AJVR, Vol 64, No. 2, February 2003 235 R ecurrent airway obstruction (RAO), also known as heaves, is a common disease in horses and is char- acterized by reversible airway obstruction, bron- chospasm, and airway hyperreactivity attributable to inflammation of the airways, production of mucus, and thickening of airway walls. 1 Horses that have an exac- erbation of RAO are characterized by flared nostrils, coughing, and marked expiratory efforts. Even during clinical remission, horses with RAO have residual, sub- clinical airway inflammation, obstruction, and hyper- responsiveness. 2 Clinicians have mentioned that a sub- set of horses with RAO tends to be thin, perhaps even cachectic. It is unknown whether increased work of breathing contributes to weight loss in these horses. Although the pathologic mechanism of chronic obstructive pulmonary disease (COPD) in humans differs substantially from that of horses with RAO, 2 it may be useful to mention that humans with COPD similarly have frequent weight loss. There are multiple putative contributors to COPD-associated weight loss, including increased resting energy expenditure (REE) attributable to increased oxygen consumption needed to overcome airway obstruction. 3,4 Pharmacologic treatment with β 2 -agonist bronchodilators, however, increases REE in humans with COPD despite bron- chodilation. 5-6,a Horses with RAO are frequently treated with β 2 -agonist receptors (β 2 -ARs), especially aerosolized albuterol, to effect bronchodilation. 7 Despite several studies on the effects of β 2 -ARs on air- way and lung mechanics in horses with airway obstruction during rest 7-9 and in clinically normal hors- es during exercise, 10-12 to the authors’ knowledge, there have not been any studies on the effect of β 2 -ARs on oxygen consumption or REE in horses with airway obstruction during rest. Airway obstruction in humans with COPD is often largely irreversible, 13 as opposed to horses with RAO. Therefore, it may be that albuterol sulfate (as a β 2 -AR bronchodilator) has a greater abili- ty to reverse airway obstruction in horses with RAO, which consequently results in decreased energy expen- diture in these horses. Open-flow indirect calorimetry, a modification of open-flow respirometry that has been commonly used in exercising horses, 12,14-17 has been used extensively to measure REE in humans that are in critical condition 18 and as a tool to investigate weight loss in humans with COPD. By measuring oxygen consumption (V • O 2 ) and carbon dioxide production (V • CO 2 ), the abbreviated Weir formula 19 can be used to calculate the number of kilocalories needed by a resting individual each day. This technique is considered to be extremely accurate and reliable in humans, 18,20 but its use in veterinary medicine has been largely confined to small ani- mals. 21,22 Its use for measurement of REE in large ani- mals at rest 23,24 has been hampered by technical limita- tions of sensitivity and reproducibility. We had 3 objectives in the study reported here. First, because of the difficulty of inserting instruments Received March 18, 2002. Accepted August 30, 2002. From the Lung Function Laboratory, Department of Clinical Sciences, School of Veterinary Medicine, Tufts University, North Grafton, MA 01536 (Mazan, Hoffman, Kuehn); and the Physics Department, School of Arts and Sciences, Bridgewater State College, Bridgewater, MA 02325 (Deveney). Supported by the Tufts Hills Research Fund. Address correspondence to Dr. Mazan. Effect of aerosolized albuterol sulfate on resting energy expenditure determined by use of open-flow indirect calorimetry in horses with recurrent airway obstruction Melissa R. Mazan, DVM; Andrew M. Hoffman, DVM, DVSc; Heike Kuehn, DVM; Edward F. Deveney, PhD Objective—To evaluate effects of sedation on stabili- ty of resistance of the respiratory system (RRS) and measures of resting energy expenditure (REE) by use of open-flow indirect calorimetry (IC) and treatment with aerosolized albuterol on REE in horses with recurrent airway obstruction (RAO). Animals—9 clinically normal horses and 8 horses with RAO. Procedure—In phase 1, RRS was measured by using forced oscillometry (FOT) in 5 clinically normal horses before and after sedation with xylazine. In phase 2, REE was measured in 4 clinically normal horses between 20 and 25 minutes and again 35 to 40 min- utes after sedation with xylazine. In phase 3, IC was performed between 20 and 25 minutes and FOT was performed between 30 and 35 minutes after xylazine administration in 8 horses with RAO; after administra- tion of 450 μg of albuterol, IC and FOT were repeated. Results—In phase 1, RRS values were significantly lower 5 and 10 minutes after sedation. In phase 2, diminishing sedation did not significantly affect REE. In phase 3, there was a significant decrease in mean RRS (1.15 ± 0.25 vs 0.84 ± 0.14 cm H 2 0/L/s) and REE (30.68 ± 17.89 vs 27.46 ± 16.54 kcal/kg/d) after albuterol administration. Conclusions and Clinical Relevance—FOT and IC are useful in obtaining repeatable measurements of RRS and REE, respectively, in sedated horses. Concurrent bronchodilation and decreased REE after albuterol administration suggest that increased work of breathing as a result of airway obstruction may contribute to increased energy demands in horses with RAO. 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