Abstract
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 purposes of this four-part, prospective, pilot study were to develop and describe a repeatable method to measure the diaphragm thickness, using ultrasonography, in Thoroughbred racehorses. A standardized 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 three occasions 1week apart, by a single observer to determine the intraobserver repeatability, and by a second observer on one occasion to assess interobserver reproducibility. The diaphragm was observed in all intercostal spaces (ICS) from 7 to 17 on the left side, and 6 to 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 three occasions (P<.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 standardized location allowed examination and measurement of the diaphragm with good repeatability.
Highlights
The diaphragm is one of the primary respiratory muscles,[1] working principally during inspiration,[2] and has an important role in exercise
In a single Thoroughbred racehorse (6 years old, gelding, 489 kg), the diaphragm was observed in all intercostal spaces (ICS) from ICS 7 to 17 on the left side and ICS 6 to 17 on the right side inclusive
The thickness measurements recorded by observer 1 were larger than the measurements by observer 2, with a mean bias of 0.031 cm (Figure 7). This preliminary study was intended to provide a detailed description of a repeatable technique for quantifying effects of training in performance horses using ultrasonographic imaging and measurement of equine diaphragm thickness
Summary
The diaphragm is one of the primary respiratory muscles,[1] working principally during inspiration,[2] and has an important role in exercise. Reported methods investigating the equine diaphragm include invasive measurement of blood flow and oxygen extraction in research horses,[3] providing a global assessment of respiratory function, or have focused on the imaging diagnosis of a diaphragmatic rent.[4] A new technique for non-invasive and repeatable imaging assessment of the diaphragm in the equine athlete is required, and will enable evaluation of the response of the respiratory muscles to exercise and training. The diaphragm is a dome-shaped structure, with a large tendinous center and a muscular periphery.[5] The oxygen requirements of the equine diaphragm during strenuous exercise are equivalent to those of the vigorously working locomotor muscles.[6] The increases in both blood flow (23-fold) and oxygen extraction (threefold) of the diaphragm, recorded during exercise evaluation in ponies, were correlated with running speed.[3] Overall, the respiratory musculature commands a similar proportion (∼14-20%) of cardiac output, in both horses and humans, during maximal exercise.[7]
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