Residency · Residency · Critical Care

Prone Positioning and Neuromuscular Blockade in ARDS

Prone Positioning

Physiological Rationale

The benefits of prone positioning in ARDS derive from the fundamental heterogeneity of lung injury and the influence of gravity on pleural pressure distribution. In the supine position, a gravitational pleural pressure gradient causes compression and atelectasis of dependent dorsal lung regions while ventral regions become overdistended, creating a pattern of simultaneous injury from both atelectrauma and volutrauma. Turning the patient prone redistributes pleural pressure more uniformly across the lung, producing several synergistic physiological benefits. Alveolar recruitment in dorsal lung regions, which contain the majority of lung parenchyma, is the most significant mechanism. The more homogeneous distribution of ventilation reduces regional stress and strain gradients, minimizing the shear forces that drive ventilator-induced lung injury. Improved secretion drainage by gravity enhances airway clearance. The removal of cardiac weight from the dorsal lung regions eliminates the cardiac fossa effect, which in the supine position compresses the left lower lobe beneath the heart. Decreased chest wall compliance in the ventral regions redirects ventilation to the now-non-dependent dorsal lung, and improved right ventricular function results from decreased pulmonary vascular resistance.

Hemodynamic Effects

Prone positioning has important cardiovascular effects that extend beyond respiratory physiology. The reduction in pulmonary vascular resistance improves right ventricular function, which is particularly relevant given that acute cor pulmonale is present in 22 to 25 percent of patients with moderate-to-severe ARDS. In these patients, prone positioning serves a directly therapeutic hemodynamic role. Cardiac output is generally maintained or improved in the prone position, and some patients demonstrate decreased vasopressor requirements. Careful attention to abdominal positioning is essential: the abdomen must be allowed to hang freely to prevent compression of the inferior vena cava, which would impair venous return and negate the hemodynamic benefits.

<image>Cross-sectional CT-based illustration of lung aeration in supine versus prone position. Left panel: supine patient cross-section showing dense consolidation and atelectasis in dependent (dorsal) lung regions with overdistension in non-dependent (ventral) regions. Pleural pressure gradient shown with arrows indicating higher pressure dorsally. Heart compressing left lower lobe. Right panel: same patient in prone position showing redistribution of aeration with improved dorsal recruitment, reduced ventral overdistension, and more uniform pleural pressure gradient. Heart displaced anteriorly, no longer compressing lung. Include V/Q matching diagrams below each panel showing improved correlation in prone position. Color-coded lung regions: blue (atelectatic), yellow (normally aerated), red (overdistended).</image>

PROSEVA Trial (2013) — Landmark Evidence

The PROSEVA trial is the landmark randomized controlled trial that established the mortality benefit of prone positioning in severe ARDS. This multicenter study enrolled 466 patients with severe ARDS, defined by a PaO2/FiO2 below 150 on FiO2 of 0.6 or greater and PEEP of 5 cmH2O or higher. A stabilization period of 12 to 24 hours of optimized supine ventilation preceded enrollment to exclude patients who rapidly improved. The intervention group received prone positioning for at least 16 hours per day, initiated within 12 to 24 hours of ARDS diagnosis. The results were striking: 28-day mortality was 16 percent in the prone group versus 32.8 percent in the supine group, yielding a number needed to treat of 6. This benefit persisted at 90 days, with mortality of 23.6 percent versus 41 percent. Prone sessions were continued until PaO2/FiO2 improved to 150 or above on FiO2 of 0.6 or below and PEEP of 10 or below when assessed in the supine position for at least 4 hours after a proning session. An important contextual detail is that all patients received low tidal volume ventilation, neuromuscular blockade during the first 48 hours, and relatively high PEEP, representing a comprehensive ARDS management bundle.

Indications and Timing

Prone positioning is indicated for patients with moderate-to-severe ARDS, specifically those with PaO2/FiO2 below 150 on PEEP of 5 cmH2O or higher and FiO2 of 0.6 or greater. Initiation should occur within 12 to 36 hours of ARDS diagnosis, and delay should be avoided as early implementation is associated with better outcomes. Each prone session should last a minimum of 16 hours, with some centers extending to 18 to 20 hours or longer. Daily proning should continue until sustained improvement in oxygenation is demonstrated in the supine position. Earlier trials that used shorter prone durations of less than 12 hours per day, including those by Gattinoni in 2001 and Guerin in 2004, failed to demonstrate mortality benefit, underscoring the importance of adequate duration.

