Prone positioning

Contents

Mechanisms of Benefit

The dorsal lung is subject to compression in the supine position — from the weight of the heart, abdominal contents, and oedematous lung itself. This causes atelectasis and recruits poorly to ventilation. Prone positioning addresses multiple pathological mechanisms simultaneously:

1. Recruitment of dorsal atelectasis

Gravity no longer compresses the dorsal lung. Dependent atelectatic regions open, increasing functional residual capacity and improving gas exchange.

2. Redistribution of perfusion

Pulmonary blood flow follows gravity — in supine ARDS it is directed to the dorsal (atelectatic) lung, worsening V/Q mismatch. In prone, perfusion remains distributed to the now-dorsal aspects but these are now the better-ventilated non-dependent zones, improving matching.

3. More homogeneous ventilation distribution

In supine ARDS, the transpulmonary pressure gradient across the lung is high (apex to base). Prone positioning reduces this gradient, distributing tidal volume more uniformly across lung units — reducing regional overdistension of ventral lung and cyclic collapse-reopening of dorsal units.

4. Reduction of VILI

The more homogeneous stress and strain distribution reduces ventilator-induced lung injury (atelectrauma and volutrauma) across the lung.

5. Reduced cardiac compression of the lung

The heart lies anteriorly (non-dependent) in prone, reducing its compressive effect on the lower lobes.

Patient Selection and Criteria

PROSEVA-aligned criteria (standard practice):

  • Moderate-severe ARDS: PaO₂/FiO₂ <150 mmHg on PEEP ≥5 cmH₂O and FiO₂ ≥0.6
  • Criteria met for >12 hours (allows assessment of whether ARDS is established)
  • Mechanically ventilated

Relative contraindications:

  • Spinal instability
  • Open chest or abdomen
  • Facial or pelvic fractures preventing positioning
  • Significantly raised intracranial pressure
  • Haemodynamic instability refractory to vasopressors
  • Major thoracic or abdominal surgery within 14 days
  • Pregnancy
  • Massive haemoptysis

None of these are absolute — clinical judgement required.

PROSEVA Trial

Guérin et al., NEJM 2013 — the landmark trial establishing prone positioning as standard of care in severe ARDS.

Feature Detail
Design Multicentre RCT, France and Spain; n=466
Population Moderate-severe ARDS (P:F <150), MV <36h, criteria met for 12–24h
Intervention Prone ≥16h/day vs supine
Primary outcome 28-day mortality
Result 16.0% prone vs 32.8% supine (p<0.001); RR 0.49
NNT ~6

Key features of the PROSEVA protocol that may explain the dramatic result:

  • Long proning sessions (≥16h)
  • Low tidal volume ventilation in both groups (lung-protective)
  • Standardised weaning criteria from prone

PROSEVA stands as one of the most impactful positive intervention trials in modern critical care.

Practical Considerations

Preparation

  • Check all lines, tubes, and catheters are secured before turning
  • Ensure ETT is well-fixed and position marked
  • Plan positioning: at minimum 4–5 staff
  • Switch to closed-circuit suction; continuous SpO₂ and cardiac monitoring

Turning procedure

  1. Pre-oxygenate; check ventilator settings
  2. Apply foam padding to pressure points (forehead, chin, chest, pelvis, knees, feet)
  3. Logroll to lateral then prone — ensure ETT does not kink or migrate
  4. Verify tube position, ventilator function, and haemodynamics immediately after turning

Duration

  • Minimum 16 hours per session (PROSEVA protocol)
  • Return to supine for nursing care, airway management, procedures

Assessing response

  • Improvement in PaO₂/FiO₂ by ≥20 mmHg after 1–4h of prone = responder
  • Three sessions without response = consider discontinuation
  • Most patients show sustained P:F improvement on return to supine after successful sessions

Returning to supine

  • Logroll carefully
  • Reassess haemodynamics, ventilator settings, tube position

Complications

Complication Notes
Accidental extubation or ETT obstruction Most serious; check tube position immediately after turning
Loss of CVC or arterial line Secure all lines before turning; have emergency access plan
Pressure injuries Forehead, cheeks, chin, ears, chest, knees — dedicated foam padding and repositioning
Periorbital and facial oedema Gravity-dependent; improves on return to supine
Haemodynamic instability Usually transient on turning; brief deterioration acceptable; persistent instability → return to supine
Brachial plexus stretch injury Avoid extreme arm positions (swimmer's position with caution; alternate side each session)
Reverse bowel obstruction Abdominal distension from gas redistribution; rare
Corneal abrasion Close eyes, tape if necessary

Proning in Non-ARDS Settings

Awake prone positioning (APP)

During the COVID-19 pandemic, APP was widely used in non-intubated patients with hypoxaemia. Physiological mechanisms are the same. Multiple observational studies and some RCT data suggest APP can transiently improve oxygenation and may reduce intubation rates, though the evidence is more modest than in mechanically ventilated ARDS. Tolerability limits session duration. Monitoring and positioning challenges are different from mechanically ventilated patients.

