Application and techniques
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Prone ventilation, sometimes called prone positioning or proning, is a method of mechanical ventilation with the patient lying face-down (prone). It improves oxygenation in most patients with acute respiratory distress syndrome (ARDS) and reduces mortality. [1] The earliest trial investigating the benefits of prone ventilation occurred in 1976. [2] Since that time, many meta-analyses and one randomized control trial, the PROSEVA trial, have shown an increase in patients' survival with the more severe versions of ARDS. [3] There are many proposed mechanisms, but they are not fully delineated. The proposed utility of prone ventilation is that this position will improve lung mechanics, improve oxygenation, and increase survival. Although improved oxygenation has been shown in multiple studies, this position change's survival benefit is not as clear. [4] [5] [6] Similar to the slow adoption of low tidal volume ventilation utilized in ARDS, many believe that the investigation into the benefits of prone ventilation will likely be ongoing in the future. [7]
The purpose of prone ventilation is to better facilitate lung mechanics to improve ventilation/perfusion ratio mismatches in ARDS. [8]
By redistributing pulmonary blood flow, oxygen levels can increase from low ventilated areas to higher ventilation. [4] [5] [6] The physiologic mechanism can be explained by a gravity-dependent increase in pleural pressure when supine compared to prone. In the prone position, the lungs' dorsal aspects have less pleural pressure, which alleviates forces trying to collapse the alveoli. When there is less pleural pressure, the alveoli can stay open and thus increase surface area for ventilation. Because there are more alveoli dorsally than ventrally, a prone position allows for more dorsal alveoli to stay open and thus increase the amount of ventilation available to be perfused. [8] Another benefit of prone ventilation may come from reduced VALI (Ventilator-associated lung injury). Proning and the redistribution of dependent fluid lead to more homogenous compliance of the lung and thus minimizes the barotrauma that usually occurs from more heterogeneous lungs and the repeated opening and closing of alveoli associated with it produces. [9] An observational study in 2007 found a reduction in IL-6, a marker of systemic inflammation, in the prone ventilation group compared to the supine ventilation. This reduction in inflammation was attributed to a decrease in barotrauma and a rapid decrease in the need for high FiO2, reducing the number of reactive oxygen species contributing to ongoing inflammation in the lung. [10]
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The studies that have found survival benefit of prone ventilation derived benefit only from patients with severe ARDS defined as a Horowitz index of less than 200–150 mm Hg. [3] [11] A meta-analysis published in 2017 suggested that patients only benefit from prone ventilation when they are in a prone position for longer than 12 hours a day. [12] A trained staff and the resources to move/monitor patients is important
During the 2020 COVID-19 pandemic, awake high flow nasal cannula in the prone position, awake proning, was utilized to keep patients from being intubated. [13] A retrospective analysis showed that the number needed to treat and keep people off the ventilator was 6. [14] This significantly reduced amount of required ventilators allowing for the use of ventilators in those in critical condition. The Society of Critical Care Medicine gave prone ventilation a weak recommendation in The Surviving Sepsis Campaign COVID-19 panel. [15] The panel cited the few studies that showed morality benefit from prone ventilation in ARDS and that this was a low-cost intervention; however, they cautioned the use due to the necessity of needing competent staff and complications that can occur if done incorrectly.
Special precautions must be in place for prone ventilation in children because of their risk of sudden infant death syndrome (SIDS). [16] An updated Cochrane meta-analysis (2022) found low certainty evidence of benefit in oxygen saturation with prone positioning of mechanically ventilated preterm infants with ARDS but due to the increased risk of SIDS hospitalized infants and children should only be placed in this position with cardiorespiratory monitoring. [17]
There are many complications of proning patients. Most of the complications occur because of the intrinsic position and the effect of gravity on body parts unaccustomed to its effects. Some complications have occurred because of the logistics of increased time that staff members need to monitor and help patients in this disabling position. [18] Complications include increased endotracheal tube displacement and even accidental extubation, loss of vascular lines, pressure sores, brachial plexopathy, enteral feeding intolerance, facial edema, and injury. [19] [20] [21] [22] [23]
Respiratory failure results from inadequate gas exchange by the respiratory system, meaning that the arterial oxygen, carbon dioxide, or both cannot be kept at normal levels. A drop in the oxygen carried in the blood is known as hypoxemia; a rise in arterial carbon dioxide levels is called hypercapnia. Respiratory failure is classified as either Type 1 or Type 2, based on whether there is a high carbon dioxide level, and can be acute or chronic. In clinical trials, the definition of respiratory failure usually includes increased respiratory rate, abnormal blood gases, and evidence of increased work of breathing. Respiratory failure causes an altered mental status due to ischemia in the brain.
