Written by Clay Smith
The pediatric trachea is elliptical at the infraglottic level, with the transverse diameter being narrowest. The endotracheal tube (ETT) size formula, (age/4) + 3.5, with a cuffed tube makes more sense anatomically.
Why does this matter?
Classic teaching is that we should use the formula (16+age)/4 or (age/4) + 4 to calculate the uncuffed pediatric ETT size. Also, the trachea is reportedly cone shaped, narrowing below the cricoid. Two questions arise. Is this formula right compared to actual measurements in living children (as opposed to cadaver studies)? And is this understanding of pediatric tracheal anatomy correct?
Ellipse not cone…smaller than the formula
This was a retrospective, single-center study based in Japan that looked at CT reconstructions of the trachea in 86 children 1 month to 15 years old who had been otherwise referred for CT c-spine imaging. The trachea was found to be circular at the cricoid but elliptical in the infraglottic area, with the narrowest diameter in the transverse dimension. So, it’s not a pure cone shape. Also, at its narrowest, the usual formula (above) was an overestimate. This is important, because an oversized tube may cause excess pressure on the tracheal wall and lead to injury, scarring, and stenosis. The authors recommend using the cuffed ETT formula: (age/4) + 3.5 or using the usual formula and selecting a half size down. This would be more generalizable if it was an international, multi-center sample. More and more evidence suggests cuffed ETTs have advantages over uncuffed. This is yet another reason to select a smaller tube that’s cuffed. Anatomically, it will fit better. Any leak can be fixed by adding air to the cuff. And per other studies using cuffed tubes, outcomes are probably better.
Tracheal Size and Morphology on the Reconstructed CT Imaging. Pediatr Crit Care Med. 2019 Aug;20(8):e366-e371. doi: 10.1097/PCC.0000000000001996.
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Reviewed by Thomas Davis