Mechanical ventilation or assisted ventilation is the medical term for using a ventilator machine 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.
Mechanical ventilation is termed invasive if it involves an instrument to create an airway that is placed inside the trachea. This is done through an endotracheal tube or nasotracheal tube. For non-invasive ventilation in people who are conscious, face or nasal masks are used. The two main types of mechanical ventilation include positive pressure ventilation where air is pushed into the lungs through the airways, and negative pressure ventilation where air is pulled into the lungs. There are many specific modes of mechanical ventilation, and their nomenclature has been revised over the decades as the technology has continually developed.
History
The Greek physician Galen may have been the first to describe mechanical ventilation: "If you take a dead animal and blow air through its larynx [through a reed], you will fill its bronchi and watch its lungs attain the greatest distention." In the 1600s, Robert Hooke conducted experiments on dogs to demonstrate this concept. Vesalius too describes ventilation by inserting a reed or cane into the trachea of animals. These experiments predate the discovery of oxygen and its role in respiration. In 1908, George Poe demonstrated his mechanical respirator by asphyxiating dogs and seemingly bringing them back to life. These experiments all demonstrate positive pressure ventilation.
To achieve negative pressure ventilation, there must be a sub-atmospheric pressure to draw air into the lungs. This was first achieved in the late 19th century when John Dalziel and Alfred Jones independently developed tank ventilators, in which ventilation was achieved by enclosing a patient's body within a chamber of sub-atmospheric pressure. This machine came to be known colloquially as the Iron lung, which went through many iterations of development. The use of the iron lung became widespread during the polio epidemic of the 1900s.
The first functional positive pressure ventilator was invented by Carl Gunnar Engström and was first used by Bjørn Aage Ibsen in Blegdamshospitalet, Copenhagen, Denmark in 1952. Denmark was experiencing a poliomyelitis epidemic, and there was only one iron lung in the hospital. The first patient to be saved by the new machine was 12-year old Vivi Ebert. Ibsen gave the patent on free use and licensed it to AGA, Sweden, to enable quick serial production of the new machines. The positive pressure ventilator quickly superseded the iron lung in Europe by 1953, but the older machine persisted in the United States.
Early ventilators were control style with no support breaths integrated into them and were limited to an inspiration to expiration ratio of 1:1. In the 1970s, intermittent mandatory ventilation was introduced as well as synchronized intermittent mandatory ventilation. These styles of ventilation had control breaths that patients could breathe between.
Uses Mechanical ventilation is indicated when a patient's spontaneous breathing is inadequate to maintain life. It may be indicated in anticipation of imminent respiratory failure, acute respiratory failure, acute hypoxemia, or prophylactically. Because mechanical ventilation serves only to provide assistance for breathing and does not cure a disease, the patient's underlying condition should be identified and treated in order to liberate them from the ventilator.
Common specific medical indications for mechanical ventilation include:
Surgical procedures Acute lung injury, including acute respiratory distress syndrome (ARDS), trauma, or COVID-19 Pneumonia Pulmonary hemorrhage Apnea with respiratory arrest Hypoxemia Acute severe asthma requiring intubation Obstruction, such as a tumor Acid/base derangements such as respiratory acidosis Neurological diseases such as muscular dystrophy, amyotrophic lateral sclerosis (ALS), Guillain–Barré syndrome, myasthenia gravis including transient neonatal myasthenia gravis, etc. Newborn premature infants with neonatal respiratory distress syndrome Respiratory failure due to paralysis of the respiratory muscles caused by botulism
Mechanical ventilation is typically used as a short-term measure. It may, however, be used at home or in a nursing or rehabilitation institution for patients that have chronic illnesses that require long-term ventilatory assistance.
Risks and complications Mechanical ventilation is often a life-saving intervention, but carries potential complications. Common complications of positive pressure ventilation stemming directly from ventilator settings include volutrauma, barotrauma, and atelectrauma. These mechanisms contribute to ventilator-induced lung injury (VILI). Patients with pre-existing lung injury, in particular patients with acute respiratory distress syndrome (ARDS), are especially vulnerable to additional ventilator-associated lung injury. Excessive pressure or volume delivered by the ventilator can also cause pneumothorax, subcutaneous emphysema, pneumomediastinum, and pneumoperitoneum. Other complications of mechanical ventilation include diaphragm atrophy, decreased cardiac output, and oxygen toxicity.
