Acute Respiratory Distress Syndrome

Learning objectives

Recognize ARDS
Treatment of ARDS
Anesthetic management of ARDS

Definition and mechanism

Acute respiratory distress syndrome (ARDS) is a life-threatening acute onset of respiratory failure
Characterized by hypoxemia, stiff lungs, alveolar edema, endothelial cell damage, and neutrophil infiltration
Multiple risk factors trigger the acute onset
ARDS Berlin definition:

Timing	Within 1 week of a known clinical insult or new or worsening respiratory symptoms
Chest imaging	Bilateral opacities - not fully explained by effusions, lobar/lung collapse or nodules
Origin of edema	Respiratory failure not fully explained by cardiac failure or fluid overload Need objective assessment (e.g. echocardiography) to exclude hydrostatic edema if no risk factor present
Oxygenation Mild Moderate Severe	200 mmHg < PaO2/FiO2 ≤ 300 mmHg with PEEP or CPAP ≥ 5 cmH2O 100 mmHg < PaO2/FiO2 ≤ 200 mmHg with PEEP ≥ 5 cmH2O PaO2/FiO2 ≤ 100 mmHg with PEEP ≥ 5 cmH2O

Signs and symptoms

Severe shortness of breath
Labored and unusually rapid breathing
Low blood pressure
Confusion and extreme tiredness

Risk factors

Direct	Indirect
Pneumonia Aspiration of gastric contents Inhalation injury Pulmonary contusion Pulmonary vasculitis Drowning	Non-pulmonary Sepsis Major Trauma Pancreatitis Severe Burns Non-cardiogenic shock Drug overdose Multiple transfusions or Transfusion-related acute lung injury (TRALI)

Complications

Deep vein thrombosis
Gastrointestinal bleeding
Barotrauma
Pneumothorax
Nosocomial infection
Delirium
Pulmonary fibrosis
Refractory respiratory failure requiring prolonged dependence on mechanical ventilation
prolonged stay in ICU
Need for tracheostomy
Muscle weakness
Sepsis
Ambulatory dysfunction

Treatment

Supplemental provision of oxygen
Optimize PEEP: esophageal pressure, PV curves, lung ultrasound
Protective mechanical ventilation
Non-conventional therapies: prone position, high-frequency oscillatory ventilation, and extracorporeal membrane oxygenation
Consider:
- Extracorporeal lung support (ECLS) techniques including ECMO as possible rescue therapy in patients with severe hypoxemic and hypercapnic respiratory failure
- Steroids: methylprednisolone
- Neuromuscular blocking agent cisatracurium or inhaled NO as these could improve oxygenation

Anesthetic management

Volume-regulated volume control (VCV), pressure-regulated volume control (PRVC), or pressure-controlled (PCV) mode.

Hemodynamic management in ARDS

Hemodynamic instability is often present in ARDS
The right ventrile (RV) is most directly affected by positive-pressure mechanical ventilation and ARDS
Therpaeutc strategies should be directed at preventing and treating RV dysfunction
- Optimize RV preload
- Optimize RV systolic function
- Reduce RV afterload
- Maintain an adequate systemic blood pressure and coronary perfusion
- Treat underlying disease
Spectrum of pulmonary vascular dysfunction in acute respiratory distress syndrome (ARDS) and principal treatment strategies:

MOD, multiple organ dysfunction; PAH, pulmonary arterial hypertension; NE, norepinephrine; PPV, pulse pressure variation; PPMV, positive-pressure mechanical ventilation; RVEDA, right ventricular end-diastolic area, LVEDA, left ventricular end-diastolic area; DPAW, airway driving pressure; DPTP, transpulmonary driving pressure; PPLAT, plateau pressure; PEEP, positive end-expiratory pressure; EIT, electrical impedance tomography; ECMO, extracorporeal membrane oxygenation.