ARDS (Acute Respiratory Distress Syndrome)

• ARDS is a syndrome that results from widespread loss of alveolar function due to alveolar consolidation and permeability derived pulmonary oedema causing lung collapse.
• Lung compliance is often reduced, alveolar shunt & dead space increase, while airway resistance remains well preserved.
• ARDS pathophysiology variations depend on lung infiltrate distribution (focal/diffuse) and whether risk factors are pulmonary or extrapulmonary.
• Significant pathology differences can occur between entities.
• Pathology includes pronounced alveolar collapse, fibrinous exudative material, alveolar wall oedema, and increased collagen content.
• ARDS insights have been highlighted by presentations of ARDS induced by vitamin E acetate from vaping constituents and the COVID-19 pandemic.
• Advances in lung imaging and patient biology have improved ARDS definition based on lung imaging criteria.
• Ability to clear alveolar fluid and correlation with outcome varies by subtype (focal/non-focal or diffuse).
• The presence of inflammatory biomarkers (hyperinflammatory phenotype 2 versus phenotype 1) differently associates with treatment choices impacting patient outcome.
• A predominant feature of ARDS can be shunt (bronchopneumonia), increased airways resistance (asthma exacerbation), or reduced elastic recoil (asthma, COPD).
• Pathophysiologic features that predominate in ARDS, like low lung compliance, are less frequently encountered, which could imply ventilatory management considerations.
• Invasive MV can worsen ARDS severity with increasing shear stress in stiff lungs, specifically volutrauma, barotrauma, and biotrauma.
• Lung ventilation that limits tidal volume to 6-8 mls/Kg predicted body weight and plateau pressures under 30 cm H2O boosts patient survival in ARDS.
• The approach to optimizing settings for ventilation parameters, such as oxygen titration (conservative versus liberal), PEEP (lower versus higher) and lung recruitment needs further optimization.
• Individualizing ventilator settings at the bedside may be achieved with strategies that improve ARDS patient outcomes.
• PEEP optimal setting response for oxygenation and/or lung compliance to titration may be helpful.
• The LIVE study highlights the potential for personalized ventilation in ARDS.
• Patients were randomized by ARDS infiltrate (focal versus diffuse) versus a control group receiving a protective ventilation approach.
• While there was no outcome difference, a high rate of misclassification had a benefit in the personalized ventilation arm upon reclassification.
• Lung ventilation input parameters should be matched to the underlying lung physiology instead of using "one size fits all."
• Using dynamic indices (driving pressure) to guide bedside tidal volume titration has been gaining popularity.
• The best way to manage invasive MV in patients with uAHRF needs further study, and high tidal volume ventilation is harmful even in healthy lungs.
• No additional benefit is gained with very low tidal volumes in uAHRF patients, or from ARDSnet lung protective strategy if lung compliance is in the highest quartile.
• Prone positioning of invasively ventilated patients with confirmed moderate to severe ARDS is standard treatment.
• Prone positioning remains underutilized.
• COVID innovations resulted in extending prone positioning to awake patients receiving non-invasive respiratory support for invasive ventilation.
• The role of the prone intervention in non-COVID patients requires study.
• Evidence supporting neuromuscular blockade use among moderate to severe ARDS patients is mixed.
• There are no current data on muscle relaxant uses in uAHRF patients.

PEEP in ARDS

• PEEP is a key ventilatory treatment for ARDS in critically ill patients.
• Ashbaugh and colleagues first formally described PEEP's beneficial effect in reversing hypoxemia in ARDS in 1967.
• Overall, PEEP increases the intrathoracic pressure, variably distributed to the lung and chest walls depending on their respective compliance.
• PEEP prevents of alveolar collapse by counteracting elevated surface tension caused by surfactant impairment.
• PEEP also superimposes pressure due to increased lung weight, as well as chest wall recoil.
• Beneficial effects avoid alveolar over deflation during expiration and reduce intrapulmonary shunt.
• Technically, PEEP is preventative of alveolar derecruitment, not recruitment.
• Increased PEEP is associated to an increased inspiratory pressure.
• Recruitment Maneuvers (RM) may be applied in combination with PEEP titration under the rationale that the pressure required to open an alveolus is higher than the pressure which avoids derecruitment (hysteresis).
• Gas exchange restoration and EELV (end-expiratory lung volume) increases when PEEP is applied.
• The use of PEEP decreases of lung strain and improves respiratory system compliance.
• In turn, reduce the driving pressure for the same tidal volume, which promotes robust association between a lower DP and a higher survival rate.
• PEEP also minimizes ventilator-induced lung injury (VILI).
• PEEP minimizes VILI by reducing atelectrauma (cyclic alveolar opening/closing during tidal ventilation) and lung heterogeneity and stress.
• Harmful PEEP effects occur if set inappropriately on the respiratory and cardiovascular systems.
• High PEEP can increase the right atrial pressure, which decreases gradients to venous return, decreasing preload to the right, then left heart and finally, overall cardiac output.
• PEEP will affect the lung regions with a higher compliance.
• Consolidated areas within ARDS cases will reopen poorly regardless of high PEEP.
• Well-ventilated areas are prone to overdistension.
• PEEP may increase pulmonary vascular resistance, increasing the risk of the right afterload.
• Cor pulmonale occurs with PEEP increasing the right afterload.
• If alveolar exceeds capillary pressure, alveolar dead space increases due to capillary occlusion, causing an reduced carbon dioxide clearance.

