Article: Mechanical ventilation

In architecture 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.

Bag valve mask

In medicine, mechanical ventilation is a method to assist or replace spontaneous breathing. Mechanical ventilation can be life-saving and is a mainstay of CPR, intensive care medicine, and anesthesia.

History

The iron lung was used through much of the middle 20th century, mostly for long-term ventilation. It was refined and used largely as a result of the polio epidemic that struck the world in the 1950s.

The machine is effectively a large elongated tank, which encases the patient up to the neck. The neck is sealed with a rubber gasket so that the patient's face (and airway) are exposed to the room air.

By means of a pump, the air is withdrawn mechanically to provide inspiration and released to room pressure to allow expiration.

Thus the patient inhales room air by a means of negative pressure applied to the patient's thoracic area. There are large portholes for nurse or home assistant access. Patients could remain in these iron lungs for years at a time quite successfully. Some are still in use, notably with the Polio Wing Hospitals in England such as St Thomas' (by Westminster in London) and the John Radcliffe in Oxford.

The patients can talk and eat normally, and can see the world through a well-placed series of mirrors.

A smaller device known as the cuirass was invented to place onto the chest wall like a giant plumber's suction plunger. It was prone to falling off and caused severe chafing and skin damage and was not used as a long term device.

In recent years this device has re-surfaced as a modern polycarbonate shell with multiple seals and a high pressure oscillation pump. It has mostly been effective with children and is still in use in domiciliary ventilation in West England and Wales.

Clinical use

Mechanical ventilation is used when natural (spontaneous) breathing is absent (apnea) or insufficient. This may be the case in cases of intoxication, cardiac arrest, neurological disease or head trauma, paralysis of the breathing muscles due to Guillain-Barré syndrome, Myasthenia Gravis, spinal cord injury, or the effect of anaesthetic and muscle relaxant drugs. Various pulmonary diseases (i.e. pulmonary edema, COPD) or chest trauma (i.e. ARDS, broken ribs), cardiac disease such as congestive heart failure, sepsis and shock may also necessitate ventilation.

Depending on the situation, mechanical ventilation may be continued for a few minutes or many years. While returning to spontaneous breathing is rarely a problem in routine anaesthesia, weaning an intensive care patient from prolonged mechanical ventilation can take weeks or even months. Some patients never adequately regain the ability to breathe and require permanent mechanical ventilation. This is often the case with severe brain injury, spinal cord injury, or neurological disease.

Techniques

Nasotracheal intubation

Invasive ventilation

Invasive ventilation refers to the application of ventilation via an artifical airway. This airway may be an endotracheal tube ( a tube passed through the mouth or nose into the trachea) or a tracheostomy tube ( a tube inserted through the neck into the trachea). Invasive ventilation is required when a patient is unable to breathe for themselves, which is to say they cannot breathe enough to eliminate carbon dioxide from their lungs. This may be due to a disease process such as pneumonia, trauma to the chest wall, during an operation due to anaesthetic drugs or a neuromuscular problem such a high spinal injury (see Christopher Reeve).

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, a negative pressure 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.

Modes of ventilation

The ventilation mode describes the characteristics of the inspiratory pressure or flow program and determines if a patient may augment the volume or breathing rate through his or her own efforts. There are several ventilation modes, the most important are:

• Controlled Mechanical Ventilation (CMV)
  • Pressure Controlled Ventilation (PCV)
  • Volume Controlled Ventilation (VCV)
• Intermittent Positive Pressure Ventilation (IPPV);
• Pressure Support Ventilation (PSV);
  • All breaths are spontaneous breath types meaning that the ventilator delivers breaths only in response to patient effort. Thus appropriate apnea alarms must be set on the ventilator.
• Synchronized Intermittent Mandatory Ventilation (SIMV);
• Bi-level Positive Airway Pressure (BIPAP).
• Continuous Positive Airway Pressure (CPAP)
• Inspiratory Positive Airway Pressure (IPAP)

Non-invasive ventilation

This refers to all modalities that assist ventilation without the use of an endotracheal tube.

Indications

Initially this technique was used for patients with COPD to avoid intubation, but recent studies have suggested its merit in facilitating weaning, early extubating and cardiogenic pulmonary edema.

Generally speaking patients elligable for this treatment should be:

• dyspneic due to hypoxic, hypercapnic or mixed respiratory failure
• dysplaying physical signs of respiratory muscle weakness (exhaustion)
• tachypneic, respiratory rate greater than 25
• hemodynamically stable
• able to tolerate or submit to the particular mode of ventilation

Modalities

Several types of interfaces are possible. A nasal, oronasal (i.e. venturi mask), or full-face mask. The oronasal masks frequently cause nasal bridge ulcers which is a clear disadvantage.

Connection to ventilators

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 airway. These are designed to provide a passage of air to the pharynx through the nose or mouth, respectively. A face mask does not, however, 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 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.
• Cricothyrotomy - Patients that require emergent airway management that fail tracheal intubation sometimes require that an airway be inserted through a surgical opening in the cricoid cartilage. This is similar to a tracheostomy but can be performed emergently. [1]
• Tracheostomy - When patients require mechanical ventilation for more several weeks, and oral tubes may damage mucosal lining which increases the risks of secondary infection. Tracheostomy provides the most suitable access to the patient's trachea. A tracheostomy is a surgically created passage to 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 who are expected to be difficult to wean from mechanical ventilation, i.e., patients who have little muscular reserve.