Effectiveness of substantial shortening of the endotracheal tube for decreasing airway resistance and increasing tidal volume during pressure-controlled ventilation in pediatric patients: a prospective observational study

The endotracheal tubes (ETTs) used for children have a smaller inner diameter. Accordingly, the resistance across ETT (R_ETT) is higher. Theoretically, shortening the ETTs can decrease total airway resistance (R_total), because R_total is sum of R_ETT and patient’s airway resistance. However, the effectiveness of ETT shortening for mechanical ventilation in the clinical setting has not been reported. We assessed the effectiveness of shortening a cuffed ETT for decreasing R_total, and increasing tidal volume (TV), and estimated the R_ETT/R_total ratio in children. In anesthetized children in a constant pressure-controlled ventilation setting, R_total and TV were measured with a pneumotachometer before and after shortening a cuffed ETT. In a laboratory experiment, the pressure gradient across the original length, shortened length, and the slip joint alone of the ETT were measured. We then determined the R_ETT/R_total ratio using the above results. The clinical study included 22 children. The median ETT percent shortening was 21.7%. Median R_total was decreased from 26 to 24 cmH₂O/L/s, and median TV was increased by 6% after ETT shortening. The laboratory experiment showed that ETT length and the pressure gradient across ETT are linearly related under a certain flow rate, and approximately 40% of the pressure gradient across the ETT at its original length was generated by the slip joint. Median R_ETT/R_total ratio were calculated as 0.69. The effectiveness of ETT shortening on R_total and TV was very limited, because the resistance of the slip joint was very large.