Professor Arthur T. Johnson

umd logo

Airflow Perturbation Device

The airflow perturbation device (APD) is a medical instrument developed for noninvasive measurement of respiratory resistance. This device can be very useful for measurements on small children, unconscious or uncooperative patients, and animals.

Publications & Abstracts: Airflow Perturbation Device

Comparison of Expiratory Isovolume Pressure-Flow Curves With the Stop-Flow Versus the Esophageal-Balloon Method

Respiratory Care 2011 56(7): 969 –975

D.C. Coursey
S.M. Scharf
A. T. Johnson

BACKGROUND: Expiratory isovolume pressure-flow curves allow determination of flow limitation and airway resistance, but obtaining an isovolume pressure-flow curve requires placing an esophageal balloon. The stop-flow method of obtaining isovolume pressure-flow curves is easy and noninvasive. OBJECTIVE: To compare the stop-flow and esophageal-balloon methods by measuring the differences between the pressures and flows at which flow limitation first occurs. METHODS: In 5 healthy subjects we used the esophageal-balloon method and the stop-flow method at 25%, 50%, and 75% of vital capacity (VC), and constructed isovolume pressure-flow curves showing the pressure at which the flow became limited during forced expiration. RESULTS: The mean calculated pleural pressure at flow limitation with the stop-flow method was 2.7 times and 1.6 times that via the esophageal-balloon method at 25% of VC and 50% of VC, respectively. The maximum flow at flow-limitation with the stop-flow technique was 0.7 times and 0.6 times that via the esophageal-balloon method at 25% of VC and 50% of VC, respectively. We also calculated the resistance (the inverse of the slope of the line to the point of flow limitation), but there were large variations in the resistance values, so there was no statistically significant relationship between the stop-flow and esophageal-balloon methods. CONCLUSION: The stop-flow method showed potential to noninvasively obtain isovolume pressure-flow curves.


Physiol. Meas. 31 (2010) 921-934
doi: 10.1088/0967-3334/31/7/004

D.C. Coursey
S.M. Scharf
A. T. Johnson

The airflow perturbation device (APD) perturbs flow and mouth pressure during regular breathing. Ratios of mouth pressure perturbation magnitudes to flow perturbation magnitudes were used to calculate inspiratory, expiratory and average respiratory resistances. Resistance measurements with the APD were compared to pulmonary resistances directly measured with an esophageal balloon. Six healthy subjects were tested during tidal breathing when known external resistances were added during inspiration, during expiration and during both inspiration and expiration. When the baseline averaged balloon measured pulmonary resistance was subtracted from the baseline averaged APD measured resistance, the difference between them was 0.92 ± 1.25 (mean ± SD) cmH2O L–1 s–1. Compared to the magnitude of the known increase in the added resistance, the APD measured resistance increased by 79%, whereas directly measured pulmonary resistance increased only by 56%. During addition of external resistances to both inspiration and expiration, the changes in inspiratory and expiratory pulmonary resistance were only 36% and 62% of the added resistance, respectively. On the other hand, the APD inhalation and exhalation resistance measured between 82% and 76% of the added resistance. We conclude that the APD detects changes in external resistance at least as well as, and probably better than, classical measurements of pulmonary resistance.

Keywords: respiratory resistance, pulmonary resistance, pulmonary function, external rigid resistance, airflow perturbation device.


BioMedical Engineering OnLine 2008, 7:28

Erika R Lopresti
Arthur T Johnson
Frank C Koh
William H Scott
Shaya Jamshidi
Nischom K Silverman

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.

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.

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.

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.



International Journal of Medical Implants and Devices, Vol 1, Nos. ¾, pp 137-151, 2005

Arthur T. Johnson
William H. Scott
Estelle Russek-Cohen
Frank C. Koh
Nischom K. Silverman
Karen M. Coyne

Background: In its current form, the Airflow Perturbation Device (APD) is a new device for measuring respiratory resistance. It is convenient, small, and requires little more than normal breathing. Because of this the APD can be a useful medical diagnostic tool for pulmonary function.

Method of Approach: Normal ranges of data have been obtained for over 900 subjects ranging in age from 2 to 88.

Results: The reduction of respiratory resistance with age is clearly evident in children; adults can maintain normal values of 2.5 to 3.5 cm H2O*sec/L throughout their lives. Other relationships between resistance and height, weight, or body mass index were also explored. Exhalation resistances were almost greater than inhalation resistances, as expected.


  1. The Airflow Perturbation Device can be used easily for people of all ages.
  2. Normal respiratory resistance values have been established for children and adults. These will be refined as more data become available.
  3. Resistance in the exhalation direction is generally larger than resistance in the inhalation direction, except, perhaps in small children.
  4. Data appear to conform to a log normal probability distribution.
  5. Male adults have average resistance values about 80% of those of adult females.

Keywords: respiratory resistance, children, adults, respiration, pulmonary, airflow perturbation device (APD).


In-Home Hand-Held Device to Measure Respiratory Resistance

1st Transdisciplinary Conference on Distributed Diagnosis and Home Healthcare (D2H2) 12-15 (2-4 April 2006)

J. Vossoughi
A.T. Johnson
N.K. Silverman  

A small hand-held Airflow Perturbation Device (APD) has been developed that is capable of noninvasively evaluating respiratory resistance. The APD has several advantages over commercially available spirometers. The APD is small, compact, self-contained, and low cost. It can measure respiratory resistance noninvasively in inhalation and exhalation accurately and reliably. Because of its size, cost, and ease of operation, it can be a suitable diagnostic device for home use.



Proceedings of the IEEE 32nd Annual Northeast Bioengineering Conference. 205–206 (2006)

J. Vossoughi
A. Johnson
M. Goldman
N. Silverman
E. Lopresti
P. Oo  

A mouthpiece used in spirometry is modified to produce a partial air leak while the patient is breathing into the device. Tubes were mounted on the left and right sides of the standard mouthpiece. Three different size tubes were used to produce high, moderate, and low leaks through the mouthpiece. The subjects were asked to use the Airflow Perturbation Device (APD) under identical conditions; with no leak (control), low, moderate, and high leak mouthpieces. Their average respiratory resistances were evaluated in each case. The results show that respiratory resistance decreases as more leak was allowed. It is therefore important that the respiratory technician takes additional steps to ensure that the patient completely holds and seals the mouthpiece with his/her lips so that all the breathing takes place through the mouthpiece and no leak is allowed from around the lips.

Keywords: respiratory resistance, children, adults, respiration, pulmonary, airflow pertubation device (APD)


Respiratory resistance measured by an airflow perturbation device

Physiol. Meas. 20 (1999) 21-35. (PDF)

Christopher G Lausted
Arthur T Johnson

The airflow perturbation device (APD) is an instrument for the measurement of respiratory resistance. The APD is small, lightweight fast and requires no special breathing manoeuvres. Airflow perturbation determines resistance by superimposing a periodic signal onto spontaneous breathing with a variable resistance device. Respiratory impedance is the ratio of magnitude of pressure perturbation to magnitude of flow perturbation, and respiratory resistance is the in-phase portion of respiratory impedance. The APD was tested to determine its responses to repeated resistance measurements and to changes in resistance. A mechanical model test showed that the APD could detect increased resistance levels, but overestimated resistance when resistance increased with flow. Tests with human subjects showed that the APD gave results consistent from day to day, was able to detect added resistances and gave resistance values correlated with airway resistance values obtained by body plethysmography. Accelerometers placed on the chests of the subjects showed that perturbations extend to the chest surface. Thus, the APD must measure total respiratory resistance.

Keywords: respiration, resistance