Mechanical ventilation

Mechanical ventilation, assisted ventilation or intermittent mandatory ventilation (IMV), is the medical term for artificial ventilation where mechanical means are used to assist or replace spontaneous breathing. This may involve a machine called a ventilator, or the breathing may be assisted manually by a suitably qualified professional, such as an anesthesiologist, paramedic or other first responder, or in some parts of the United States, by a respiratory therapist (RT), by compressing a bag valve mask device.
Mechanical ventilation is termed "invasive" if it involves any instrument inside the trachea through the mouth, such as an endotracheal tube, or the skin, such as a tracheostomy tube.
 Face or nasal masks are used for non-invasive ventilation in appropriately selected conscious patients.
The two main types of mechanical ventilation include "positive pressure ventilation" where air (or another gas mix) is pushed into the lungs through the airways, and "negative pressure ventilation" where air is usually, in essence, sucked into the lungs by stimulating movement of the chest. Apart from these two main types, there are many specific modes of mechanical ventilation, and their nomenclature has been revised over the decades as the technology has continually developed.
Uses.
Mechanical ventilation is indicated when the patient's spontaneous breathing 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 serves only to provide assistance for breathing and does not cure a disease, the patient's underlying condition should be identified and treated in order to resolve over time. In addition, other factors must be taken into consideration because mechanical ventilation is not without its complications One of the main reasons why a patient is admitted to an ICU is for delivery of mechanical ventilation. Monitoring a patient in mechanical ventilation has many clinical applications: Enhance understanding of pathophysiology, aid with diagnosis, guide patient management, avoid complications and assessment of trends. In general, mechanical ventilation is initiated to protect the airway/reduce work of breathing and/or correct blood gases.
Common specific medical indications for use include:
Mechanical ventilation 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.
Risks/Complications.
Mechanical ventilation is often a life-saving intervention, but carries potential complications including pneumothorax, airway injury, alveolar damage, ventilator-associated pneumonia, and ventilator-associated tracheobronchitis. Other complications include diaphragm atrophy, decreased cardiac output, and oxygen toxicity. One of the primary complications that presents in patients mechanically ventilated is acute lung injury (ALI)/acute respiratory distress syndrome (ARDS). ALI/ARDS are recognized as significant contributors to patient morbidity and mortality.
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.
Mechanical ventilation is often associated with many painful procedures and the procedure itself can be uncomfortable. For infants who require opioids for pain, the potential side effects of opioids include problems with feeding, gastric and intestinal mobility problems, the potential for opioid dependence, and opioid tolerance.
Technique.
Due to the anatomy of the human pharynx, larynx, and esophagus and the circumstances for which ventilation is needed, additional measures are often required to secure the airway during positive-pressure ventilation in order to allow unimpeded passage of air into the trachea and avoid air passing into the esophagus and stomach. The common method is by insertion of a tube into the trachea: intubation, 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 maneuvres, an oropharyngeal airway or laryngeal mask airway may be employed. If the patient is able to protect his/her own airway and non-invasive ventilation or negative-pressure ventilation is used, then an airway adjunct may not be needed.
Pain medicine such as opioids are sometimes used in adults and infants who require mechanical ventilation. For preterm or full term infants who require mechanical ventilation, there is no strong evidence to prescribe opioids or sedation routinely for these procedures, however, some select infants requiring mechanical ventilation may require pain medicine such as opioids. It is not clear if clonidine is safe or effective to be used as a sedative for preterm and full term infants who require mechanical ventilation.
Modes of mechanical ventilation.
Mechanical ventilation utilizes several separate systems for ventilation referred to as the mode. Modes come in many different delivery concepts but all modes fall into one of three categories; volume-cycled, pressure-cycled, spontaneously cycled. In general, the selection of which mode of mechanical ventilation to use for a given patient is based on the familiarity of clinicians with modes and the equipment availability at a particular institution.
Positive pressure.
The design of the modern positive-pressure ventilators were based mainly 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. Then, 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.
Negative pressure machines.
Negative pressure mechanical ventilators are produced in small, field-type and larger formats. The prominent design of the smaller devices is known as the cuirass, a shell-like unit used to create negative pressure only to the chest using a combination of a fitting shell and a soft bladder. In recent years this device has been manufactured using various-sized polycarbonate shells with multiple seals, and a high-pressure oscillation pump in order to carry out biphasic cuirass ventilation. Its main use has been in patients with neuromuscular disorders that have some residual muscular function. The latter, larger formats are in use, notably with the polio wing hospitals in England such as St Thomas' Hospital in London and the John Radcliffe in Oxford.
The larger units have their origin in the iron lung, also known as the Drinker and Shaw tank, which 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, in effect, 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.
Intermittent abdominal pressure ventilator.
Another type is the "intermittent abdominal pressure ventilator" that applies pressure externally via an inflated bladder, forcing exhalation, sometimes termed exsufflation. The first such apparatus was the Bragg-Paul Pulsator. The name of one such device, the Pneumobelt made by Puritan Bennett has to a degree become a generic name for the type.
