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 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.”
Plethysmography: The horse’s breathing pattern and lung function may also be assessed using a technique called Open Pleth, which was developed at the Cummings School by Dr. Andrew Hoffman. This system evaluates the synchrony of chest and abdominal breathing, while determining the horse’s effort needed to overcome breathing resistance. During this test, two elastic bands that contain a sinusoidal conducting wire are temporarily placed around the animal’s chest and abdomen. Stretch and contraction of these bands due to normal breathing movements are measured as voltage alterations and quantify the changes in circumference and volume of the chest and abdomen during breathing. Simultaneously, we measure the animal’s airflow at the level of the nostrils, thus rapidly assessing asynchrony of the patient’s breathing pattern due to airway obstruction and breathing muscle (diaphragmatic) dysfunction. This system is portable and allows lung function testing in the horse’s home environment or barn. The Hospital for Large Animals offers an ambulatory service of lung function testing for groups of horses on selected days of the month, if requested by your referring veterinarian. For further information please contact Drs. Bedenice or Mazan at (508) 839 7926.
FRC is the measure of the amount of gas that remains in the lungs at the end of a breath. It is thus lower in patients with stiff lungs as well as airway inflammation, while FRC increases in patients with impairments during exhalation or with gas trapping. This test can be quickly and easily performed in awake clinical patients, while breathing a low concentration of helium gas. It is most commonly utilized to monitor and prognosticate progression of lung disease and evaluate the patient’s treatment response.
Another diagnostic we offer is histamine bronchoprovocation, a challenge test that assesses the response of the breathing system to an agent that temporarily and lightly constricts the airways (e.g. inhaled histamine). This test thus mimics the response of horses to triggers such as mast cell degranulation, dust, cold air or exercise when histamine is released into the lung. Airway hyperreactivity (AHR) is, by definition, an exaggerated narrowing response to a bronchoconstrictive stimulus, which was first found in horses with heaves and is considered a hallmark of IAD. The clinical symptoms associated with AHR are thought to include coughing (increased sensitivity) and exercise intolerance, where bronchoconstriction causes uneven airflow distribution in the lung as well as low oxygen levels.