The protocol lasts 45 minutes, divided into 3 periods of 15 minutes. Patients with severe pulmonary edema are randomly assigned in 2 groups: 1/ “Early CPAP” (n=63): CPAP alone (T0-T15); CPAP + medical treatment (T15-T30); medical treatment alone (T30-T45); 2/ “Late CPAP” (n=61): medical treatment alone (T0-T15); medical treatment + CPAP (T15-T30); medical treatment alone (T30-T45). Primary endpoint : effect of early CPAP on a dyspnea clinical score and on arterial blood gases. Secondary endpoints : incidence of tracheal intubation, inotropic support, and in-hospital mortality.