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ARDS - A Comprehensive Review for MD Anesthesia

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1. Definition and Historical Background

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2. Berlin Definition (2012) - Diagnostic Criteria

Berlin Severity Classification

Key exam point: Mild ARDS replaced the old "ALI" (PaO2/FiO2 <300). Overall mortality ranges from 35% (mild) to 46% (severe).

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3. Etiology / Risk Factors

Direct (Pulmonary) Causes

• Pneumonia (most common direct cause)
• Aspiration of gastric contents
• Pulmonary contusion
• Inhalation injury
• Near drowning

Indirect (Extrapulmonary) Causes

• Sepsis (most common overall cause)
• Major trauma / multiple transfusions (TRALI)
• Acute pancreatitis
• Burns
• Drug overdose (amiodarone, chemotherapy agents, radiation)
• Cardiopulmonary bypass

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4. Pathophysiology

Phase 1 - Exudative Phase (Days 1-7)

• Initial V/Q mismatch followed by progressive shunt
• Inflammatory cytokines (TNF-α, IL-1, IL-6, IL-8) recruit neutrophils to alveolar-capillary interface
• Neutrophil-mediated release of proteases, reactive oxygen species, and platelet-activating factor damages type I pneumocytes and capillary endothelium
• Loss of alveolar-capillary barrier integrity → protein-rich edema floods alveolar spaces
• Diffuse Alveolar Damage (DAD) - pathologic hallmark with hyaline membrane formation
• Type II pneumocyte dysfunction: reduced surfactant production → alveolar collapse, reduced compliance
• Gross lung: heavy, red, consolidated (hepatization)

Phase 2 - Proliferative / Fibroproliferative Phase (Days 7-21)

• Type II pneumocyte proliferation (repair attempt)
• Fibroblast activation → early fibrosis
• Some patients recover; others progress

Phase 3 - Fibrotic Phase (>21 days)

• Extensive collagen deposition, obliteration of alveolar architecture
• Severe loss of compliance, pulmonary hypertension
• Increased dead space and inability to wean from ventilator

Key Physiological Consequences

• Hypoxemia: primarily from intrapulmonary shunt (V/Q = 0 units)
• Low compliance (stiff lungs) → high plateau pressures at normal tidal volumes
• Increased dead space ventilation (V/Q = ∞ units) → hypercapnia in severe disease
• Pulmonary hypertension → RV strain

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5. Radiological Findings

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6. Management: The Stepwise Approach

A. Treat the Underlying Cause

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B. Lung-Protective Ventilation (LPV) - ARDSNet Strategy

• Males: 50 + 2.3 × (height in inches - 60)
• Females: 45.5 + 2.3 × (height in inches - 60)

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C. PEEP Titration

• Overdistension of aerated lung → volutrauma/barotrauma
• Impaired venous return → reduced cardiac output

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D. Adjunct Therapies for Moderate-Severe ARDS (PaO2/FiO2 <150)

1. Prone Positioning

• Improves V/Q matching by recruiting dependent (dorsal) lung regions, reduces dorsoventral transpulmonary pressure gradient
• PROSEVA trial (2013): Prone positioning for ≥16 hr/day in severe ARDS (PaO2/FiO2 <150) reduced 28-day mortality from 32.8% to 16.0%
• Initiate within 36 hours of moderate-severe ARDS
• Continue for ≥16 hr/day until PaO2/FiO2 >150 on PEEP ≤10, FiO2 ≤0.6
• Murray & Nadel's, Current Surgical Therapy 14e

2. Neuromuscular Blockade (NMB)

• Cisatracurium infusion for 48 hours in moderate-severe ARDS (ACURASYS trial, 2010) improved 90-day survival and reduced pneumothorax
• Proposed mechanisms: improved ventilator synchrony, reduced oxygen consumption, decreased lung and systemic inflammation
• ROSE trial (2019) did NOT confirm mortality benefit with routine NMB
• Current practice: NMB reserved for refractory hypoxemia or ventilator dyssynchrony

3. Inhaled Pulmonary Vasodilators

• Inhaled NO (iNO) or inhaled prostacyclin (iloprost): Improve V/Q matching by selective pulmonary vasodilation in ventilated regions
• Improve oxygenation transiently but no mortality benefit demonstrated
• Use as bridge therapy or for RV failure

