Heliox-induced changes in the breathing mechanics of ponies during exercise.

The changes occurring in breathing mechanics of exercising ponies when heliox is substituted for air are reported. Respiratory airflow, tidal volume (VT) and transpulmonary pressure (Ptp) were simultaneously measured in 5 healthy ponies (255 ± 15 kg) at rest (breathing air) and while they exercised for 6 min on a treadmill. During exercise they first breathed air (2 min), than heliox (2 min) and then air again (2 min). Tidal volume (VT), respiratory rate (fR), minute volume (VE), maximum Ptp changes, total pulmonary resistance (RJ, mechanical work of Breathing per liter (Wrm l'), mechanical work per min (Wrm), dynamic compliance and pulmonary inertance were calculated. Heliox breathing significantly increased f,, and VE while VT remained unchanged. RL was reduced by 50% of lis "air value", Wrm I-' decreased, while Wrm remained unchanged. Pulmonary inertance dropped to 30% of its "air value". These observations suggest that in ponies, turbulent resistance and pulmonary inertance could be mainly responsible for the increases of RL and Wrm 1' during exercise