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