The equine diaphragm: a novel technique for repeatable ultrasound measurement

The diaphragm is an important respiratory muscle, playing a key role during exercise. In humans, diaphragm thickness increases in response to training and is correlated with inspiratory strength. In order to assess respiratory strength in the horse, new evaluation techniques are required and measurement of diaphragm thickness, in a non-invasive and repeatable manner, is a possible approach. The purpose of this prospective, descriptive study was to develop a repeatable method to measure the diaphragm thickness, using ultrasonography, in Thoroughbred racehorses. A standardised technique was developed whereby the ultrasound transducer was positioned 1cm below a line between the cranioventral aspect of the tuber coxae and olecranon. The diaphragm thickness was measured on 3 occasions one week apart, by a single observer to determine the intra-observer repeatability, and by a second observer on one occasion to assess inter-observer reproducibility. The diaphragm was observed in all ICS from 7-17 on the left side, and 6-17 on the right side in a single horse. The thickest measurement (1.42cm), obtained from 11 horses, was at ICS 11 on the left-side during inspiration. The narrowest measurement (0.56cm) was obtained at ICS 16 on the right-side during expiration. There was no significant difference between the measurements obtained by a single observer on 3 occasions (p<0.05). This is the first study to provide a detailed description of ultrasonographic imaging and measurement of the equine diaphragm. The novel technique developed to position the ultrasound transducer in a standardised location allowed examination and measurement of the diaphragm with good repeatability.