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Effects of tension of the girth strap on respiratory system mechanics in horses at rest and during hyperpnea induced by administration of lobeline hydrochloride

Andrew M. Hoffman DVM, DVSc1, Lisa G. Swanson DVM2, Susan J. Bruns MSc3, Heike Kuehn Dr Med Vet4, and Daniela Bedenice Dr Med Vet5
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  • 1 Department of Clinical Sciences, Massachusetts School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.
  • | 2 Department of Clinical Sciences, Massachusetts School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.
  • | 3 Department of Clinical Sciences, Massachusetts School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.
  • | 4 Department of Clinical Sciences, Massachusetts School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.
  • | 5 Department of Clinical Sciences, Massachusetts School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.

Abstract

Objective—To determine whether tension of the girth strap of a saddle would sufficiently affect rib motion and reduce lung volume to alter pulmonary resistance in horses.

Animals—10 healthy adult horses.

Procedure—We used classical techniques to measure the effects of tightening a girth strap (15 kg of tension) on pulmonary dynamics during eupnea and hyperpnea in horses. Respiratory impedance was evaluated by use of oscillometry, and resistance and reactance data were partitioned into lung and chest wall components. Rib cage and abdominal contributions to tidal volume and minute ventilation were measured by use of respiratory inductance plethysmography. Effects of strap tension on functional residual capacity (FRC) were measured during eupnea by use of a helium-dilution technique. In a subgroup of 6 horses, we also measured transdiaphragmatic pressures during eupnea and hyperpnea induced by administration of lobeline hydrochloride (0.2 mg/kg, IV).

Results—Pulmonary resistance measured by use of oscillometry but not by use of classical methods was significantly increased by the tension of the girth strap. However, the increase in pulmonary resistance could not be explained by a decrease in FRC. Motion of the rib cage was significantly reduced during eupnea and hyperpnea. However, ventilatory variables (tidal volume, minute ventilation, and peak flows), FRC, and transdiaphragmatic pressures were unaltered by strap tension.

Conclusions and Clinical Relevance—Although tension of the girth strap caused measurable changes in respiratory mechanics (loss of rib motion and increased pulmonary resistance), there was no evidence that ventilation was limited. (Am J Vet Res 2005;66:1167–1174)

Abstract

Objective—To determine whether tension of the girth strap of a saddle would sufficiently affect rib motion and reduce lung volume to alter pulmonary resistance in horses.

Animals—10 healthy adult horses.

Procedure—We used classical techniques to measure the effects of tightening a girth strap (15 kg of tension) on pulmonary dynamics during eupnea and hyperpnea in horses. Respiratory impedance was evaluated by use of oscillometry, and resistance and reactance data were partitioned into lung and chest wall components. Rib cage and abdominal contributions to tidal volume and minute ventilation were measured by use of respiratory inductance plethysmography. Effects of strap tension on functional residual capacity (FRC) were measured during eupnea by use of a helium-dilution technique. In a subgroup of 6 horses, we also measured transdiaphragmatic pressures during eupnea and hyperpnea induced by administration of lobeline hydrochloride (0.2 mg/kg, IV).

Results—Pulmonary resistance measured by use of oscillometry but not by use of classical methods was significantly increased by the tension of the girth strap. However, the increase in pulmonary resistance could not be explained by a decrease in FRC. Motion of the rib cage was significantly reduced during eupnea and hyperpnea. However, ventilatory variables (tidal volume, minute ventilation, and peak flows), FRC, and transdiaphragmatic pressures were unaltered by strap tension.

Conclusions and Clinical Relevance—Although tension of the girth strap caused measurable changes in respiratory mechanics (loss of rib motion and increased pulmonary resistance), there was no evidence that ventilation was limited. (Am J Vet Res 2005;66:1167–1174)