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David R Schwartz, MD
Atul Malhotra, MD
Robert M Kacmarek, PhD, RRT
Polly E Parsons, MD
Geraldine Finlay, MD
INTRODUCTION — Prone ventilation refers to mechanical ventilation with the patient lying in the prone position. It improves oxygenation in most patients with acute respiratory distress syndrome (ARDS), although the mechanism is uncertain.
The physiologic effects, clinical outcomes, and application of prone ventilation are reviewed here. Patient selection is also discussed. Approaches to mechanical ventilation and supportive care for patients with ALI or ARDS are described separately. (See "Mechanical ventilation in acute respiratory distress syndrome" and "Supportive care and oxygenation in acute respiratory distress syndrome".)
PHYSIOLOGIC EFFECTS — The improvement of oxygenation during prone ventilation is multifactorial (table 1). The most important factors are probably the optimization of ventilation and perfusion, although changes in the distribution of extravascular lung water and secretions may also play a role.
Ventilation — Prone positioning improves ventilation via its effect on pleural pressure and lung compression. Increased functional residual capacity (FRC) has also been proposed, but changes in FRC have not been a dominant finding in most studies of prone ventilation [1-3].
Pleural pressure — The alveolar distending pressure is estimated by the transpulmonary pressure (Ptp), which is defined as the difference between the airway pressure (Paw) and pleural pressure (Ppl):
Ptp = Paw - Ppl
When an individual is supine, the dorsal pleural pressure is greater than ventral pleural pressure. As a result, the ventral transpulmonary pressure exceeds the dorsal transpulmonary pressure and there is greater expansion of the ventral alveoli than the dorsal alveoli.
This effect is exaggerated in supine patients with acute respiratory distress syndrome (ARDS), probably because the difference between the dorsal and ventral pleural pressures is increased by the excess lung weight. The result is overinflation of the ventral alveoli and atelectasis of the dorsal alveoli (image 1) [4,5].
Prone positioning reduces the difference between the dorsal and ventral pleural pressures, making ventilation more homogeneous [6]. This leads to a decrease in alveolar overinflation and alveolar collapse [7]. This should minimize stress and strain on alveoli, limiting injury from overdistention and cyclic atelectasis. (See "Mechanical ventilation in acute respiratory distress syndrome".) In addition, prone ventilation should promote opening of alveoli that had collapsed during both initial standard supine ventilation (ie, recruitment) and following supine positioning during a prone ventilation strategy. The result is improved ventilation and oxygenation, which many patients sustain even after they return to the supine position [1,7-9].
Compression — When an individual is supine, the heart compresses the posterior lung parenchyma and the diaphragm compresses the posterior-caudal lung parenchyma. The latter is caused by the abdominal contents displacing the diaphragm caudally, which can be exacerbated by a loss of diaphragmatic tone due to sedation and/or paralysis [10]. Compression by either the heart or the diaphragm may cause regional lung collapse and increase hypoxemia (figure 1) [11].
During prone ventilation, the heart becomes dependent and pre-load increases, which decreases posterior compression of the lung parenchyma and may increase the cardiac index [6,12-15]. In addition, the diaphragm is displaced, decreasing compression of the posterior-caudal lung parenchyma [16]. These effects improve ventilation and oxygenation [17].
Perfusion — 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. This improves when the patient is moved into the prone position because the previously dependent lung continues to receive the majority of the blood flow as alveoli reopen, while the newly dependent lung continues to receive the minority of the blood flow as alveoli begin to collapse [18].
It was previously hypothesized that prone ventilation permits the redistribution of blood flow to aerated lung. However, there is little evidence to support this belief, with most studies indicating that the blood flow pattern remains virtually unchanged upon turning prone [19,20]. Even with similar degrees of dependent atelectatic alveoli, the shunt fraction and oxygen tension will be higher in the prone patient.
CLINICAL OUTCOMES
Oxygenation — The majority of trials have consistently shown that prone ventilation increases arterial oxygen tension (PaO2) in most patients with acute respiratory distress syndrome (ARDS), allowing a reduction in the fraction of inspired oxygen (FiO2) [2,21-24]. Among patients whose oxygenation improves during prone ventilation, some continue to have improved oxygenation for hours after they return to the supine position and many improve each time prone ventilation is repeated (figure 2) [8,9,25].
The best predictor of a sustained increase in PaO2 during prone ventilation is improved oxygenation during a brief trial, as illustrated by an uncontrolled trial of 13 patients with moderate to severe ARDS [26]. In the trial, a 10 mmHg increase in PaO2 over the first 30 minutes of prone ventilation predicted a sustained increase in PaO2 over the next two hours. In contrast, patients whose PaO2 did not increase during the first 30 minutes of prone ventilation showed no subsequent improvement in their oxygenation.
