oa Prone positioning in ARDS: physiology, evidence and challenges

Introduction: Prone position has been used since the 1970s as a rescue therapy to treat severe hypoxemia in patients with acute respiratory distress syndrome (ARDS). Despite numerous observational and randomized controlled trials showing the effectiveness of prone position in improving oxygenation, mortality benefit was demonstrated only recently in the PROSEVA study¹. Intensivists taking care of patients with ARDS should be aware about the physiological changes during prone ventilation, the latest evidence available and challenges that can be encountered in managing such patients. Physiology of prone position ventilation: When a person is supine, the weight of the ventral lungs, heart, and abdominal viscera increase dorsal pleural pressure. This compression reduces transpulmonary pressure in the dorsal lung regions. The increased mass of the edematous ARDS lung further increases the ventral-dorsal pleural pressure gradient and reduces regional ventilation of dependent dorsal regions. The ventral heart is estimated to contribute approximately an additional 3 to 5 cm of water pressure to the underlying lung tissue. In addition to the weight of the heart, intraabdominal pressure is preferentially transmitted through the diaphragm, further compressing dorsal regions. Although these factors tend to collapse dependent dorsal regions, the gravitational gradient in vascular pressures preferentially perfuses these regions, yielding a region of low ventilation and high perfusion, manifesting clinically as hypoxemia. Placing a person in the prone position reduces the pleural pressure gradient from nondependent to dependent regions, in part through gravitational effects and conformational shape matching of the lung to the chest cavity² [Figure 1]. Clinical evidence: A few large randomized clinical trials, conducted over a period of 15 years, investigated the possible benefit of prone position on ARDS outcome [Table 1]. The improvements in oxygenation apparent in most trials were not associated with improvements in mortality, suggesting that oxygenation is not itself the source of improved survival with prone positioning. Most recently, the PROSEVA study group¹ enrolled 466 subjects with moderate-to-severe ARDS. Mortality at 28 and 90 days was significantly lower with prone position versus supine position (16% vs 33%, respectively, p < 0.001, and 24% vs 41%, respectively, p < 0.001). Challenges: There are only a few absolute contraindications to prone positioning, such as unstable vertebral fractures and unmonitored or significantly increased intracranial pressure. Hemodynamic instability and cardiac rhythm disturbances are some of the relative contraindications. The common complications of prone positioning are pressure ulcers, ventilator-associated pneumonia and endotracheal tube obstruction. More serious fatal events such as accidental extubation is rare (zero to 2.4% prevalence). A recent meta-analysis of the safety and efficacy of the maneuver showed that it is safe and inexpensive but requires teamwork and skill. Reports in the literature suggest that the incidence of adverse events is significantly reduced in the presence of trained and experienced staff. Thus, centers with less experience may have difficulty managing complications, but nursing care protocols and guidelines can mitigate this risk⁴. Conclusion: Prone position ventilation in patients with moderate-to-severe ARDS improves hypoxemia, provides mortality benefit and is relatively safe.