Mechanical ventilation or assisted ventilation is the medical term for using a machine called a ventilator to fully or partially provide artificial ventilation. Mechanical ventilation helps move air into and out of the lungs, with the main goal of helping the delivery of oxygen and removal of carbon dioxide. Mechanical ventilation is used for many reasons, including to protect the airway due to mechanical or neurologic cause, to ensure adequate oxygenation, or to remove excess carbon dioxide from the lungs. Various healthcare providers are involved with the use of mechanical ventilation and people who require ventilators are typically monitored in an intensive care unit.
Tidal volume is the volume of air moved into or out of the lungs in one breath. In a healthy, young human adult, tidal volume is approximately 500 ml per inspiration at rest or 7 ml/kg of body mass.
Extracorporeal membrane oxygenation (ECMO), also known as extracorporeal life support (ECLS), is an extracorporeal technique of providing prolonged cardiac and respiratory support to persons whose heart and lungs are unable to provide an adequate amount of oxygen, gas exchange or blood supply (perfusion) to sustain life. The technology for ECMO is largely derived from cardiopulmonary bypass, which provides shorter-term support with arrested native circulation. The device used is a membrane oxygenator, also known as an artificial lung.
Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin coloration (cyanosis). For those who survive, a decreased quality of life is common.
Liquid breathing is a form of respiration in which a normally air-breathing organism breathes an oxygen-rich liquid (such as a perfluorocarbon), rather than breathing air, by selecting a liquid that can hold a large amount of oxygen and is capable of CO2 gas exchange.
Permissive hypercapnia is hypercapnia in respiratory insufficient patients in which oxygenation has become so difficult that the optimal mode of mechanical ventilation is not capable of exchanging enough carbon dioxide. Carbon dioxide is a gaseous product of the body's metabolism and is normally expelled through the lungs.
Ventilator-associated pneumonia (VAP) is a type of lung infection that occurs in people who are on mechanical ventilation breathing machines in hospitals. As such, VAP typically affects critically ill persons that are in an intensive care unit (ICU) and have been on a mechanical ventilator for at least 48 hours. VAP is a major source of increased illness and death. Persons with VAP have increased lengths of ICU hospitalization and have up to a 20–30% death rate. The diagnosis of VAP varies among hospitals and providers but usually requires a new infiltrate on chest x-ray plus two or more other factors. These factors include temperatures of >38 °C or <36 °C, a white blood cell count of >12 × 109/ml, purulent secretions from the airways in the lung, and/or reduction in gas exchange.
Alveolar lung diseases, are a group of diseases that mainly affect the alveoli of the lungs.
High-frequency ventilation is a type of mechanical ventilation which utilizes a respiratory rate greater than four times the normal value and very small tidal volumes. High frequency ventilation is thought to reduce ventilator-associated lung injury (VALI), especially in the context of ARDS and acute lung injury. This is commonly referred to as lung protective ventilation. There are different types of high-frequency ventilation. Each type has its own unique advantages and disadvantages. The types of HFV are characterized by the delivery system and the type of exhalation phase.
Ventilator-associated lung injury (VALI) is an acute lung injury that develops during mechanical ventilation and is termed ventilator-induced lung injury (VILI) if it can be proven that the mechanical ventilation caused the acute lung injury. In contrast, ventilator-associated lung injury (VALI) exists if the cause cannot be proven. VALI is the appropriate term in most situations because it is virtually impossible to prove what actually caused the lung injury in the hospital.
