To evaluate whether early use of airway pressure release ventilation would improve oxgyenation , improve lung compliance and shorten the time of mechanical ventilation compared to BIPAP ventilation in ARDS patients

Acute respiratory distress syndrome (ARDS) is an extremely dangerous lung condition that leads to low blood oxygen levels, and is commonly caused by sepsis, pneumonia, aspiration, and trauma. Despite great improvements in mechanical ventilation in recent years, the mortality rate of ARDS is still high. Low tidal volume ventilation (LTVV), optimum positive end-expiratory pressure, permissive hypercapnia, lung recruitment, and the prone position are common treatments for ARDS .

Lung recruitment maneuvers (LRMs) are a common element of the "open lung" approach to lung-protective ventilation. Given that atelectasis increases alveolar stress and strain during tidal ventilation, LRMs may mitigate ventilator-induced lung injury (VILI) and reduce the considerable risk of death for patients with ARDS. However, the transient application of high airway pressures may impair cardiovascular function and traumatize the pulmonary epithelium (barotrauma). Consequently, the use of LRMs is controversial .

APRV and BIPAP are modes of mechanical ventilation that allow unrestricted spontaneous breathing independent of ventilator cycling, using an active expiratory valve. Both modes are pressure-limited. Ventilation occurs via the time-cycled switching between two set pressure levels .

Bilevel positive airway pressure is a partial support mode that employs pressure-controlled, time-cycled ventilation set at two levels of continuous positive airway pressure (CPAP) with unrestricted spontaneous breathing. Which can occur at any phase of the mechanical ventilatory cycle. Bilevel positive airway pressure is able to modulate the inspiratory effort by modifying the frequency of controlled breaths .

In traditional biphasic ventilation, the inspiratory to expiratory ratio is not inverse. This ratio often distinguishes biphasic ventilation from APRV. Ventilator parameters set in this mode include the inspiratory pressure limit (Phigh), inspiratory time (Thigh), PEEP level (Plow), rate, and fraction of inspired oxgyen (FiO2). On some ventilators, pressure support may be added to augment the patient's spontaneous breaths .

Airway pressure release ventilation (APRV) is defined as a continuous positive airway pressure (CPAP) with a brief intermittent release phase based on the open lung concept; it also perfectly fits the ARDS treatment principle. Despite this, APRV has not been widely used in the past, rather only as a rescue measure for ARDS patients who are difficult to oxygenate .

APRV is a form of pressure controlled intermittent mandatory ventilation that is designed to allow unrestricted spontaneous breathing through the breath cycle, especially during Timehigh (Thigh).it is also applied using I:E ratios much greater than 1:1 or inverse ratio ventilation, usually relying on short expiratory times and gas trapping to maintain end-expiratory lung volume. APRV improve oxygenation and decrease the amount of alveolar derecruitment during the Timelow (Tlow) .

All preexisting data did not address the question for which APRV was being promoted, is that APRV should be used as initial mode of mechanical ventilation for patients with ARDS. Multiple reports and studies in animals and humans have not helped answer this question. Not only is there paucity in the number of high quality trials in humans, but there is a lack of consistency on how APRV is applied .

Over recent years, with an increased understanding of the pathophysiology of ARDS, APRV has been re-proposed to improve patient prognosis. Nevertheless, this mode is still not routinely used in ARDS patients .

Compared to the classical ventilation, APRV has been shown to provide lower peak pressure, better oxygenation, less circulatory loss, and better gas exchange without deteriorating the hemodynamic condition of the ARDS patient .

Inclusion Criteria:

• Patients who meet all ARDS criteria (according to Berlin definition).
• Patients who had received endotracheal intubation and mechanical ventilation for <48h prior to inclusion.

Exclusion Criteria:

• Age below 18 years and above 65 years.
• Presence of significant chronic pulmonary disorder (chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis, suppurative lung disease or diffuse alveolar hemorrhage) or documented barotrauma.
• Presence of cardiac disorder (rheumatic or ischemic heart disease).
• Presence of intracranial hypertension (suspected or confirmed).
• History of malignancies or patients on immunosuppressive drugs.
• Pregnancy or presence of neuromuscular disorder known to prolong intubation.

To ensure ultimate patients' privacy and disclosure , IPD might not be shared , but data will be available with the corresponding author on special request .