Breathing during exercise in dogs--passive or active?

D. M. Ainsworth, C. A. Smith, K. S. Henderson and J. A. Dempsey Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York 14850, USA. The activation patterns of the costal and crural diaphragm and transversus abdominis muscle and their relationship to esophageal pressure (Pes) changes and footplant were examined in five chronically instrumented dogs which breathed at high frequencies at rest and during exercise. In two tracheostomized dogs, measurements were made of diaphragmatic length via sonomicrometry and of airflow and were related to diaphragmatic electrical activity and Pes. Dogs exhibited either a high-frequency breathing pattern, characterized by Pes changes occurring at 2-6 Hz, or a mixed-frequency breathing pattern, characterized by low-amplitude Pes oscillations (4-6 Hz) superimposed on a slower breathing rate of 0.5-1 Hz. Regardless of the type of breathing pattern elected or of the various breathing-to-stride frequency ratios observed during exercise, decreases in Pes were always associated with phasic electromyographic activity of the costal and crural diaphragm and with phasic diaphragmatic muscle shortening. The transversus abdominis electromyographic activity coincided with an increasing Pes from peak negative values in resting dogs and exhibited both an expiratory and a locomotory modulation during exercise. Although footplant may have contributed to some airflow generation when dogs utilized the mixed-frequency pattern, these data demonstrate that the movement of air into and out of the lungs in stationary or exercising dogs requires phasic neural activation of the diaphragm and other respiratory muscles.