Acute respiratory distress syndrome: Prone ventilation in adults

INTRODUCTION — 

Prone ventilation may be used for the treatment of acute respiratory distress syndrome (ARDS).

The rationale for, indications and contraindications, and procedural technique of prone ventilation are reviewed here. The diagnosis, management, and prognosis of ARDS are described separately.

●(See "Acute respiratory distress syndrome: Epidemiology, pathophysiology, pathology, and etiology in adults".)

●(See "Acute respiratory distress syndrome: Clinical features, diagnosis, and complications in adults".)

●(See "Acute respiratory distress syndrome: Ventilator management strategies for adults".)

●(See "Acute respiratory distress syndrome: Fluid management, pharmacotherapy, and supportive care in adults".)

●(See "Acute respiratory distress syndrome: Prognosis and outcomes in adults".)

DEFINITION(S)

●Prone ventilation – Prone ventilation refers to the delivery of invasive mechanical ventilation with the patient lying prone. It is not a mode of mechanical ventilation. Modes of mechanical ventilation are discussed separately. (See "Modes of mechanical ventilation".)

●Awake pronation – Prone positioning in spontaneously breathing patients is termed awake pronation and is discussed separately. (See "COVID-19: Respiratory care of the nonintubated hypoxemic adult (supplemental oxygen, noninvasive ventilation, and intubation)", section on 'Awake pronation'.)

RATIONALE — 

Prone positioning improves gas exchange and lung compliance by ameliorating the ventral-dorsal transpulmonary pressure (Ptp) difference, reducing dorsal lung compression, and improving lung perfusion (figure 1).

•The effects of supine positioning – When an individual is supine, the dorsal Ppl is greater than the ventral Ppl. As a result, the ventral Ptp exceeds the dorsal Ptp and there is a greater expansion of the ventral alveoli compared with the dorsal alveoli. This effect is exaggerated in supine patients with ARDS, probably because the difference between the dorsal and ventral Ppl is increased by excess lung weight. The result is a tendency towards overinflation of the ventral alveoli and atelectasis of the dorsal alveoli (image 1) [1,2].

•The effects of prone positioning – Prone positioning reduces the difference between the dorsal and ventral Ptp, making ventilation more homogeneous [3], and thereby leading to a decrease in ventral alveolar overinflation and dorsal alveolar collapse [4]. As a result, reduced alveolar distension limits ventilator-associated lung injury from overdistention and cyclic atelectasis. Prone ventilation also recruits (ie, opens) alveoli that had collapsed during supine ventilation, a process that may continue over time when the patient is lying prone while receiving appropriate positive end-expiratory pressure. The result is improved ventilation and oxygenation, which many patients sustain even after they return to the supine position [4-7]. (See "Acute respiratory distress syndrome: Ventilator management strategies for adults".)

●Reduced lung compression – Prone positioning can favorably affect lung compression by both the heart and the diaphragm.

•The effects of supine positioning – When an individual with ARDS is supine, the heart compresses the medial posterior lung parenchyma and the diaphragm compresses the posterior-caudal lung parenchyma. The latter is caused by the abdominal contents displacing the diaphragm caudally (ie, cranially), which can be exacerbated by a loss of diaphragmatic tone due to sedation and/or paralysis or increased abdominal pressure [8]. Compression by either the heart and/or the diaphragm may exaggerate dependent lung collapse in the supine position, increasing hypoxemia (ie, worsening shunt) and ventilator-associated lung injury [9].

•The effects of prone positioning – During prone ventilation, the heart becomes dependent, lying on the sternum, potentially decreasing medial posterior lung compression. In addition, the diaphragm is displaced caudally (especially in obesity and when the abdomen is left unsupported), decreasing compression of the posterior-caudal lung parenchyma [10]. These effects improve ventilation and oxygenation [11].

●Improved lung perfusion – Improved perfusion of the dependent portions of the lung is thought to be partially responsible for the improved oxygenation seen with prone ventilation.

•The effects of supine position – In ARDS, there is substantial ventilation-perfusion mismatch in the supine position since blood flow and alveolar collapse are both greatest in the dependent portions of the lung.