Contraindications

True absolute contraindications to prone positioning are remarkably few, including an open abdomen, spinal instability, and facial or anterior body burns that preclude positioning. Many conditions traditionally considered contraindications are in fact relative: hemodynamic instability, which may actually improve with proning; elevated intracranial pressure, which requires close monitoring but does not preclude proning; recent sternotomy; and pregnancy, which can be accommodated with appropriate modifications. The intensivist should approach perceived contraindications critically, as denying patients this proven mortality-reducing intervention based on relative concerns requires careful justification.

Practical Implementation

Safe prone positioning requires a coordinated team effort with a minimum of 3 to 5 trained personnel. A pre-prone checklist should confirm that the endotracheal tube is secured with its depth documented, all lines and tubes are identified and protected, eye protection is applied, and skin protection is placed over pressure points including the forehead, chin, and chest. Arm positioning follows the "swimmer" configuration, with alternating arms placed overhead and repositioned every 2 hours to prevent brachial plexus injury. The head of bed should be elevated in reverse Trendelenburg at 15 to 20 degrees to reduce facial and airway edema and decrease aspiration risk. Enteral feeding should continue in the prone position, with post-pyloric access preferred though gastric feeding remains acceptable. Monitoring includes continuous pulse oximetry, arterial line monitoring, and maintenance of ventilator settings unchanged initially. The most common complications include pressure injuries to the face, chest, and pelvis, occurring in 30 to 40 percent of patients; accidental extubation in 0.5 to 1 percent; transient desaturation during the turning process; corneal abrasion; brachial plexus injury; and enteral feeding intolerance.

Response Assessment

PaO2/FiO2 improvement is typically observed within 1 to 4 hours of prone positioning. Patients are categorized as persistent responders if they maintain improved oxygenation when returned to the supine position, non-persistent responders if oxygenation deteriorates upon return to supine, or non-responders if no improvement is seen after 4 to 6 hours in the prone position. A critical clinical principle is that proning should not be discontinued based solely on lack of oxygenation response: the mortality benefit demonstrated in PROSEVA was not solely mediated by oxygenation improvement but also by reductions in ventilator-induced lung injury, hemodynamic improvements, and other mechanisms.

Neuromuscular Blockade in ARDS

Physiological Rationale

The use of neuromuscular blockade in ARDS is supported by several complementary physiological mechanisms. Complete paralysis eliminates patient-ventilator dyssynchrony, ensuring consistent delivery of lung-protective ventilation without injurious patient efforts. Respiratory muscle activity in critically ill patients can consume up to 25 percent of total oxygen consumption, and eliminating this metabolic cost reduces overall oxygen demand. Neuromuscular blockade prevents expiratory muscle recruitment, which can cause derecruitment and atelectrauma by actively compressing the lungs below functional residual capacity. Perhaps most importantly in severe ARDS, paralysis prevents patient self-inflicted lung injury by eliminating the large transpulmonary pressure swings that can result from vigorous spontaneous breathing efforts. There is also evidence suggesting that neuromuscular blockade may reduce biotrauma through decreased inflammatory mediator release.

ACURASYS Trial (2010)

The ACURASYS trial, a multicenter randomized controlled trial, enrolled 340 patients with PaO2/FiO2 below 150 within 48 hours of ARDS onset. The intervention group received cisatracurium besylate as a 15 mg bolus followed by a 37.5 mg/hr infusion for 48 hours, while the control group received deep sedation with a placebo infusion. The primary outcome of 90-day mortality was 31.6 percent in the cisatracurium group versus 40.7 percent in the control group. While the unadjusted comparison did not reach statistical significance at a p-value of 0.08, the adjusted hazard ratio of 0.68 was significant at p equals 0.04. Additional benefits included improved oxygenation, fewer pneumothoraces, and more ventilator-free days, without a significant difference in ICU-acquired weakness at day 28. Important limitations included the use of deep sedation in both groups, which differs from current practice, and challenges with blinding given the obvious paralytic effects of the study drug.

