Acute chest syndrome (ACS) is the leading cause of admission to intensive care and the leading cause of death in patients with sickle cell disease. Irrespective of the cause of ACS, there is an heterogeneity in pulmonary ventilation/perfusion ratios, leading to worsening of the disease.

Efficiency of awake prone positioning (APP) in acute respiratory failure (ARF) was particularly highlighted during the COVID-19 pandemic. Several physiological factors contribute to this benefit including an improvement in ventilatory drive and gas exchange.

The investigator hypothesize that APP could lead to clinical improvement in ACS in terms of oxygenation and ventilatory drive, by improving the heterogeneity of ventilation

• Several physiological mechanisms contribute to the benefit of APP during ARF. In addition to hypoxemia improvement, there is also an effect on ventilatory drive, notably in terms of polypnea, ROX index and inspiratory effort.
• Considering that hypoxemia in ACS contributes to the physiopathological process: deoxygenation of haemoglobin S - red blood cells falciformisation - vaso-occlusive event, APP could be an additional therapy in severe ACS. In addition, improving ventilation-perfusion ratios, mainly by recruiting dorsal zones, could be particularly useful in ACS, where pulmonary damage predominates in gravito-dependent zones.

Inclusion Criteria:

• Age >18 years
• Major sickle cell anemia (SS, SC, Sβ)
• Admission in intensive care unit for ACS
• Registered in the French social insurance regime.
• Written, informed consent

Exclusion Criteria:

• Pregnant or breastfeeding women
• Immediate need for intubation
• Impaired vigilance status (Glasgow scale score < 12)
• Pneumothorax
• Haemodynamically unstable
• Thoracic trauma admission
• Severely obese with body-mass index higher than 40 kg/m²
• EIT contraindication: pacemaker, automatic implantable defibrillators, skin lesions facing the EIT belt, unstable rachis fracture or medullary lesions