The use of the standard exercise test to establish the clinical significance of mild echocardiographic changes in a Thoroughbred poor performer.

A 4-year-old Thoroughbred racehorse exhibited post-race distress and collapse, and this study aimed to determine the cause through a standard exercise test, concluding that the horse exhibited a dynamic cardiac insufficiency during exercise.

Testing Process

• The horse was referred to the Onderstepoort Veterinary Academic Hospital, which typically cares for animals with significant and complex medical needs. The team noticed post-race distress and collapse in the horse, but initial diagnostic work did not reveal any obvious abnormalities.
• To discover any underlying problems, the team decided to perform a standard exercise test. Such a test is commonly used to assess the cardiovascular system’s response to increased work, particularly in racehorses.

Results of the Exercise Test

• The horse started to show oxygen desaturation at rather slow speeds, where the PO2 (Partial Pressure of Oxygen) was measured at a low 82.3 mm Hg.
• Also, at a blood pH of 7.28, the horse’s PCO2 (Partial Pressure of Carbon Dioxide) had dropped significantly. This suggested a combined metabolic acidosis and respiratory alkalosis – conditions typically indicating severe disturbances in the acid-base balance of the body.

Diagnosis and Conclusion

• The source of the horse’s distress was attributed to severe hypoxia, an oxygen deficiency, and an associated hypocapnoea, a reduced level of carbon dioxide in the blood. These were confirmed by blood gas analyses that showcased unexpectedly low levels of PO2 and PCO2 during strenuous exercise.
• Upon cessation of exercise, the horse’s arterial oxygenation normalized relatively swiftly, bringing the PO2 up to 106.44 mm Hg. However, the lower-than-normal carbon dioxide level, or hypocapnoeic alkalosis, lasted until the horse’s breathing returned to its usual pattern.
• The researchers concluded that the horse had a condition that bore resemblance to high-altitude pulmonary edema, usually seen in humans at high altitudes, with respiratory changes and compensation similar to acute altitude sickness. They attributed this to a combination of aortic stenosis (a narrowing of the valve in the large blood vessel branching off the heart) and a mitral valve insufficiency (a condition that prevents the mitral valve in the heart from closing tightly).
• The findings from this study provide evidence of a dynamic cardiac insufficiency present during exercise and highlight the need for an extensive diagnostic regime to identify causes of poor performance. They also solidify the importance of the standard exercise test as an integral part of this evaluation process.