ARDS/Mechanical Ventilation

Pathophysiology:

Scattered, nonhomogeneous alveolar damage that leads to oxygenation (V/Q mismatch) problems

Diagnosis:

• Onset: within one week of a known clinical insult, or new or worsening respiratory symptoms
• Imaging: bilateral infiltrates on CXR not fully explained by effusions, nodules, or lung collapse
• Origin: respiratory failure not fully explained by cardiac failure or fluid overload. An objective assessment (eg ECHO) is required to exclude pulmonary edema if no ARDS risk factors are present
• Oxygenation: (on ventilator settings that include PEEP or CPAP >5cm H2O)
  • Mild ARDS: PaO2/FiO2 ratio 200-300 mmHg
  • Moderate ARDS: PaO2/FiO2 ratio 100-200 mmHg
  • Severe ARDS: PaO2/FiO2 ratio <100 mmHg

Etiology:

• Direct lung injury: Pneumonia, aspiration, pulmonary contusion, fat emboli, near-drowning, inhalational injury, post lung transplantation, or hematopoietic stem cell transplant
• Indirect lung injury: Sepsis, severe trauma, shock, drug overdose, DIC, pancreatitis, cardiopulmonary bypass, transfusion of blood products (TRALI)

Management:

• Mechanical ventilation: 
  • Goal: Maintain adequate gas exchange until the inflammation and edema subside and minimize ventilator-induced lung injury
  • ARDSnet protocol (ARMA Trial): low tidal volume (4-6 ml/kg) and low airway pressure (Pplat <30 mmHg)
• Additional therapeutic considerations
  • "Conservative" fluid management: FACTT trial showed that it improves oxygenation and shortens the duration of mechanical ventilation and intensive care but does not improve 60 day mortality. Goal CVP <4, PCWP <8. Excluded patients with hypotension, pressures, HD, oliguric renal failure.
  • Early neuromuscular blockade in severe ARDS: ACURASYS trial (single randomized trial) showed the use of cisatracurium in patients with severe ARDS resulted in a reduction in 90 day mortality and an increase in ventilator free days. ROSE trial did not reproduce this mortality benefit.
  • Prone positioning PROSEVA randomized trial showed a reduction in mortality in patients with severe ARDS; recommended for patients with P/F < 100, consider if P/F < 150
  • ECMO and high frequency oscillatory ventilation: further studies are required to evaluate high frequency oscillatory ventilation and extra-corporeal membrane oxygenation. There are no consensus guidelines but consider in P/F <80.
  • Steroids should NOT be initiated in late ARDS (14 days or longer). The impact of earlier steroid therapy on mortality is uncertain, as the DEXA-ARDS trial showed reduced mortality and improved liberation from the vent but previous studies were less clear.
  • Recombinant surfactant does not improve survival or ventilator free days
  • Low-dose nitric oxide temporarily improves oxygenation but not mortality
• Overall care
  • Identify and treat underlying causes
  • Ensure adequate nutrition (preferably enteral)
  • Provide GI and DVT prophylaxis
  • Prevent and treat nosocomial infections early

Indications for intubation

• Is there failure of airway maintenance or protection?
  • Upper airway obstruction, airway protection
• Is there a failure of oxygenation or ventilation?
  • Uncorrectable hypoxemia (pO2 <70 mmHg on 100% O2 NRB)
  • Hypercapnea (pCO2 >55 mmHg) with acidosis [clinical judgement for pCO2 in COPD]
  • Ineffective respiration (max inspiratory force <25 cm H2O)
• Does the anticipated clinical course require intubation?
  • Fatigue (RR>35 with increasing pCO2)

Initial ventilator settings

• ARDS
  • Initial TV 6ml/kg PBW (range 4-8 ml/kg)
  • Ventilator rate 14-22 breaths per minute
  • Initial PEEP of 5 cm H2O, up to 24 cm H2O
• Non-ARDS
  • Initial TV between 6-8 ml/kg PBW reasonable, 
  • Ventilator rate 12-16 breaths per minute
  • PEEP between 3-5 cm H2O

Ware LB, Matthay MA. The acute respiratory distress syndrome. N Engl J Med 2000; 342:1334-49.
https://www.uptodate.com/contents/acute-respiratory-distress-syndrome-clinical-features-diagnosis-and-complications-in-adults
https://www.uptodate.com/contents/ventilator-management-strategies-for-adults-with-acute-respiratory-distress-syndrome