Thoracoabdominal motion during hypercapnia, hypoxia, and exercise

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Abstract
We measured, in 11 healthy volunteers, the contributions of rib cage and abdomen–diaphragm compartments to increased ventilation caused by hypercapnia, hypoxia, and exercise to determine whether different stimuli produce similar or different patterns of ventilation with respect to the motion of rib cage and abdominal compartments. Progressive hypcroxic hypercapnia and progressive isocapnic hypoxia were induced by rebreathing methods and graded exercise performed on a treadmill, and compartmental tidal volume (VT) was measured by respiratory inductive plethysmography. For each stimulus, the wide range of VT responses among individuals was determined primarily by the range of rib cage contributions to VT, the abdominal compartment VT response slopes accounting for less of this range. There were no significant differences between hypercapnia and hypoxia in either rib cage or abdominal contributions to ventilation (for both, p < 0.3). However, exercise rib cage and abdominal contributions to ventilation were significantly different from those during chemically driven breathing: for the rib cage compartment, p < 0.0001 and for the abdominal compartment, p < 0.05. Whereas, in 8 of 10 subjects the rib cage contribution to VT during exercise was similar to or exceeded that during rebreathing, in 7 of 10 subjects the abdomen–diaphragm contribution fell below that measured during both hypercapnia and hypoxia. There was a significant correlation between hypercapnia and hypoxia in the VT contribution of each compartment at equivalent levels of ventilation (rib cage, p < 0.0001; abdomen, p < 0.0005), but there was no significant correlation in the VT contribution of either compartment between exercise and hypercapnia or exercise and hypoxia. The pattern of compartmental contributions to ventilation during treadmill exercise differs significantly from that observed during chemically driven breathing.