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Mechanical ventilationط
From Wikipedia, the free encyclopedia
In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing.
This may involve a machine called a ventilator or the breathing may be assisted by a physician or other suitable person compressing a bag or set of bellows. Traditionally divided into negative-pressure ventilation, where air is essentially sucked into the lungs, or positive pressure ventilation, where air (or another gas mix) is pushed into the trachea.
It can be used as a short term measure, for example during an operation or critical illness (often in the setting of an intensive care unit). It may be used at home or in a nursing or rehabilitation institution if patients have chronic illnesses that require long-term ventilatory assistance.
Owing to the anatomy of the human pharynx,larynx and oesophagus and the circumstances for which ventilation is required then additional measures are often required to "secure" the airway during positive pressure ventilation to allow unimpeded passage of air into the trachea and avoid air passing into the oesophagus and stomach. Commonly this is by insertion of a tube into the trachea which provides a clear route for the air. This can be either an endotracheal tube, inserted through the natural openings of mouth or nose or a tracheostomy inserted through an artificial opening in the neck. In other circumstances simple airway manouevres, an oropharyngeal airway or laryngeal mask airway may be employed. If the patient is able to protect their own airway such as in non-invasive ventilation or negative pressure ventilation then no airway adjunct may be needed.
Mechanical ventilation is often a life-saving intervention, but carries many potential complications including pneumothorax, airway injury, alveolar damage, and ventilator-associated pneumonia.[citation needed].
In many healthcare systems prolonged ventilation as part of intensive care is a limited resource (in that there are only so many patients that can receive care at any given moment). It is used to support a single failing organ system (the lungs) and cannot reverse any underlying disease process (such as terminal cancer). For this reason there can be (occasionally difficult) decisions to be made about whether it is suitable to commence someone on mechanical ventilation. Equally many ethical issues surround the decision to discontinue mechanical ventilation.
1 History
1.1 Negative pressure machines
1.2 Positive pressure machines
2 Indications for use
3 Types of ventilators
4 Modes of ventilation
4.1 Conventional ventilation
4.1.1 Breath termination
4.1.2 Breath initiation
4.2 High Frequency Ventilation (HFV)
4.3 Non-invasive ventilation (Non-invasive Positive Pressure Ventilation or NIPPV)
4.4 Proportional Assist Ventilation (PAV)
4.5 Adaptive Support Ventilation (ASV)
4.6 Neurally Adjusted Ventilatory Assist (NAVA)
5 Choosing amongst ventilator modes
6 Initial ventilator settings
6.1 Tidal volume, rate, and pressures
6.2 Sighs
6.3 Initial FiO2
6.4 Positive end-expiratory pressure (PEEP)
6.5 Positioning
6.6 Sedation and Paralysis
6.7 Prophylaxis
7 Modification of settings
7.1 When to withdraw mechanical ventilation
8 Connection to ventilators
9 Terminology
10 See also
11 References
12 Further reading
13 External links


The Roman physician Galen may have been the first to describe mechanical ventilation: "If you take a dead animal and blow air through its larynx [through a reed], you will fill its bronchi and watch its lungs attain the greatest distention."[1] Vesalius too describes ventilation by inserting a reed or cane into the trachea of animals[2]. In 1908 George Poe demonstrated his mechanical respirator by asphyxiating dogs and seemingly bringing them back to life.[3]

Negative pressure machines

An Iron Lung
Main article: Iron Lung
The iron lung, also known as the Drinker and Shaw tank, was developed in 1929 and was one of the first negative-pressure machines used for long-term ventilation. It was refined and used in the 20th century largely as a result of the polio epidemic that struck the world in the 1940s. 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.
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.
In the iron lung by means of a pump, the air is withdrawn mechanically to produce a vacuum inside the tank, thus creating negative pressure. This negative pressure leads to expansion of the chest, which causes a decrease in intrapulmonary pressure, and increases flow of ambient air into the lungs. As the vacuum is released, the pressure inside the tank equalizes to that of the ambient pressure, and the elastic coil of the chest and lungs leads to passive exhalation. However, when the vacuum is created, the abdomen also expands along with the lung, cutting off venous flow back to the heart, leading to pooling of venous blood in the lower extremities. There are large portholes for nurse or home assistant access. The patients can talk and eat normally, and can see the world through a well-placed series of mirrors. Some could remain in these iron lungs for years at a time quite successfully.
Today, negative pressure mechanical ventilators 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 prominent device used is a smaller device known as the cuirass. The cuirass is a shell-like unit, creating negative pressure only to the chest using a combination of a fitting shell and a soft bladder. Its main use is in patients with neuromuscular disorders who have some residual muscular function. However, 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 in order to carry out biphasic cuirass ventilation.