Although not validated by high quality evidence, the following factors may also predict improved oxygenation during prone ventilation:
●Patients with diffuse pulmonary edema and dependent alveolar collapse appear more likely to improve their PaO2 during prone ventilation than patients with predominantly anterior abnormalities, marked consolidation, and/or fibrosis [27].
●Patients with an extrapulmonary cause for their ARDS seem more likely to increase their PaO2 during prone ventilation than patients with a pulmonary cause [28].
●Patients with elevated intraabdominal pressure appear more likely to increase their PaO2 during prone ventilation than patients with normal intraabdominal pressure [29].
●Patients whose chest wall compliance decreases when moving from the supine to the prone position are likely to improve their PaO2 during prone ventilation [2].
Mortality — Prone ventilation may improve mortality in some patients with severe ARDS. While numerous randomized trials and one meta-analysis reported no mortality benefit with prone positioning for ARDS in general [30,31], some studies suggested that the subgroup of patients with severe ARDS may benefit [32-34]. One observational study of 218 patients with severe ARDS (PaO2:FiO2 ratio <150 mmHg) reported a mortality of 19 percent, well below the expected mortality for this population [35]. Additionally, a meta-analysis of the subgroups of patients with severe ARDS (PaO2:FiO2 <100 mmHg) from seven randomized trials (555 patients) found that prone ventilation reduced mortality (53 versus 63 percent, RR 0.84, 95% CI 0.74-0.96) [36].
The benefit of prone ventilation in this subpopulation is further supported by a single large randomized trial of early (within 33 hours of intubation), high-dose (17 consecutive hours) prone ventilation for severe ARDS (PROSEVA) [24]. This trial of 466 patients receiving low tidal volume mechanical ventilation for severe ARDS (PaO2:FiO2 <150 mmHg, FiO2 ≥0.6, PEEP ≥5 cm H2O), reported that, compared to patients ventilated in the supine position, patients receiving prone ventilation (average time spent prone: 73 percent) had a reduction in 28-day mortality (16 versus 33 percent; hazard ratio [HR], 0.39; 95% CI, 0.25-0.63) and 90-day mortality (24 versus 41 percent; HR, 0.44; 95% CI, 0.29-0.67, respectively). The mortality benefit occurred without excess risk of complications. (See 'Indications' below and 'Duration' below and 'Risks' below.)
Important issues to note in this trial (PROSEVA) that may have affected the outcome or limit its generalizability are the following:
●The intervention was applied to a highly selective group of patients that represent only a minority of patients with ARDS. Of the 1434 patients with severe ARDS screened, 858 were not eligible on the basis of exclusion criteria alone (see below). An additional 107 patients were excluded prior to randomization due to improvement after a predefined 12 to 24 hour window, organizational issues, or withdrawal by their physician. Thus, the mortality benefit likely applies to a small select group of patient with ARDS.
●The list of exclusion criteria was lengthy and included elevated intracranial pressure, spinal or other fracture instability, massive hemoptysis, deep venous thrombosis treated for less than two days, a mean arterial pressure less than 65 mmHg, anterior chest tube with leaks, chronic oxygen-dependent respiratory failure, use of NIV inhaled nitric oxide or almitrine bismesylate, or extracorporeal membrane oxygenation (ECMO) before inclusion (table 2). Thus, patient selection for prone positioning appears to be important in determining a good outcome.
●Despite randomization, patient groups were not matched. Patients ventilated in the supine position had higher sequential organ failure assessment scores (SOFA) and were receiving more pressors and neuromuscular blocking agents, suggesting they were a sicker population of patients that may have biased the outcome.
●Study site staff had extensive experience (five years) with prone ventilation so that the same benefits may not apply to facilities with untrained staff.
Although this study suggests mortality benefit of early, high-dose prone ventilation in a select population of severe ARDS, a second randomized trial will be needed to validate this benefit. The indications and application of prone positioning are discussed separately. (See 'Patient selection' below and 'Duration' below.)
Other — There is no evidence that prone ventilation prevents organ system dysfunction [32] or reduces the ICU length of stay [31,33]. Prior studies suggested that it does not shorten the duration of mechanical ventilation [30,31]. However, the large randomized PROSEVA trial suggested improvement in ventilator–free days (14 versus 10 ventilator-free days at day 28) and time to extubation (85 versus 65 percent successful extubations at day 90) in the prone position group [24].
PATIENT SELECTION — We believe that prone positioning is likely effective in select patients with severe ARDS who fail to improve with standard-of-care supine low tidal volume ventilation strategies. Our view is based upon the proven physiological effects of prone ventilation and the evidence that suggests a mortality benefit in the highly select group of severely ill patients with ARDS. The benefit appears to be optimal when prone positioning is high-dose (12 to 18 consecutive hours per day) and applied early (up to 36 hours after intubation). (See 'Mortality' above and 'Indications' below and 'Duration' below.)