Monitoring.
In ventilated patients, pulse oximetry it is commonly used when titrating FIO2. A reliable target of Spo2 is greater than 95%.
Different strategies exist to find the level of PEEP in these patients with ARDS guided by esophageal pressure, Stress Index, static airway pressure-volume curve. In such patients, some experts recommend limiting PEEP to low levels (~10cmH2O). In patients who have diffused loss of aeration, PEEP can be used provided it does not cause the plateau pressure to rise above the upper inflection point.
Most modern ventilators have basic monitoring tools. There are also monitors that work independently of the ventilator which allow for measuring patients after the ventilator has been removed, such as a T tube test.
Withdrawal from mechanical ventilation.
Timing of withdrawal from mechanical ventilation—also known as weaning—is an important consideration. People who require mechanical ventilation should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation, and this should be assessed continuously. There are several objective parameters to look for when considering withdrawal, but there are no specific criteria that generalizes to all patients.
The Rapid Shallow Breathing Index (RSBI, the ratio of respiratory frequency to tidal volume (f/VT), previously referred to as the "Yang Tobin Index" or "Tobin Index" after Dr. Karl Yang and Prof. Martin J. Tobin of Loyola University Medical Center) is one of the best studied and most commonly used weaning predictors, with no other predictor having been shown to be superior. It was described in a prospective cohort study of mechanically ventilated patients which found that a RSBI > 105 breaths/min/L was associated with weaning failure, while a RSBI < 105 breaths/min/L predicted weaning success with a sensitivity, specificity, positive predictive value and negative predictive value of 97%, 64%, 78%, 95% respectively.
Types of ventilators.
Ventilators come in many different styles and method of giving a breath to sustain life. There are manual ventilators such as bag valve masks and anesthesia bags that require the users to hold the ventilator to the face or to an artificial airway and maintain breaths with their hands. Mechanical ventilators are ventilators not requiring operator effort and are typically computer-controlled or pneumatic-controlled. Mechanical ventilators typically require power by a battery or a wall outlet (DC or AC) though some ventilators work on a pneumatic system not requiring power. There are a variety of technologies available for ventilation, falling into two main (and then lesser categories), the two being the older technology of negative-pressure mechanisms, and the more common positive-pressure types.
Common positive-pressure mechanical ventilators include:
Breath delivery mechanisms.
Trigger.
The trigger is what causes a breath to be delivered by a mechanical ventilator. Breaths may be triggered by a patient taking their own breath, a ventilator operator pressing a manual breath button, or by the ventilator based on the set breath rate and mode of ventilation.
Cycle.
The cycle is what causes the breath to transition from the inspiratory phase to the exhalation phase. Breaths may be cycled by a mechanical ventilator when a set time has been reached, or when a preset flow or percentage of the maximum flow delivered during a breath is reached depending on the breath type and the settings. Breaths can also be cycled when an alarm condition such as a high pressure limit has been reached, which is a primary strategy in pressure regulated volume control.
Limit.
Limit is how the breath is controlled. Breaths may be limited to a set maximum circuit pressure or a set maximum flow.
Breath exhalation.
Exhalation in mechanical ventilation is almost always completely passive. The ventilator's expiratory valve is opened, and expiratory flow is allowed until the baseline pressure (PEEP) is reached. Expiratory flow is determined by patient factors such as compliance and resistance.
Artificial airways as a connection to the ventilator.
There are various procedures and mechanical devices that provide protection against airway collapse, air leakage, and aspiration:
Mechanism.
In theory, the transairway pressure is simply the difference between the pressure at airway opening and the pressure in the alveoli. That is,
where "P"TA is the transairway pressure, "P"AO is the pressure at the airway opening, and "P"ALV is the pressure in the alveoli.
The following are further parameters related to ventilation.
When 100% oxygen (1.00 "Fi") is used initially for an adult, it is easy to calculate the next "Fi" to be used, and easy to estimate the shunt fraction. The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation. In normal physiology, gas exchange (oxygen/carbon dioxide) occurs at the level of the alveoli in the lungs. The existence of a shunt refers to any process that hinders this gas exchange, leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the "left heart" (which ultimately supplies the rest of the body with unoxygenated blood). When using 100% oxygen, the degree of shunting is estimated as:
For each difference of 100 mmHg, the shunt is 5%. A shunt of more than 25% should prompt a search for the cause of this hypoxemia, such as mainstem intubation or pneumothorax, and should be treated accordingly. If such complications are not present, other causes must be sought after, and positive end-expiratory pressure (PEEP) should be used to treat this intrapulmonary shunt. Other such causes of a shunt include:
Mechanical dead space is a further important parameter in ventilator design and function. It is defined as the volume of gas breathed again as the result of use in a mechanical device. The following is an example calculation for mechanical dead space:
History.
The Greek 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." Vesalius too describes ventilation by inserting a reed or cane into the trachea of animals. In 1908 George Poe demonstrated his mechanical respirator by asphyxiating dogs and seemingly bringing them back to life.