Four methods of measuring tidal volume during high-frequency oscillatory ventilation

Area of Science:

• Respiratory Physiology
• Medical Device Technology

Background:

• Accurate measurement of tidal volume is essential for mechanical ventilation.
• High-frequency oscillatory ventilation (HFOV) presents unique challenges for accurate volume monitoring.

Purpose of the Study:

• To assess the accuracy of four different methods for measuring tidal volume during simulated HFOV.
• To compare the performance of pneumotachometers, a Pitot tube, an ultrasound flowmeter, and a hot wire anemometer.

Main Methods:

• An in vitro study using a Sensormedics 3100B HFOV ventilator and an 8.0-mm endotracheal tube.
• Four devices were tested: 3 differential pressure pneumotachometers, a modified Pitot tube, an ultrasound flowmeter, and an adult hot wire anemometer.
• Measurements were taken at various frequencies and pressure amplitudes, with and without frequency-specific calibration.

Main Results:

• Before calibration, all devices showed systematic bias in tidal volume measurements.
• After frequency-specific calibration, a Hans Rudolph pneumotachometer (mean error 0.2%) and a hot wire anemometer (mean error -1.1%) demonstrated the highest accuracy and precision.
• The hot wire anemometer maintained accuracy under varied clinical conditions (Fio2 1.0, 37°C, 80% humidity, mean airway pressure 20 cm H2O, I:E ratio 1:2).

Conclusions:

• Tidal volume measurement during HFOV can be achieved with multiple techniques.
• Frequency-specific calibration is critical for enhancing the accuracy and precision of these measurements.
• Hot wire anemometry is a suitable clinical tool for HFOV due to its stable performance, minimal dead space, and independence from mean airway pressure.