ARDS (1) Flashcards
Whats is ARDS?
This is clinical syndrome precipitated by precipitated by a number of clinical conditions and it causes acute lung inflammation
What are the possible causes of ARDS?
- Pneumonia
- Sepsis
- Trauma
- PE
- Aspiration
- Vasculitis
Histologic appearance of ARDS lungs?
- Diffused alveolar damage
- Hyaline membrane formation
- Oedema
- Cell necrosis or fibrosis
What was the 1994 definition of ARDS?
- Acute hypoxaemia
- PO2 to FiO2 ratio of < 200mmHg regardless of PEEP
- Bilateral infiltrates on CXR
- Pulmonary wedge pressure of < 18mmHg or no clinical signs of cardiogenic pulmonary oedema
Whats the current Berlin definition of ARDS?
Changes to Berlin criteria for ARDS?
- Acute onset ARDs within 7 days of insult
- Mild, moderate & severe ARDS
- Measurement of PO2/FiO2 with PEEP of 5
- Absence of CCF or pulmonary oedema
- B/L opacities of CXR & CT
Classification of severity of ARDS according to PO2/FiO2 ratio?
- Mild 200 - 300 mmHg
- Moderate 100 - 200 mmHg
- Severe < 100 mmHg
Tidal volume setting on the ventilator in ARDS?
6ml/kg (Predicted body weight)
Intervals for checking PO2/FiO2 ratio on a particular ventilator setting?
- 24hrs after ARDS onset
- After FiO2 & PEEP settings
Classify ARDS?
- Pulmonary - Direct lung insult
- Extra-pulmonary ARDS - Indirect insult
What are the causes of pulmonary ARDS?
- Pneumonia
- Aspiration
- Inhalation injury
- Pulmonary contusions
- Vasculitis
- Near drowning
What are the causes of non-pulmonary ARDS?
- Sepsis
- Shock
- Pancreatitis
- Trauma
- TRALI
- Burns
- Drug overdose
What is the percentage of pulmonary ARDS?
It is about 60%
What are the characteristics of ARDS?
- Reduction in lung compliance
- Diffuse alveolar damage
- Pulmonary capillary congestion
Characteristics of ARDS in covid-19 patients?
- Diffused alveolar damage
- Capillary congestion
- Micro-thrombi formation
What is diffused alveolar damage?
- Presence of hyaline membranes
- Interstitial oedema
- Cell necrosis & proliferation
- Fibrosis
Phases of histological changes in ARDS?
- Exudative
- Proliferative
- Fibrosis
Characteristics of of the exudative phase in ARDS?
- Capillary congestion
- Intra-alveolar oedema
Characteristics of of the proliferative phase in ARDS?
- Proliferation of alveolar type 2 cells
- Fibroblasts
- Resolution or disease progression leading to fibrosis
Incidence of diffused alveolar damage?
Increases with severity of disease - Mild, moderate and severe
How can the onset and evolution of DAD be mitigated?
- Protective lung ventilation - Low TV
What is the baby lung in ARDS?
- Large amount of non-aerated lung
- Reduced volume of aerated lung
- Reduced lung compliance
What are the components of end-expiratory lung volume induced by PEEP ?
- Recruited volume - New lung regions
- Hyperinflated volume - Hyperinflating already open lungs
What is the recruitment to inflation ratio?
Ratio equal or higher than 0.5 corresponds to high lung recruitability
What is the characteristic of baby lung?
- Small lung
- Normal elasticity
Intrapulmonary causes of hypoxaemia in ARDS?
- Shunt
- Dead space
- Impaired gas diffusion
Mechanism contributing to hypoxaemia in ARDS can be divided into ?
- Epithelial damage & increased shunt
- Endothelial damage & increased deadspace
- Interstitial damage & impaired gas diffusion
Characteristics of epithelial damage in ARDS causing shunt?
- Primary cause of hypoxaemia
- Surfactant deficiency & loss of volume
- Alveolar oedema
- Lung collapse - Intrapulmonary shunt
- Alteration in V/Q ratio
Characteristics of endothelial damage in ARDS causing increased dead-space?
