The effect of reversible left recurrent laryngeal neuropathy on the metabolic cost of locomotion and peak aerobic power in thoroughbred racehorses.

The effect of left recurrent laryngeal neuropathy (LRLN) on the metabolic cost of locomotion (MCL) and peak aerobic power (VO2peak) was evaluated in four trained Thoroughbred racehorses. Oxygen consumption (VO2), carbon dioxide production (VCO2), venous lactate concentrations (LAC), and heart rate (HR) were measured during a treadmill exercise test (TET). Each horse performed the exercise test four times, alternating between normal upper airway function and reversibly induced LRLN. Subcutaneous infusion of 2% mepivicaine, a local anesthetic, into the region were the left recurrent laryngeal nerve passes caudal to the cricoid cartilage was used to induce LRLN. The induction of LRLN did not alter the relationship between VO2 and treadmill speed at exercise intensities where VO2 was less than VO2peak (< 9 m/sec). However, a 15.3% reduction in VO2peak (Normal = 165.3 +/- 3.4, LRLN = 140.0 +/- 3.2 mL/kg/min +/- SE, P < .001) occurred at higher treadmill speeds in horses with induced LRLN. A significant group (Normal v LRLN) by treadmill speed effect was found for LAC and R only at treadmill speeds where VO2 = VO2peak. Peak lactate (LACpeak) did not change after the induction of LRLN. The relationship between HR and treadmill speed increased in horses with induced LRLN at exercise intensities where VO2 < VO2peak. Peak heart rate (HRpeak) remained unchanged. Performance as indicated by the maximum number of speed intervals completed (STEPmax) decreased 7% in horses with induced LRLN (Normal = 9.1 +/- 0.04, LRLN = 8.5 +/- 0.2 minutes +/- SE, P < .04). A comparison of paired exercise test measurements showed no evidence of a training effect, or decreased performance caused by a learned response, over the course of the experiment.(ABSTRACT TRUNCATED AT 250 WORDS)