Impulse Oscillometry: What Can It Tell Us About Lung Function?

Respiratory function tests such as spirometry are routinely integrated to the evaluation of respiratory patients in human medicine. The application of respiratory function tests to horses has been limited by the lack of adapted equipment, the unfeasibility of voluntary respiratory manoeuvres and the invasiveness of some procedures. But more recently, the potential use of these techniques in equine practise has broadened with the up-coming of effective and more manageable systems (Hoffman 2002). Evaluating respiratory mechanics allows us to make objective measurements of the functional impact of a disease, determine its severity and reversibility, follow its evolution with time and quantify improvement with therapy. These tests are therefore particularly valuable in horses where respiratory diseases are highly prevalent and noninfectious airway inflammation constitutes the most commonly diagnosed respiratory problems in that species. They usefully complete the information gained from diagnostic imagery and respiratory sample analysis.

Respiratory mechanics can be evaluated by different means, the aim being to obtain pressure, flow and volume signals from the respiratory system. These signals allow the calculation of parameters such as airway resistance or pulmonary compliance that reflect specific mechanical properties of the respiratory system and may be modified by disease or breathing strategy. Conventional techniques are based on the measurement of signals generated by spontaneous breathing and the work of respiratory muscles. They are generally more invasive (e.g. oesophageal balloon technique) and a regular breathing pattern is a prerequisite to reliable measurements. Other techniques such as oscillometry produce external pressure signals (i.e. oscillations) superimposed on the patient’s respiration to elicit a pressure and flow response from the respiratory system. The advantages of these techniques are their noninvasiveness and the reproducibility of the oscillations. Another substantial advantage is that the mechanical measurements may be expressed in the frequency- domain rather than in the time-domain, providing additional indications regarding the level of dysfunction. [...]