Bench-to-bedside review: Weaning failure – should we rest the respiratory muscles with controlled mechanical ventilation?

Controlled mechanical ventilation (CMV) is a mode of ventilator support in which each breath is triggered by the ventilator's timer using a respiratory rate set by the clinician. The characteristics of the breath are also set by the clinician, i.e. pressure or flow controlled, volume, flow or time cycled. Because the respiratory muscles are not contracting, the minute ventilation is fully controlled by the ventilator, which takes full responsibility for inflating the respiratory system.CMV is traditionally used in severely ill patients who cannot tolerate partial ventilatory support (e.g., acute respiratory distress syndrome, septic shock, multiple organ failure), in cases of overt patient-ventilator dysynchrony, and in the immediate postoperative period. CMV is also used when weaning fails (especially T-piece weaning) to rest the respiratory muscle before the next weaning attempt. This review will summarize recent evidence concerning the deleterious effects of CMV on respiratory muscle function and discuss the use of CMV during weaning failure.Animal models have been used to unravel the effects of CMV that are beneficial for the respiratory muscles: reversal of respiratory muscle fatigue [1], prevention of muscle fiber injury during a short-term (four hours) model of sepsis [2], and restoration of perfusion to vital organs in shock states when blood flow is 'stolen' by the intensely working respiratory muscles [1,3].Accumulating experimental evidence suggests, however, that CMV can also induce dysfunction of the diaphragm, resulting in decreased diaphragmatic force generating capacity, diaphragmatic atrophy, and diaphragmatic injury, also called ventilator-induced diaphragmatic dysfunction (VIDD) [4].In the intact diaphragm of various animal species (including primates) studied in vivo after a period of CMV, transdiaphragmatic pressure generation caused by phrenic nerve stimulation declines at both submaximal and maximal stimulation frequencies (20 to 100 Hz) in a