4. Recruitment Maneuvers (RM)

• Brief application of high airway pressure (e.g., 40 cm H2O for 40 seconds) to open collapsed alveoli
• ART trial (2017) showed increased mortality with high-pressure RM + high PEEP strategy - now generally not recommended routinely

5. Conservative Fluid Strategy

• ARDS Network Fluid and Catheter Treatment Trial (FACTT): Conservative fluid management reduced duration of mechanical ventilation and ICU stay without worsening kidney function in patients not in shock
• Goal: lowest intravascular pressure that maintains adequate perfusion; use diuresis to achieve fluid balance

6. Corticosteroids

• High-dose steroids - NOT beneficial (5 large RCTs failed to show mortality reduction; may increase harm)
• Low-dose methylprednisolone (1 mg/kg/day) in late ARDS (fibroproliferative phase): Some benefit in improving oxygenation but inconclusive mortality data
• NOT routinely recommended based on current evidence - Murray & Nadel's Textbook of Respiratory Medicine

7. High-Frequency Oscillatory Ventilation (HFOV)

• No longer recommended - OSCAR and OSCILLATE trials showed no benefit; OSCILLATE showed increased mortality with HFOV
• Abandoned as rescue therapy

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E. Rescue Therapies for Refractory ARDS

Veno-Venous ECMO (VV-ECMO)

• CESAR trial (2009): Transfer to ECMO center improved survival (63% vs. 47%) in severe ARDS
• EOLIA trial (2018): 60-day mortality 35% (ECMO) vs. 46% (control) - p=0.07; significant crossover from control to ECMO
• 2026 network meta-analysis (PMID 41781628): VV-ECMO + LPV remains the most effective rescue strategy for long-term mortality
• Indications (Gattinoni criteria): PaO2/FiO2 <80 despite optimal ventilation, severe hypercapnia (pH <7.15), or air leak syndrome
• Target "ultra-protective" ventilation on ECMO: Vt 3-4 mL/kg, Pplat ≤20 cm H2O, RR 4-10

Extracorporeal CO2 Removal (ECCO2R)

• Lower-flow system to correct hypercapnia, allow ultra-low Vt
• Under investigation (SUPERNOVA trial data)

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7. Anesthesia-Specific Considerations

Intraoperative Ventilation in ARDS Patients

• Continue LPV strategy in the OR (Vt 6 mL/kg PBW, Pplat ≤30, PEEP titrated)
• Monitor driving pressure intraoperatively - correlates with postoperative pulmonary complications
• Avoid disconnection from PEEP (causes alveolar derecruitment)
• Prone-position surgery requires multidisciplinary coordination, careful padding, and airway security

Sedation in Mechanically Ventilated ARDS

• A 2026 meta-analysis (PMID 41380284) compared inhaled vs. IV sedation agents
• Propofol or midazolam infusions are standard; dexmedetomidine for lighter sedation and early waking
• ABCDEF bundle (Assess, Breathe, Choose, Delirium, Early mobility, Family) improves outcomes
• Target lightest effective sedation (RASS -1 to -2 for most patients; deeper for prone/NMB)

One-Lung Ventilation (OLV) and ARDS Risk

• OLV creates de facto intrapulmonary shunt - ARDS patients poorly tolerate this
• If ARDS present, OLV has high risk; use lowest FiO2 tolerated, CPAP to non-dependent lung
• Intraoperative acute lung injury: use Vt 4-6 mL/kg, PEEP 5-10 cm H2O, avoid Pplat >30

ARDS After Cardiac Surgery

• Cardiopulmonary bypass activates complement, cytokines - 0.5-2% incidence of post-bypass ARDS
• Management identical to standard ARDS protocol
• TRALI from FFP/platelets: clinical presentation identical to ARDS; supportive management

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8. Key Landmark Trials Summary

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9. Monitoring in ARDS

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10. Differences: ARDS vs. Cardiogenic Pulmonary Edema

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11. Outcomes and Post-ARDS Sequelae

• Overall ICU mortality: 27-46% based on severity
• Survivors have significant long-term morbidity: pulmonary fibrosis, muscle weakness, PTSD, cognitive impairment, depression
• "Post-intensive care syndrome (PICS)" - physical, cognitive, and psychological impairment
• Return to work may take months to years; exercise tolerance may remain permanently reduced

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Recent Evidence Updates (2025-2026)

• Driving pressure-limited vs. conventional LPV (PMID 41345693): Systematic review confirms driving pressure-limited strategies are non-inferior and may reduce harm compared to conventional LPV.
• Network meta-analysis of adjuncts + LPV for long-term mortality (PMID 41781628): VV-ECMO combined with LPV had the greatest effect on long-term mortality.
• Inhaled vs. IV sedation in ARDS (PMID 41380284): Inhaled agents (isoflurane, sevoflurane) may offer bronchodilator benefit; evidence still evolving.

