Techniques of non-invasive Lung Function Testing
In the clinical setting, three main non-invasive methods are currently used at Tufts to test lung function in awake large animals: Forced Oscillation Techniques (FOT), plethysmography and measurement of functional residual capacity (FRC), which is an estimate of lung elasticity.
Forced oscillation techniques (FOT) are non-invasive tests to measure the resistance of the breathing system and thus evaluate airway narrowing. Different frequencies (1-7 Hz) of small air-pulses are directed into the patient’s airways via a face mask, while a computer captures changes in pressure and flow of these waves during spontaneous breathing. This calculates respiratory system resistance (Rrs).

A horse with airway inflammation undergoing FOT lung function testing at Tufts' Hospital for Large Animals.
A high Rrs often indicates airway narrowing due to thickening of bronchial walls and / or mucous accumulation in the lower lung. In horses with lower airway inflammation (e.g. IAD), we commonly observe a frequency dependence of resistance. This means that higher values for resistance are recorded at the lower oscillatory frequencies (1-2 Hz), a finding that is indicative of bronchoconstriction. Higher oscillation frequencies (> 2 Hz) provide information concerning central airway resistance. Baseline respiratory resistance measurements using Forced Oscillation Techniques can also be combined with bronchoprovocation tests to determine airway hyperreactivity or “twitchiness.” Continue reading →