Indications — There are no universally accepted indications for the initiation and timing of prone ventilation. In the one randomized trial that showed a mortality benefit (PROSEVA), there was a 12 to 24 hour stabilization period before the initiation of proning in the patients with severe ARDS that met eligibility criteria for the trial [24]. Thus, in our clinical practice, we use the supine position with lung protective ventilatory strategies as the routine initial management for patients with ARDS. We reserve prone positioning as an early rescue therapy for refractory hypoxemia or severe ARDS, provided select exclusion criteria are met (table 2) and other ventilatory strategies with a known mortality benefit have been tried. (See "Mechanical ventilation in acute respiratory distress syndrome" and 'Mortality' above and 'Duration' below.)
The degree of hypoxemia and severity of ARDS that warrants prone ventilation has been variably defined. The only randomized trial to show benefit (PROSEVA) defined severe ARDS as those having a PaO2:FiO2 ratio <150 mmHg with a FiO2 ≥0.6 and PEEP ≥5 cm H2O. An alternative reasonable definition is a PaO2:FiO2 of ≤100 mmHg with a PaO2 ≤60 mmHg despite optimization of the ventilator settings on FIO2 of 1 [24,31,36].
We maintain a low threshold for initiating prone ventilation early (up to 36 hours) in the course of mechanical ventilation for ARDS. This is based on the observation from PROSEVA and additional trials that early initiation of prone ventilation was most effective, as well as the physiologic rationale that collapsed lung units are likely to be opened (ie, recruited) most easily during the acute exudative phase of ARDS [24,33].
The optimal duration of prone positioning is discussed separately. (See 'Duration' below.)
Contraindications — Spinal instability is an absolute contraindication to prone ventilation. Hemodynamic and cardiac abnormalities (eg, pacemaker) are considered relative contraindications, since immediate access for cardiopulmonary resuscitation is limited during prone ventilation. Thoracic and abdominal surgeries are also considered relative contraindications, although prone ventilation has been accomplished safely during the early postoperative period (table 2).
Exclusion criteria in PROSEVA, that reported benefit in severe ARDS were many, and included elevated intracranial pressure, spinal or other fracture instability, massive hemoptysis, deep venous thrombosis treated for less than two days, a mean arterial pressure less than 65 mmHg, anterior chest tube with leaks, chronic oxygen-dependent respiratory failure, use of NIV inhaled nitric oxide or almitrine bismesylate or extracorporeal membrane oxygenation (ECMO) before inclusion. Although not true contraindications, these criteria may need to be replicated to see benefit outside of a trial setting.
APPLICATION
Positioning — There is no standard method for moving a patient from the supine to the prone position. Our intensive care unit uses a log roll, which is described step-by-step in the table (table 3).
A commercially available bed, the Roto-Prone, facilitates prone positioning, minimizes risk during turning, and provides some continuous rotation if desired. One retrospective cohort study of 61 surgical and trauma patients with ALI/ARDS compared prone positioning with the Roto-Prone bed to supine positioning [37]. The study found non-statistically significant improvement in oxygenation, ventilator-free days, and hospital days among patients positioned prone. It also found a statistically significant decrease in mortality among patients positioned prone. Randomized trials are needed to investigate the efficacy and safety of this bed and to justify widespread use.
Regardless of the technique, moving the patient into the prone position is labor intensive requiring a coordinated effort among respiratory therapists and nurses for each turn [38]. 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. An experienced clinician who can reintubate the patient if necessary should also be present. There are commercially available devices that can assist the movement, including one type of bed that can both initiate and maintain the prone position [39].
Once the patient is in the prone position, the head of their bed should be elevated 30 to 45 degrees. This may facilitate gastric emptying, as well as minimizing facial and ocular edema.
Prone ventilation — Mechanical ventilation in the prone position is similar to that employed when the patient is supine. (See "Mechanical ventilation in acute respiratory distress syndrome".)
Peak and plateau airway pressures tend to increase immediately after a patient is placed in the prone position, but typically decline with time. The initial increase is likely related to decreased chest wall compliance, while the subsequent decrease is probably due to progressive alveolar recruitment [40]. Paralysis is not required and potentially harmful, since it could exacerbate supradiaphragmatic alveolar collapse [10,41].
Prone ventilation does not require additional monitoring, although the need for endotracheal suctioning should be assessed with increased frequency after the patient is placed prone because large quantities of secretions may drain into the endotracheal tube. Electrocardiographic leads should be placed on the back. All involved staff should know how to quickly put the patient back into the supine position, if necessary, since this is required if cardiopulmonary resuscitation becomes necessary.
