Using Impulse Oscillometry in Clinical Practice

Aaron B. Holley, MD


April 27, 2016

The first paper on impulse oscillometry (IOS) was published in the 1950s.[1] IOS is an effort-independent test that requires minimal cooperation from the patient and provides multiple measures of respiratory mechanics during normal tidal breathing, including resistance (R), reactance (X), and impedance (Z).[2] This article reviews the use of IOS in the adult population and includes discussion of reference values and disease-specific changes.

Spirometric References

R, X, and Z are more difficult to conceptualize than spirometric measurements, such as airflow (V) and volume, so it's important to start with a simple physiology review.

R is equal to pressure (P) divided by V (R = P/V). Whereas V can be measured using a pneumotach placed at the mouth, the drop in P across the respiratory system (Pmouth – Palveoli) is required to calculate the system's resistance, and Palveoli is difficult to obtain. An esophageal catheter[3] or body plethysmography[4] can be used, but the former is invasive and the latter technique requires the patient to perform complicated breathing maneuvers, so neither is ideal for routine clinical practice. Alternatively, IOS and "interrupter" techniques can estimate R at different points within the respiratory system by measuring P and V at the mouth.[2,5]

The IOS machine consists of a pneumotach and a pressure transducer connected in series, with a speaker at one end and a mouthpiece at the other. The patient forms a tight seal around the mouthpiece, and the speaker sends sound waves into the respiratory system at different frequencies. Typical frequency ranges are between 4 and 26 cycles/second (Hz).

The resistance to a given sound wave traveling into the respiratory system is calculated using the P and V values obtained by the pneumotach and pressure transducer in line with the system. Sound waves at 4 Hz have longer wavelengths and travel to the distal airways. Therefore, the P/V measured by the IOS machine during a sound-wave pulse at 4 Hz estimates the resistance at the distal airways. Resistance to sound waves at 20 Hz (shorter wavelength) is more likely to reflect the mechanics of the upper airway because the waves don't travel as far. This is an oversimplification, and several authors have written more detailed reviews of IOS physiology.[2,6]

Finally, a brief explanation of R, X, and Z is in order. R was explained above, and airway R is largely, but not entirely, determined by cross-sectional area (the Poiseuille law). X is a surrogate for lung elastance, which is the inverse of compliance. Anything that reduces lung compliance (eg, obesity, interstitial lung disease, small-airways disease) should produce a more negative value for X. Z is the combination of R and X and is less commonly used clinically.

The Benefits of Using IOS

There are several theoretical benefits to using IOS, as opposed to or in conjunction with standard spirometry, to evaluate the performance of an individual's respiratory system. First, as mentioned, it is effort-independent. Patients who are acutely ill or cannot follow instructions have difficulty performing forced maneuvers. Approximately 20% of elderly patients cannot provide acceptable and reproducible spirometry.[7] IOS can provide an objective measurement of respiratory function for these patients.

Second, because R and X values are obtained during tidal breathing, IOS provides a more "real-world" assessment of respiratory function. Humans do not use the forced maneuvers needed for spirometry during normal daily activities. Artifacts from gas compression and small-airways closure due to increased thoracic pressures also limit the inferences made from standard spirometry.[8]

Finally, R and X provide information that is not available using spirometry. IOS is particularly sensitive for detecting small-airways dysfunction.[9]