ARDS (2) Flashcards
Characteristics of ARDS on histological examination?
- Diffused alveolar damage
- Oedema
- Cell necrosis
- Fibrosis
What was the initial definition of ARDS?
- Acute onset hypoxia
- PaO2 to FiO2 ration < 200mmHg
- Bilateral infiltrates on CXR
- PAWP < 18mmHg or cardiogen pul oedema
What is the current Berlin definition of ARDS?
- Acute onset ARDS within 7 days of insult
- Degree of hypoxia based on PaO2/FiO2
- PEEP of at least 5
- Absence of fluid overload or CCF
- Infiltrates on CXR & CT
Classification of ARDS based on degree of hypoxaemia (PaO2/FiO2 ratio)?
- Mild - 201 and 300
- Moderate - 101 and 200
- Severe - <100 mmHg
Characteristics of ARDS?
- Precipitated by underlying factors
Distinguishing between types of ARDS?
- Pulmonary
- Extra-pulmonary
What is pulmonary ARDS?
Direct insult to the lung affecting pulmonary epithelium
What is extra-pulmonary ARDS?
Indirect lung injury caused by inflammatory mediators acting on vascular endothelium
What are the causes of pulmonary ARDS?
- Pneumonia (Main cause)
- Aspiration pneumonitis
- Inhalation injury
- Pulmonary contusion
- Pulmonary vasculitis
- Near drowning
What are the causes of extra-pulmonary ARDS?
- Non-pulmonary sepsis
- Non-cardiogenic shock
- Pancreatitis
- Major trauma
- TRALI
- Burns
- Drug overdose
What are the mimics of ARDS?
- Vasculitis (Alveolar haemorrhage)
- Drug induced
- Eosinophilic pneumonia
- Interstitial pneumonia
- Lung mets (Cancer)
What are the phases of ARDS pathophysiology?
- Exudative
- Proliferative
- Fibrosis
Features of the exudative phase of ARDS?
- Capillary congestion
- Alveolar oedema
Features of the proliferative phase of ARDS?
- Proliferation of alveolar Type-2 cells
- Proliferation of fibroblasts
- This phase can lead to resolution or formation of fibrosis
What are the intrapulmonary causes of hypoxaemia in ARDS?
- Shunt
- Dead space
- Impairment of gas diffusion
What are the mechanisms contributing to hypoxaemia in ARDS?
- Epithelial damage and increased shunt
- Endothelial damage and increased dead space
- Interstitial damage and impaired diffusion
Explain the mechanism of epithelial damage and increased shunt?
- Primary cause of hypoxaemia in ARDS
- Loss of lung volume - Surfactant deficiency
- Alveolar oedema
- Lung collapse
- Intraplulmonary shunt
Explain the mechanism of Endothelial damage and increased dead space ?
- Abnormalities in pulmonary blood flow
- The lung region is usually well ventilated
- Surrogate for dead space is ventilatory ratio
Explain the mechanism of Interstitial damage and impaired gas diffusion?
- Interstitial oedema
- Hyaline membrane
- Fibrosis - Thickened alveolar-capillary memebrane
What are the components of ventilator induced lung injury ? (VILI)
- Volutrauma
- Barotrauma
- Atelectrauma
- Biotrauma
Risk factors of VILI?
- High end-inspiratory lung volume
What is volutrauma?
This is high end-inspiratory lung volume
Factors promoting volutrauma?
- Lung strain
What is lung strain?
This is the ratio of VT to end-expiratory lung volume. Excess in strain results from high VT or high PEEP which brings the end-expiratory lung volume closer to TLC
What are the risk factors for barotrauma?
- Lung hyper-inflation
What is atelectrauma?
This is due to the repeat opening and closing of the small airway over the breathing cycle at low tidal volumes.
Factors promoting atelectrauma?
- Surfactant deficiency
- Alveolar instability
- Oedema
- Increased permeability
- Inflammation
- Structural damage
- Fibrin accumulation
- Fibrosis
How can atelectrauma be be prevented?
- Application of high PEEP
What is biotrauma?
- Release of inflammatory mediators
- Oxygen toxicity
What are the major determinants of VILI and in-hospital mortality ?
- Driving pressure (Plateau pressure - PEEP)
- Mechanical power
Phenotype of Covid-19?
- Type L
Characteristics of L-type covid-19?
- Low elastance (i.e high compliance)
- Low ventilation/perfusion ratio
- Low lung weight
- Low recruitability
Characteristics of type-H ARDS sub-phenotype?
- High elastance (Low compliance)
- High right to left shunt
- High lung weight
- High recruitability
What is the ventilatory strategy for the L-type phenotype?
- High VT 8-9ml/kg
- Low PEEP
What is the ventilatory strategy for the H-type phenotype?
- Low VT
- High PEEP
- Prone ventilation
What is compliance ? How is it calculated
- Tidal volume divided by driving pressures
- Combination of lung/chest wall compliance
- Chest wall compliance > lung compliance
What is the average value of the respiratory system compliance?
30ml/cmH2O or less
Factors increasing respiratory system resistance?
- Low lung volumes
Methods of measuring chest wall compliance?
- Oesophageal pressure
What is the influence of compliance on plateau pressure and driving pressure
- Relevant in intra-abdominal pathology
What are the methods of oxygenating patients ?
- CPAP
- HFNC
- NIV
What is the ROX index?
- Ratio between sats & FiO2
- High respiratory rate
- Predicts therapy failure in pts on HFNC
What is the HACOR scale?
Predicts NIV failure by evaluating;
- HR
- Acidosis
- GCS
- RR
What is the driving pressure?
- VT / Compliance
- > 15cmH2O is high risk for mortality
What is the volume of oxygen and flow rate that can be delivered during ECMO?
- 400ml O2/min
- Flow 6L/min
Characteristics of the Extracorporeal CO2 removal?
- CO2 clearance 150-200ml/min
- Veno-venous blood flow (1-2L/min)
What are the characteristics of the High Frequency Oscillatory Ventilation?
- 900 cycles/min
What are the advantages of iNO in ARDS ?
- Selectively dilates pulmonary vessels
- Improves oxygenation (preserved HPV)
- Reduction of PVR
What are the components of the total peak pressure? This can be separated in VCV
- Resistive (Peak pressure - Plateau pressure)
- Elastic (plateau pressure)
What is End-Expiratory Lung Volume ?
- Amount of aerated lung at the end of exp
- Evaluates collapse or over-distention
- Assessment of the effect of PEEP
- Its underestimated by bedside
- Circuit leaks can compromise measurement
What is acute cor-pulmonale?
- Increased RV afterload
- Complication of ARDS
- Increased HPV & compression pul capillaries
- Increased PVR
- Dilated RV & septal dyskinesia
