MECHANICAL VENTILATION

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

Positive and negative pressure ventilation

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

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.

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.

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. 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 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.

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.

• life support
• safety engineering