Dynamic hyperinflation and frequency-related changes in energy demand are bound to set limits on respiratory rate responses to C02. Alveolar hypoventilation may, thus, be one mechanism that enables the respiratory pump to cope with a large intrinsic load. Whether and how long such a breathing strategy can be sustained without resulting in overt pump failure remains unclear. Nevertheless, C02 retention (relative to C02RT) may allow the pressure output of respiratory muscles (ie, intrinsic load and drive) to fall to a lower level, not unlike that seen in patients without respiratory distress.
This hypothesis is consistent with the findings of Jabour et al who developed a highly specific weaning index based on gas exchange, tidal volume, and effort. This index, which helps to discriminate between weaning success and failure, is based on an estimate of the respiratory muscle power output that would be required to maintain a PaC02 of 40 mm Hg. The reason we did not find differences in parameters of load and drive is that patients are not constrained to defend a PfcC02 of 40 mm Hg or C02RT, as the case may be.
Nevertheless, the added load of maintaining COsSB near C02RT would have raised respiratory drive and power requirements of patients in group 2 above those of group 1.