The control of breathing during weaning from mechanical ventilation

Using the recruitment threshold technique, we measured the CO₂ responsiveness of the unloaded respiratory pump in 14 mechanically ventilated patients prior to weaning. The CO₂ recruitment threshold (CO₂RT) was compared with the arterial CO₂ tension during unassisted breathing (CO₂SB) and with the PaCO₂ during mechanical ventilation (CO₂MV) at machine settings determined by the primary physician. Based on these comparisons, we tested the hypotheses that (1) patients without weaning-induced respiratory distress (group 1) maintain CO₂SB near CO₂RT, (2) patients with weaning-induced respiratory distress (group 2) retain CO₂SB above CO₂RT, thereby manifesting incomplete load compensation, and (3) CO₂MV is ventilator setting dependent and provides insufficient information about the ventilatory requirement during weaning. Respiratory distress was prospectively defined as sustained tachypnea (rate≥30) or intense dyspnea (Borg scale rating) and limited weaning in nine of 14 patients. The average CO₂RT was 40 mm Hg in both groups. All patients in group 1 maintained CO₂SB near CO₂RT (p>0.1). Seven of nine patients in group 2 retained CO₂ by ≥3 mm Hg above CO₂RT (p<0.01). There was no significant difference between CO₂MV and CO₂SB in either group. We conclude that CO₂RT provides a better reference of the adequacy of ventilatory load compensation during weather than CO₂MV.