Kids tracheal tubes – formulas galore

March 30, 2011 by  
Filed under All Updates, ICU, Kids, Resus, Ultrasound

An ultrasound study of paediatric airways showed sonographic measurement to be a better predictor of tracheal tube size (using a formula – derived and then validated – to estimate external tube diameter) than traditional formulae for selecting the internal tube diameter based on age. Since the measurements, taken at the lower edge of the cricoid cartilage, were made after patients were paralyzed, and were performed without ventilation or positive end-expiratory pressure to minimize fluctuation in tracheal diameter, taking about 30 seconds, this is not something I anticipate applying in critical care practice. However, the paper does provide a good opportunity to revise some of the existing formulae. They used:

(1) The Cole formula for uncuffed tubes: ID (intenal diameter) in mm= (age in years)/4 + 4

(2) The Motoyama formula for cuffed ETTs in children aged 2 yr or older: ID in mm = (age in years)/4 + 3.5

(3) The Khine formula for cuffed ETTs in children younger than 2 yr: ID in mm = (age in years)/4 + 3.0

The formula established in the study was:

  • cuffed ETT outer diameter (OD) = 0.46 x (subglottic diameter) + 1.56
  • uncuffed ETT OD = 0.55 x (subglottic diameter) + 1.16

Age in months also correlated with optimal ETT size in mm, although the correlation was weaker than for subglottic diameter:

  • cuffed ETT OD = 0.027 x (age) + 5.2
  • uncuffed ETT OD = 0.030 x (age) + 5.4
BACKGROUND: Formulas based on age and height often fail to reliably predict the proper endotracheal tube (ETT) size in pediatric patients. We, thus, tested the hypothesis that subglottic diameter, as determined by ultrasonography, better predicts optimal ETT size than existing methods.

METHODS: A total of 192 patients, aged 1 month to 6 yr, who were scheduled for surgery and undergoing general anesthesia were enrolled and divided into development and validation phases. In the development group, the optimal ETT size was selected according to standard age-based formulas for cuffed and uncuffed tubes. Tubes were replaced as necessary until a good clinical fit was obtained. Via ultrasonography, the subglottic upper airway diameter was determined before tracheal intubation. We constructed a regression equation between the subglottic upper airway diameter and the outer diameter of the ETT finally selected. In the validation group, ETT size was selected after ultrasonography using this regression equation. The primary outcome was the fraction of initial cuffed and uncuffed tube sizes, as selected through the regression formula, that proved clinically optimal.

RESULTS: Subglottic upper airway diameter was highly correlated with outer ETT diameter deemed optimal on clinical grounds. The rate of agreement between the predicted ETT size based on ultrasonic measurement and the final ETT size selected clinically was 98% for cuffed ETTs and 96% for uncuffed ETTs.

CONCLUSIONS: Measuring subglottic airway diameter with ultrasonography facilitates the selection of appropriately sized ETTs in pediatric patients. This selection method better predicted optimal outer ETT diameter than standard age- and height-based formulas.

Prediction of Pediatric Endotracheal Tube Size by Ultrasonography
Anesthesiology. 2010 Oct;113(4):819-24

Comments

3 Responses to “Kids tracheal tubes – formulas galore”

  1. Christopher on March 30th, 2011 09:32

    I think the question we have in the field is not tube size but rather cuff inflation volume! Easy for adults, 10 mL until the pilot balloon feels solid. For kids do we start with 5 mL? Less? Tube size based?

    All we have is a Broselow tape and cuffed tubes, no manometers to get the 20 cmH2O or less required to seal the airway.

    Are there any formulas for this?

  2. Cliff on March 30th, 2011 11:33

    Good question! I think less – in fact less even for adults check out this table http://www.ncbi.nlm.nih.gov/pmc/articles/PMC535565/table/T3/ which comes from the article ‘Endotracheal tube cuff pressure in three hospitals, and the volume required to produce an appropriate cuff pressure’ at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC535565/.

    I think for kids especially a manometer is a good idea

    Cliff

  3. Christopher on March 31st, 2011 10:35

    Uh oh, field experience shows we fill the cuff with a full syringe! Likely we need to adjust our thinking and probably grab some more studies to get our protocols in order. Thank you for the link.