- Increased physiological dead-space
- Abnormal blood flow in ventilated lungs
- Large dead-space is ass. with high mortality
How is ventilation ratio calculated to achieve dead-space ventilation?
Ventilation ratio = actual MV x PaCO2 / Expected MV x PaCO2
OR
Ventilation ratio = MV x PaCO2 (mmHg) / PBW x 100 x 37.5 (mmHg)
The ventilation ratio is limited by the fact that ?
It implies a constant CO2 production
Characteristics of interstitial damage and impaired gas diffusion ?
The is impaired gas diffusion due to below;
- Interstitial oedema
- Hyaline membrane
- Fibrosis & thickening of alveolar-capillary membrane
Aetiology of lung oedema in ARDS?
- Epithelial - Primary/Direct lung injury
- Endothelial - Secondary/Indirect injury
Extrapulmonary causes of hypoxaemia in ARDS?
- Haemodynamics
- Low mixed venous oxygen due to low CO
- Low cardiac output
- Pulmonary HTN & Positive pressure can open PFO with intra-cardiac shunt
Shunting in ARDS?
- This is usually right to left with incidence of 20%
- Poor oxygenation response to increased PEEP
Hypoxaemia in ARDS is predominantly caused by ?
- Increased volumes of non-aerated perfused lungs
Types of ARDS in covid patients?
- Typical
- Atypical
What is the major cause of death in patients with ARDS who have covid 19?
Acute hypoxaemic respiratory failure
What are the characteristics of Covid ARDS?
- Severe hypoxaemia despite preserved lung compliance
- Capillary congestion
- Micro-thrombosis
- Angio-genesis (new blood vessels)
- High dead-space
What are the components of ventilator induced lung injury ?
- Volutrauma
- Barotrauma
- Atelectrauma
- Biotrauma
What are the characteristics of volutrauma ?
- Increased end-inspiratory lung volumes
- ## Lung strain - Promotes volutrauma
Define strain in ventilator induced lung injury?
This is the ratio of TV to end-expiratory lung volume due to high TV or PEEP which brings the end-expiratory lung volume closer to total lung capacity
What is lung strain?
This is changes in lung volumes due to PEEP & TV
What is lung stress?
This is the distribution of forces caused by strain
Risk factors for barotrauma?
- High lung volumes
- Over-distention due to hyper-inflation
Components of ultraprotective lung ventilation?
- TV < 6ml/kg PBW (Up to 4ml/kg PBW)
- Extra-corporeal CO2 remoaval
what are the characteristics of atelectrauma?
- Repeat opening & closure of small airway
- The above with low TV
- Enhances lung inflammation
- Increased local trans-pulmonary pressures
Atelectrauma lung injury is amplified by ?
- Surfactant deficiency
- Alveolar instability
- Increased permeability & oedema
- Inflammation & structural damage
- Accumulation of fibrin & fibrosis
What are the characteristics of biotrauma?
- Release of inflammatory mediators from injured cells
Impact of high FiO2 on injured cells ?
Cellular toxicity
How is driving pressures calculated ?
Plateau pressure - PEEP
What is a major determinant of VILI?
- Driving pressures
- Mechanical power
What is mechanical power?
This is metric to quantify the risk of VILI which reflects the energy delivered by the ventilator to the respiratory system per unit time expressed in J/min
How is mechanical power (MP) calculated?
MP = 0.098 x RR x VT x (PEEP + 0.5 x D. pressure + [Ppeak - Pplateau])
What is the new model for mechanical power?
MP = (4 x D. Pressure ) + RR
Targets for lung protective ventilation?
- Low VT
- High PEEP
- Pplateau < 30cmH2O
- Driving pressures < 15 cmH2O
What are the ventilatory strategies in focal ARDS?
- VT of 8ml/kg PBW
- Low PEEP
- Prone ventilation
What are the ventilatory strategies in non-focal ARDS?
- VT of 6ml/kg PBW
- Recruitment maneuver
- High PEEP
Phenotypes of covid 19 ?
- Type (L)
- Type (H)
What are the types of ARDS?
