Respiratory muscle recruitment in awake ponies during exercise and CO2 inhalation

We measured respiratory muscle electromyograms (EMG), inspiratory (I) and expiratory (E) airflow patterns and functional residual capacity (FRC) in six ponies at rest, during treadmill walking at 1.8 mph-5, 10, and 15% grades, and during 2, 4, and 6% CO2 inhalation. There were several similarities in the responses to exercise and CO2 inhalation. The shapes of the I and E flow patterns were not changed appreciably from the respective control patterns during either condition. Mean diaphragm EMG increased from control (P less than 0.05) at 1.8 mph-10 and 15% grades, and during 4 and 6% inhalation. However, mean transversus abdominis EMG did not change significantly from control (P greater than 0.10) during either condition. Exercise did not have an effect (P greater than 0.05) on FRC, and there was only a slight (P less than 0.05) increase in FRC (100 ml or 2%) during 6% CO2 inhalation. Based on the fact that we did not find major differences between exercise and CO2 inhalation in mean diaphragm and transversus abdominis EMG, I and E flow patterns, and FRC we conclude that factor(s) other than the ventilatory stimulus and the non-ventilatory functions of the respiratory muscles are important determinants of the pony's respiratory muscle recruitment pattern under these conditions.