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

OBJECTIVE:Assess the accuracy of four different methods of measuring tidal volume during simulated high-frequency oscillatory ventilation. DESIGN:In vitro study. SETTING:Research laboratory. SUBJECTS:Three differential pressure pneumotachometers, a modified Pitot tube, an ultrasound flowmeter, and an adult hot wire anemometer. INTERVENTIONS:Each device was placed in series with a Sensormedics 3100B high-frequency ventilator and an 8.0-mm endotracheal tube attached to a 48.9-L plethysmograph. Inspiratory/expiratory ratio was fixed at 1:1 and mean airway pressure at 10 cm H2O. Tidal volumes were calculated at each combination of frequency (f3, 4, 6, 8, 10, 12 Hz) and pressure amplitude (ΔP30, 60, 90 cm H2O) by digital integration of the sampled flow signals from each device and compared with those calculated from pressure changes within the plethysmograph. The protocol was repeated after incorporation of frequency-specific calibrations to the flow-measuring algorithm of each device. The hot wire anemometer was further evaluated at Fio2 of 1.0, 37°C, 80% humidity, mean airway pressure of 20 cm H2O, and an inspiratory/expiratory ratio of 1:2. MEASUREMENTS AND MAIN RESULTS:Tidal volumes were 36–305 mL. Bland-Altman analysis demonstrated that each device exhibited systematic bias before frequency-specific adjustment. After frequency-specific adjustment of the flow-measuring algorithm, the two most accurate and precise devices were the Hans Rudolph pneumotachometer, which exhibited a mean error of 0.2% (95% confidence interval, −3.0% to 3.4%), and the hot wire anemometer, which had a mean error of −1.1% (95% confidence interval, −5.5% to 3.3%). The hot wire anemometer remained accurate at Fio2 1.0, 37°C, 80% humidity, mean airway pressure of 20 cm H2O, and an inspiratory/expiratory ratio of 1:2. CONCLUSIONS:Tidal volume can be measured during high-frequency oscillatory ventilation using a variety of techniques. Frequency-specific calibration improves the accuracy and precision of tidal volume measurements. Hot wire anemometry exhibits stable performance characteristics across the range of temperature, humidity, Fio2, and inspiratory/expiratory ratios encountered clinically, has a small deadspace, is unaffected by mean airway pressure, and is therefore suitable for clinical applications.