Endotracheal tube size selection
Investigators have tried to use anatomical predictors for endotracheal
tube selection in the dog however none of these methods have been found
to be accurate (Tong & Pang, 2019). There are recommendations for
endotracheal tube selection in adult humans as well as for paediatric
patients (Karmali & Rose, 2020; Miller et al., 2014). It is interesting
to note that the recommended inner diameter endotracheal tube size for
human adults is approximately 50 to 60% of the tracheal at the level of
the cricoid assuming the average tracheal diameter at this point
(Karmali & Rose, 2020). There are differences in outer diameter
dimensions based upon the different manufacturers of endotracheal tubes
and this may come into play when selecting an appropriate size of
endotracheal tube. (Karmali & Rose, 2020).
There are currently no recommended endotracheal tube selection criteria
for horses. The large variety of horse breeds and chondrodysplastic
breeds make this difficult. The large tidal volumes required for horses,
encourage the use of the largest endotracheal tube possible to
facilitate the use of mechanical ventilation. Using relatively small
endotracheal tubes (c.f. human recommendations) may result in increases
in airway resistance and reductions in the alveolar minute ventilation
and subsequently cause elevations in PaCO2. Another
issue is the higher cuff pressures in these relatively smaller silicone
endotracheal tubes could result in higher tracheal-cuff interface
pressure, especially with overinflation (Richardson & McMillan, 2017).
This needs further research into what is the idea endotracheal tube size
that does not injure the respiratory tract and can still facilitate
ventilation.