Respiratory Mechanics

• Compliance and Driving Pressure of the Respiratory System helps determine PEEP effectiveness.
• Compliance of the respiratory system has been proposed as a target for PEEP titration since 1975.
• Suter and Falke showed PEEP increase was associated with FRC, compliance, and oxygenation.
• Using Cpl,rs as a target to improve PEEP hinges on it reflecting alveolar recruitment, as opposed to overdistension causing compliance loss.
• Compliance mainly relies on the amount of lung recruited volume and the opening pressures of poorly inflated areas.
• Chest wall and abdominal mechanics are assumed to be unaffected by PEEP application.
• Intra-tidal alveolar opening should be considered to calculate compliance.
• PEEP optimization relies of a driving pressure that balances the delivered tidal volume and compliance, where values are proportional to end expiratory lung volume and the baby lung size, linked to mortality.
• Recent analyses show that changes in mortality are based on ventilator changes and PaO2/FiO2 and driving pressure.

Pressure-Volume Curve

• Pressure-Volume (PV) Curve and Lung Volume Measurements can dictate PEEP usage.
• Some authors have suggested using the lower inflection point of the pressure- volume curve as a threshold above which PEEP should be set, minimizing atelectrauma.
• This requires deep sedation and neuromuscular blocking, and uncertainty in identifying an inflection point plus variable opening pressures.
• Alveolar closing pressure is lower than the opening pressure, so the PV loop shows relevant hysteresis and provides a rationale for recruitment prior to increasing PEEP from the expiratory limb, as opposed to use of inspiratory limb.
• The lungs in ARDS collapses at high pressure levels following the gravitational gradient (from dependent to non-dependent regions).
• PEEP titration can also measure gained aerated volume because of PEEP change.
• EELV expands proportionally, and if its increase exceeds that compliance, new lung units are aerated.
• A method by Chen includes a PEEP decrease to measure compliance ratio to calculate the lung recruitability.

Stress index

• The Stress Index (SI) is used when assessing lung mechanics and potential lung injury during mechanical ventilation.
• The ventilator settings can identify causes ventilator-induced lung injury and the shape extracted from the pressure-time during inspiration.
• Flow curves represent inspiratory flow waveforms and volume-controlled ventilation.
• Breaths start with rapid flow increase then become constat dropping off during expiration.
• Airway curve measures pressure over the inspiration period.
• Curve is influenced by the Stress Index (SI): value
• SI < 1 : is a pressure curve concave with slow rising airway pressure.
• The indicates alveolar collapse or insufficient pressure to keep them open with a need for adjustments to avoid under inflation.
• SI = 1 : is a pressure curve linear with a normal rase.
• Optimal compensation is achieved with no collapse.
• This signifies a normal ventilator setting. Safe strategy
• SI > 1 : Is pressure upwards curve due to alvelo overdistention.
• This required ventilator reduction to avoid volutrauma.
• The Sl can be calculated based on the flow.
• the equation indicates a normal lung compliance.
• The > 1 with overal inflation and <1 inadequate pressure
• 1 may require higher pressures or end expiratoy pressures.

• 1 require reduced ventilation.

• =1 for minimum stress.

ARDS vs ALI

• ARDS and ALI are connected conditions which can result to respiratory distress.
• The conditions very significantly in severity and the clinic approaches.
• ARDS can cause a severe long injury with significant inflammation with lack of of inflammation
• ARDS can causes gas exchange and can be caused by the pneumonia sepsis,aspiration an multiple blood transfusion.
• According the Berlin definition the diagnosis includes an acute onset od symptoms with a less then 3000mmhg.
• The ALI represent a less sever form. It is diagnosed the the falls between 200 and 300.
• ALI management stratigies include the underlining reasons of supportive care and the ARDS has more intensive intervention, particulary mechanical
• While ALL mechanical may be necessart with can achieve adquete with the more invasive support and the
• CPAP MAY BE USED.
• High mortity higher with AL typically better out come .
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Refractory Hypoxemia

• Refractory Hypoxemia is when meeting the Berlin definition criteria with the ARDS in convenrtion lung and a high ICU mortality.

• despite these stratigies lung injury may persist that can worsen with and or

• The mortality can be from Multiorgan failer with the amount the partice die to to refractory hypoxmia.

• There is no refractory to to date .

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