ROSE Trial (2019)

The ROSE trial, a larger multicenter study enrolling 1006 patients with moderate-to-severe ARDS, compared cisatracurium infusion at the same dose as ACURASYS with deep sedation against the current standard of light sedation targeting a RASS of 0 to negative 1, with neuromuscular blockade reserved for specific indications. The primary outcome of 90-day mortality showed no difference: 42.5 percent versus 42.8 percent. The trial was stopped early for futility, and the NMB group experienced higher rates of cardiovascular adverse events. The key message from ROSE is that routine 48-hour NMB infusion with heavy sedation does not improve outcomes compared to a light sedation strategy with NMB used only when specifically indicated.

Reconciling ACURASYS and ROSE

The apparent discrepancy between these two trials is explained by fundamental differences in their design, particularly the sedation strategy in the control arms. In ACURASYS, both arms received deep sedation, meaning the trial essentially compared deep sedation with paralysis versus deep sedation alone. In ROSE, the control arm received light sedation, which represents a substantially different and arguably better standard of care than the deep sedation used in ACURASYS. The ROSE control arm had lower-than-expected mortality, reflecting improvements in ARDS management since the ACURASYS era. Higher PEEP levels in ACURASYS may also have created a physiological context more favorable to NMB. Both trials demonstrated the safety of short-course cisatracurium, with no increase in ICU-acquired weakness at 48 hours of use. The current interpretation favors light sedation with targeted NMB for specific indications as the preferred strategy.

<image>Side-by-side comparison infographic of the ACURASYS and ROSE trials. For each trial, show: study design boxes (population, intervention, comparator), primary outcome with Kaplan-Meier survival curves, and key secondary outcomes (oxygenation, ventilator-free days, ICU-acquired weakness rates). Below the trials, a reconciliation panel highlighting key differences: sedation strategy in control arm, PEEP levels, era of practice, and overall mortality rates. Final box with "Current Practice Recommendation" showing light sedation target with indications for as-needed NMB use (refractory dyssynchrony, dangerous transpulmonary pressures, facilitation of prone positioning).</image>

Current Indications for NMB in ARDS

Based on the totality of evidence, neuromuscular blockade in ARDS is currently indicated for refractory patient-ventilator dyssynchrony that persists despite optimized sedation and ventilator settings, severe refractory hypoxemia with PaO2/FiO2 below 80 that has not responded to other interventions, facilitation of prone positioning particularly during initial sessions, dangerously high transpulmonary pressures resulting from strong spontaneous efforts raising concerns for P-SILI, and uncontrolled respiratory drive with risk of barotrauma. When used, the duration should be limited to 24 to 48 hours whenever possible, with daily reassessment of the continued need for paralysis.

Pharmacology of Neuromuscular Blocking Agents

Cisatracurium, a benzylisoquinolinium compound, is the preferred neuromuscular blocking agent in the ICU because of its unique pharmacokinetic profile. It undergoes Hofmann elimination, a spontaneous organ-independent degradation process that occurs at physiological pH and temperature, making its metabolism independent of hepatic or renal function. It is dosed at 0.15 to 0.2 mg/kg as a bolus followed by 1 to 3 mcg/kg/min as an infusion, produces no histamine release, and causes no vagolysis, resulting in hemodynamic stability. Rocuronium, an aminosteroid compound dosed at 0.6 to 1.2 mg/kg as a bolus and 0.6 to 1.2 mg/kg/hr as an infusion, undergoes hepatic metabolism and renal excretion, with accumulation and prolonged paralysis occurring in hepatic and renal dysfunction. Its key advantage is reversibility with sugammadex through selective molecular binding. Vecuronium, another aminosteroid, is similar to rocuronium but has a shorter duration and produces an active metabolite, 3-desacetylvecuronium, that accumulates in renal failure.

Monitoring Depth of Paralysis

Train-of-four monitoring using peripheral nerve stimulation is the standard method for assessing the depth of neuromuscular blockade. Stimulation of the ulnar nerve at the wrist with observation of the adductor pollicis response is the most common technique, though facial nerve stimulation monitoring the orbicularis oculi and posterior tibial nerve stimulation are alternatives. The target is 1 to 2 out of 4 twitches, representing deep blockade without complete paralysis. A train-of-four count of 0 out of 4 indicates excessive blockade requiring dose reduction. Clinical assessment, including observation for spontaneous respiratory efforts and triggered breaths on the ventilator, supplements peripheral nerve stimulation. Limitations of TOF monitoring include variability related to electrode placement, edema, and hypothermia.