In many healthcare systems, prolonged ventilation as part of intensive care is a limited resource. For this reason, decisions to commence and remove ventilation may raise ethical debate and often involve legal orders such as do-not-resuscitate orders.
Mechanical ventilation is often associated with many painful procedures and the ventilation itself can be uncomfortable. For infants who require opioids for pain, the potential side effects of opioids include problems with feeding, gastric and intestinal mobility problems, the potential for opioid dependence, and opioid tolerance.
Withdrawal from mechanical ventilation Timing of withdrawal from mechanical ventilation—also known as weaning—is important. Patients should be considered for weaning as soon as they are able to support their own ventilation and oxygenation, and this should be assessed continuously. The following objective parameters can guide this decision:
Spontaneous breathing trials are conducted to assess the likelihood of a patient being able to maintain stability and breath on their own without the ventilator. This is done by changing to a continuous spontaneous ventilation (CSV) mode that provides support only to compensate for the added resistance of the endotracheal tube.
The Rapid Shallow Breathing Index (RSBI, the ratio of respiratory frequency to tidal volume (f/VT), previously referred to as the "Yang Tobin Index" or "Tobin Index" after Dr. Karl Yang and Prof. Martin J. Tobin of Loyola University Medical Center) is one of the best studied and most commonly used weaning predictors, though it is limited by moderate sensitivity and poor specificity. It was described in a prospective cohort study of mechanically ventilated patients which found that a RSBI > 105 breaths/min/L was associated with weaning failure.
A cuff leak test may be performed before extubation in patients at high risk of post-extubation stridor caused by laryngeal edema. The cuff of the endotracheal tube is deflated, and little or no air leakage around the tube may indicate upper airway narrowing.
Physiology The function of the lungs is to provide gas exchange via oxygenation and ventilation. This phenomenon of respiration involves the physiologic concepts of air flow, tidal volume, compliance, resistance, and dead space. Other relevant concepts include alveolar ventilation, arterial PaCO2, alveolar volume, and FIO2. Alveolar ventilation is the amount of gas per unit of time that reaches the alveoli and becomes involved in gas exchange. PaCO2 is the partial pressure of carbon dioxide of arterial blood, which determines how well carbon dioxide is able to move out of the body. Alveolar volume is the volume of air entering and leaving the alveoli per minute. Mechanical dead space is another important parameter in ventilator design and function, and is defined as the volume of gas breathed again as the result of use in a mechanical device.
Due to the anatomy of the human pharynx, larynx, and esophagus and the circumstances for which ventilation is needed, additional measures are required to secure the airway during positive-pressure ventilation in order to allow unimpeded passage of air into the trachea and avoid air passing into the esophagus and stomach. The common method is by insertion of a tube into the trachea. Intubation, which provides a clear route for the air can be either an endotracheal tube, inserted through the natural openings of mouth or nose, or a tracheostomy inserted through an artificial opening in the neck. In other circumstances simple airway maneuvers, an oropharyngeal airway or laryngeal mask airway may be employed. If non-invasive ventilation or negative-pressure ventilation is used, then an airway adjunct is not needed.
Pain medicine such as opioids are sometimes used in adults and infants who require mechanical ventilation. For preterm or full term infants who require mechanical ventilation, there is no strong evidence to prescribe opioids or sedation routinely for these procedures, however, some select infants requiring mechanical ventilation may require pain medicine such as opioids. It is not clear if clonidine is safe or effective to be used as a sedative for preterm and full term infants who require mechanical ventilation.
When 100% oxygen (1.00 FIO2) is used initially for an adult, it is easy to calculate the next FIO2 to be used, and easy to estimate the shunt fraction. The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation. In normal physiology, gas exchange of oxygen and carbon dioxide occurs at the level of the alveoli in the lungs. The existence of a shunt refers to any process that hinders this gas exchange, leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart, which ultimately supplies the rest of the body with de-oxygenated blood. When using 100% oxygen, the degree of shunting is estimated as 700 mmHg - measured PAO2. For each difference of 100 mmHg, the shunt is 5%. A shunt of more than 25% should prompt a search for the cause of this hypoxemia, such as mainstem intubation or pneumothorax, and should be treated accordingly. If such complications are not present, other causes must be sought after, and positive end-expiratory pressure (PEEP) should be used to treat this intrapulmonary shunt. Other such causes of a shunt include:
alveolar collapse from major atelectasis alveolar collection of material other than gas, such as pus from pneumonia, water and protein from acute respiratory distress syndrome, water from congestive heart failure, or blood from hemorrhage
Technique Modes