1. purpose of study: optimal minute could be the prediction of successful weaning and become the new weaning parameter.

2. study design: inclusion criteria: investigators will perform this study at our respiratory care center. Patients who had been maintained on mechanical ventilator in excess of 3 weeks before respiratory care center admission and all previous weaning attempts had ailed.

   Exclusion criteria: Patients do not have spontaneous breath. Terminal cancer stage and unstable hemodynamics condition.

3. study duration: 2016/01/01~12/31

Investigators will preform this study at our respiratory care center. Initially, Investigators collected all physiological parameters and baseline characteristics of patients with prolong mechanical ventilator. Investigators use Gold-Galileo ventilator and patients use adaptive support ventilation mode. In adaptive support ventilation, the clinician enters a target minute volume , using a parameter called minute ventilation percentage. The minute ventilation percentage setting be initially set at 100% (the 100%minute ventilation setting), which provides a target minute ventilation of 0.1 L/min/kg of ideal body weight . Investigators observed respiratory frequency of patients, the minute ventilation percentage was increased every 5 min until the mandatory breath began to appear and spontaneous rate is zero. If the minute ventilation percentage had exceeded 250%, but patients do not have mandatory rate appeared, the optimal minute ventilation of the patient is means above 250%. Investigators also measure other weaning parameters. Finally, Investigators analyze the prediction of optimal minute ventilation as a weaning parameter.

Inclusion Criteria:

• patients who had been maintained on mechanical ventilation in excess of 3 weeks before respiratory care center admission and all previous weaning attempts had failed.

Exclusion Criteria:

• Patients do not have spontaneous breath. Patients do not use invasive mechanical ventilator. Terminal cancer stage and hemodynamics unstable.

All patients are included in this study if they had been maintained on mechanical ventilation in excess of 3 weeks before respiratory care center admission, and all previous weaning attempts had failed.

Patients are eligible for respiratory care center admission : hemodynamics stability, no vasoactive drug infusion for 24 hours or more before transfer, stable oxygen requirements(FIO2<40%,PERP<10 cmH2O), no hepatic failure, no requirement for surgical intervention within the 2 weeks, .