APRV mode of ventilation will result in an improved partial pressure of arterial oxygenation/ fraction of inspired oxygen (P/F ratio) on day 3 of mechanical ventilation. Sub hypotheses: APRV will be associated with a reduced amount of sedation used during the ICU stay in patients with respiratory failure. APRV will be associated with a reduction in the amount of vasoactive medication used for blood pressure support in patients with respiratory failure.