Aspects of respiratory function during exercise in the thoroughbred horse in health and disease.

Abstract

The horse's enormous respiratory reserve enables it to increase the respiratory load on the lungs many fold during exercise, hence low-grade respiratory disease is difficult to detect at rest, although of vital importance during heavy exercise. Diagnostic exercise testing is, therefore, of great interest and this thesis describes its application to the investigation of respiratory disease in the Thoroughbred horse. Horses were exercised on a treadmill whilst respiratory flow rates were recorded using ultrasound flow transducers, respired gas concentrations measured by mass spectrometry and blood gas analysis performed. The exercise test involved a warm up followed by two minute canters/gallops at increasing workloads separated by ten minute walks. Validation studies confirmed the suitability of the methods for these studies. A steady state for respiratory variables was reached by ninety seconds of canter/gallop. The effects of training on exercising respiratory function were investigated by exercise testing horses following a sedentary period and after a fifteen week training programme. State of training did not complicate clinical interpretation of some variables, e.g tidal volume, respiratory frequency and minute ventilation, whilst peak flow rates were stable at the highest workloads. Arterial oxygen tension and blood pH during exercise were affected by training but other blood gas variables and end-tidal gas tensions were not. The effect of influenza on respiratory function was studied by challenging partially immune horses with H3N8 influenza, inducing infection similar to that seen in vaccinated racehorses. Twenty one days after infection most horses showed no changes in respiratory function but 2/9 horses altered breathing strategy during canter and another showed altered acid-base and blood lactate responses. Horses with low-grade respiratory disorders showed differences in respiratory function to the experimental animals but the wide normal range of pulmonary function tests limited their one-off diagnostic value. Serial testing may prove to be a more sensitive method.