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 metabo lic cost of locomotion (MCL) and peak aerobic power (VO(2)peak) was ev aluated in four trained Thoroughbred racehorses. oxygen consumption (V O2), carbon dioxide production (VCO2()), venous lactate concentrations (LAG), and heart rate (HR) were measured during a treadmill exercise test (TET). Each horse performed the exercise test four times, alterna ting between normal upper airway function and reversibly induced LRLN. Subcutaneous infusion of 2% mepivicaine, a local anesthetic, into the region where the left recurrent laryngeal nerve passes caudal to the cricoid cartilage was used to induce LRLN. The induction of LRLN did n ot alter the relationship between VO2 and treadmill speed at exercise intensities where VO2 was less than VO(2)peak (<9 m/sec). However, a 1 5.3% reduction in VO(2)peak (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 trea dmill speed effect was found for LAC and R only at treadmill speeds wh ere VO2 = VO(2)peak. Peak lactate (LACpeak) did not change after the i nduction of LRLN. The relationship between HR and treadmill speed incr eased in horses with induced LRLN at exercise intensities where VO2 < VO(2)peak 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 exerci se test measurements showed no evidence of a training effect, or decre ased performance caused by a learned response, over the course of the experiment. The results of this study indicate that alterations in ven tilation caused by LRLN cause a significant reduction in VO(2)peak, bu t do not cause an increase in the metabolic cost of locomotion at exer cise intensities where VO2 is less than VO(2)peak. (C) Copyright 1995 by The American College of Veterinary Surgeons