- Focal ARDS
- Non-focal ARDS
Characteristics of type L phenotype of covid 19?
- Low elastance (High compliance)
- Low V/Q ratio
- Low lung weight
- Low recruitability
Characteristics of type H phenotype of covid 19?
- High elastance (Low compliance)
- High right to left shunt
- High lung weight
- High recruitability
Ventilatory strategies for type L phenotype of covid 19?
- High VT (8-9ml/kg) PBW
- Low PEEP
Ventilatory strategies for type H phenotype of covid 19?
- Low VT
- High PEEP
- Prone
What are the benefits of hypercapneic acidosis in ARDS patients ?
- Attenuation of pulmonary inflammation
- Attenuation of oxidative stress
What is the dangerous level of PO2 in critically unwell patients?
> 120mmHg
What is the ideal oxygenation target in critically unwell patients?
- Sats > 90%
- PO2 of 7-8
How is respiratory compliance calculated ?
Tidal volume / Driving pressures = x ml/cmH2O
Assessment of chest wall compliance and its effect on plateau pressures?
Measurement of oesophageal pressures
Comparison between chest and lung compliance including their effects ?
Chest wall compliance higher than lung compliance.
Impact of chest wall compliance is negligible on plateau & driving pressures
Alteration in chest wall compliance is important in patients with intra-abdominal pathology or obese patients causing ARDS
What are the predictors of NIV failure ?
- VT of > 9ml/kg PBW
- Tidal swings of oesophageal pressures
- High inspiratory efforts
- After 1 hour of therapy
What is the ROX index?
- This predicts therapy failure in patients on HFNC & NIV
- This is the ratio between SO2/FiO2 & RR indicative of high risk for intubation
Threshold for ROX index in patients on HFNC?
Patients with ROX index of > 4.88 after 2, 6 & 12 hrs of HFNC are less likely to be intubated
What is the HACOR scale?
Used to predict failure during face mask NIV and a high score predicts high risks for failure
What are the components on the HACOR scale?
- HR
- Acidosis
- GCS
- Oxygenation
- RR
What is the threshold for HACOR score?
A score >5 after 1 hour of NIV has diagnostic accuracy of 85% for NIV failure
What are the commonly used composite scores used in predicting therapy failure for HFNC & NIV?
- HFNC = ROX
- NIV = HACOR
What is the main advantage of VCV compare to PCV in terms of respiratory mechanics?
The possibility of separating resistive (peak pressure minus plateau pressure) and elastic (plateau pressure ) components of total (peak) pressure by applying an end-inspiratory occlusion.
When using PCV, what issues can be identified when there is a drop in VT?
- Reduced compliance: Lung collapse, atelectases)
- Increased resistance: Tracheal secretions, PTX
When adjusting ventilatory settings, what is the best approach to use when setting VT?
- Using driving pressures
- Driving pressures indirectly reflect lung strain
How is lung strain calculated?
VT/compliance
What is the predictive value for driving pressure associated with high mortality ?
Pressures > 15 cmH2O
The response of PEEP may differ depending on the underlying pathology ?
- Pulmonary or Extra-pulmonary
- Timing (early or late ARDS)
- Localization of infiltrates ( diffuse or lobar)
- Potential for recruitment - Sub phenotype
What is the importance of sedation & NM blockade?
- Resting of respiratory muscles
- Decrease of oxygen consumption by muscle
- Cisatracurium is the NMB of choice
What is the comparison between the anterior, posterior and apical parts of the lungs?
- Posterior lung has larger volume
- Anterior & apical smaller
What is the benefit of prone ventilation?
- Better ventilation of dorsum of lung
- Improved ventilation
- Reduction in intrapulmonary shunt
- Reduction in VILI
- Haemodynamic stability
- Reduction in afterload
- Lower pulmonary vascular resistance
- Increased in CO - Preload dependent pt
What is the threshold for prone ventilation in relation to PO2/FiO2 ratio?
100 - 150 mmHg
What is the minimum duration of prone ventilation?
16 hours
When to discontinue prone ventilation?