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Goal-Directed Fluid Therapy (GDFT) - A Comprehensive Review for MD Anesthesia

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1. Concept and Historical Background

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2. The Problem with Traditional Fluid Management

Too Little (Hypovolemia):

• Decreased cardiac output → tissue hypoperfusion
• Acute kidney injury, bowel ischemia, anastomotic leak
• Increased risk of SSI, multi-organ dysfunction

Too Much (Fluid Overload):

• Interstitial edema → impaired tissue oxygen delivery (despite increased intravascular volume)
• Anastomotic leakage, pulmonary edema, pneumonia
• Wound infection, postoperative ileus, prolonged hospitalization
• Excess fluids increase body weight by 3-6 kg, impairing early mobilization
• Dilutional coagulopathy (especially with saline-based fluids)

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3. Definition of GDFT

1. Detecting fluid responsiveness using dynamic hemodynamic variables
2. Administering fluid only when the patient is on the steep part of the Frank-Starling curve (i.e., when a bolus will meaningfully increase SV/CO)
3. Avoiding both under- and over-resuscitation

• Heart rate, MAP
• Stroke Volume (SV), Stroke Volume Index (SVI)
• Cardiac Output (CO), Cardiac Index (CI)
• Pulse Pressure Variation (PPV)
• Stroke Volume Variation (SVV)
• Plethysmographic Variability Index (PVI)

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4. Physiological Basis: Frank-Starling Mechanism and Fluid Responsiveness

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5. Static vs. Dynamic Predictors of Fluid Responsiveness

Static Indicators (Unreliable - Now Largely Abandoned)

CVP and PCWP are no longer recommended as sole guides for fluid resuscitation. - Barash Clinical Anesthesia, 9e

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Dynamic Indicators (Validated and Preferred)

• Inspiration → ↑ intrathoracic pressure → ↓ RV preload → ↓ RV stroke volume
• After 2-3 beats → ↓ LV filling (expiration phase) → ↓ LV stroke volume
• The cyclic variation in SV with the respiratory cycle indicates preload dependence

A. Pulse Pressure Variation (PPV)

• PPV >13% is predictive of fluid responsiveness (sensitivity ~72%, specificity ~89%)
• As fluid is administered, PPV decreases (patient moves to flat part of Starling curve)

B. Stroke Volume Variation (SVV)

• SVV >10-13% indicates fluid responsiveness
• More direct than PPV as it measures actual SV change
• Requires continuous CO monitoring device (e.g., FloTrac/Vigileo, LiDCO, PiCCO)

C. Systolic Pressure Variation (SPV)

• SPV >10 mmHg suggests fluid responsiveness
• Delta-down component (fall in SBP below apneic baseline) is the most preload-sensitive component
• Historically the first dynamic index used

D. Inferior Vena Cava (IVC) Distensibility Index

• dIVC >18% (mechanically ventilated) predicts fluid responsiveness
• Measured via ultrasound (subcostal or TTE view)
• Collapsibility index used in spontaneously breathing patients (>50% collapse = responsive)

E. Plethysmographic Variability Index (PVI)

• Non-invasive photoplethysmography-based (pulse oximeter waveform)
• PVI >14% predicts fluid responsiveness
• A 2025 meta-analysis (PMID 40778974) confirms PVI-guided GDFT reduces intraoperative fluid use and postoperative complications

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Limitations of Dynamic Indices

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6. Tests for Fluid Responsiveness

A. Fluid Challenge (Mini-Fluid Challenge)

• Administer 100-250 mL IV fluid over 1-2 minutes
• Measure CO/SV before and after
• ≥10-15% increase in CO/SV = fluid responsive
• Limitations: irreversible if patient develops edema

B. Passive Leg Raise (PLR) Test

• Raise legs to 45° from supine position (transfers ~250-500 mL from lower limbs as autotransfusion)
• Simultaneously measure CO by continuous CO monitor or echocardiography
• ≥10% increase in CO = fluid responsive
• Completely reversible (legs lowered, fluid returns to venous reservoir)
• Valid in spontaneously breathing patients, arrhythmias, low tidal volume ventilation
• The most versatile and reversible fluid responsiveness test available

