Control of breathing patterns and abdominal muscles during graded loads and tilt

Tilting from supine to upright purportedly enhances both segmental and pulmonary proprioceptive feedback, whereas an expiratory threshold load (ETL) preferentially enhances pulmonary feedback. To test this we studied 13 adults when supine and 60° and 90° head up. We measured tidal volume, inspiratory duration (TI), and expiratory duration (TE) from flow; estimated end- expiratory lung volume from inspiratory capacity; and determined burst amplitudes and durations from abdominal electromyograms (EMGs). ETLs were incremented from 0 (control) to 25 cmH₂O in 5-cmH₂O steps. Tidal volume was significantly increased by ETL but was unaffected by body position. Every load prolonged TE, whereas TI remained unchanged. When subjects were supine, abdominal EMGs were silent but became tonically active when subjects were upright. During ETL, abdominal activity became rhythmical and phase locked to expiration. Bursts amplitudes were enhanced with each increment in ETL, but burst durations did not change even though TE was prolonged. The altered breathing pattern and active expiration augmented inspiratory flow and decreased end-tidal PCO₂. Responses were greatest when subjects were 90° head up. The load-related increments in abdominal recruitment, with no change in burst durations, fit the concept of two central pattern generators: one controlling pattern and the other controlling rhythm of the central respiratory drive.