Respiratory Failure and Acute Respiratory Distress Syndrome (ARDS) Flashcards
Acute respiratory failure is due to impaired gaseous exchange either __ or __ or both
Respiratory failure can occur within ____!
Impaired oxygenation, impaired CO2 elimination
Minutes to hours
Types of respiratory failure
Type 1 (hypoxaemic): PaO2 < 50mmHg
Type 2 (hypercapnic): PaCO2 > 45mmHg
Or both
Mechanism of type 1 respiratory failure
Example of causes
Mechanisms
1. Ventilation perfusion (V/Q) mismatch
2. Alveolar hypoventilation
3. Shunt (physiological in alveolar; anatomical proximal to lung)
4. Diffuse limitation
5. Low inspired oxygen fracture
Causes (all may also cause type 2)
1. COPD / asthma
2. Pneumonia
3. Pulmonary oedema
4. ARDS
5. Pulmonary fibrosis
6. Obesity
7. Pulmonary embolism
Mechanism of type 2 respiratory failure
Examples of causes
Mechanisms: related to alveolar hypoventilation
1. Reduced central respiratory drive
2. Chest wall restriction
3. Airway abnormalities - dead space, increased work of breathing, fatigue
4. Neuromuscular disease
RR related or TV related
Causes
1. Severe asthma / COPD
2. Drug overdose
3. Myasthenia gravis
4. Cervical cord injuries
5. Brainstem injuries
6. Obesity and hypoventilation
7. Kyphoscoliosis
Goals of therapy for acute respiratory failure
- Hypoxaemia - immediate threat
- Hypercarbia better tolerated - unless severe acidosis -> risk of cardiac arrest and death
Aim:
1. Improve oxygenation
2. Enhance CO2 removal or buffer the blood
What are the indications for mechanical ventilation or ETT?
- Bradypnoea or apnoea or respiratory arrest
- ARDS
- Respiratory muscle fatigue
- Obtunded or comatose
- PaO2 < 55mHg despite oxygenation
- PaCO2 > 50mmHg with pH < 7.2 despite NIV
Berlin’s Criteria for ARDS (2012)
ARDS New Global Definition (2023)
- Timing - respiratory symptoms within 1 week of known insult, or worsening symptoms
- Imaging - bilateral opacities consistent with pulmonary oedema; not explained by pleural effusions, collapse or nodules
- Origin - not fully explained by heart failure or fluid overload
- Oxygen impairment (PF ratio) moderate to severe
- Mild: 200-300 on PEEP > 5 cmH2O
- Moderate: 100-200 on PEEP > 5 cmH2O
- Severe: < 100 on PEEP > 5 cmH2O
ARDS NGD 2023 - additional optional criteria for less fortunate worlds
5. SpO2/FiO2 ratio < 315 with sats < 97%
Pathogenesis of ARDS
- Injury phase - increased permeability of endothelial and epithelial barriers
- Accumulation of protein-rich oedema fluid in interstitium and alveolar space - Diffuse alveolar damage - proteins, neutrophils and necrotic debris packed into dense eosin-staining hyaline membrane
- Fibroproliferative phase - epithelium regenerates and heals leading to fibrosis
Risk factors for development of ARDS
- Gastric aspiration
- Pneumonia
- Sepsis
- Trauma
- Blood transfusion
- Pancreatitis
- Fat embolism
- Near drowning
- Alcoholism
Ironically - diabetes reduces likelihood of ARDS
Ventilation strategies in ARDS
ARDS Network - 9% absolute risk reduction
1. Low tidal volume ventilation (4-8mL/kg)
- Ideally 6mL/kg
2. Plateau pressure < 30 cmH2O
3. Neuromuscular blockade first 48 hours
4. Prone positioning
5. Previously high frequency oscillatory ventilation (HFOV) - discredited in recent RCTs
- Canada study: increased mortality in ARDS (except only in severe ARDS)
- UK study: no difference in mortality
Prone positioning in ARDS
Proning Severe ARDS Patients study:
- Reduction in 28- and 90-day mortality in severe ARDS with proning of 16 hours a day, with low TV and plateau pressure
Watch out for complications of proning
What is the common mortality cause in ARDS?
What is MODS?
Leading cause of death in ARDS: MODS
(Ironically unsupportable respiratory failure is a less common cause of death)
Multiorgan dysfunction syndrome (MODS)
- Incremental physiologic derangement in major organs (liver, gut, kidney, brain, CVS, haematologic)
- Reversible or irreversible organ failure
Causes of ARDS
A. Pulmonary
B. Extrapulmonary
