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

thumb|262x262px|Hospital staff examine a patient in an Iron lung tank respirator during the polio epidemic. The machine creates a negative pressure around the thoracic cavity, thereby causing air to rush into the lungs to equalize intrapulmonary pressure.

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.

thumb|262x262px|Engström 150

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.

  • 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. A common complication of positive pressure ventilation stemming directly from the ventilator settings include volutrauma and barotrauma. Others include pneumothorax, subcutaneous emphysema, pneumomediastinum, and pneumoperitoneum. Another well-documented complication is ventilator-associated lung injury which presents as acute respiratory distress syndrome. Other complications include diaphragm atrophy, decreased cardiac output, and oxygen toxicity. One of the primary complications that presents in patients mechanically ventilated is acute lung injury (ALI)/acute respiratory distress syndrome (ARDS). ALI/ARDS are recognized as significant contributors to patient morbidity and mortality.

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 an important consideration. People who require mechanical ventilation should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation, and this should be assessed continuously.

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, while a RSBI < 105 breaths/min/L.

A cuff leak test is done to detect if there is airway edema to show the chances of post-extubation stridor. This is done by deflating to the cuff to check if air begins leaking around the endotracheal tube. Other relevant concepts include alveolar ventilation, arterial PaCO2, alveolar volume, and F<sub>I</sub>O<sub>2</sub>. 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.

thumb|245x245px|Image of endotracheal tube placement required to connect a patient's physiologic airway to the ventilator.

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 F<sub>I</sub>O<sub>2</sub>) is used initially for an adult, it is easy to calculate the next F<sub>I</sub>O<sub>2</sub> 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. A relatively new ventilation mode is flow-controlled ventilation (FCV). FCV is a fully dynamic mode without significant periods of 'no flow'. It is based on creating a stable gas flow into or out of the patient's lungs to generate an inspiration or expiration, respectively. This results in linear increases and decreases in intratracheal pressure. In contrast the conventional modes of ventilation, there are no abrupt drop intrathoracic pressure drops, because of the controlled expiration. Further, this mode allows to use thin endotracheal tubes (~2 – 10&nbsp;mm inner diameter) to ventilate a patient as expiration is actively supported. In general, the selection of which mode of mechanical ventilation to use for a given patient is based on the familiarity of clinicians with modes and the equipment availability at a particular institution.

Types of Ventilation

thumb|[[Carl Gunnar Engström invented in 1950 one of the first intermittent positive pressure ventilator, which delivers air straight into the lungs using an endotracheal tube placed into the windpipe.]]

Positive pressure

The design of the modern positive-pressure ventilators were based mainly on technical developments by the military during World War II to supply oxygen to fighter pilots in high altitude. Such ventilators replaced the iron lungs as safe endotracheal tubes with high-volume/low-pressure cuffs were developed. The popularity of positive-pressure ventilators rose during the polio epidemic in the 1950s in Scandinavia and the United States and was the beginning of modern ventilation therapy. Positive pressure through manual supply of 50% oxygen through a tracheostomy tube led to a reduced mortality rate among patients with polio and respiratory paralysis. However, because of the sheer amount of man-power required for such manual intervention, mechanical positive-pressure ventilators became increasingly popular. The prominent design of the smaller devices is known as the cuirass, a shell-like unit used to create negative pressure only to the chest using a combination of a fitting shell and a soft bladder. In recent years this device has been manufactured using various-sized polycarbonate shells with multiple seals, and a high-pressure oscillation pump in order to carry out biphasic cuirass ventilation. Its main use has been in patients with neuromuscular disorders that have some residual muscular function. The latter, larger formats are in use, notably with the polio wing hospitals in England such as St Thomas' Hospital in London and the John Radcliffe in Oxford. The name of one such device, the Pneumobelt made by Puritan Bennett has to a degree become a generic name for the type.

Oscillator

thumb|3100A Oscillator

The most commonly used high frequency ventilator and only one approved in the United States is the 3100A from Vyaire Medical. It works by using very small tidal volumes by setting amplitude and a high rate set in hertz. This type of ventilation is primarily used in neonates and pediatric patients who are failing conventional ventilation.

