What is Acute Respiratory Distress Syndrome?
acute diffuse inflammatory lung injury leading to increase pulmonary vascular permeability, increased lung weight, and loss of aerated lung tissue
What is the Berlin 2012 criteria for ARDS?
- ACUTE: Symptom onset within 1 week of known clinical insult
- Pulmonary Edema: Bilateral opacities consistent with pulmonary edema
- Respiratory failure not fully explained by fluid overload or cardiac failure (may use echo to exclude)
- Hypoxemia: Ratio of arterial oxygen tension to fraction inspired oxygen is <300mmHg
What is mild ARDS?
PaO2/FiO2 200-300mmHg on ventilatory setting with PEEP> 5cmH20
What is moderate ARDS
PaO2 /FiO2 100-200mmHg with ventilator settings of PEEP 5cmH2O
What is severe ARDS?
PaO2/FiO2 < 100mmHg on ventilator settings including PEEP > 5 cm H20
What are the causes of ARDS?
- Direct
- infection
- Contusion from blunt trauma
- Aspiration
- Smoke inhalation
- Near drowning
- Indirect (non-pulmonary)
- Sepsis
- Major trauma
- Prolonged hemorrhage
- Embolism (fat/amniotic fluid/thrombotic)
- Burns
- Pancreatitis
- DIC
- Massive blood transfusion
- Cardiopulmonary bypass
What are the phases of ARDS?
- Injury
- Exudative
- Proliferative
- Fibrotic
- Resolution
What are the phases of ARDS?
- Injury
- Exudative
- Proliferative
- Fibrotic
- Resolution
Describe the immediate exudative phase of ARDS.
- capillary endothelial injury and alveolar epithelial injury mediated by cytokines
- Increase permeability leading to widespread pulmonary edema + alveolar collapse
Describe proliferative phase of ARDS.
Proliferation and activation of type 2 pneumocytes and fibroblasts
Describe the fibrotic phase of ARDS.
Infiltration of fibroblasts, replacing alveoli and alveolar ducts with fibrosis
Effects of ARDS pathophysiology on respiratory function
- Hypoxemia
- Lung collapse and consolidation
- Decreased compliance
- Increased minute ventilation
- Increased work of breathing
- Pulmonary hypertension
Management of ARDS
- Treat precipitating condition
- Nutritional support
- Strict fluid management (sedation + diuresis if stable)
- Prophylaxis for stress ulcer
- Prevention of nosocomial pneumonia
- Ventilation strategy
- Muscle relaxant
What are the mechanical ventilation strategies for ARDS?
- Post- ARMA lung protective ventilation protocol:
- Low tidal volume 6ml/kg (predicted body weight)
- Permissive hypercapnia
- Pplat < 30 cm H20
- High PEEP 10-20cmH2O
- Inverse ratio ventilation
What did the ARMA trial conclude?
- MC RCT
- Traditional TV. 12ml /kg vs low TV 6ml/kg
- Outcome:
- less in-hospital mortality
- significant reduction in duration of MV
What are the salvage therapies for refractory ARDS?
Prone positioning
Neuromuscular blockade
Recruitment manoeuvres
Unconventional ventilator modes (Airway Pressure Release Ventilation, High frequency oscillatory ventilation)
What is prone positioning beneficial?
Recruit collapsed dorsal alveolar to improve V/Q mismatch
What are complications of prone positioning?
- Pressure ulcers
- Airway obstruction
- Increase abdominal pressure with liver/renal dysfunction
- Loss of venous access
- Dislodgement of endotracheal tube
Contraindications to prone positioning
- Severe facial / neck trauma
- Elevated ICP
- Pelvic/spinal instability
- High probability of CPR
What are the benefits of neuromuscular blocking agents?
Abolish patient’s inspiratory and expiratory efforts to improve patient-ventilatory synchrony
Minimize muscle oxygen consumption
What is a recruitment manoeuvre?
Transient sustained increase transpulmonary pressure to attempt to open previously collapsed alveoli
What are some methods of recruitment manoeuvres?
- CPAP mode: applied pressure of 30-40cmH20 for 30-40 seconds
- Stepwise: incremental PEEP with constant driving pressure
Pharmacological therapies for ARDS
- Surfactant therapy
- Inhaled vasodilators (NO, prostacyclins)
- Steroids
What is high frequency oscillatory ventilation?
- low tidal volume (1-4ml/kg)
- high RR and mean airway pressure (3-15 Hz)
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