Biochemical changes in three-day-event horses at the beginning, middle and end of Phase C and after Phase D.

Blood samples were collected 12-16 h before the Speed and Endurance test, immediately after steeplechase, midway through Phase C (4 km marker), at the end of Phase C and immediately after cross-country from 36 horses that completed a 3-day-event at the CCI* (n = 19) or CCI** (n = 17) level. Packed cell volume (PCV), plasma concentrations of sodium, potassium, chloride, ionized calcium, pH and lactate; and serum concentrations of total protein (TP), albumin, total calcium, alkaline phosphatase (AP), gamma glutamyl transferase (GGT), creatine kinase (CK), aspartate aminotransferase (AST), glucose and creatinine were measured. No differences were noted in any biochemical measurements between the CCI* and CCI** horses at any sampling time, despite differences in speed and length of various phases of the Speed and Endurance test. For all horses (n = 36), biochemical parameters changed significantly over time (P < 0.01). The PCV, calcium, potassium, lactate, total protein and albumin concentrations were significantly increased over pre-exercise concentrations immediately after Phase B. During Phase C, these parameters decreased towards pre-exercise concentrations, thereby supporting the concept that Phase C is a recovery phase. In contrast, chloride concentration decreased, and the creatinine concentration and CK increased compared to pre-exercise concentrations from the end of Phase B to the end of Phase C. These changes probably resulted from fluid and electrolyte losses in sweat, reduced renal blood flow and continued muscular activity. Many parameters did not change significantly between the 4 km marker (midway point on Phase C) and the end of Phase C. However, there were a few notable exceptions: potassium, chloride, lactate and glucose concentrations decreased, whereas pH and creatinine concentrations increased significantly from the 4 km marker to the end of Phase C. The most significant benefit of the extra distance from the 4 km marker to the end of Phase C was that it facilitated further dissipation of lactate concentrations prior to the start of Phase D.