mechanical ventilation

mechanical ventilation, the use of an automated machine to move air in and out of the lungs, doing the work of breathing for patients who have stopped breathing or whose breathing is sporadic. Mechanical ventilation machines keep the airways open, allowing for the continual exchange of oxygen and carbon dioxide in the terminal air sacs of the lungs.

Although some patients require long-term ventilation, in most cases mechanical ventilation is intended for short-term use to provide assistance with breathing while underlying conditions are diagnosed and treated. For example, during surgery when a patient is under general anesthesia, mechanical ventilation may be used to maintain breathing. It also may be used to prevent aspiration or to stabilize patients who are undergoing treatment for such conditions as acute respiratory distress syndrome, chronic obstructive pulmonary disease (COPD), muscular dystrophy, pneumonia, sleep apnea, or trauma to the respiratory system.

Noninvasive and invasive ventilation

There are two types of mechanical ventilation: noninvasive and invasive. Noninvasive ventilation takes the form of nasal masks or face masks, which can be used by patients who are conscious and experiencing mild to moderate difficulty in breathing. Such masks require little or no specialized training to administer. Noninvasive mechanical ventilation machines, such as continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BiPAP) devices, can be used at home. These machines are often used for chronic respiratory conditions, such as COPD, congestive heart failure, or obstructive sleep apnea.

Invasive mechanical ventilation involves the use of either an endotracheal or a nasotracheal tube, which is inserted through the mouth or nose and into the trachea to maintain an open airway. This type of ventilation is often administered by a specialist, such as an anesthesiologist or a respiratory therapist, and it requires full-time monitoring. Patients undergoing invasive mechanical ventilation often require pain medication or a sedative, which can help alleviate discomfort and anxiety and reduce oxygen consumption. Some patients also need a feeding tube. The need for mechanical ventilation is a leading cause of admission to an intensive care unit (ICU); indeed, some 20 to 70 percent of patients in an ICU require invasive ventilation at any given time.

Mechanism

Most modern ventilators, both invasive and noninvasive, use positive pressure to push air into the lungs. This differs from natural breathing, in which air is drawn into the lungs through negative pressure created by the diaphragm and chest muscles. Once the ventilator breath ceases, airway pressure drops, and the chest walls automatically recoil, pushing the breath out through passive exhalation.

Historically, ventilators were designed to mimic natural breathing via negative pressure ventilation. In this process, a machine is used to create a vacuum around the chest, causing the lungs to expand and intrapulmonary pressure to decrease, which increases airflow into the lungs. When the vacuum is released, pressure equalizes, and the elastic recoil of the chest leads to deflation of the lungs and passive exhalation. The iron lung is one example of this type of machine.

Risks

Although often lifesaving, the application and monitoring of mechanical ventilation, especially invasive methods, can be costly and demanding on labor and resources. Moreover, they can lead to medical complications, including ventilator-associated pneumonia, which occurs when pathogenic bacteria—or, less commonly, viruses or fungi—enter the lungs through devices used for ventilation. Prolonged use of ventilation also increases the risk of bacterial infections such as sinusitis and bacterial tracheitis; lung injuries, such as barotrauma and alveolar overdistention; and such conditions as acute respiratory distress syndrome, oxygen toxicity, and cardiac irregularities.

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History

The idea of sending air into the lungs via artificial means was first formally introduced by ancient Greek physician Galen, in his studies of respiratory anatomy and physiology in the 2nd century ce. In De usu partium corporis humani (Eng. trans. On the Usefulness of the Parts of the Body), he describes inflating the lungs of animals by using bellows. In the 16th century Belgian physician Andreas Vesalius referred to the use of bellows to revive animals in his work De humani corporis fabrica libri septem (“The Seven Books on the Structure of the Human Body”). In the following century English physicist Robert Hooke also used the principles of positive pressure ventilation—in this case to revive a dog with damaged lungs.

In the late 19th and early 20th centuries negative pressure ventilators predominated. One of the first body-enclosing negative pressure ventilators was patented by American Alfred Jones in 1864. Such machines posed challenges to patient care, owing to difficulties in accessing the patient’s body. To address this problem, in 1908 American physician Peter Lord patented a respirator room, in which a patient could lie with the head outside the room. Although respirator rooms capable of accommodating multiple patients were also invented, the motors and other equipment needed to run them were expensive and cumbersome.

In 1927 Philip Drinker and Louis Agassiz Shaw at Harvard University introduced the iron lung, which used negative pressure to facilitate breathing. For a time the machine was used to treat individuals affected by paralytic polio and certain other breathing conditions. About the mid-20th century, with the development during World War II of military technology to provide oxygen for fighter pilots at high altitudes, there occurred significant advancements in positive pressure ventilation. In subsequent decades efforts were made to improve lung protection in patients undergoing positive pressure ventilation, and ventilators for home use, such as CPAP and BiPAP machines, were introduced.