Jun 15, 2020
EMRA*Cast host Dr. Alex Kaminsky takes a deep dive on the topic of Acute Respiratory Distress Syndrome (ARDS) with Dr. Sean Hickey and Dr. Evan Leibner.
Broad Strokes: ARDS is diffuse interstitial pulmonary edema secondary to increased pulmonary vasculature permeability. This causes a disruption in gas exchange.
More precise definition: Acute Respiratory Distress Syndrome (ARDS) is an acute diffuse, inflammatory lung injury, leading to increased pulmonary vascular permeability, increased lung weight, and loss of aerated lung tissue with hypoxemia and bilateral radiographic opacities, associated with increased venous admixture, increased physiological dead space and decreased lung compliance.
Etiology of injury pattern must be from a non-primary cardiopulmonary source.
Phases of Histologic Patterns:
Broad Strokes: Histologically in the lungs "diffuse alveolar damage" (DAD)
Initial response to injury:
Exudative phase, immune cell mediated damage to alveoli.
- Protein rich fluid buildup in the interstitium and alveoli.
- Inflammatory cytokines released leading to recruitment of macrophages/T-cells
-Any injury during this stage is worsened by stretch on alveoli by mechanical ventilation
Second phase is proliferative phase
- Beginning of healing process.
- Edema begins to be reabsorbed.
- Alveoli gradually regains integrity and function.
- Fibrotic phase
- Does not occur in all ARDS patients; linked with higher mortality
MOST COMMON CAUSES:
The Berlin Criteria
*Either objective assessment (echo) which excludes hydrostatic edema, or risk factors for ARDS (sepsis, pancreatitis, trauma, pneumonia).
Evaluation of Oxygenation: The P:F Ratio
Can be used as a rough guide to whether there is a significant A-a gradient present.
PaO2 should = FiO2 x 500 (e.g. 0.21 x 500 = 105 mmHg)
Mild ARDS (PF 200-300): Consider BiPAP in patients who are awake, alert, protecting their airway and oxygenating/ventilating.
Majority Will Require Mechanical Ventilation
Key Point: Goal is to optimize gas exchange, but avoid barotrauma, volutrauma, atelectotrauma and biotrauma. This is accomplished by low tidal volumes and dry volume status as tolerated.
ARDSnet and Clinical Considerations
Pro-tip from Evan: Pick your ideal vent mode based on comfort. There are lots of fancy and potentially optimal ventilator modes, such as Bi-level, oscillatory modes, and otherwise. Unfortunately, these are nuanced and can leave to harmful outcomes if doctors and RT’s are not intimately familiar.
2. High vs low PEEP tables have not been shown to affect mortality. Must be careful as high PEEP can reduce cardiac output.
3. To maintain the minute ventilation with LTV you must increase the RR.
However, by increasing the RR too much you can cause auto-PEEP. Thus, permissive hypercapnia can be tolerated unless acidotic (pH <7.15)
Amato MB, Meade MO, Slutsky AS, et al. Driving pressure and survival in the acute respiratory distress syndrome. N Engl J Med. 2015;372(8):747–55.
Wiedemann HP, et al. "Comparison of two fluid-management strategies in acute lung injury". The New England Journal of Medicine. 2006. 354(24):2564-75.
National Heart, Lung, and Blood Institute PETAL Clinical Trials Network. "Early neuromuscular blockade in the acute respiratory distress syndrome". The New England Journal of Medicine. 2019. 380(21):1997-2008.
APRV: Essentially a persistent inspiratory phase by ventilator, with “exhallation” phase where the vent triggers “off.” To allow for passive exhalation. Theory is it keeps alveoli open for longer to participate in recruitment and gas exchange and then gives longs a “break.”
Writing Group for the Alveolar Recruitment for Acute Respiratory Distress Syndrome Trial (ART) Investigators. Effect of Lung Recruitment and Titrated Positive End-Expiratory Pressure (PEEP) vs Low PEEP on Mortality in Patients With Acute Respiratory Distress Syndrome: A Randomized Clinical Trial. JAMA. 2017;318(14):1335–1345. doi:10.1001/jama.2017.14171
Guérin C, Reignier J, Richard JC, et al. Prone positioning in severe acute respiratory distress syndrome. N Engl J Med. 2013;368(23):2159‐2168. doi:10.1056/NEJMoa1214103
Especially Consider if single organ system is affected in otherwise non-terminal patients
Peek GJ, et al. "Efficacy and economic assessment of conventional ventilatory support versus extracorporeal membrane oxygenation for severe adult respiratory failure (CESAR): A multicentre randomised controlled trial.". The Lancet. 2009. 374(9698):1351-1363.