Pressure loss caused by pediatric endotracheal tubes during high-frequency-oscillation-ventilation

In a physical model of a pediatric respiratory system we measured the pressure drop across ETTs of 3 and 4mm inner diameter (ID) when we varied frequency, mean airway pressure and pressure amplitude of high-frequency-oscillation-ventilation (HFOV). Depending on ventilator settings the relative loss of mean pressure amplitude caused by the ETT ranged from 3.3% to 24.7% for ETT 4mm ID, respectively, from 23.8% to 51.8% for 3mm ID. In addition to the well-described flow dependency, ventilation frequency affected ETT resistance. Due to this frequency dependence, calculation of the pressure drop across the ETT using Rohrer's or Blasius-Itos' approach underestimated the true pressure drop significantly (p<0.001). Based on the experimental results, nomograms for graphical determination of the pressure drop across the ETT during HFOV were developed. We conclude that the pressure drop across the ETT during HFOV is dependent on ETT size, pressure amplitude and ventilation frequency. Calculation of this pressure drop with conventional methods is inaccurate. The high-frequency-resistance of the ETT might protect the lungs from excessive pressure amplitudes during HFOV.