2.1 Protective Ventilation & ARDSNet

Protective Ventilation & ARDSNet

Acute Respiratory Distress Syndrome (ARDS) is a form of non-cardiogenic pulmonary edema characterized by severe inflammatory lung injury, diffuse alveolar damage, and profound hypoxemia. Because the lungs in ARDS are heterogeneously consolidated, the volume of functional, aerated lung tissue is severely reduced (the 'baby lung' concept). Standard mechanical ventilation can easily cause ventilator-induced lung injury (VILI) through alveolar overdistension (volutrauma) and high airway pressures (barotrauma).

Berlin Definition of ARDS

ARDS is classified by the Berlin Criteria:

• Onset: Acute, within 1 week of a known clinical insult.
• Chest Imaging: Symmetrical bilateral opacities not fully explained by effusions, lobar collapse, or nodules.
• Origin of Edema: Respiratory failure not fully explained by heart failure or fluid overload (confirmed via echocardiography or a PAOP <= 18 mmHg).
• Severity of Hypoxemia: Classified by PaO2/FiO2 ratio on PEEP >= 5 cm H2O: Mild (201-300 mmHg), Moderate (101-200 mmHg), and Severe (<= 100 mmHg).

The ARDSNet Protocol: Core Strategies

1. Calculate Predicted Body Weight (PBW)

Tidal volumes must be calculated using PBW, never actual body weight, because lung volume is determined by biological sex and height, not adipose tissue or fluid retention.

• Males: $\text{PBW (kg)} = 50 + 2.3 \times (\text{Height in inches} - 60)$
• Females: $\text{PBW (kg)} = 45.5 + 2.3 \times (\text{Height in inches} - 60)$

Worked Example: For a 5'4" (64 inches) female patient: $\text{PBW} = 45.5 + 2.3 \times (64 - 60) = 45.5 + 9.2 = 54.7 \text{ kg}$

2. Initial Settings and Tidal Volume Titration

• Initial Vt: Set at 8 mL/kg PBW in Volume Control or Pressure Control mode.
• Reduction: Reduce the tidal volume by 1 mL/kg PBW every 2 hours until the target of 6 mL/kg PBW is reached (acceptable range is 4 to 8 mL/kg). For our 54.7 kg female patient, the target Vt is approximately 330 mL (6 mL/kg), with a minimum range down to 220 mL (4 mL/kg).
• Set the initial respiratory rate (RR) to match the patient's baseline minute ventilation (typically 15 to 20 breaths/min).

3. Airway Pressure Targets

• Plateau Pressure (Pplat): Target $\le 30 \text{ cm H2O}$. Pplat represents static alveolar pressure and is measured by performing an inspiratory hold (0.5 to 1.0 second) in a passive patient.
  • If Pplat > 30 cm H2O: Decrease the Vt in 0.5 to 1.0 mL/kg steps (minimum of 4 mL/kg).
  • If Pplat < 25 cm H2O and Vt is < 6 mL/kg: The Vt can be increased to optimize ventilation.
• Driving Pressure (dP): Calculated as $\text{dP} = \text{Pplat} - \text{PEEP}$. Target is strictly < 15 cm H2O, as lower driving pressures are strongly associated with improved survival.

4. Oxygenation and PEEP Titration

The target oxygenation range is a PaO2 of 55 to 80 mmHg or an SpO2 of 88% to 95%. This is achieved by adjusting PEEP and FiO2 in tandem using the standardized ARDSNet Lower PEEP/Higher FiO2 table:

A Higher PEEP/Lower FiO2 strategy can be considered in patients with moderate-to-severe ARDS who demonstrate alveolar recruitment response without hemodynamic compromise.

5. Permissive Hypercapnia & Acid-Base Management

To maintain lung-protective settings (low Vt and low Pplat), the clinician must accept a respiratory acidosis, a strategy known as permissive hypercapnia.

• pH Target: 7.30 to 7.45 is normal, but 7.15 to 7.30 is tolerated.
• If pH falls between 7.15 and 7.30, increase the RR (up to a maximum of 35 breaths/min) to clear CO2.
• If pH falls below 7.15, the Vt can be increased in 0.5 mL/kg steps (as long as Pplat remains <= 30 cm H2O), or sodium bicarbonate infusion can be considered.
• Absolute Contraindications: Elevated intracranial pressure (CO2 causes cerebral vasodilation, increasing ICP), severe right ventricular failure (CO2 increases pulmonary vascular resistance), severe hemodynamic instability, and severe metabolic acidosis.

Alveolar Recruitment and PEEP Titration

In moderate-to-severe ARDS, recruitment maneuvers (e.g., applying a continuous airway pressure of 40 cm H2O for 40 seconds) can be used to open collapsed alveoli. This is followed by a decremental PEEP titration:

1. Set PEEP high (e.g., 20 cm H2O).
2. Decrease PEEP by 2 cm H2O every 2 to 3 minutes.
3. Calculate static compliance ($C_{stat} = V_t / [P_{plat} - PEEP]$) at each step.
4. The PEEP level that yields the highest compliance (and lowest driving pressure) is the optimal PEEP. Set the ventilator PEEP 2 cm H2O above this level to prevent derecruitment.

• Hazards: Severe hypotension (due to reduced venous return), barotrauma (pneumothorax), and transient desaturation.

ACCS Exam Traps

• Trap 1: Selecting a tidal volume based on actual body weight. If a question gives a patient's actual weight (e.g., 100 kg) and height (e.g., 5'6"), calculate the PBW first, then select the Vt (approx. 350 mL).
• Trap 2: Allowing permissive hypercapnia in a patient with a traumatic brain injury and suspected high ICP. In this scenario, you must maintain a normal pH and PaCO2.