Prone Positioning in ARDS Patients

Clinical Significance of Prone Positioning

The American Thoracic Society recommends early application of prone positioning in severe ARDS patients, as it is associated with improved 28-day outcomes compared to non-responders, with a mortality benefit most pronounced in patients with severe ARDS (PaO₂/FiO₂ <150 mmHg) 1
Prone positioning serves as an indicator to maintain prolonged prone positioning (≥12 hours per day) in patients with severe ARDS, with physiological benefits including improved ventilation-perfusion matching, increased end-expiratory lung volume, and decreased ventilator-induced lung injury through more uniform distribution of tidal volume 1, 2

Current guidelines recommend early application of prone positioning in severe ARDS patients (PaO₂/FiO₂ <150 mmHg) within 48 hours of starting mechanical ventilation, with a recommended duration of more than 12 hours per day 1, 2, 3
Meta-analyses show prone positioning reduced mortality in trials with prone duration greater than 12 hours per day (RR, 0.74; 95% CI, 0.56–0.99) 1

The American College of Chest Physicians recommends lung-protective ventilation strategies with limited tidal volumes (4-8 ml/kg PBW) for both early and late responders, with evidence suggesting that the limitation of tidal volume is necessary for mortality benefit from prone positioning 3, 4, 5, 6
Prone positioning and PEEP have an additive effect on improving oxygenation 3

The European Society of Intensive Care Medicine recommends continuing prone positioning until there is improvement in oxygenation (PaO₂/FiO₂ ≥150) under de-escalated ventilation (PEEP ≤10 cmH₂O and FiO₂ ≤0.6) 4 hours after returning to supine position, and terminating prone positioning therapy after two unsuccessful attempts (lack of improvement in oxygenation) 3

Prone positioning is associated with risks including endotracheal tube obstruction (RR, 1.76; 95% CI, 1.24–2.50) and pressure sores (RR, 1.22; 95% CI, 1.06–1.41), as well as potential increases in intra-abdominal pressure from 12±4 mmHg to 14±5 mmHg 1, 3
Special considerations are needed for patients with increased intracranial pressure, obesity or recent abdominal surgery, and concurrent ECMO therapy, which requires experienced centers 3

Prone positioning improves oxygenation and reduces mortality in severe ARDS through more homogeneous distribution of ventilation, improved ventilation-perfusion matching, reduced alveolar shunt, and decreased ventilator-induced lung injury risk 7, 8, 9
Redistribution of lung densities occurs with prone positioning, causing recruitment of well-perfused dorsal regions that are often collapsed in supine position 7, 9
Prone position creates a more even distribution of the gravitational gradient in pleural pressure, leading to better distribution of ventilation to the dorsal areas of the lungs 10, 11
Reduced alveolar shunt is achieved through improved recruitment of dorsal lung regions and better ventilation-perfusion matching 9

Improved lung recruitment is accompanied by increased chest wall elastance, but this change is usually beneficial 7, 9
More homogeneous ventilation distribution enhances alveolar ventilation and CO₂ elimination 9

Prone positioning significantly reduces mortality in severe ARDS when applied early and for prolonged periods 9

The use of prone positioning for at least 17 hours/day in severe ARDS patients is an independent factor associated with improved survival in multivariate analysis, as seen in a study published in Intensive Care Medicine 12, 13, 14
Earlier meta-analyses showed equivocal results due to shorter prone durations (7-8 hours/day vs. ≥12 hours) and inclusion of less severe ALI patients rather than severe ARDS, as noted in studies published in Intensive Care Medicine 12, 15
The critical distinction from earlier studies lies in the use of prolonged prone positioning (≥12 hours) in severe ARDS patients, which demonstrates a clear mortality benefit, as discussed in Intensive Care Medicine 12, 15

Prone positioning reduces mortality through multiple mechanisms, including more homogeneous distribution of ventilation and improved ventilation-perfusion matching, as explained in Intensive Care Medicine 12
The prone position also leads to a more even distribution of gravitational gradient in pleural pressure and reduces compression of lung segments by the heart, as described in Intensive Care Medicine 12

The patient's head should be positioned at the level of the anesthetist's xiphoid process or nipple when lying on an operating table for optimal ergonomics during intubation, as recommended by the American Society of Anesthesiologists 16
The anesthetic machine should be positioned so all controls are within a 45° arc from the midline, allowing the anesthetist to maintain proper shoulder and elbow positioning, according to the Anaesthesia journal 16

Prone positioning is generally hemodynamically well tolerated and may positively affect right ventricular load, as stated by the European Society of Intensive Care Medicine 17
Volume status should be optimized prior to positioning, though ongoing vasopressor therapy is not a contraindication, as recommended by the European Society of Intensive Care Medicine 17
Mean arterial pressure (MAP) may decrease with prone positioning, particularly when combined with increased PEEP in patients with normal lung compliance, as reported in the Critical Care journal 18

Intra-abdominal pressure increases from 12 ± 4 mmHg to 14 ± 5 mmHg in prone position, as observed in a study published in the Intensive Care Medicine journal 17
Despite increased rates of hypoxic hepatitis and renal failure in some studies, obese patients do not experience more complications overall and may have greater oxygenation improvement, according to the Intensive Care Medicine journal 17
Intracranial pressure (ICP) increases significantly with prone positioning, as reported in studies published in the Critical Care and Intensive Care Medicine journals 17, 18
Individual risk-benefit assessment is required for patients with acute cerebral lesions, considering both potential ICP elevation and oxygenation benefits, as recommended by the European Society of Intensive Care Medicine 17

PEEP and prone positioning have additive effects on improving oxygenation, as stated by the European Society of Intensive Care Medicine 17
For ARDS patients requiring prone positioning, minimum duration of 12 hours is necessary for mortality benefit, as recommended by the European Society of Intensive Care Medicine 17
Neglecting to optimize volume status before positioning may exacerbate hemodynamic changes, as reported in the Intensive Care Medicine journal 17
In patients with intracranial pathology, increasing sedation may be necessary if ICP rises acutely with positioning, as recommended by the Critical Care journal 18