How proning works to reduce acute respiratory distress and improve outcomes in coronavirus infections
Thousands of patients suffering from severe cases of Covid-19 are sharing a common treatment in hospitals throughout the world. Rather than receiving intricate medicaitons or vaccines, they are simple being placed face-down in their hospital bed in a practice known as proning. This change in position is often enough to improve lung functionality and reduce the impact of respiratory distress in Covid-19 cases.
The Lungs and Gravity
The lungs are remarkably complex in functionality. As the primary interface for gas exchange in the body, the lungs are a dynamic organ that responds to minute changes in the internal and external environments to maximize oxygen transfer and carbon dioxide removal. As the body inspires, air is pulled through the airways into a network of microscopic sacs called alveoli, where on the others side of a thin barrier millions of capillaries no wider than a single blood cell are waiting with red blood cells lined up ready to receive the incoming oxygen. Based on the demand, position and health of the lungs, this network of blood vessels dilate and constrict to maximize gas transfer all under the orchestration of hormone and nerve signals. In one breath, the lungs saturate the blood in the pulmonary system with oxygen to be transferred throughout the body to meet metabolic demand.
The lungs are positioned within the thoracic cage, but are not technically attached to anything other than the airways and blood vessels that supply them. When you inspire, muscles that expand the thoracic cavity — the diaphragm and accessory muscles surrounding the ribs — pull the chest wall away from the lungs. Between the lung tissue itself and the inner wall of the chest is an enclosed space called the pleural cavity. When the chest wall draws away from the lungs, the pressure in the pleural cavity (intrapleural pressure) drops to the point that it is less than the pressure inside the alveoli (intra-alveolar pressure), which forces the lung tissue to expand as if they were being sucked toward the chest wall. As the lung tissue expands, the alveoli open and draw air into them.
For all their elegance and delicacy, the lungs are also subject to some more crude forces. As an organ, the lungs are relatively heavy, weighed down mostly by the weight of blood that circulates through them. In an upright person gravity pulls the lungs downward, pressing the tissue down against the diaphragm. Blood within the pulmonary circulation also responds to the force of gravity, pooling in the lower vessels of the lung tissue. As a result, certain areas of the lungs are better perfused — meaning they have more adequate blood supply — than others. Similarly, the fine alveoli that contain air are under greater compression forces in the lower lungs than the upper.
The same effects of gravity are in play when the body is laying down. If a person lays supine — face up — then blood will tend to pool toward the back of the thoracic cavity. Blood will similarly pool toward the front of the thoracic cavity when a person is laying face down — prone — though not quite to the same extent. In this position, another factor imparts unequal pressure on the lungs — the heart. Resting slightly off-center of the middle of the thoracic cavity, the heart is a dense, blood-drenched muscle roughly the size of your fist. The heart is almost entirely surrounded by the lungs with the exception of the anterior-most area and depending on the position, the heart may or may not rest itself on the lungs further compressing the underlying tissue. This is particularly true in the supine position, where gravity pulls the heart toward the back of the chest wall where it weighs on lung tissue.
Acute Respiratory Distress Syndrome (ARDS)
ARDS is defined as any condition in which breathing and/or respiration is acutely limited or at risk. For patients with severe Covid-19 cases, the vigorous immune response that follows a coronavirus infection causes swelling and damage to the lung tissue. Accumulating fluid and swollen tissue between air and blood make breathing a major task, often requiring that severely ill patients be put on mechanically-assisted ventilation.
Ventilators come with their own set of risks for ill patients ranging from secondary infection to further damage of lung tissue. Placing patients on ventilators is necessary to maintain oxygenation, but is considered a last choice effort. ARDS is one of the most ominous outcomes of Covid-19 patients, and placing patients on mechanical ventilation signals a patient at significant risk. Some doctors are turning to proning patients to improve lung function and potentially keep them off ventilators.
Proning — or placing a patient laying face down — minimizes the effects of gravity on the lungs. In the supine position — the position most hospital patients assume — lung tissue toward the back of the lungs becomes compressed under the weight of the lungs, blood and the nearby heart. For patients who spend weeks in this position, these regions can suffer chronically reduced lung capacity, thus exacerbating an already distressed respiratory system. Placing patients in the prone position relieves the effects of gravity and opens up new regions of lung tissue for air and gas exchange. In this position, the heart no longer weighs heavily on the lung tissue, but instead rests against the sternum. Lung tissues in the back of the thoracic cavity is relieved of the burdon of gravity and is better capably of expansion and gas exchange. Some patients with ARDS have seen marked improvements in oxygen saturations from proning alone.
Patients cannot spend all day in this position however. Proning limits the ability of hospital workers to communicate with patients, perform physical exams and administer medications. Likewise, it is simply not comfortable for patients to lay face day all the time. So, hospitals practice proning in shifts, having patients laying face down for a portion of the day and returning to the face up position for the rest. Some guidelines have patients rotating through various positions to improve lung functioning.
ARDS is the most dangerous outcome of coronavirus infections accounting for a vast percentage of deaths. While the hunt for more effective treatments, hospital care and vaccines continues, the simple practice of proning patients seems to offer improvements in lung functionality and better outcomes. This simple practice is useful in almost all Covid-19 patients, though some contraindications do exist. The practice relies simply on the anatomy of the human lungs to optimize breathing during a coronavirus infection.