2.2 Prone Positioning Protocols

Prone Positioning Protocols

Prone positioning is a highly effective, evidence-based therapy used to treat refractory hypoxemia in patients with moderate-to-severe Acute Respiratory Distress Syndrome (ARDS). By rotating the patient from the supine to the prone (face-down) position, clinicians can optimize gas exchange, recruit collapsed alveoli, and reduce ventilator-induced lung injury (VILI).

Physiological Mechanisms of Prone Positioning

In the supine position, the heart, mediastinal structures, and abdominal contents exert gravity-dependent pressure on the dorsal regions of the lungs. This compression, combined with the vertical pleural pressure gradient, leads to significant atelectasis in the dorsal lung zones. However, pulmonary perfusion remains directed to these dorsal regions due to gravity and the anatomical distribution of the pulmonary vasculature. This results in a massive ventilation-perfusion (V/Q) mismatch (shunt).

When the patient is turned prone, several physiological changes occur:

• Homogenization of Pleural Pressure: The vertical pleural pressure gradient becomes much more uniform, relieving the dorsal lung units from compression.
• Alveolar Recruitment: The dorsal lung units (which contain the largest portion of the lung's parenchyma) are recruited, while the ventral units remain open.
• Improved V/Q Matching: Because blood flow remains directed toward the dorsal lung regions, match between ventilation and perfusion is dramatically improved, reducing the intrapulmonary shunt and improving PaO2.
• Reduction in VILI: The distribution of mechanical stress and strain is homogenized throughout the lungs, preventing regional alveolar overdistension.
• Right Ventricular (RV) Protection: Prone positioning decreases pulmonary vascular resistance, reducing RV afterload and improving cardiac output.

Clinical Indications (PROSEVA Trial Criteria)

Based on the landmark PROSEVA trial, prone positioning should be initiated early in patients who meet the following criteria:

• Severe ARDS: Defined as a PaO2/FiO2 ratio < 150 mmHg.
• Ventilator Settings: Evaluated after 12 to 24 hours of optimization with an FiO2 >= 0.60, a PEEP >= 5 cm H2O, and a tidal volume of 6 mL/kg PBW.
• Timing: Initiated within 12 to 24 hours of meeting severity criteria.

Protocol Duration and Discontinuation

• Session Duration: Patients must remain in the prone position for at least 16 consecutive hours daily. Returning the patient to the supine position too early negates the mortality benefit.
• Discontinuation Criteria: Proning can be discontinued when the patient exhibits sustained improvement in oxygenation: a PaO2/FiO2 ratio >= 150 mmHg on a PEEP <= 10 cm H2O and an FiO2 <= 0.60, measured at least 4 hours after returning to the supine position.

Step-by-Step Execution of the Turn

Prone positioning is a high-risk procedure that requires a minimum of 5 to 6 trained personnel.

• The Airway Lead: The clinician at the head of the bed (typically the respiratory therapist or anesthesiologist) is solely responsible for securing the endotracheal tube, monitoring the ventilator circuit, and directing the team's movements using a clear, synchronized count (e.g., "1, 2, 3, slide; 1, 2, 3, turn").
• Preparation Steps:
  1. Pre-oxygenate the patient with 100% FiO2.
  2. Stop enteral feedings 1 to 2 hours prior to the turn and aspirate gastric contents to prevent emesis and aspiration.
  3. Secure all invasive lines (arterial, central venous, dialysis, Foley). Position them to avoid tension or displacement during the turn.
  4. Apply protective silicone dressings to bony prominences and pressure points (forehead, chin, shoulders, sternum, anterior iliac crests, knees).
  5. Place new ECG electrodes on the patient's back.
• The Turn (Envelope Method): The patient is slid to the side of the bed opposite the direction of the turn, rotated onto their side (lateral decubitus), and then rolled onto their abdomen onto supporting chest and pelvic bolsters.

Management in the Prone and Supine Phase

• The Swimmer's Position: The patient's head is turned to one side. The arm on the side the head is facing is abducted and flexed (raised), while the opposite arm is extended alongside the body. The head turn and arm positions must be alternated every 2 hours to prevent brachial plexus stretch injury and pressure ulcers.
• Elevation: Maintain the head of the bed in a 5 to 10-degree reverse Trendelenburg position. This reduces facial, conjunctival, and upper airway edema.
• Eye Care: Apply ophthalmic lubricant and tape the eyelids shut to prevent corneal abrasions and drying.
• Post-Prone Care and Monitoring: After turning the patient back to the supine position (supination), the clinical team must carefully assess for signs of clinical improvement or decay. The patient's skin must be inspected for stage 1 or 2 pressure ulcers. Secretions that were mobilized during the prone phase must be aggressively suctioned. A repeat arterial blood gas should be drawn within 4 hours to evaluate if the patient is a responder to proning. If the PaO2/FiO2 ratio remains improved (>150 mmHg) on lower settings, proning can be suspended; otherwise, daily prone cycles should continue.

Contraindications to Prone Positioning

• Absolute Contraindications:
  • Unstable spinal fractures (cervical, thoracic, or lumbar).
  • Open abdomen or recent complex abdominal surgery.
  • Severe, uncontrolled hemodynamic instability (MAP < 65 mmHg despite maximum vasopressor support).
  • Elevated intracranial pressure (>20 mmHg).
• Relative Contraindications:
  • Late-stage pregnancy.
  • Massive hemoptysis.
  • Recent tracheostomy (<24 hours old) or facial trauma.
  • Untreated deep vein thrombosis.

ACCS Exam Traps

• Trap 1: Suggesting a short duration of proning (e.g., 4 to 6 hours) in a patient with a P/F ratio of 100. Always select a duration of at least 16 hours.
• Trap 2: Assigning line management or general commands to the RT during a turn. The RT's role is strictly at the head of the bed, securing the ETT and leading the count.