•The effects of prone positioning – Ventilation/perfusion matching improves in the prone position. As alveoli reopen, the previously dependent lung continues to receive the majority of the blood flow while the newly dependent lung continues to receive the minority of the blood flow as alveoli begin to collapse [12]. In addition, increased lung recruitment and reduction in hypoxic pulmonary vasoconstriction may also improve cardiac output by increasing right ventricular (RV) preload, decreasing RV afterload, and decreasing pulmonary vascular resistance [3,13-17].

●Others – Other purported beneficial changes include increased functional residual capacity and altered distribution of extravascular lung water and secretions [5,18,19].

INDICATIONS — 

Prone ventilation is almost exclusively used in patients with refractory ARDS. Despite the benefits and increased use of prone positioning during the COVID-19 pandemic, data suggest underutilization, especially among those with non-COVID-related severe ARDS [20,21].

Use of prone positioning in nonintubated COVID-19 patients is discussed separately. (See "COVID-19: Respiratory care of the nonintubated hypoxemic adult (supplemental oxygen, noninvasive ventilation, and intubation)", section on 'Awake pronation'.)

ARDS unresponsive to lung-protective ventilation — Prone ventilation is an option for patients with ARDS who, despite lung-protective ventilation, have moderate to severe hypoxemia (eg, partial arterial oxygen tension [PaO₂]/fraction of inspired oxygen [FiO₂] <150 mmHg) and/or require unacceptably high ventilator settings to achieve adequate gas exchange (eg, plateau pressure >30 cm H₂O). Prone ventilation increases oxygenation and improves mortality in this subgroup of ARDS patients (see 'Efficacy' below). Other strategies that are options in this subgroup of ARDS patients are discussed separately. (See "Extracorporeal life support in adults: Management of venovenous extracorporeal membrane oxygenation (V-V ECMO)", section on 'Severe acute respiratory failure'.)

The degree of hypoxemia and severity of ARDS that warrants prone ventilation has been variably defined. We use a PaO₂/FiO₂ ratio <150 mmHg on a FiO₂ ≥0.6 and positive end-expiratory pressure (PEEP) ≥5 cm H₂O since this was the criteria used in a major randomized trial (PROSEVA) that demonstrated efficacy [22]. These criteria include patients with severe ARDS and patients on the worse end of moderate ARDS (table 1). Determining ARDS severity is described separately topic. (See "Acute respiratory distress syndrome: Clinical features, diagnosis, and complications in adults", section on 'Clinical diagnosis'.)

Prone ventilation is not typically used in patients with mild ARDS since data are limited [23].

Efficacy — Prone ventilation improves oxygenation and mortality in patients with moderate to severe ARDS. The benefits appear to be consistent among those with ARDS from many etiologies and in those with underlying lung disease or obesity [24-28].

●Oxygenation – Trials have consistently shown that in most patients with ARDS (up to 70 percent), prone ventilation increases PaO₂, allowing a reduction in the FiO₂ [18,22,29-31]. Among patients whose oxygenation improves while prone, oxygen levels may continue to improve in the supine position and improve further when prone ventilation is repeated (figure 2) [6,7,32,33]. The definition of what is considered a response is described below. (See 'Assessing the response' below.)

●Mortality – Prone ventilation reduces mortality in patients with moderate to severe ARDS (ie, PaO₂/FiO₂ <150 mmHg) who are managed with low tidal volume ventilation (LTVV) [34-41].

•A 2021 meta-analysis of five trials including over 900 adults with moderate to severe ARDS (median PaO₂/FiO₂ 118 mmHg) reported that prone ventilation resulted in a 27 percent reduction in hospital mortality when compared with LTVV alone (risk ratio 0.73, 95% CI 0.56-0.96) [39]. The included trials have used different definitions of ARDS, thresholds for initiation, and durations of proning, which may explain the moderate heterogeneity.