High Frequency Jet Ventilation

The first type of high frequency ventilator made for neonates and the only jet type is made by Bunnell Incorporated. It works in conjunction with a separate CMV ventilator to add pulses of air to the control breaths and PEEP. Usually, a double lumen tube is inserted for this procedure, but even a bronchial blocker may be used. To maintain a sufficient gas exchange and avoid trauma in the ventilated lung, tidal volume and respiratory rate should be adjusted, which may lead to higher mechanical stress and inflammatory reactions. Furthermore, a hypoxic vasonstriction reflex in the non-ventilated lung is triggering several physiological changes that may compromise the hemodynamic situation of the patient.

Monitoring

One of the main reasons why a patient is admitted to an ICU is for delivery of mechanical ventilation. Monitoring a patient in mechanical ventilation has many clinical applications: Enhance understanding of pathophysiology, aid with diagnosis, guide patient management, avoid complications, and assess trends.

In ventilated patients, pulse oximetry is commonly used when titrating FIO2. A reliable target of Spo2 is greater than 95%.

The total PEEP in the patient can be determined by doing an expiratory hold on the ventilator. If this is higher than the set PEEP, this indicates air trapping.

The plateau pressure can be found by doing an inspiratory hold. This shows the actual pressure the patient's lungs are experiencing.

Loops can be used to see what is occurring in the patient's lungs. These include flow-volume and pressure-volume loops. They can show changes in compliance and resistance.

Functional Residual Capacity can be determined when using the GE Carestation.

Modern ventilators have advanced monitoring tools. There are also monitors that work independently of the ventilator which allow for measuring patients after the ventilator has been removed, such as a Tracheal tube test.

Types of ventilators

thumb|SMART BAG MO Bag-Valve-Mask Resuscitator

Ventilators come in many different styles and method of giving a breath to sustain life. but pressurized protection suits for caregivers are recommended due to the risks of poorly fitting masks emitting contaminating aerosols.

  • Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration. A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea. In most cases, tubes with inflatable cuffs are used for protection against leakage and aspiration. Intubation with a cuffed tube is thought to provide the best protection against aspiration. Tracheal tubes inevitably cause pain and coughing. Therefore, unless a patient is unconscious or anaesthetized for other reasons, sedative drugs are usually given to provide tolerance of the tube. Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis.
  • Supraglottic airway — a supraglottic airway (SGA) is any airway device that is seated above and outside the trachea, as an alternative to endotracheal intubation. Most devices work via masks or cuffs that inflate to isolate the trachea for oxygen delivery. Newer devices feature esophageal ports for suctioning or ports for tube exchange to allow intubation. Supraglottic airways differ primarily from tracheal intubation in that they do not prevent aspiration. After the introduction of the laryngeal mask airway (LMA) in 1998, supraglottic airway devices have become mainstream in both elective and emergency anesthesia. There are many types of SGAs available including the esophageal-tracheal combitube (ETC), laryngeal tube (LT), and the obsolete esophageal obturator airway (EOA).
  • Cricothyrotomy — Patients requiring emergency airway management, in whom tracheal intubation has been unsuccessful, may require an airway inserted through a surgical opening in the cricothyroid membrane. This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access.
  • Tracheostomy — When patients require mechanical ventilation for several weeks, a tracheostomy may provide the most suitable access to the trachea. A tracheostomy is a surgically created passage into the trachea. Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs. Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease, or in any patient expected to be difficult to wean from mechanical ventilation, i.e., patients with little muscular reserve.
  • Mouthpiece — Less common interface, does not provide protection against aspiration. There are lipseal mouthpieces with flanges to help hold them in place if patient is unable.

See also

  • , inventor of the pulmoventilateur
  • Mean airway pressure

References

  • , article on mechanical ventilation along with technical information.
  • International Ventilator Users Network (IVUN) , Resource of information for users of home mechanical ventilation.
  • Mechanical Ventilation, (detailed, illustrated slideshow presentation), by Amirali Nader, MD FCCP, Critical Care Medicine, Suburban Hospital, Johns Hopkins Medicine.