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The accuracy of the ultrasound measured transverse cricoid diameter and the epiphyseal transverse diameter of the distal radius in predicting the pediatric endotracheal tube size

Pediatric Anesthesia

Submitted January 2024 by Dr Sorcha Evans

Read by 54 Journal Watch subscribers

This single centre prospective randomised study in Egypt looked at the use of USS measurements of the transverse cricoid distance (TCD) and the epiphyseal diameter of the distal radius to predict the best fit size endotracheal tubes (ETT) in paediatric patients. This was compared to traditional methods of aged-based formulas.

The incentive for this study was the frequency of paediatric ETT exchanges in children based on aged-based formulas and the concern regarding morbidity associated with it.

The study included 100 children aged 1 – 6 years, ASA I-II for elective surgery, that did not have previous upper airway surgery or malformations, respiratory infections or anticipated difficult airway. The groups were allocated to cuff vs uncuffed ETT. Several steps occurred following this;

1. Preoperative ETT size estimated based on aged-based formulae.
2. A standardised induction protocol was followed including muscle paralysis.
3. TCD measurement was then performed during apnoea.
4. USS measurement of the epiphyseal diameter of the distal radius.
5. ETT was selected for insertion in accordance with TCD measurement and corresponding outer diameter (OD) of ETT.
6. The ETT was assessed whether it was best fit ETT, defined as an uncuffed or deflated cuffed ETT that had a leak detected at inflation pressures of 10-20 cmH20. The ETT was changed accordingly if it was not best fit.

The primary end point was the agreement between the TCD based ETT size and best fit ETT. The agreement rate was 88% and 90% for cuffed and uncuffed ETT. This was associated with ETT exchanges in 12% and 10% for cuffed and uncuffed group and was deemed non-significant.

Additionally, correlation was assessed between best fit ETT and the ETT chosen based on the two USS methods and that based on the aged-based formulas. A higher degree of positive correlation was reported using USS techniques versus traditional methods.

Points for consideration
1. TCD predicts outer diameter rather than inner diameter. Thus, meaning choosing an ETT would be dependent on the brand ETT used and less standardised. If only one brand of ETT is being utilised in an institution this would be preferable, but with recurrent issues in receiving consistent stock, this may be a challenge.
2. Delay in intubating to measure TCD may not be appropriate depending on the child and the emerging situation during induction. TCD can be over or under-estimated depending on the angle, thus it is recommended to use the mean of 3 measurements, which could contribute to further delays. Adding to this would be the initial learning curve associated with this technique and the availability of USS dependent on the institution.
3. The radial epiphyseal measurement is reported in the study as difficult and time consuming. However, this could be performed on an awake child allowing pre-planning. This also may be a better technique than TCD in paediatric patients that are obese or have short necks.
4. The methodology of not inflating the cuff on the cuffed ETT, does not coincide with what I would believe would be most anaesthetists practice. The reason for the cuff is to reduce potential leaks and thus ETT exchanges and concerns associated with this.
5. USS measurements of TCD and the epiphyseal diameter of the distal radius measurement may be useful in the planning of complex airway management. For example, to calculate the maximum ETT size feasible for single lung isolation using a foley catheter.

Take home message
This is an interesting paper, describing USS techniques to size ETTs in paediatric patients. This demonstrates potential and may have a role in preoperative planning for select patients. However, this will not influence my current routine practice in utilising aged based formulaes and microcuff ETTs.

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