Duration — The optimal duration of prone positioning is unknown. Most studies have used either repeated sessions of prone ventilation lasting six to eight hours per day [30,32] or prolonged prone ventilation lasting 17 to 20 hours (figure 2) [9,24,31,33], with similar results. In the one randomized study that showed a mortality benefit for prone positioning in severe ARDS (PROSEVA), the mean duration of time in the prone position was 17 hours per day with an average of four sessions per patient [24]. Prone ventilation was continued for the study period for up to 28 days. It was stopped for continued improvement in oxygenation (PaO2:FiO2 ≥150 mmHg, FiO2 ≤0.6, PEEP ≤10 cm H2O) maintained for at least four hours after the end of the last prone session.
We believe that minimizing the frequency of turning severely ill patients decreases the likelihood of complications. Supported by data from PROSEVA, we prefer to maintain prone ventilation for longer periods (12 to 18 hours), with position changes as needed for interim nursing care and interventions. Cessation of proning is reasonable after signs of improved oxygenation or for acute emergencies, prolonged interventions or surgical procedures.
Risks — Prone positioning increases the risk of certain complications (table 2). However, estimates of the frequency of such complications have been variable [24,31,42] :
●In a randomized trial of 342 patients, the following adverse events were more likely among patients undergoing prone ventilation than among patients undergoing conventional supine ventilation: Increased need for sedation or paralysis (80 versus 56 percent), hypotension or arrhythmias (72 versus 55 percent), transient oxyhemoglobin desaturation (64 versus 51 percent), airway obstruction (51 versus 34 percent), vomiting (29 versus 13 percent), loss of venous access (16 versus 4 percent), and displacement of the endotracheal tube (11 versus 5 percent) [31].
●In contrast, a review of 240 patients who underwent prone ventilation (>746 prone cycles) reported far fewer adverse events: Hemodynamic instability (1.1 percent per prone cycle), inadvertent extubation (0.4 percent per prone cycle), decreased oxyhemoglobin saturation (0.3 percent per prone cycle), and apical atelectasis (0.3 percent per prone cycle) [42]. In addition, each of the following events occurred with an incidence of only 0.1 percent per prone cycle: obstructed endotracheal tube, kinked endotracheal tube, obstructed chest tube, dislodged central venous catheter, dislodged femoral hemodialysis catheter, compressed tubing infusing vasoactive medications, and transient episodes of supraventricular tachycardia.
Transient hemodynamic instability and oxyhemoglobin desaturation related to turning the patient can be minimized by providing adequate sedation and preoxygenating with a fraction of inspired oxygen (FiO2) of 1.0 prior to moving the patient [8].
Skin breakdown over pressure points and dependent facial edema can occur. These adverse events can be minimized by frequent repositioning and soft padding.
Enteral feeding during prone ventilation is frequently complicated by emesis and/or increased residual gastric volumes [43]. The rate of enteral feeding should be reduced during prone ventilation, in order to minimize the likelihood of complications such as aspiration. (See "Nutrition support in critically ill patients: Enteral nutrition".)
Although not proven, centers experienced in prone positioning may have fewer complications. The only randomized controlled trial (PROSEVA) to show benefit of prone positioning for severe ARDS was performed in 25 centers by staff with five years experience in proning [24]. In that trial, the rate of expected complications (eg, unplanned extubation, endotracheal tube obstruction, hemoptysis, arterial desaturations, bradycardia and severe hypotension) was no different between the groups. Supine patients had a higher rate of cardiac arrest, which could be explained by the higher SOFA scores in this group. However, the lack of difference in other complications suggests that ICUs experienced in turning patients frequently may protect against complications.
SUMMARY AND RECOMMENDATIONS
●Prone ventilation refers to mechanical ventilation with the patient lying in the prone position. It increases oxygenation in most patients with acute respiratory distress syndrome (ARDS). Initial data suggests that a mortality benefit is limited to a select group of patients with severe ARDS. (See 'Clinical outcomes' above.)
●For most patients with ARDS, we recommend using supine ventilation rather than prone ventilation as the initial ventilation strategy for patients with ARDS (Grade 1B). (See 'Indications' above.)
●For patients with refractory hypoxemia or severe ARDS (PaO2:FiO2 ratio <150 mmHg with a FiO2 ≥0.6 and PEEP ≥5 cm H2O) despite use of lung protective ventilatory strategies, we suggest a trial of prone ventilation, provided exclusion criteria are met (Grade 2B). (See 'Patient selection' above.)
●We prefer to maintain prone ventilation for 12 to 18 hours, with position changes as needed for interim nursing care and interventions. Cessation of proning is reasonable after signs of improved oxygenation or for acute emergencies, prolonged interventions, or surgical procedures. (See 'Application' above.)
REFERENCES
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