•The sentinel study reporting mortality benefit, PROSEVA [22], has driven most protocolized use of proning since its publication. In PROSEVA, after a 12- to 24-hour initial stabilization period of LTVV in the supine position, 466 patients with moderate to severe ARDS (PaO₂/FiO₂ <150 mmHg, FiO₂ ≥0.6, PEEP ≥5 cm H₂O) were randomized to receive prone ventilation together with LTVV or continued LTVV in the supine position. LTVV in the prone position resulted in a reduction in 28-day mortality compared with LTVV alone (16 versus 33 percent; hazard ratio [HR] 0.39, 95% CI 0.25-0.63). The mortality benefit was maintained at 90 days (24 versus 41 percent; HR 0.44, 95% CI 0.29-0.67) and occurred without excess risk of complications. In addition, patients in the prone group needed less rescue therapy, including extracorporeal membrane oxygenation (1 versus 2.6 percent) or inhaled nitric oxide (10 versus 16 percent). Pronation also resulted in more ventilator-free days at day 28 (14 versus 10 ventilator-free days) and successful extubations (85 versus 65 percent at day 90).

Limitations included lengthy exclusion criteria and extensive staff experience, which may impact generalizability. Perhaps favoring the outcome, more patients in the prone group received neuromuscular blocking agents and patients in the supine group were sicker, although the mortality benefit was maintained when adjusted for these variables. In addition, PEEP was not optimized before pronation per the ARDSNet protocol (table 2) (average PEEP <10 cm H₂O).

CONTRAINDICATIONS — 

Contraindications have evolved as experience with pronation has increased. When assessing suitability, we use clinical judgment to individualize prone ventilation.

Absolute — Absolute contraindications to prone ventilation are listed in the table (table 3).

For patients whose spine has been stabilized with surgery, we consult with the operating surgeon before prone positioning is instituted. Patients with atlanto-occipital joint dysfunction may be stabilized using a neck collar.

While not ideal, some case reports suggest successful use in patients with chest wall trauma and traumatic brain injury, the latter being followed with an intracranial pressure monitor [42].

Relative — Relative contraindications are listed in the table (table 3).

Pregnancy, especially late-term, was previously listed as an absolute contraindication since pregnant individuals were excluded from major trials. However, case reports and experience during COVID-19 suggest success with prone ventilation [43-45]. Proper positioning to limit abdominal and pelvic compression and continuous monitoring of fetal heart tones may facilitate pronation during pregnancy.

Thoracic and abdominal surgeries are also considered relative contraindications, although prone ventilation has been accomplished safely during the early postoperative period.

Other relative contraindications include difficult airway or difficult intubation and massive hemoptysis.

The following are not contraindications:

●Kidney replacement therapy – This applies to replacement therapy through femoral, jugular, or subclavian catheters.

●Obesity – In one randomized trial that demonstrated a mortality benefit, the median body mass index was 28, ranging from 21 to 36 [22]. However, turning patients with obesity may pose more procedural challenges.

●Extracorporeal membrane oxygenation – These data are provided separately. (See "Extracorporeal life support in adults: Management of venovenous extracorporeal membrane oxygenation (V-V ECMO)", section on 'Prone positioning'.)

PRONE PROCEDURE — 

Most institutions have policies in the practice of prone ventilation.

Timing of initiation — When indicated, we initiate prone ventilation within 36 hours of mechanical ventilation, provided there are no contraindications (table 3). The greatest efficacy is seen when pronation is initiated early; in addition, collapsed lung units are likely to be opened (ie, recruited) most easily during the early (ie, exudative) phase of ARDS (table 1) [22,46]. (See 'Efficacy' above and 'Rationale' above.)

Positioning — Most intensive care units (ICUs) support manual pronation procedures with their own protocol since there is no standard method for it. We use a log roll, which is described step-by-step in the table (table 4). While some protocols provide neuromuscular blockade (for ventilator synchrony and safety) prior to turning, we do not since data suggest it is not necessary [47]. This video is also freely available.

While prone, the patient's position is changed every 2 to 4 hours (eg, arms in the swimmer's crawl position, or down by their side, face turned to the opposite side).

