Airway-Pressure Measurement during High Frequency Ventilation

Abstract

Because of the difficulty of measuring small tidal volumes during high frequency ventilation (HFV), it is necessary to accurately monitor airway pressures to quantitate ventilator settings and provide a basis for comparison with conventional ventilation. The accuracy of a pressure-measuring system is a function of its frequency response. We evaluated the frequency response of two types of measurement systems. Method: One system incorporated various lengths (14, 28, 33, and 33.5 cm) of 16-gauge polyvinyl catheter used to measure distal endotracheal tube (ET) pressures. The other system made use of the pressure-monitoring channel of several sizes (9-, 7-, 5-, and 3-mm ID) of Mallinckrodt Hi-Lo Jet tubes. In addition to the sine wave frequency response, we determined a triangular wave response for both systems, which is more clinically relevant to high frequency jet ventilation. A sinusoidal pressure waveform (amplitude 2 cm H₂O, frequency 1-50 Hz) and a triangular pressure waveform (amplitude 25 cm H2O, frequency 1-8 Hz, duty cycle 25%) were used as reference signals. Results: For both systems, the amplitude of the measured pressure waveform decreased relative to the reference amplitude as the frequency of the pressure signal increased. The 9- and 7-mm-ID ET tubes attenuated the measured pressure signal less than did the 35.5- and the 33-cm 16-gauge catheters. The 5-mm-ID ET tube showed about the same response as the 28-cm catheter. However, the 3-mm-ID ET tube attenuated the signal substantially more than did the 14-cm length catheter. For both systems, the triangular waveform was attenuated more than the sine wave at all frequencies. The measured pressure signal for both systems exhibited a phase lag relative to the reference waveform. Phase lag was minimal at 1 Hz but approached -150 degrees at 50 Hz. Conclusions: The results of this study emphasize that during HFV, the frequency response of a pressure-measurement system must be appropriate for the frequency and waveform of the signals measured. This implies that the accuracy of the system will be affected by the mode as well as the frequency of ventilation. (Respir Care 1985;30:750-758.)