The discovery of prone positioning in ARDS
I still remember the first time I “helped” prone a patient with acute respiratory distress syndrome (ARDS) during critical care fellowship. The prone team, led by our critical care nurses, respiratory therapists and technicians, efficiently performed a 180-degree turn of a patient from the supine to prone position, producing a dramatic improvement in arterial oxygen saturation. As I watched, I remember feeling like the least important person in the room. The role of the physician during a prone positioning maneuver is largely to replace a dislodged endotracheal tube. But, the breathing tube did not fall out. In fact, it rarely does. Experienced teams can prone hundreds of patients without dislodging a single endotracheal tube (0-2.4% prevalence) or entangling the complex matrix of intravenous, arterial and bladder catheters and lines. One can argue prone positioning is one of the greatest inventions in critical care medicine. So, whose idea was it?
Prone positioning was discovered by a woman.
She was a nurse.
This should surprise no one who has participated in proning critically ill patients.
While working in a small community ICU in southeastern Michigan, Margaret A. Piehl, RN observed that oxygenation in ARDS patients improved when placed in the prone position. In a seminal paper published in Critical Care Medicine in 1976, she first described the effect of “extreme position changes” on arterial oxygenation in an observational study of five patients with acute respiratory distress syndrome (ARDS).
In the decades following this publication, leaders in critical care medicine, including Luciano Gattinoni, Laurent Papazian and Claude Guerin, tirelessly studied prone positioning to determine the optimal timing, dose (duration) and ARDS population that will benefit the most. Over forty years of clinical trials confirmed that prone positioning improves arterial oxygenation, but evidence of a mortality benefit was lacking until the publication of the PROSEVA trial in the New England Journal of Medicine in 2013. The PROSEVA trial demonstrated that early application of prone positioning in ARDS patients with the most severe hypoxemia (moderate to severe ARDS with PaO2 to FiO2 ratio of 150 mm Hg or less) produced a 17 percent absolute reduction in mortality. This study established that prone positioning has a greater impact on survival than any other ARDS therapy, including lung protective, low tidal volume ventilation and continuous intravenous infusion of neuromuscular blockade.
The physiologic benefit of placing a severely hypoxemic patient with ARDS in the prone position is both elegant and simple. The supine position diminishes functional residual capacity (FRC), or the volume of air in the lungs at the end of exhalation. The thorax is compressed by the upward movement of the diaphragm from the hydrostatic pressure exerted by the abdomen. Like a Slinky, the shape of the lung distorts under its own weight and the weight of the heart due to gravitational forces that compress the dorsal segments of the lung. As a result, the smallest fraction of gas content is found in the dorsal portions of the lungs where the majority of blood flow is directed in the supine position, diminishing the oxygen carrying capacity of the blood. Conversely, when a patient is placed in the prone position, the compressive force exerted by the heart is redirected toward the sternum, taking the weight of the heart off the lungs. The prone position reduces the intrapulmonary shunt fraction through recruitment of dorsal alveoli, producing more homogeneous distribution of inspired gas during each tidal breath, and protecting the lungs from stress and strain that can cause ventilator-induced lung injury (VILI). Prone positioning also redistributes perfusion to better ventilated regions of the lung (i.e. improves V/Q matching). And, as Margaret Piehl observed, prone positioning assists in pulmonary secretion clearance.
As a medical community, we will continue to search for evidence-based treatments for coronavirus. Meanwhile, a simple turn to prone position will improve oxygenation and save the lives of countless patients with ARDS from COVID-19 pneumonia. It is time we give credit where credit is due, to our critical care nurses, who are undoubtedly leading prone teams across the world.
Kallet RH. A comprehensive review of prone position in ARDS. Resp Care 2015;60(11):1660-1687.
Piehl MA and Brown RS. Use of extreme position changes in acute respiratory failure. Critical Care Med 1976;4(1):13-14.
PROSEVA trial: Prone positioning in severe ARDS. New Engl J Med 2013;368(23):2159-2168.