Influence of second-order data filtering on common forced expiration indices

Area of Science:

• Pulmonary Function Testing
• Biomedical Engineering
• Respiratory Physiology

Background:

• Forced expiratory manoeuvres are crucial for diagnosing respiratory conditions.
• Flow meters and pressure transducers used in these tests often function as second-order filters.
• The dynamic characteristics of this equipment can impact the accuracy of measured indices.

Purpose of the Study:

• To determine the necessary dynamic characteristics of spirometry equipment for accurate forced expiration index measurement.
• To assess the impact of second-order filtering on common forced expiration indices.

Main Methods:

• Acquired 125 flow-time curves from 25 healthy subjects under optimal conditions.
• Applied varying degrees of second-order filtering to the recorded curves.
• Compared indices derived from filtered and unfiltered curves to evaluate measurement accuracy.

Main Results:

• With an optimal damping ratio (r=0.7), key indices like forced expiratory volume in one second (FEV1), maximum mid-expiratory flow rate (MMEF), and maximal expiratory flow at 25% of forced vital capacity (MEF25) were accurately measured at resonant frequencies (fn) as low as 3 Hz.
• MEF50 required fn of 5 Hz, peak expiratory flow rate (PEFR) required 20 Hz, and MEF75 required >20 Hz for accurate measurement.
• Deviations from the optimal damping ratio (r=0.7) necessitate higher resonant frequencies for accurate measurements.

Conclusions:

• Specific dynamic characteristics, particularly resonant frequency and damping ratio, are critical for accurate spirometry.
• Equipment with an optimal damping ratio of 0.7 can maintain accuracy for several key indices even at lower resonant frequencies.
• These findings provide guidance for selecting and calibrating spirometry equipment to ensure reliable pulmonary function testing.