Regardless of the technique, moving the patient into the prone position is labor-intensive, requiring a well-prepared and -coordinated effort among ICU staff for each turn [48]. The respiratory therapist ensures the stability of the endotracheal tube, one nurse protects the vascular access lines, and at least two other staff members turn the patient (higher if the patient has a high number of devices or is on extracorporeal membrane oxygenation [ECMO]). An experienced clinician who can reintubate the patient (if necessary) should also be present. All involved staff should know how to quickly put the patient back into the supine position if necessary.

Transient hemodynamic instability and oxyhemoglobin desaturation related to turning the patient are frequent; we minimize this phenomenon by providing adequate sedation and preoxygenation with a fraction of inspired oxygen of 1 prior to moving the patient [6].

We do not use commercially available beds that can initiate, maintain, and facilitate prone positioning, and are thought to minimize risk during turning. Comparative data with manual prone positioning protocols are limited and do not justify the widespread use of dedicated proning beds [48-50].

Session duration — Most studies have demonstrated similar results with either repeated sessions of prone ventilation lasting six to eight hours per day [51,52] or prolonged prone ventilation lasting 16 to 20 hours per day (figure 2) [7,22,46,53]. Based upon a major trial showing a mortality benefit (PROSEVA), we maintain prone ventilation for 16 to 20 hours per day, with position changes as needed for interim nursing care and interventions. In PROSEVA, the mean duration of time in the prone position was 17 hours per day, with an average of four sessions in total per patient [22].

Cessation of a prone ventilation session is appropriate for acute emergencies, prolonged interventions, or surgical procedures. Occasionally, session duration is determined by staffing availability.

Ventilatory strategies — The ventilatory strategy for ARDS patients in the prone position is similar to that in supine patients (ie, a lung-protective strategy with low tidal volume ventilation and positive end-expiratory pressure optimization (table 2)). This strategy is discussed separately. (See "Acute respiratory distress syndrome: Ventilator management strategies for adults" and 'Efficacy' above.)

Prone positioning is rarely combined with other methods of ventilation (eg, noninvasive ventilation, high-frequency ventilation) or therapies for improving oxygenation (eg, inhaled nitric oxide, ECMO), and case reports suggest success [54-57]. (See "Noninvasive ventilation in adults with acute respiratory failure: Benefits and contraindications" and "High-frequency ventilation in adults" and "Acute respiratory distress syndrome: Fluid management, pharmacotherapy, and supportive care in adults", section on 'Inhaled pulmonary vasodilators'.)

Peak and plateau airway pressures may increase immediately after proning but typically decline with time. The initial increase is likely related to decreased chest wall compliance and the mobilization of secretions [58]. The subsequent decrease is probably due to progressive alveolar recruitment.

Routine care

Sedation — Many patients who undergo prone ventilation require increased sedation, and some require neuromuscular blockade. Further details are provided separately. (See "Sedative-analgesia in ventilated adults: Management strategies, agent selection, monitoring, and withdrawal" and "Sedative-analgesia in ventilated adults: Medication properties, dose regimens, and adverse effects" and "Neuromuscular blocking agents in critically ill patients: Use, agent selection, administration, and adverse effects".)

Monitoring — Prone ventilation does not require additional monitoring except for the following:

●We check the need for more frequent endotracheal suctioning since, in some patients, large quantities of pulmonary secretions may drain into the endotracheal tube.

●Electrocardiographic leads should be placed on the back, mirroring their positioning on the anterior chest wall [59,60].

For patients with tracheostomy, the need for prone ventilation is rare. Nonetheless, specially designed disposable prone position head cushions with a mirror can be used for airway access during suctioning.

Feeding — Once prone, we resume enteral tube feeds at the previous rate. We place the patient's head in a slightly elevated position, monitor for high residual volumes, and have a low threshold to administer prokinetic agents. Before repositioning back to the supine position, we temporarily stop tube feeding, empty the stomach, and resume feeds when supine. However, this practice is not universal. Further data on the management of gastrointestinal complications of enteral feeding are provided separately. (See "Nutrition support in critically ill adult patients: Enteral nutrition", section on 'Monitoring and management of complications'.)

