Compartmental chest wall volume changes during volitional normocapnic hyperpnoea

During increased ventilation, inspiratory rib cage muscles have been suggested to take over part of diaphragmatic work after the diaphragm fatigues. We investigated the extent to which this proposed change in muscle recruitment is associated with changes in the relative contribution of chest wall compartments to tidal volume (VT). Thirteen healthy subjects performed 1 h of fatiguing normocapnic hyperpnoea. Chest wall volumes were assessed by optoelectronic plethysmography. While breathing frequency increased (43 ± 3 to 56 ± 5 breaths min−1, p = 0.006) and VT decreased during normocapnic hyperpnoea (2.6 ± 0.2 to 1.9 ± 0.1 l, p < 0.001), the relative contribution of chest wall compartments to VT remained unchanged (pulmonary rib cage: 48 ± 9 versus 51 ± 14%; abdominal rib cage: 24 ± 4 versus 23 ± 9%; abdomen: 28 ± 8 versus 26 ± 9%; all p > 0.05). In conclusion, fatiguing respiratory work is not associated with a change in compartmental contribution to VT, even in the presence of a change in breathing pattern.