7.1 Acute Respiratory Distress Syndrome (ARDS) & Lung-Protective Ventilation

Acute Respiratory Distress Syndrome (ARDS) & Lung-Protective Ventilation

Acute Respiratory Distress Syndrome (ARDS) represents a severe form of acute diffuse lung injury characterized by inflammation, increased pulmonary vascular permeability, and widespread alveolar damage. In critical care transport, managing patients with ARDS requires a deep understanding of pulmonary mechanics, the Berlin Definition, and strict adherence to lung-protective ventilation protocols to minimize ventilator-induced lung injury (VILI).

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Pathophysiology of ARDS

ARDS is triggered by either direct or indirect pulmonary insults that initiate an intense systemic inflammatory cascade:

• Direct Insults: Aspiration of gastric contents, severe pneumonia (viral, bacterial, fungal), near-drowning, pulmonary contusion, inhalation of toxic gases, or fat embolisms.
• Indirect Insults: Severe sepsis or septic shock, non-thoracic trauma with shock, acute pancreatitis, massive blood transfusion (Transfusion-Related Acute Lung Injury - TRALI), or cardiopulmonary bypass.

The Three Pathological Phases

1. Exudative Phase (Days 1–7):
   • Release of pro-inflammatory cytokines (IL-1, IL-6, TNF-alpha) causes neutrophilic infiltration into pulmonary capillaries.
   • Endothelial and alveolar epithelial damage disrupts the alveolar-capillary membrane.
   • Protein-rich fluid, neutrophils, and red blood cells flood the alveolar spaces, degrading pulmonary surfactant.
   • Microvascular thrombosis and loss of surfactant cause widespread alveolar collapse (atelectasis), severe intrapulmonary shunting ($V/Q = 0$), and marked compliance loss.
   • Formation of microscopic hyaline membranes lining alveoli.
2. Proliferative Phase (Days 7–21):
   • Proliferation of Type II pneumocytes, cellular repair attempts, and myofibroblast infiltration.
3. Fibrotic Phase (Day 21+):
   • Extensive remodeling, collagen deposition, pulmonary fibrosis, and persistent pulmonary hypertension.

Inflammatory Insult (Direct or Indirect)
       │
       ▼
Neutrophil Activation & Cytokine Cascade
       │
       ▼
Alveolar-Capillary Membrane Disruption
       │
       ├───────────────────────────────┐
       ▼                               ▼
Surfactant Inactivation         Proteinaceous Edema
       │                               │
       ▼                               ▼
Alveolar Collapse (Atelectasis)  Hyaline Membrane Formation
       │                               │
       └───────────────┬───────────────┘
                       ▼
      Refractory Hypoxemia (V/Q Shunt)

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Diagnostic Criteria: The Berlin Definition

Established in 2012, the Berlin Definition standardizes the clinical criteria required to diagnose ARDS and classify its severity:

Severity Classification by Oxygenation Index

The $PaO_2 / FiO_2$ ($P/F$) ratio is calculated by dividing the arterial partial pressure of oxygen ($PaO_2$ in mmHg) by the fractional concentration of inspired oxygen ($FiO_2$ as a decimal).

$\text{P/F Ratio} = \frac{PaO_2 (\text{mmHg})}{FiO_2 (\text{decimal})}$

• Mild ARDS: $200 \text{ mmHg} < P/F \le 300 \text{ mmHg}$ (with $PEEP \ge 5 \text{ cmH}_2\text{O}$)
• Moderate ARDS: $100 \text{ mmHg} < P/F \le 200 \text{ mmHg}$ (with $PEEP \ge 5 \text{ cmH}_2\text{O}$)
• Severe ARDS: $P/F \le 100 \text{ mmHg}$ (with $PEEP \ge 5 \text{ cmH}_2\text{O}$)

Clinical Calculation Example: A patient on a mechanical ventilator with $FiO_2 = 0.80$ (80%) and $PEEP = 10 \text{ cmH}_2\text{O}$ has an ABG showing $PaO_2 = 64 \text{ mmHg}$. $\text{P/F Ratio} = \frac{64}{0.80} = 80 \text{ mmHg}$ Because the $P/F$ ratio is $< 100 \text{ mmHg}$ on PEEP $\ge 5$, this patient meets criteria for Severe ARDS.

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ARDSNet Protocol & Lung-Protective Ventilation

Historical mechanical ventilation strategies utilized large tidal volumes ($10-15 \text{ mL/kg}$), leading to high mortality rates due to Ventilator-Induced Lung Injury (VILI). The landmark ARDSNet ARMA trial demonstrated a dramatic reduction in mortality when using low tidal volume ventilation.

Mechanisms of Ventilator-Induced Lung Injury (VILI)

1. Volutrauma: Over-distension of alveoli from excessive tidal volumes.
2. Barotrauma: Alveolar rupture and gas leak (pneumothorax, pneumomediastinum) from excessive peak/plateau pressures.
3. Atelectotrauma: Repeated shear stress caused by the cyclic opening and collapse of unstable alveoli during inspiration and expiration.
4. Biotrauma: Release of systemic inflammatory mediators from mechanical trauma to lung tissue, driving multi-organ dysfunction syndrome (MODS).

                  ┌─────────────────────────────────────────┐
                  │ Mechanisms of VILI in Heterogeneous ARDS│
                  └────────────────────┬────────────────────┘
                                       │
         ┌─────────────────────────────┼─────────────────────────────┐
         ▼                             ▼                             ▼
    Volutrauma                    Barotrauma                  Atelectotrauma
(Alveolar Overdistension)    (High Transpulmonary P)       (Cyclic Recruitment)
         │                             │                             │
         └─────────────────────────────┼─────────────────────────────┘
                                       ▼
                                   Biotrauma
                       (Systemic Cytokine Release / MODS)

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Step-by-Step ARDSNet Ventilator Management

Step 1: Calculate Ideal Body Weight (IBW)

Tidal volumes must NEVER be calculated based on actual body weight. Alveolar volume is determined by height and biological sex, not adipose tissue.