Limited data suggest that feeding in the prone position is safe, although data are conflicting regarding increased emesis rates [61-64]. One study of 69 patients ventilated in the prone position evaluated the use of a protocol consisting of continuous feeding with a rate increase of 25 mL every six hours, 25-degree head elevation, and prophylactic erythromycin (250 mg intravenously every six hours). Compared with the period before protocol implementation, the new protocol allowed faster titration to the nutrition target without increased gastric residuals, vomiting, or ventilator-associated pneumonia [63]. Further data are needed before we would routinely adopt this protocol.

Mouth, eye, and skin care — Mouth and skin care regimens are mostly similar to those in supine patients.

We place foam pads/dressings on bony prominences of the face and cheek, knees, and shoulders. Mouth care regimens for the prevention of ventilator-associated pneumonia are described separately. (See "Risk factors and prevention of hospital-acquired and ventilator-associated pneumonia in adults", section on 'Maintain oral care and toothbrushing'.)

The eyes may be covered with gauze to prevent corneal erosions and conjunctivitis.

Procedures — Most procedures are performed in the supine position, although some experts have successfully performed bronchoscopy during prone ventilation [65]. Similarly, the patients should be supine for planned transport to imaging suites and to other units. Clinicians should be aware that alveolar derecruitment may occur while supine. Cardiopulmonary resuscitation in prone patients is discussed separately. (See "COVID-19: Arrhythmias and conduction system disease", section on 'Patients requiring cardiopulmonary resuscitation (CPR)'.)

ASSESSING THE RESPONSE — 

Our approach is the following:

●Measure oxygenation and compliance parameters – To assess the response, we typically obtain an arterial blood gas and measure lung compliance (tidal volume/plateau pressure minus positive end-expiratory pressure [PEEP]) while supine one hour before pronation and one to four hours after pronation. However, the optimal timing of assessment is unknown and protocols differ. For example, some clinicians obtain these measures at the end of each supination and pronation session.

●Response – We consider a response to be a sustained improvement in gas exchange (eg, >10 mmHg partial arterial oxygen tension [PaO₂] on stable ventilator settings, 10 to 20 point increase in PaO₂/fraction of inspired oxygen [FiO₂] ratio) and/or evidence of alveolar recruitment (eg, increase in lung compliance), which reduces the risk of ventilator-induced lung injury. While improvement in both parameters is ideal, we continue pronation if a response is seen in one parameter more than the other [66].

A response is typically noted during the first hour of the initial trial, but longer periods (eg, 12 to 18 hours) are appropriate to ensure a response, provided no life-threatening hypoxemia is present. If a response is noted, we repeat pronation sessions until indications for cessation are met. (See 'Cessation of pronation' below.)

Predictors of a response are poorly studied but may include the following [18,67-69]:

•Later phase of ARDS (eg, proliferative phase) (figure 3)

•Diffuse pulmonary edema and dependent alveolar collapse

•Extrapulmonary cause for ARDS

•Elevated intra-abdominal pressure

•Reduction in chest wall compliance during pronation

●No or limited response – We consider a failed response as no change in the patient's gas exchange and lung mechanics or a worsening of gas exchange or cardiovascular status. When this occurs, we return the patient to the supine position and pursue alternate strategies for improving oxygenation (eg, extracorporeal membrane oxygenation, high-level PEEP). These strategies are discussed separately. (See "Extracorporeal life support in adults in the intensive care unit: Overview" and "Acute respiratory distress syndrome: Ventilator management strategies for adults", section on 'Refractory patients'.)

CESSATION OF PRONATION — 

We follow the PROSEVA protocol [22] and stop prone ventilation when all of the following parameters are maintained for at least four hours while supine after the end of the last prone session:

●Partial arterial oxygen tension/fraction of inspired oxygen (FiO₂) ratio ≥150 mmHg

●FiO₂ ≤0.6

●Positive end-expiratory pressure ≤10 cm H₂O

While most patients only require pronation for the first few days (eg, 4 to 7 days), some take weeks to improve. (See 'Assessing the response' above.)

COMPLICATIONS — 

The complications of prone ventilation are listed in the table (table 5).