Positive pressure machines

The design of the modern positive-pressure ventilators were mainly based on technical developments by the military during World War II to supply oxygen to fighter pilots in high altitude. Such ventilators replaced the iron lungs as safe endotracheal tubes with high volume/low pressure cuffs were developed. The popularity of positive-pressure ventilators rose during the polio epidemic in the 1950s in Scandinavia and the United States and was the beginning of modern ventilation therapy. Positive pressure through manual supply of 50% oxygen through a tracheostomy tube led to a reduced mortality rate among patients with polio and respiratory paralysis. However, because of the sheer amount of man-power required for such manual intervention, mechanical positive-pressure ventilators became increasingly popular.
Positive-pressure ventilators work by increasing the patient's airway pressure through an endotracheal or tracheostomy tube. The positive pressure allows air to flow into the airway until the ventilator breath is terminated. Subsequently, the airway pressure drops to zero, and the elastic recoil of the chest wall and lungs push the tidal volume -- the breath—out through passive exhalation.
This is an example of a neonatal (infant) ventilator.

Indications for use

Mechanical ventilation is indicated when the patient's spontaneous ventilation is inadequate to maintain life. It is also indicated as prophylaxis for imminent collapse of other physiologic functions, or ineffective gas exchange in the lungs. Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease, the patient's underlying condition should be correctable and should resolve over time. In addition, other factors must be taken into consideration because mechanical ventilation is not without its complications (see below)
Common medical indications for use include:
Acute lung injury (including ARDS, trauma)
Apnea with respiratory arrest, including cases from intoxication
Chronic obstructive pulmonary disease (COPD)
Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) > 50 mmHg and pH < title="Thoracic diaphragm" href="http://en.wikipedia.org/wiki/Thoracic_diaphragm">diaphragm due to Guillain-Barré syndrome, Myasthenia Gravis, spinal cord injury, or the effect of anaesthetic and muscle relaxant drugs
Increased work of breathing as evidenced by significant tachypnea, retractions, and other physical signs of respiratory distress
Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) < title="Hypotension" href="http://en.wikipedia.org/wiki/Hypotension">Hypotension including sepsis, shock, congestive heart failure
Neurological diseases such as Muscular Dystrophy Amyotrophic Lateral Sclerosis

Types of ventilators

Ventilation can be delivered via:
Hand-controlled ventilation such as:

SMART BAG MO Bag-Valve-Mask Resuscitator
Bag valve mask
Continuous-flow or Anaesthesia (or T-piece) bag
A mechanical ventilator. Types of mechanical ventilators include:
Transport ventilators. These ventilators are small, more rugged, and can be powered pneumatically or via AC or DC power sources.
ICU ventilators. These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure). This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time). Many ICU ventilators also incorporate graphics to provide visual feedback of each breath.
NICU ventilators. Designed with the preterm neonate in mind, these are a specialized subset of ICU ventilators which are designed to deliver the smaller, more precise volumes and pressures required to ventilate these patients.
PAP ventilators. these ventilators are specifically designed for non-invasive ventilation. this includes ventilators for use at home, in order to treat sleep apnea.

Modes of ventilation

Conventional ventilation:
The modes of ventilation can be thought of as classifications based on how to control the ventilator breath. Traditionally ventilators were classified based on how they determined when to stop giving a breath. The three traditional categories of ventilators are listed below. As microprocessor technology is incorporated into ventilator design, the distinction among these types has become less clear as ventilators may use combinations of all of these modes as well as flow-sensing, which controls the ventilator breath based on the flow-rate of gas versus a specific volume, pressure, or time.

Breath termination:
In a volume-cycled ventilator the ventilator delivers a pre-set volume of gas with each breath. Once the specified volume of breath is delivered, the positive pressure is terminated after a certain specified time period. Both pressure and volume modes of ventilation have their respective limitations. Many manufacturers provide a mode or modes that utilize some functions of each. These modes are flow-variable, volume-targeted, pressure-regulated, time-limited modes (for example, pressure-regulated volume control — PRVC). This means that instead of providing an exact tidal volume each breath, a target volume is set and the ventilator will vary the inspiratory flow at each breath to achieve the target volume at the lowest possible peak pressure. The inspiratory time limits the length of the inspiratory cycle and therefore the I:E ratio. Pressure regulated modes such as PRVC or Auto-flow (Draeger) can most easily be thought of as turning a volume mode into a pressure mode with the added benefit of maintaining more control over tidal volume than with strictly pressure-control.

Breath initiation:
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath. Similar to the termination classification noted above, microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications. Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above.
Assist Control (AC). In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath. Traditional assist-control used only a pre-set tidal volume—when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV. However, the initiation timing is the same—both provide a ventilator breath with every patient effort. In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic. Although a maximum rate is not usually set, an alarm can be set if the ventilator cycles too frequently. This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt).
Synchronized Intermittent Mandatory Ventilation (SIMV). In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 seconds - thus a respiratory rate of 12 results in a 5 second cycle time). Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor. When the ventilator senses the first patient breathing attempt within the cycle, it delivers the preset ventilator breath. If the patient fails to initiate a breath, the ventilator delivers a mechanical breath at the end of the breath cycle. Additional spontaneous
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