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Mechanical ventilation

Mechanical ventilation is employed to assist, or in some cases replace, spontaneous breathing. Mechanical ventilation can be life-saving and is a mainstay of resuscitation, intensive care medicine, and anesthesia.

History

The iron lung was used through much of the 20th century, mostly for long-term ventilation.

Indications

Mechanical ventilation is used when natural (spontaneous) breathing is absent (apnea) or insufficient. This may be the case in cases of intoxication, circulatory arrest, neurological disease or head trauma, paralysis of the breathing muscles due to spinal cord injury, or the effect of anesthetic or muscle relaxant drugs. Various pulmonary diseases or chest trauma, cardiac disease such as congestive heart failure, sepsis and shock may also necessitate ventilation.

Depending on the clinical situation, mechanical ventilation may be continued for a few minutes or many months. While returning to spontaneous breathing is rarely a problem in routine anesthesia, weaning an intensive care patient from prolonged mechanical ventilation can take weeks or even months. Some patients do not regain the ability to breathe by themselves sufficiently and therefore require permanent mechanical ventilation. This is often the case with severe brain injury, spinal cord injury, or neurological disease.

Techniques

Positive and negative pressure ventilation

While the exchange of oxygen and carbon dioxide between the bloodstream and the pulmonary airspace works by diffusion and requires no external work, air must be moved into and out of the lungs to make it available to the gas exchange process. In spontaneous breathing, an underpressure is created in the pleural cavity by the muscles of respiration, and the resulting gradient between the atmospheric pressure and the pressure inside the thorax generates a flow of air. This is imitated by the negative-pressure ventilation that is employed in iron lungs. An iron lung works by creating an underpressure in a chamber which encloses the body and is sealed at the neck. With the patient's airways open, the resulting gradient to the atmospheric pressure serves to inflate the lungs.

All other techniques of ventilation are positive pressure ventilation techniques, meaning that air is forced into the lungs by an external overpressure.

Mouth-to-mouth and bag-mask systems

Arguably the simplest form of mechanical ventilation is the mouth-to-mouth or mouth-to-nose technique that is used in bystander cardiopulmonary resuscitation. This technique is however limited as it is not possible to ventilate the patient with oxygen-enriched air: on the contrary, only approximately 16 percent oxygen (in contrast to 21 percent in ambient air or up to 100 percent by mechanical ventilators) can be achieved. There is also a possible risk of disease transmission through exchange of body fluids. Mechanical devices such as a bag-mask-valve system are therefore preferred where available.

A bag-mask-valve system consists of a face mask that is pressed over the patient's nose and mouth to achieve a tight seal, an elastic bag that can be manually compressed to deliver air to the patient, and a valve to direct air flow. A source of oxygen can be connected to a reservoir attached to the bag to achieve a higher concentration of oxygen than that of ambient air. This simple technique can be sufficient to maintain ventilation (and consequently, the life of an apneic patient) for up to several hours.

Mechanical ventilators

In anesthesia and intensive care, mechanical ventilators are routinely used. Ventilators allow various modes of mechanical ventilation ranging from assisted spontaneous breathing to fully controlled ventilation. In some cases, a patient can breathe almost naturally, receiving only an occasional "push" of air to augment individual breaths. This is termed assisted (or augmented) ventilation. Assisted ventilation modes are used in anesthesia and in the process of weaning the patient from controlled ventilation.

In sicker patients, the degree of ventilator-driven respiration can be increased, and if necessary, the ventilator can take over the work of breathing entirely (controlled ventilation). Modern ventilators allow a continuous adaptation of the degree of mechanical assistance according to the patient's individual demands.

The lungs of ventilated patients have a tendency to collapse partially, leading to impaired gas exchange. Therefore, many ventilation modes allow the use of PEEP (positive end-expiratory pressure). With PEEP, there is a residual overpressure at the end of a breathing cycle that keeps the lungs inflated.

Securing the patient's airways

Mechanical ventilation will be unsuccessful and dangerous unless the patient's airways are patent, meaning air can flow unimpeded back and forth into the lungs. It is also necessary to avoid air leakage so that air flow and pressure are maintained at the values set.

Another great risk is that of aspiration pneumonia. Aspiration is when stomach contents come back up the esophagus and enter the trachea to enter the lungs. When stomach contents get into the lungs, the patient can actually drown due to the volume of gastic material, or, with less material, suffer damage to the lung tissue due to the acid content of the stomach. Measures to prevent aspiration depend on the situation and the individual patient - endotracheal intubation is often necessary to protect against this.

There are various procedures and mechanical devices that provide protection against airway collapse, air leakage, and aspiration:

Face mask - In resuscitation and for minor procedures under anesthesia, a face mask is often sufficient to achieve a seal against air leakage. Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal tubes. These are designed to provide a passage of air to the pharynx through the nose or mouth, respectively. A face mask does, however, not provide protection against aspiration. Face masks are also used for "non-invasive ventilation" in conscious patients. Non-invasive ventilation is aimed at minimizing patient discomfort and ventilation-related disease. It is often used in cardiac or pulmonary disesase.

Laryngeal mask airway - The laryngeal mask airway (LMA), causes less pain and coughing than a tracheal tube. However, unlike tracheal tubes it does not seal against aspiration, making careful individualised evaluation and patient selection mandatory.

Tracheal intubation or, 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 anesthetized for other reasons, sedative drugs are usually given to provide tolerance of the tube.

Tracheostomy - When patients require mechanical ventilation for more than days or a few weeks, tracheostomy provides the most suitable access to the patient's airways. A tracheostomy is a surgically created access to the trachea. Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs.

(Note: the terminology for this procedure can be confusing. Often "tracheotomy" is used to denote the surgical procedure and "tracheostomy" the result of the procedure)

Ventilation-associated lung injury and protective ventilation

In most cases of mechanical ventilation, the patient's prognosis is determined by the underlying disease and its response to treatment. However, ventilation itself can cause significant problems that may prolong intensive care and sometimes lead to permanent injury and death. It is therefore desirable to limit mechanical ventilation to the shortest appropriate time.

Infectious complications, particularly pneumonia, occur in many patients who remain intubated for more than a few days. Tracheal intubation interferes with the natural defenses against lung infection, particularly with the process of "mucociliary clearance". This is a continuous transport of airway secretions from the lungs to the upper airways that serves to remove bacteria and foreign bodies. It is thought that the intubation-related disruption of this transport mechanism is a major factor in the development of pneumonia.

There is evidence that oxygen in higher concentrations may contribute to injury of lung tissue in ventilated patients. It is therefore recommended to set ventilators to deliver the lowest appropriate concentration of oxygen. However, in patients with severely impaired pulmonary gas exchange, high oxygen concentrations may be necessary for survival.

Most techniques of ventilation rely on an overpressure being applied to the lungs. In diseased lungs this may lead to further tissue injury caused by excessive mechanical stress (overdistension, shear forces, high peak pressure) and aggravated by inflammatory processes. Such mechanically induced lung injury can lead to severe impairment of the pulmonary gas exchange, thereby necessitating even more aggressive ventilation.

"Protective ventilation" is a collective term for strategies to minimize ventilation-associated lung injury, many of which rely on sophisticated ventilator settings to reduce overdistension of the lungs.

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