The overall rate of complications appears to be similar to that of supine patients [22]. However, rates may be center-specific or influenced by patient selection and staff experience [22,53,70-72]. In PROSEVA, the rate of expected complications was similar between the groups [22], although staff had at least five years of experience.

Pressure point-related adverse effects, such as skin breakdown, dependent facial and ocular edema, and brachial plexus neuropathy, can be minimized by frequent repositioning and soft padding [71]. Risk factors for pressure ulcers mostly relate to time spent prone, but male sex, age ≥60 years, poor nutrition, and body mass index <28.4 may also increase the risk [71].

SOCIETY GUIDELINE LINKS — 

Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Acute respiratory failure and acute respiratory distress syndrome in adults" and "Society guideline links: Assessment of oxygenation and gas exchange".)

SUMMARY AND RECOMMENDATIONS

●Definition – Prone ventilation refers to the delivery of invasive mechanical ventilation with the patient lying prone. Prone positioning improves gas exchange and lung compliance by ameliorating the ventral-dorsal transpulmonary pressure difference, reducing dorsal lung compression, and improving lung perfusion (figure 1). (See 'Definition(s)' above and 'Rationale' above.)

●Indications – For patients with acute respiratory distress syndrome (ARDS) who, despite lung-protective ventilation, have moderate to severe hypoxemia (eg, partial arterial oxygen tension [PaO₂]/fraction of inspired oxygen [FiO₂] <150 mmHg) and/or require unacceptably high ventilator settings to achieve adequate gas exchange (eg, plateau pressure >30 cm H₂O), we recommend prone ventilation rather than low tidal volume ventilation alone (Grade 1B). Prone ventilation increases oxygenation and improves mortality in this subgroup of ARDS patients. Contraindications to prone ventilation are listed in the table (table 3). (See 'Indications' above and 'Contraindications' above.)

●Prone procedure – Institutions should have policies regarding prone ventilation. (See 'Prone procedure' above.)

•We implement prone ventilation early in the course of ARDS (within the first 36 hours) and use a log roll method, which is described in the table (table 4). We maintain the prone position for 16 to 20 consecutive hours. Lung-protective ventilatory strategies are typically used (table 2). (See 'Timing of initiation' above and 'Positioning' above and 'Ventilatory strategies' above and 'Session duration' above.)

•Routine care is similar to supine patients except increased sedation is usually needed, there is a lower threshold to suction patients, electrocardiography leads are placed on the back, and tube feeds are held and the stomach emptied before resupination. Most procedures and transport require resupination. (See 'Routine care' above and 'Procedures' above.)

●Assessing the response – We consider a response to be a sustained improvement in gas exchange (eg, >10 mmHg PaO₂ on stable ventilator settings, 10 to 20 point increase in PaO₂/FiO₂ ratio) and/or evidence of alveolar recruitment (eg, increase in lung compliance), which reduces the risk of ventilator-induced lung injury. A response is typically noted during the first hour of the initial trial but may be delayed. (See 'Assessing the response' above.)

•If a response is noted, we repeat pronation sessions until indications for cessation are met.

•If there is a limited or no response, we return the patient to the supine position and pursue alternate strategies for improving oxygenation. (See "Acute respiratory distress syndrome: Ventilator management strategies for adults", section on 'Refractory patients'.)

●Cessation of proning – We typically base the decision to cease proning primarily on the oxygenation parameters used in the PROSEVA trial, namely when all of the following parameters are maintained for at least four hours while supine after the end of the last prone session:

•PaO₂/FiO₂ ≥150 mmHg

•FiO₂ ≤0.6

•Positive end-expiratory pressure ≤10 cm H₂O

●Complications – Complications are listed in the table (table 5). The overall rate of complications appears to be similar to that of supine patients, but rates vary and may be center-specific or influenced by patient selection and staff experience. (See 'Complications' above.)

ACKNOWLEDGMENTS — 

The UpToDate editorial staff acknowledges David R Schwartz, MD, and Robert Kacmarek, PhD, RRT, who contributed to earlier versions of this topic review.