When PO2/FiO2 ratio is > 150mmHg and PEEP of < 10 cmH2O & FiO2 < 60% in the supine position
What are the complications of prone ventilation?
- Accidental extubation
- Vascular access issues
- ## Pressure injury
What is the major contra-indication to prone ventilation?
- Unstable spine
What is a recruitment manoeuvre ?
This is the transient increase in transpulmoary pressures in order to open collapsed alveoli
Methods of performing recruitment manoeuvres ?
- Sigh breaths
- Extended sigh breaths
- Increased inspiratory pressures
- Increased PEEP
- Sustained inflation
- Staircase recruitment
What are the major indications for ECMO?
- FiO2 > 90%
- PO2/FiO2 ratio < 80 mmHg
- CO2 retention > 80mmHg
- Peakp > 30 cmH2O
What is the basic mechanism of VV-ECMO?
- Double lumen catheter - IVC to oxygenator
- Oxygenated blood returned to RA
- About 400 ml O2/min can be transfused
- Blood flow usually 6L / min
Extra-coporeal CO2 clearance ?
- Rate of 150-200ml/min
- Blood flow 1000-2000ml/min
- Ultra-protective lung ventilation
- Reduction of cytokine production
What is high frequency oscillatory ventilation?
- Constant mean airway pressure
- Oscillating pressure variation at high rates
- Rates of up to 900 cycles/min
- Increases end-expiratory lung volumes - Preventing de-recruitment or overdistention
- Not recommended in ARDS patients
What are the diseases which mimic ARDS where steroids are indicated ?
- Vasculitis (Pulmonary haemorrhage
- Drug induced pneumonitis
- Organized pneumonia
- Acute eosinophilic pneumonia
Mortality and the use of steroids in ARDS?
Mortality was higher when used 2 weeks after the onset of pneumonia
Mortality lower in patients at the proliferative stage
Effective in reversing the inflammatory stage but irreversible once fibrosis occurs
Steroids are beneficial at lower doses but at higher doses are associated with higher mortality
Time of onset to formation of fibrosis in ARDS?
Usually 7 days
Superior steroids in the management of ARDS?
Dexamethasone tends to be better than methylprednisolone
What is methylprednisolone?
- Synthetic glucocorticoid/corticosteroids
- Anti-inflammatory & immunosuppressive effects
What are the indications for methylprednisolone?
- TENS
- SJS
- Cancer associated hypercalcaemia
What is the primary mineralocorticoid?
- Aldosterone
- Production: Zona glomerulosa /cortex /adrenal
- Salt & water retention
- Potassium & Hydrogen ion secretion
- Action in the collecting duct
- Principal cells - Secretion of potassium
- Hydrogen ion secretion - lumenal membrane of intercalated cells
- Increases blood pressure & volume
Stimulants for the secretion of aldosterone?
- Angiotensin II (Principally)
- Adrenocorticotrophic hormone
- Local potassium levels
Mode of action of mineralocorticoids?
- Nuclear receptors - Cytosol
Synthetic mineralocorticoid includes?
Fludrocortisone
What is Addison’s disease?
- Salt-wasting syndrome
- Hypoaldosteronism
Mineralocorticoid antagonism includes?
- Spironoloactone
- Eplerenone
What are the components of peak airway pressure?
- Resistive
- Elastic
What is peak airway pressure dependent on?
Inspiratory flow rate
How is resistive peak airway pressure calculated?
Peak pressure minus plateau pressure
How is elastic peak airway pressure calculated?
Plateau pressure
Measurement of plateau pressure?
- Determined at zero flows
- Manual inspiratory hold
What is the driving pressure?
The difference between PEEP and plateau pressure
How is physiological dead space calculated?
Alveoloar + Apparatus + Anatomical dead space
Measurement of end-expiratory lung volume?
Gas dilution method - Nitrogen and Helium used
ARDS secondary to trauma?
Patients have better outcome compared to other causes of ARDS
What is the main cause of death in patients with ARDS
- Complicated sepsis
- MOF
- Refractory hypoxaemia
When will fibrosis be noted in ARDS?
As early as the second week