• Male IBW (kg) = $50 + 0.91 \times [\text{Height (cm)} - 152.4]$ (or $50 + 2.3 \times [\text{Height (inches)} - 60]$)
• Female IBW (kg) = $45.5 + 0.91 \times [\text{Height (cm)} - 152.4]$ (or $45.5 + 2.3 \times [\text{Height (inches)} - 60]$)

Step 2: Set Initial Ventilator Parameters

• Mode: Volume Assist/Control (AC/VC) or Pressure Assist/Control (AC/PC).
• Initial Tidal Volume ($V_T$): Set to $6 \text{ mL/kg}$ IBW.
• Respiratory Rate (RR): Adjust to match minute ventilation needs (typically $18-24 \text{ breaths/min}$, up to a max of $35 \text{ breaths/min}$).
• Oxygenation Goal: $PaO_2 55 - 80 \text{ mmHg}$ or $SpO_2 88 - 95%$.

Step 3: Measure and Titrate Plateau Pressure ($P_{plat}$)

Plateau pressure reflects alveolar static pressure and is measured by performing an inspiratory hold maneuver for $0.5 - 1.0 \text{ second}$ on a volume-controlled mode.

• Goal: Maintain $P_{plat} \le 30 \text{ cmH}_2\text{O}$.
• If $P_{plat} > 30 \text{ cmH}_2\text{O}$: Decrease $V_T$ in steps of $1 \text{ mL/kg}$ IBW (down to a minimum of $4 \text{ mL/kg}$ IBW).
• If $P_{plat} < 25 \text{ cmH}_2\text{O}$ and $V_T < 6 \text{ mL/kg}$: Increase $V_T$ by $1 \text{ mL/kg}$ IBW until $P_{plat} > 25 \text{ cmH}_2\text{O}$ or $V_T = 6 \text{ mL/kg}$.
• Driving Pressure ($\Delta P$): Calculated as $\Delta P = P_{plat} - PEEP$. A target $\Delta P \le 15 \text{ cmH}_2\text{O}$ is strongly associated with reduced hospital mortality.

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PEEP and $FiO_2$ Titration Ladder

Positive End-Expiratory Pressure (PEEP) prevents alveolar de-recruitment at end-expiration. In ARDS, high PEEP strategies recruit collapsed alveoli and improve oxygenation without requiring toxic $FiO_2$ concentrations ($> 0.60$).

Use the standardized ARDSNet Lower PEEP / Higher $FiO_2$ or Higher PEEP / Lower $FiO_2$ titration tables:

Lower PEEP / Higher $FiO_2$ Ladder

Higher PEEP / Lower $FiO_2$ Ladder (Preferred in Moderate-to-Severe ARDS)

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Permissive Hypercapnia

Reducing tidal volume to $4-6 \text{ mL/kg}$ frequently causes alveolar hypoventilation and respiratory acidosis. Permissive hypercapnia is the deliberate acceptance of elevated arterial carbon dioxide levels ($PaCO_2 = 50-70 \text{ mmHg}$) to prioritize lung protection over normal blood gas chemistry.

• pH Threshold: Arterial $\text{pH}$ must be maintained $\ge 7.20$.
• Buffering: If $\text{pH} < 7.20$ despite maximizing respiratory rate ($RR \le 35$), consider slow intravenous Sodium Bicarbonate ($NaHCO_3$) or THAM (tris-hydroxymethyl aminomethane) infusions.
• Absolute Contraindications to Permissive Hypercapnia:
  1. Traumatic Brain Injury (TBI) or acute intracranial hypertension ($CO_2$ causes cerebral vasodilation, increasing intracranial pressure).
  2. Severe right ventricular failure or acute cor pulmonale ($CO_2$ causes pulmonary vasoconstriction).
  3. Severe hemodynamic instability or severe metabolic acidosis.

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Advanced Rescue Therapies in Transport

1. Prone Positioning

Prone positioning is recommended for patients with early severe ARDS ($P/F \le 150 \text{ mmHg}$ despite PEEP $\ge 10 \text{ cmH}_2\text{O}$ and $FiO_2 \ge 0.60$).

• Physiological Mechanism: Dorsal lung regions comprise the largest portion of lung parenchyma. In the supine position, gravity and cardiac mass collapse these dorsal regions. Proning homogenizes transpulmonary pressure gradients, recruits dorsal alveoli, and improves $V/Q$ matching without overdistending ventral areas.
• Trial Evidence: The PROSEVA study showed a 28-day mortality reduction ($16%$ vs $32.8%$) when proning was applied for $\ge 16 \text{ hours/day}$.
• Transport Considerations:
  • ETT depth verification prior to and immediately after rotation.
  • Securement of all central lines, arterial lines, and chest tubes.
  • Use of specialized foam positioning pads to protect eyes, face, and peripheral nerves (e.g., brachial plexus).

2. Neuromuscular Blockade (NMBAs)

In early severe ARDS ($P/F < 150 \text{ mmHg}$), continuous infusion of non-depolarizing neuromuscular blockers (e.g., Cisatracurium besylate $37.5 \text{ mg/hr}$) for the first 48 hours eliminates patient-ventilator dyssynchrony (breath stacking, reverse triggering), reduces oxygen consumption, and lowers transpulmonary pressure spikes.

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