COVID-19 and ARDS

Prone positioning criteria and benefits apply equally to COVID-19-associated ARDS once established. Prone should be offered to mechanically ventilated patients meeting P:F <150 criteria.

Viva Questions

1. Explain the physiological mechanisms by which prone positioning improves gas exchange in ARDS.

Prone positioning addresses several co-existing pathological mechanisms in ARDS. First, dorsal atelectasis: in supine ARDS, the dependent posterior lung is compressed by cardiac weight, abdominal contents, and oedematous lung itself, causing atelectasis. Prone positioning removes this compressive force, recruiting atelectatic alveoli and increasing FRC. Second, V/Q matching: pulmonary blood flow is gravity-dependent and preferentially perfuses the dorsal lung. In supine, the dorsal lung is atelectatic — maximally perfused but poorly ventilated (shunt). In prone, perfusion still preferentially goes posteriorly (now the dorsal, non-dependent aspect), but these zones are now better ventilated, improving V/Q matching. Third, more homogeneous transpulmonary pressure distribution: the superimposed pressure gradient from apex to base is reduced in prone, producing more uniform tidal volume distribution, reducing regional overdistension and atelectrauma. Together these mechanisms explain the dramatic improvements in oxygenation and, via reduced VILI, the survival benefit seen in PROSEVA.

2. Describe the PROSEVA trial and explain why it was so impactful.

PROSEVA (Guérin 2013, NEJM) randomised 466 patients with moderate-severe ARDS (P:F <150 on PEEP ≥5 and FiO₂ ≥0.6 for >12h) to prone positioning for ≥16h/day versus supine care. 28-day mortality was 16.0% in the prone group versus 32.8% in the supine group — a relative risk reduction of 50% and an NNT of approximately 6. This is one of the largest mortality effects seen in any critical care RCT. Prior smaller and underpowered trials had shown mixed results, partly because they used shorter prone sessions (8h), did not restrict to severe ARDS, and used higher tidal volumes. PROSEVA succeeded by using longer sessions (≥16h), recruiting a more severely affected population, and mandating lung-protective ventilation in both groups. The magnitude of benefit has led to prone positioning being firmly embedded in ARDS management guidelines.

3. A patient is in prone for their first session — after 2 hours their SpO₂ has improved from 88% to 96% and P:F has risen from 105 to 175 mmHg. What do you do next?

This patient is responding well — a P:F improvement >20 mmHg confirms a physiological response to prone. I would continue the session for the full 16 hours as per PROSEVA protocol rather than returning to supine early. The full session maximises alveolar recruitment and may improve sustained recruitment on return to supine. I would reassess all pressure areas at regular intervals, maintain current ventilator settings (low tidal volume, adequate PEEP), and continue monitoring closely. After 16 hours, return to supine, reassess P:F — if it remains above 150 mmHg after 4 hours supine and the underlying ARDS pathology has improved, further proning may not be required. If P:F falls again below 150, plan subsequent sessions. Document the session, response, complications, and planned frequency.

4. What are the contraindications to prone positioning and how absolute are they?

Contraindications reflect situations where prone positioning cannot be safely executed or may cause harm. Absolute in practice: unstable spinal injury (cannot maintain alignment); open chest or abdomen (risk of organ herniation or wound disruption). Relative: haemodynamic instability refractory to vasopressors (turning may precipitate cardiac arrest — optimise before attempting; transient instability during turning is acceptable); raised intracranial pressure (prone increases IAP and may reduce venous drainage if not positioned carefully); facial fractures or recent major craniofacial surgery; pregnancy. There are very few truly absolute contraindications. Clinical judgement must weigh the potential survival benefit (large, NNT ~6 in PROSEVA) against the specific risk in each patient. For example, a patient with a recent spinal decompression might be able to be proned with specialist physiotherapy and neurosurgical input. Relative contraindications should prompt problem-solving, not automatic exclusion.