ACUTE RESPIRATORY DISTRESS SYNDROME LEARNING OBJECTIVES Define ALI/ARDS Name common precipitants of ALI/ARDS Describe the ARDSNet low tidal volume ventilation strategy Discuss the available evidence regarding the appropriate level of PEEP in ARDS Describe the evidence regarding the utility of steroids in late ARDS

You are called to evaluate Ms. D, a 56 year old woman on the Neurosurgical floor who is postoperative day #5 from a craniectomy for a left frontal mass. Her post-operative course was complicated by witnessed aspiration on POD #2 for which she was started on ampicillin/sulbactam. Chest radiograph at that time revealed a right middle lobe opacity consistent with aspiration. Her oxygen requirements have been slowly increasing since that time (from 2 L/min to 5 L/min O2 via nasal cannula) and this evening she desaturated to 82% on 5 L/min O2 and was placed on a non-rebreather. She complains of dyspnea and a dry cough. On exam, temperature is 36.8˚C, BP 132/72, HR 114, RR 28 and O2 sat is 92% on a nonrebreather mask. The craniectomy appears to be healing well. JVP is not elevated. Heart is tachycardic, but regular. Lungs have coarse crackles bilaterally. Abdomen is soft and nondistended. Trace pitting edema is present bilaterally. Neurologic exam is stable over the last several days. ABG: 7.30/48/57/22 on a non-rebreather CXR demonstrates diffuse bilateral opacities, no pneumothorax Recent pre-operative TTE with EF 67%, no regional wall motion abnormalities, no evidence of diastolic dysfunction What are the five causes of hypoxemia and what do you think is the cause of Ms. D’s hypoxemia? Causes of hypoxemia Low FiO2 Hypoventilation V/Q mismatch Shunt Diffusion abnormality

Normal A-a O2 difference Elevated A-a O2 difference

The A-a O2 difference can be calculated by: PAO2 = [FiO2 x (Patm – 47 mm Hg)] – (PaCO2/0.8) A-a O2 difference = PAO2 – PaO2 Here, the A-a difference is 596 torr. A normal A-a O2 difference is (age/4) + 4, or L shunt, this raises the question of pulmonary shunt physiology.

How is acute respiratory distress syndrome (ARDS) defined and what are common precipitants? Do you think Ms. D has ARDS? Definition of ARDS1 Timing: Within 1 week of a clinical insult or new/worsening respiratory symptoms Chest imaging: Bilateral opacities not fully explained by effusions, lung/lobar collapse, or nodules Origin of edema: Respiratory failure not fully explained by cardiac failure or fluid overload. Need objective assessment (i.e. echocardiography) to exclude hydrostatic edema if no risk factor present Oxygenation: - Mild: PaO2/FiO2 200-300 mm Hg with PEEP or CPAP ≥ 5 cm H2O - Moderate: PaO2/FiO2 100-199 mm Hg with PEEP ≥ 5 cm H2O - Severe: PaO2/FiO2