# Airway Resistance (Raw) Calculator

Determines the resistance of the respiratory tract to airflow during inhalation and exhalation.

Airway resistance (Raw) can be defined as the resistance of the respiratory tract to the inhalation and exhalation airflow. This can be directly measured through plethysmography or indirectly estimated based on peak inspiratory pressure, plateau pressure and flow.

As an indicator of airway function, Raw can contribute to the diagnosis and differentiation of obstructive airway diseases.

### Airway resistance (in cmH2O/L/Sec) Formula

`Raw = ΔP / V = (Peak Inspiratory Pressure – Plateau pressure) / Flow`

Peak Inspiratory Pressure (PIP)
Plateau pressure (Pplat)
Flow
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Steps on how to print your input & results:

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.

## Airway Resistance Explained

Airway resistance (Raw) can be defined as the resistance of the respiratory tract to the inhalation and exhalation airflow. This can be directly measured through plethysmography or indirectly estimated based on peak inspiratory pressure, plateau pressure and flow.

`Raw = ΔP / V = (Peak Inspiratory Pressure – Plateau pressure) / Flow`

Where: Raw is measured in cmH2O/L/Sec.

Peak inspiratory pressure (PIP) is the pressure applied to the lungs during inhalation and increases with any airway resistance (due to increased secretions, bronchospasm or decreased lung compliance. PIP values should not increase to more than 40 cmH2O (such as in acute respiratory distress syndrome) but normal increases can help increase tidal volume and CO2 elimination and decrease PaCO2, hence improve oxygenation.

Plateau pressure (Pplat) is the pressure value that is maintained constant during a portion of the inspiratory phase of the breath during no flow and so equals alveolar pressure.

As an indicator of airway function, Raw can contribute to the diagnosis and differentiation of obstructive airway diseases.

Raw reflects changes in alveolar pressure over changes in airflow representing true resistance of the airways. In a study by Topalovic et al. on a sample of 976 subjects, 651 were diagnosed with obstructive diseases and abnormal Raw values were found in 39% of the population, in which 81% had diagnosed airway obstruction. Raw values were found to be statistically relevant in differentiating asthma from COPD.

## Raw Interpretation

To date, there isn’t consensus on the normal range for airway resistance but several studies have suggested classifications of the elevated Raw levels in relation to the severity of the respiratory obstruction:

 Raw by Ries and Clausen Raw by Miller et al. Severity interpretation < 2.8 < 3.0 Normal 2.8 – 4.5 3.0 – 4.5 Mild obstruction 4.5 – 8.0 4.5 – 8.0 Moderate obstruction > 8.0 8.0 – 15.0 Severe obstruction > 15.0 Extreme obstruction

## References

Ries AL, Clausen J. Chapter: Airway Resistance. Pulmonary function testing indications and interpretation. Wilson AF, editor. Published by Grune & Stratton. 1985.

Miller WF, Scacci R, Gast LR. Laboratory evaluation of pulmonary function. Published by J.B. Lipincott Co., 1987.

Briscoe WA, Dubois AB. The relationship between airway resistance, airway conductance and lung volume in subjects of different age and body size. J Clin Invest 1958; 37: 1279-1285.

Topalovic M, Derom E, Osadnik CR, Troosters T, Decramer M, Janssens W. Airways resistance and specific conductance for the diagnosis of obstructive airways diseases. Respir Res 2015; 16: 88.

Specialty: Pulmonology

System: Respiratory

Abbreviation: Raw

Article By: Denise Nedea

Published On: June 8, 2020

Last Checked: June 8, 2020

Next Review: June 8, 2025