Sedation During NMB

Deep sedation targeting a RASS of negative 4 to negative 5 is mandatory during neuromuscular blockade. Paralysis without adequate sedation is inhumane and can result in awareness, which is associated with severe post-traumatic stress disorder. Bispectral index monitoring, targeting a value of 40 to 60, can help assess sedation depth during paralysis, though its accuracy in the ICU setting is imperfect due to EMG artifact, hypothermia, and drug effects. The sedation regimen typically combines propofol or midazolam with an opioid. The estimated risk of awareness during paralysis is 0.2 to 1 percent, and clinicians should always assume the patient can hear and feel, narrating procedures and minimizing noxious stimuli accordingly.

Complications of Prolonged NMB

While 48-hour NMB use was not associated with increased ICU-acquired weakness in either ACURASYS or ROSE, prolonged use beyond 48 hours carries additional risks. Concurrent administration of corticosteroids and NMB agents has synergistic myotoxicity, increasing the risk of critical illness myopathy. Additional risk factors for NMB-related weakness include aminoglycoside use, hyperglycemia, and prolonged administration. Drug holidays with daily cessation to reassess need and depth are recommended. Other complications include mesenteric ischemia from reduced splanchnic blood flow, corneal exposure requiring mandatory eye care with lubrication and taping, and increased venous thromboembolism risk from immobility necessitating pharmacological prophylaxis.

Combined Prone Positioning and NMB

Synergistic Approach

The combination of prone positioning and neuromuscular blockade addresses multiple mechanisms of ventilator-induced lung injury simultaneously and reflects the protocol used in the PROSEVA trial, in which 91 percent of proned patients received NMB during the first 24 hours. Neuromuscular blockade facilitates safe prone positioning by preventing patient movement during turns, eliminating coughing and bucking that could cause circuit disconnection or accidental extubation, and enabling consistent delivery of lung-protective ventilation in the prone position.

Implementation Protocol

Neuromuscular blockade should be initiated 30 to 60 minutes before the first prone session to ensure adequate paralysis during the turning procedure. NMB should be maintained during initial prone sessions and may be discontinued after the first 24 to 48 hours once a sedation strategy is established and the patient is tolerating the prone position. Subsequent prone sessions can often be accomplished without NMB once the patient and clinical team are experienced with the procedure. Some centers have implemented awake proning protocols for non-intubated patients, particularly during the COVID-19 pandemic, which do not require neuromuscular blockade.

Key Clinical Pearls

• Prone positioning is the only ARDS intervention besides low Vt ventilation with proven mortality benefit (NNT = 6)
• Initiate proning early (<36 hours) and for >= 16 hours/day — shorter sessions do not confer mortality benefit
• Do not discontinue proning based on lack of oxygenation response — the mortality benefit is not solely oxygenation-mediated
• Routine 48-hour NMB infusion is NOT recommended (ROSE trial) — use light sedation with targeted NMB for specific indications
• When NMB is used, deep sedation and BIS monitoring are mandatory to prevent awareness
• Cisatracurium is preferred over aminosteroid agents in ICU due to organ-independent Hofmann elimination
• NMB facilitates prone positioning and may be most valuable in that context rather than as standalone therapy
• Few true contraindications exist for prone positioning — do not deny patients this intervention due to perceived (but not actual) risk

References

1. Guerin C, Reignier J, Richard JC, et al. Prone positioning in severe acute respiratory distress syndrome. N Engl J Med. 2013;368(23):2159-2168.
2. Papazian L, Forel JM, Gacouin A, et al. Neuromuscular blockers in early acute respiratory distress syndrome. N Engl J Med. 2010;363(12):1107-1116.
3. National Heart, Lung, and Blood Institute PETAL Clinical Trials Network. Early neuromuscular blockade in the acute respiratory distress syndrome. N Engl J Med. 2019;380(21):1997-2008.
4. Gattinoni L, Taccone P, Carlesso E, Marini JJ. Prone position in acute respiratory distress syndrome: rationale, indications, and limits. Am J Respir Crit Care Med. 2013;188(11):1286-1293.
5. Munshi L, Del Sorbo L, Bhatt DL, et al. Prone position for acute respiratory distress syndrome: a systematic review and meta-analysis. Ann Am Thorac Soc. 2017;14(Suppl 4):S280-S288.

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