C02 retention above CO£BT can be viewed as evidence for incomplete load compensation, but it need not imply overt pump failure. We have shown that patients without weaning-induced respiratory distress usually maintain an alveolar ventilation predicted by C02RT. Only one of five patients in group 1 retained C02 during weaning relative to C02RT. The 2 mm Hg difference between COsSB and C02RT in this patient falls within the variability of the recruitment threshold measurement. Wfe have also demonstrated that most patients with weaning-induced dyspnea and/or tachypnea already retain C02 compared with C02RT 5 min after the appearance of these signs of respiratory distress.
The Determinants of COzMV
In awake mechanically ventilated subjects, the respiratory muscles remain active despite substantial reductions in PaC02 below eucapneic levels. The triggering of a machine breath with a predetermined tidal volume requires minimal effort. Compared with spontaneous breathing, this results in an uncoupling of respiratory timing, the pressure output of respiratory muscles, minute ventilation, and PaC02. The rate at which subjects trigger the ventilator is not determined simply by the necessity to defend a particular PaC02. Although little is known about the control of alveolar ventilation during assisted breathing, experiments in anesthetized humans have demonstrated that there is a considerable range of frequency and tidal volume settings over which 1:1 entrainment between a subject’s spontaneous respiratory efforts and machine-delivered breaths can be induced. Vagal afferent signals are responsible for the entrainment between the respiratory oscillator and the mechanical ventilator.