Pulmonary function tests in standing ponies: reproducibility and effect of vagal blockade

Arterial blood gas tensions, pulmonary mechanics, and lung volumes were measured in 4 sedated ponies every hour for 6 hours and in 5 ponies 4 times at 2-month intervals to assess the short- and long-term reproducibility of pulmonary function measurements. Variability in blood gas tensions was small over the short- and long-term measurement periods, whereas the variability in total respiratory resistance and functional residual capacity was small over the short term but larger over the long term. The variability in tidal volume, minute ventilation, respiratory rate, and dynamic and quasistatic compliance was relatively large over the short and long term. When data from 5 ponies were pooled, significant change did not occur in any of the variables over a 6-month period. Vagal blockade increased tidal volume and decreased respiratory rate and total respiratory resistance, but arterial blood gas tensions, minute ventilation, dynamic compliance, quasistatic compliance, functional residual capacity, and lung and thoracic cage pressure-volume curves were unaffected. Total respiratory resistance decreased with increasing lung volume, with the vagus intact. After vagal blockade, the decrease in total respiratory resistance with lung volume was minimal. Dynamic compliance was frequency independent over a range of 15 to 60 breaths/min-1, suggesting that measurable inhomogeneity of peripheral time constants did not exist in our clinically normal ponies.