• horse;
  • flow-volume loops;
  • upper airway flow mechanics;
  • airway impedance


The objective of the present study was to determine the effect of head and neck position on upper airway flow mechanics in exercising horses. Five Standardbred horses (452 ± 16.5 kg bwt; 4.7 ± 0.9 years [mean ± s.e.]) were exercised at 75% (Period A) and at 100% of maximal heart rate (Period B) with head and neck unrestrained, extended, or flexed. Airflow was measured using a facemask mounted pneumotachograph, while tracheal pressure during inhalation and exhalation (PUI and PUE) was measured using a nasotracheal catheter. With head and neck unrestrained, PUI, PUE, inspiratory and expiratory impedance (ZI, ZE), respiratory frequency (f) and indices describing the tidal breathing flow-volume loops (TBFVLs) were similar to those previously reported. There were no significant changes in the extended position, except that expiratory time (Te) and ratio of peak expiratory flow and peak inspiratory flow (PEF/PIF) were significantly increased in Periods A and B, respectively. In contrast, in the flexed position, ZI was significantly increased in Period B. Also, inspiratory flow at 50% of tidal volume (IF50) was significantly decreased, and PEF/PIF and EF50/IF50 were significantly increased. At period A, PUI was increased and Te was prolonged. We conclude that during strenuous exercise head and neck extension has little effect on upper airway flow mechanics, but that head and neck flexion causes upper airway obstruction.