Pressure drop across neonatal endotracheal tubes during high-frequency ventilation

Introduction

High-frequency ventilation (HFV) is a concept of mechanical ventilation that is mainly used in therapy of infants. The resistance of the small neonatal endotracheal tubes (ETT) causes a noticeable difference between airway pressure (proximal end of the ETT) and tracheal pressure (distal end of the ETT). The aim of this laboratory study was to evaluate the pressure drop across the ETT during HFV and to investigate whether tracheal pressure can be calculated from airway pressure using conventional methods.

Methods

A physical model of an infant's respiratory system was connected with one of two differently sized ETTs (ID 3 mm or 4 mm; Blue Line, Portex Ltd., Hythe, Kent, UK) with the positioning of the tip inside the trachea of the model. The ETT was bent along a test fixture to approximate the in-situ curvature of an ETT simulating the nasal route of intubation. An infant HF-ventilator Sensormedics 3100A (SensorMedics Corp., Yorba Linda, CA, USA) was used to ventilate the model with an I/E ratio of 1:2. We varied mean airway pressure from 8 to 16 mbar (in steps of 2 mbar), the set airway pressure amplitude from 10 to 50 mbar (in steps of 10 mbar) and the frequency to 5 Hz, 10 Hz and 15 Hz, respectively. We analyzed the pressure drop across neonatal ETTs in a physical model setup during different conditions of HFV.

Results

We found that depending on the ventilator's settings the relative loss of mean pressure amplitude caused by the ETT ranged from 23.8% up to 51.2% during the positive flow phase and from 3.3% up to 24.7% during the negative flow phase.

Additional to the well-described flow dependency of ETT resistance we found an increase of resistance caused by the HFV. Due to this effect, calculation of the ETT's pressure drop using the Rohrer or Blasius-Itos approach underestimated the true pressure drop significantly.

Conclusion

We conclude that an increased pressure drop during HFV caused by the ETT must be considered to be dependent on the size of the ETT, the ventilation frequency and the flow rate, the latter implicating a dependency on the ventilator's performance in flow delivery. For the patient's respiratory system only that part of delivered energy that is transferred to the patient's lung is of relevance. This means that decisions for setting parameters of HFV must be made from the view of tracheal pressure. For an adequate noninvasive monitoring of tracheal pressure during HFV, new methods for calculation of the pressure drop across the ETT appear crucial.