Mechanical ventilation

In architecture and climate control, mechanical or forced ventilation is the use of powered equipment, e.g. fans and blowers, to move air - see ventilation (architecture).

Bag valve mask

In medicine, mechanical ventilation is a method to artificially assist or replace spontaneous breathing when patients cannot do so on their own. In many cases, mechanical ventilation is used in acute settings -- such as in the ICU, during CPR, or during surgical anesthesia -- for a short period of time during a serious illness. For some patients who have certain chronic illnesses that require long-term ventilation assistance, they are also able to do so at home or other nursing/rehabilitation institution with the help of respiratory therapists and physicians.

History

Vesalius was the first person to describe mechanical ventilation by inserting a reed or cane into the trachea of animals and then blowing into this tube.

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 By means of a pump, the air is withdrawn mechanically to provide inspiration and released to room pressure to allow expiration.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

Using a ventilator introduces the risk of ventilator associated barotrauma. The higher the ventilator pressure, and the longer the duration of ventilation, the greater the risk.

Prolonged ventilation can lead to ventilator associated pneumonia

Techniques

Nasotracheal intubation

Invasive ventilation

Invasive ventilation refers to ventilation via an artificial 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 or they are unable to protect their own airway. 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 as a high spinal injury.

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

Within the scope of positive pressure ventilation (see above), pressure can be provided:

• Intermittently (IPPV) to simulate the normal tidal breathing pattern.
  • Greatest pressure is provided during the inspiratory phase of ventilation, determined by the Peak Inspiratory Pressure (PIP).
  • Usually a smaller positive pressure is deliverd during the expiration phase (Positive End Expiratory Pressure or PEEP) to prevent atelectasis.

• Continuously, so that inspiratory and expiratory phases of respiration are met with the same pressure resistance - see CPAP.
• High frequency oscillation. Used exclusively in neonates. This specialised form of ventilation does not require specific inspiratory or expiratory phases to ventilate the alveoli.

Within the subset of IPPV ventilation, parameters can be set by varying any 2 of time, pressure and volume. Thus there exists:

• Pressure Controlled Ventilation (PCV, or pressure ventilation). A constant inspiratory pressure is delivered for a set time.
• Volume Controlled Ventilation (VCV, or volume preset ventilation). A set tidal volume is given with each breath.

Note that any IPPV that provides PEEP will provide positive pressure throughout the respiratory cycle. Because of this PEEP, all patient initiated breaths will receive some pressure support. However, in the descriptions that follow, a supported breath refers to an inspiration supported by the higher PIP.

Patient breathing effort can either be supported (by initiating an inspiratory cycle), or ignored. Thus ventilation is further divided into:

• Controlled Mechanical Ventilation (CMV).
  • Inspiratory and expiratory cycles are determined on a preset cycling pattern. Patient respiratory efforts are ignored.
• 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.

By utilising the better features of the above 2 ventilation modes, some ventilators can deliver:

• Assist control ventilation (AC);
  • All spontaneous breaths are supported, but with a back up ventilation rate if the patient fails to breath spontaneously.
• Synchronized Intermittent Mandatory Ventilation (SIMV);
  • As with PSV, only the set number of breaths per minute are supported. Unlike PSV, the exact time between ventilation cycles is able to vary (within limits), in an attempt to co-ordinate with patient inspiratory effort.

Non-invasive methods of ventilation are discussed below.

Types of Ventilator

Intermittent Positive pressure ventilation can be delivered via:

• Hand controlled ventilation. This is always a short term measure. It can be delivered via the following devices.
  • Bag valve mask
  • Anaesthetic (or T-piece) bag
  • Neopuff in neonates
• A mechanical ventilator. Mechanical ventilators can be divided in to:
  • Transport ventilators. These ventilators are smaller, more rugged, and can be powered via:
    • The energy contained within the pressurised gas (oxygen and/or air) being delivered to the patient.
    • a portable battery.
  • ICU ventilators. These ventilators are larger and usually require mains power. This style of ventilator provides much more control of ventilation parameters. Often these can easily switch between different modes of ventilation. Usually these ventilators give more accurate feedback on various patient ventilation parameters.
    • NICU ventilators. designed with the preterm neonate in mind, these are a specialised subset of ICU ventilators which are even more accurate at delivering the smaller volumes, pressures and oxygen concentrations required to ventilate this patient subset.
  • PAP ventilators. these ventilators are specifically designed for non-invasive ventilation. this includes ventilators for use at home, in order to treat sleep apnoea.

Non-invasive ventilation

This refers to all modalities that assist ventilation without the use of an endotracheal tube. Non-invasive ventilation is aimed at minimizing patient discomfort and ventilation-related disease. It is often used in cardiac or pulmonary disesase, sleep apnea, and neuromuscular diseases.

Forms of non-invasive ventialtion include:

• Continuous positive airway pressure (CPAP).
• Bi-level Positive Airway Pressure (BIPAP). Pressures alternate between Inspiratory Positive Airway Pressure (IPAP) and a lower Expiratory Positive Airway Pressure (EPAP), triggered by patient effort. On many such devices, backup rates may be set, which deliver IPAP pressures even if patients fail to initiate a breath.
• Intermittent positive pressure ventilation (IPPV) via mouthpiece or mask

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, and in some cases as an alternative to invasive ventilation.

Generally speaking patients eligible 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.

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. Poorly fitted masks often cause nasal bridge ulcers which is a problem for some patients. Face masks are also used for non-invasive ventilation in conscious patients. A face mask does not, however, provide protection against aspiration.
• 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. Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis.
• Cricothyrotomy - Patients who require 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. [1]
• Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the patient's 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 who are expected to be difficult to wean from mechanical ventilation, i.e., patients who have little muscular reserve.

Sources and External Links

• Irwin R, Rippe J, "Intensive care medicine", 5th Edition, 2003 Lippincott Williams & Wilkins
• Marino P, "The ICU Book", 3rd Edition, 2006 Lippincott Williams & Wilkins
• Irwin R, Rippe J, "Procedures and Techniques in Intensive care medicine", 3rd Edition, 2003 Lippincott Williams & Wilkins
• International Ventilator Users Network (IVUN), Resource of information for users of home mechanical ventilation
• NIV Users Group, Group for users of noninvasive ventilation and interested parties (i.e. respiratory therapists, doctors, parents)
• Dr. Bach, a doctor experienced in use of noninvasive ventilation for patients with neuromuscular diseases (note: site is written by a third-party)
• e-Medicine, article on mechanical ventilation along with technical information