7.2 Pharmacotherapy for Mechanical Ventilation and ARDS

Pharmacotherapy in ARDS

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. The Berlin Definition categorizes ARDS as mild, moderate, or severe based on the PaO2/FiO2 (P/F) ratio.

While non-pharmacologic interventions—specifically lung-protective mechanical ventilation and prone positioning—are the mainstays of reducing mortality, several pharmacologic therapies play a crucial adjunctive role in managing ARDS.

Mechanical Ventilation Basics in ARDS

Before discussing pharmacotherapy, it is vital to understand the goals of mechanical ventilation in ARDS, as medications are often used to facilitate these goals:

• Low Tidal Volume Ventilation (LTVV): Target a tidal volume of 6 mL/kg of predicted body weight (PBW). This prevents volutrauma to the remaining healthy alveoli.
• Plateau Pressure: Maintain plateau pressure (the pressure applied to small airways and alveoli) ≤ 30 cm H2O to prevent barotrauma.
• PEEP (Positive End-Expiratory Pressure): Moderate to high PEEP is used to recruit collapsed alveoli, prevent atelectrauma, and improve oxygenation.
• Permissive Hypercapnia: Allowing PaCO2 to rise is often necessary to achieve low tidal volume goals, provided the pH remains acceptable (usually > 7.20-7.25).

Neuromuscular Blocking Agents (NMBAs)

In patients with moderate-to-severe ARDS, patient-ventilator dyssynchrony can lead to severe hypoxemia and further lung injury (patient self-inflicted lung injury). When deep sedation is insufficient to achieve synchrony and strict lung-protective ventilation, NMBAs may be employed.

Clinical Evidence

• ACURASYS Trial (2010): Demonstrated a mortality benefit with early, 48-hour continuous infusion of cisatracurium in patients with severe ARDS (P/F < 150).
• ROSE Trial (2019): Evaluated early continuous infusion of cisatracurium combined with heavy sedation versus lighter sedation without routine NMBAs. The trial found no mortality difference between the groups, challenging the routine use of NMBAs.

Current Recommendations

Routine continuous infusion of NMBAs is no longer widely recommended for all severe ARDS patients. However, short courses (≤ 48 hours) of continuous infusion (e.g., cisatracurium) are still utilized for specific indications: severe ventilator dyssynchrony despite deep sedation, facilitation of prone positioning, or persistently high plateau pressures. Patients receiving NMBAs require deep sedation (RASS -5) and regular monitoring (e.g., Train-of-Four) to ensure adequate paralysis and prevent awareness.

Corticosteroids in ARDS

The profound inflammatory cascade in ARDS provides a strong rationale for corticosteroid use. The goal is to attenuate the fibroproliferative phase and reduce systemic inflammation.

Clinical Evidence and Application

• DEXA-ARDS Trial (2020): Evaluated the early use of intravenous dexamethasone (20 mg/day for 5 days, then 10 mg/day for 5 days) in moderate-to-severe ARDS. The trial demonstrated a significant increase in ventilator-free days and a reduction in 60-day mortality.
• COVID-19 ARDS: The RECOVERY trial confirmed the mortality benefit of dexamethasone (6 mg daily for 10 days) in patients with COVID-19 requiring oxygen or mechanical ventilation.

Key Considerations

Corticosteroids are generally recommended for moderate-to-severe ARDS, provided there are no absolute contraindications (e.g., severe uncontrolled systemic fungal infection). Timing is critical; steroids should ideally be initiated early (within the first 14 days). Late initiation (after 14 days) in unresolving ARDS was historically associated with increased mortality.

Inhaled Pulmonary Vasodilators

Inhaled pulmonary vasodilators—such as inhaled nitric oxide (iNO) and inhaled epoprostenol (iEPO)—are used to selectively dilate pulmonary capillaries in well-ventilated lung regions. This improves ventilation-perfusion (V/Q) matching and acutely increases systemic oxygenation without causing systemic hypotension.

Role in Therapy

Despite providing rapid, often dramatic improvements in oxygenation, multiple randomized controlled trials have consistently shown that inhaled pulmonary vasodilators do not improve mortality or increase ventilator-free days in ARDS.

Therefore, they should not be used routinely. They are reserved exclusively as rescue therapy for life-threatening, refractory hypoxemia to buy time for other interventions (like prone positioning or ECMO) to work.

Adverse Effects and Rebound

• iNO: Can cause methemoglobinemia and renal dysfunction. Requires gradual weaning to avoid severe rebound pulmonary hypertension and hypoxemia.
• iEPO: Generally cheaper than iNO but can cause sticky residue in the ventilator circuit, potentially leading to obstruction.

Conservative Fluid Management

After the initial resuscitation phase for shock, ARDS patients often accumulate significant extravascular lung water due to increased capillary permeability.

FACTT Trial

The Fluid and Catheter Treatment Trial (FACTT) compared a conservative fluid strategy (targeting a lower CVP/PAOP using diuretics and fluid restriction) to a liberal strategy. The conservative strategy resulted in a significant increase in ventilator-free days and improved lung function, without increasing the incidence of acute kidney injury or shock.

Application

Once a patient is hemodynamically stable and off vasopressors, a conservative fluid management strategy (often utilizing furosemide) should be implemented to achieve an even or net-negative fluid balance, helping to "dry out" the lungs and facilitate extubation.