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Information about Mechanical ventilation

In medicine, mechanical ventilation is employed to assist, or in some cases
replace, spontaneous breathing. Mechanical ventilation is a mainstay of
resuscitation, intensive care medicine, and anesthesia.

Clinical use

Reasons for instituting mechanical ventilation include:

   * absent (apnea) or insufficient spontaneous breathing, which may result
     from causes such as
        o intoxication
        o neurological disease or head trauma
        o paralysis of breathing muscles due to spinal cord injury, effect
          of anesthetics or effect of muscle relaxants
   * pulmonary disease or trauma
   * cardiac disease such as congestive heart failure
   * other severe disease such as sepsis

Depending on the clinical situation, mechanical ventilation may be used for
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.

Life-critical system

Because the failure of a mechanical ventilation system may result in death,
it is classed as a life-critical system, and precautions must be taken to
ensure that mechanical ventilation systems are highly reliable. This
includes their power-supply provision. Mechanical ventilation systems
usually have alarms and manual backup mechanisms to enable hand-driven
respiration to continue in the absence of power. Some systems are also
equipped with compressed-gas tanks and backup batteries to provide
ventilation in case of power failure or of defective gas supplies.

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.

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.

Mechanical systems

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.

In anesthesia and intensive care, ventilators are routinely used. In its
simplest form, a ventilator consists of a compressible air reservoir, air
and oxygen supplies, and a set of valves and tubes. The reservoir is
pneumatically compressed several times a minute to deliver air to the
patient; when overpressure is released, the patient can exhale passively.
The oxygen content of the inspired gas can be set from 21 percent (ambient
air) to 100 percent (pure oxygen). Modern ventilators are electronically
controlled to allow exact adaptation of pressure and flow characteristics to
an individual patient's needs. Fine-tuned ventilator settings also serve to
make ventilation more tolerable for the patient.

Modes of ventilation

There are 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.
Modern ventilators allow a continuous adaptation of the degree of mechanical
assistance according to the patient's individual demands.

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 set values. 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.
   * Larygeal mask airway - Another device is the laryngeal mask airway
     (LMA), which consists of a tube with an inflatable cuff that is
     inserted into the pharynx. It causes less pain and coughing than a
     tracheal tube; however, sealing against aspiration is inferior to
     tracheal tubes, making careful patient evaluation and selection
     mandatory. The LMA is used in anesthesia and sometimes in emergency
     medicine.
   * Tracheal intubation or, colloquially, "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. 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 mechanical ventilation is required for more than
     days or a few weeks, tracheostomy provides the most convenient 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-related lung injury and protective ventilation

In most cases of mechanical ventilation, the patient's prognosis is
determined by the underlying disease and its reponse 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.

Most techniques of ventilation rely on an overpressure being applied to the
lungs. In some cases high pressures are necessary to provide the patient
with a sufficient amount of oxygen, but such pressures may severly injure
the lungs themselves and impair their ability to exchange oxygen and carbon
dioxide between air and blood. This type of injury is called "pulmonary
barotrauma". Pulmonary barotrauma can in turn necessitate more aggressive
ventilation, resulting in a vicious circle of therapy-induced additional injury.

"Protective ventilation" is a collective term for strategies to minimize
ventilation-related pulmonary injury, many of which rely on sophisticated
ventilator settings to reduce pulmonary barotrauma.

History

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

Meaning of "mechanical ventilation" in architecture terminology

In architecture, mechanical engineering, and HVAC, mechanical ventilation is
the use of powered equipment, e.g. fans and blowers, to move air; as
contrasted with the natural ventilation provided by convection and winds.

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