C. End-Expiratory Occlusion Test (EEOT)

• Occlude expiratory circuit for 15 seconds during mechanical ventilation
• Prevents cyclic decrease in preload; effective "auto-fluid challenge"
• ≥5% increase in pulse pressure or CO = fluid responsive
• Valid even during arrhythmias and spontaneous breathing

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7. Monitoring Devices for GDFT

Invasive Devices

Minimally Invasive Devices

Non-Invasive Devices

Esophageal Doppler - Key Exam Points

• FTc (corrected flow time): Normal = 330-360 ms
  • FTc <330 ms → preload deficiency → give fluid
  • FTc >360 ms → vasoplegia → consider vasopressor
• Peak velocity (PV): Reflects contractility; reduced PV = poor contractility → consider inotrope
• Validated in major abdominal, orthopedic, and cardiac surgery; reduces complications and length of stay

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8. GDFT Endpoints / Goals

Oxygen Delivery Optimization (Shoemaker Protocol - Historical)

• DO2 >600 mL/min/m²
• VO2 >170 mL/min/m²
• CI >4.5 L/min/m² These "supranormal" targets were associated with reduced mortality in Shoemaker's original work but not consistently reproduced.

Current Standard GDFT Goals (Perioperative)

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9. GDFT Algorithm (Stepwise Perioperative Approach)

START: Intraoperative patient (major surgery, high-risk)
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         v
ASSESS fluid responsiveness (SVV/PPV/PLR/FTc)
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    SVV >13% or FTc <330ms?
   YES                    NO
    |                      |
Give fluid bolus         Assess MAP
(250 mL colloid         MAP <65 mmHg?
 or 500 mL crystalloid)   |        |
         |               YES       NO
         |                |        |
    Reassess SVV       Vasopressor  Reassess
         |             (norepinephrine)  CO/SVI
    Still responsive?     |
   YES → repeat        MAP ≥65 mmHg
   NO → stop fluids        |
                      CI <2.5 L/min/m²?
                         |         |
                        YES        NO
                         |         |
                    Inotrope    Optimize analgesia
                  (dobutamine)  Temp, Hgb, etc.

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10. Fluid Types in GDFT

• Morgan & Mikhail's Clinical Anesthesiology, 7e

• Colloids: Albumin 4-5%, gelatin (Gelofusine), starch (HES - caution in sepsis/renal impairment)
• HES 130/0.4: Avoided in septic patients (increased AKI, need for RRT - CHEST trial, 6S trial)
• Albumin is the preferred colloid in sepsis and ARDS

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11. GDFT in Enhanced Recovery After Surgery (ERAS)

• GDFT is a core component of ERAS protocols for major abdominal, colorectal, esophageal, and orthopedic surgery
• In the ERAS context, the target is "zero balance" fluid strategy: 3-5 mL/kg/h crystalloid baseline + blood loss replacement 1:1.5 crystalloid ratio
• GDFT is most beneficial when blood loss is expected >1000 mL or in high-risk patients (ASA III-IV, major vascular/abdominal surgery)
• In laparoscopic/robotic surgery: benefit of GDFT is less clear (less blood loss, less fluid shift)
• Current Surgical Therapy 14e

• Pearse et al. (OPTIMISE trial, 2014): Doppler-guided GDFT with dopexamine reduced complications in high-risk surgical patients
• 2025 meta-analysis in oncologic surgery (PMID 40305698): GDFT reduced postoperative complications and hospital LOS in cancer surgeries
• 2024 meta-analysis in elderly (PMID 40994091): GDFT reduced major complications and ICU admissions in elderly surgical patients

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12. High-Risk Surgical Patients - Indications for GDFT

• ASA ≥ III with major surgery
• Expected blood loss >1000 mL
• Major abdominal, vascular, thoracic, or orthopedic surgery
• Esophagectomy (particularly sensitive to fluid excess and deficit)
• Cardiac surgery with bypass
• Septic patients undergoing emergency surgery
• Patients with pre-existing cardiac or renal dysfunction

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13. Limitations and Controversies

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14. Comparison of Fluid Strategies

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15. Key Landmark Trials in GDFT

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16. Quick Summary for Viva/MCQ

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