Diffuse alveolar damage (DAD) is a histologic term used to describe specific changes that occur to the structure of the lungs during injury or disease. Most often DAD is described in association with the early stages of acute respiratory distress syndrome (ARDS). It is important to note that DAD can be seen in situations other than ARDS (such as acute interstitial pneumonia) and that ARDS can occur without DAD.
Airway pressure release ventilation (APRV) is a pressure control mode of mechanical ventilation that utilizes an inverse ratio ventilation strategy. APRV is an applied continuous positive airway pressure (CPAP) that at a set timed interval releases the applied pressure. Depending on the ventilator manufacturer, it may be referred to as BiVent. This is just as appropriate to use, since the only difference is that the term APRV is copyrighted.
The Horowitz index or Horovitz index is a ratio used to assess lung function in patients, particularly those on ventilators. It is useful for evaluating the extent of damage to the lungs. The simple abbreviation as oxygenation can lead to confusion with other conceptualizations of oxygenation index.
Modes of mechanical ventilation are one of the most important aspects of the usage of mechanical ventilation. The mode refers to the method of inspiratory support. In general, mode selection is based on clinician familiarity and institutional preferences, since there is a paucity of evidence indicating that the mode affects clinical outcome. The most frequently used forms of volume-limited mechanical ventilation are intermittent mandatory ventilation (IMV) and continuous mandatory ventilation (CMV). There have been substantial changes in the nomenclature of mechanical ventilation over the years, but more recently it has become standardized by many respirology and pulmonology groups. Writing a mode is most proper in all capital letters with a dash between the control variable and the strategy.
Inverse ratio ventilation (IRV) is not necessarily a mode of mechanical ventilation though it may be referred to as such. IRV is a strategy of ventilating the lungs in such a way that the amount of time the lungs are in inhalation is greater than the amount of time they are in exhalation, allowing for a constant inflation of the lungs, ensuring they remain "recruited". The primary goal for IRV is improved oxygenation by forcing inspiratory time to be greater than expiratory time increasing the mean airway pressure and potentially improving oxygenation. Normal I:E ratio is 5:6, so forcing the I:E to be 2:1, 3:1, 4:1, is the source of the term for the strategy.
Within the medical field of respiratory therapy, Open lung ventilation is a strategy that is utilized by several modes of mechanical ventilation to combine low tidal volume and applied PEEP to maximize recruitment of alveoli. The low tidal volume aims to minimize alveolar overdistention and the PEEP minimizes cyclic atelectasis. Working in tandem the effects from both decrease the risk of ventilator-associated lung injury.
The pathophysiology of acute respiratory distress syndrome involves fluid accumulation in the lungs not explained by heart failure. It is typically provoked by an acute injury to the lungs that results in flooding of the lungs' microscopic air sacs responsible for the exchange of gases such as oxygen and carbon dioxide with capillaries in the lungs. Additional common findings in ARDS include partial collapse of the lungs (atelectasis) and low levels of oxygen in the blood (hypoxemia). The clinical syndrome is associated with pathological findings including pneumonia, eosinophilic pneumonia, cryptogenic organizing pneumonia, acute fibrinous organizing pneumonia, and diffuse alveolar damage (DAD). Of these, the pathology most commonly associated with ARDS is DAD, which is characterized by a diffuse inflammation of lung tissue. The triggering insult to the tissue usually results in an initial release of chemical signals and other inflammatory mediators secreted by local epithelial and endothelial cells.
Proning or prone positioning is the placement of patients into a prone position so that they are lying on their front. This is used in the treatment of patients in intensive care with acute respiratory distress syndrome (ARDS). It has been especially tried and studied for patients on ventilators but, during the COVID-19 pandemic, it is being used for patients with oxygen masks and CPAP as an alternative to ventilation.
The treatment and management of COVID-19 combines both supportive care, which includes treatment to relieve symptoms, fluid therapy, oxygen support as needed, and a growing list of approved medications. Highly effective vaccines have reduced mortality related to SARS-CoV-2; however, for those awaiting vaccination, as well as for the estimated millions of immunocompromised persons who are unlikely to respond robustly to vaccination, treatment remains important. Some people may experience persistent symptoms or disability after recovery from the infection, known as long COVID, but there is still limited information on the best management and rehabilitation for this condition.