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.