Testing limits to airflow perturbation device (APD) measurements.

BACKGROUND: The Airflow Perturbation Device (APD) is a lightweight, portable device that can be used to measure total respiratory resistance as well as inhalation and exhalation resistances. There is a need to determine limits to the accuracy of APD measurements for different conditions likely to occur: leaks around the mouthpiece, use of an oronasal mask, and the addition of resistance in the respiratory system. Also, there is a need for resistance measurements in patients who are ventilated. METHOD: Ten subjects between the ages of 18 and 35 were tested for each station in the experiment. The first station involved testing the effects of leaks of known sizes on APD measurements. The second station tested the use of an oronasal mask used in conjunction with the APD during nose and mouth breathing. The third station tested the effects of two different resistances added in series with the APD mouthpiece. The fourth station tested the usage of a flexible ventilator tube in conjunction with the APD. RESULTS: All leaks reduced APD resistance measurement values. Leaks represented by two 3.2 mm diameter tubes reduced measured resistance by about 10% (4.2 cmH2O.sec/L for control and 3.9 cm H2O.sec/L for the leak). This was not statistically significant. Larger leaks given by 4.8 and 6.4 mm tubes reduced measurements significantly (3.4 and 3.0 cm cmH2O.sec/L, respectively). Mouth resistance measured with a cardboard mouthpiece gave an APD measurement of 4.2 cm H2O.sec/L and mouth resistance measured with an oronasal mask was 4.5 cm H2O.sec/L; the two were not significantly different. Nose resistance measured with the oronasal mask was 7.6 cm H2O.sec/L. Adding airflow resistances of 1.12 and 2.10 cm H2O.sec/L to the breathing circuit between the mouth and APD yielded respiratory resistance values higher than the control by 0.7 and 2.0 cm H2O.sec/L. Although breathing through a 52 cm length of flexible ventilator tubing reduced the APD measurement from 4.0 cm H2O.sec/L for the control to 3.6 cm H2O.sec/L for the tube, the difference was not statistically significant. CONCLUSION: The APD can be adapted for use in ventilated, unconscious, and uncooperative patients with use of a ventilator tube and an oronasal mask without significantly affecting measurements. Adding a resistance in series with the APD mouthpiece has an additive effect on resistance measurements, and can be used for qualitative calibration. A leak size of at least the equivalent of two 3.2 mm diameter tubes can be tolerated without significantly affecting APD measurements.

Exercise-induced respiratory resistance changes as measured with the airflow perturbation device.

Respiratory resistance (Rrs) changes during physical labor can modify the work of breathing and thus influence the time that the labor can be performed. Ideal in vivo human respiratory performance measurements should have fine temporal resolution and minimally impact the respiratory system itself. The airflow perturbation device (APD) provides respiratory resistance measurements in this manner. Pre-exercise Rrs was compared with post-exercise Rrs, continuously monitored for 6 min in 12 non-asthmatic subjects. Rrs following exercise at 70-75% VO2max was below pre-exercise levels for 40 s into recovery (p < 0.05) and inhalation resistance differed more markedly than exhalation resistance. The same post-exercise Rrs decline was found when Rrs measurements were compared to those taken during flow rates comparable to those found late in recovery. Results indicate that (1) Rrs indeed declines following exercise in non-asthmatic subjects, probably due to bronchial dilation, (2) Rrs changes can occur on short time scales, (3) Rrs changes are not related primarily to flow rate.