Prevalence of Sigh responders. After enrollment, FiO2 will be titrated to obtain SpO2 of 90-96% and then each patient will first undergo a clinical test of PSV vs. PSV+Sigh to assess the prevalence of Sigh responders vs. non-responders in respect to improved oxygenation. After 30 minutes of clinical PSV+Sigh, SpO2/FiO2 ratio will be collected again to quantify the number of patients in whom it increased (i.e., "Sigh responders").