbilicus to xiphisternum plus 2cm (again I don’t know where that comes from), which would lead to more catheters being too high, but very few being too low, and I think, based on this article I will stick to that.
The big question that these studies pose hasn’t been asked: why are we so backward? Surely we should all be using point-of-care ultrasound to position our catheters correctly the first time. We could all choose the best formula (the best ones from these studies) and then quickly check where the catheter tip is, by ultrasound, to adjust within a few seconds if it is not right.
I say this not as a fanatic of point-of-care ultrasound, I don’t even remember how to switch on our machine, but after thinking about these 2 studies, it just seems evident that we could improve care if we had an immediate objective methods for determining catheter position. I will now have to learn how to do it myself.