the Effect of Doxapram on Diaphragmatic Excursion(DE) During Weaning From Mechanical Ventilation.

The goal of this clincal trial is to learn if doxapram is effective in weaning in adults on mechanical ventilation. The objectives of this study: * Evaluate the effect of doxapram on diaphragmatic function (assessed by DE)during weaning from mechanical ventilation. * Assess the impact of doxapram on respiratory muscle sterngth. * Mointor adverse effects associated with doxapram administration. Researchers will compare expermental group(intervenous doxapram during weaning)with control group(saline infusion)during weaning. The patients who fulfull criteria of weaning will be subjected to our clincal trial and put on spontanous breathing trial for 120mins using positive end expiratory pressure (peep)5 cmh2o,pressure support 5cm h2o and fio2 0.3.By the end of the SBTthe patient will be extubated if ventilator parameters and vital signs were maintained within 20%of baseline values. The patients will be followed up after extubation and afailed weaning will be defined as the neeed for either invasive or non invasive ventilation for 72 after extubation or failed SBT.