VSV Versus PSV as a Weaning Mode of Mechanically Ventilated Chronic Obstructive Pulmonary Disease Patients

Acute exacerbations may occur during the management of stable COPD which can negatively impact health status, rates of hospitalization and re-admission. During exacerbation, some patients need immediate admission to the respiratory intensive care unit (RICU). Ventilatory support in an exacerbation can be provided by either noninvasive (nasal or facial mask) or invasive (oro-tracheal tube or tracheostomy) ventilation. Mechanical ventilation may be complicated by barotrauma, volutrauma, and also unfavourable impacts on cardiovascular system and organ perfusion. Moreover, prolonged mechanical ventilation enhances the risk of nosocomial pneumonia. So it is Important to minimize the duration of mechanical ventilation and perform extubation as soon as possible. In recent years, development of numerous models of artificial respiration, which could support spontaneous breathing, has made it possible to gradually decrease the mechanical ventilatory support. From these new modes, PSV (pressure-support ventilation) which is a well known weaning mode will be compared in our study to a new weaning mode which is a volume support ventilation (VSV). VSV could be viewed as "PRVC for spontaneous breathing" as it delivers a variable pressure to meet a target VT. * The ventilator gives a test breath with an inspiratory pressure of 10 cmH2O above PEEP (5 cmH2O in earlier software versions) * It measures the volume delivered and calculates system Compliance * For each subsequent breath, the ventilator calculates compliance of the previous breath and adjusts the inspiratory pressure level (pressure level) to achieve the set VT on the next breath * The ventilator will not change the inspiratory pressure by more than 3 cm H2O from one breath to the next * Maximum available inspiratory pressure level is 5 cm H2O below the preset upper pressure limit (alarm will sound at this point and the breath will switch into exhalation) * The minimum pressure limit is the baseline setting (PEEP) * If apnea occurs, back up pressure control is activated and an alarm sounds * If Auto mode is on and patient becomes apneic, the mode will automatically switch to PRVC(pressure regulated volume control). In PSV patients in whom a 8 cm H2O pressure support level could be achieved,a 2-h trial of spontaneous breathing with this pressure support level will performed before extubation . In the VSV group, VT(tidal volume) will be adjusted at 6 ml/ Kg and If the patients showed good tolerance with an acceptable ABG (arterial blood gas)analysis (pH o7.35, PaO2(partial pressure of arterial oxygen)/FIO2 .150 with an FIO2(fraction of inspired oxygen) f 40%, RR (respiratory rate) f 35 breaths/min), they will ventilated for 2-h trial of spontaneous breathing and then extubated