Mechanical ventilation

Source: Wikipedia, the free encyclopedia.
This article is about medical ventilation. For the use in architecture and climate control, see Ventilation (architecture).
"Intermittent positive-pressure ventilation" redirects here; not to be confused with Intermittent positive pressure breathing.

Mechanical ventilation
Servo-u Ventilator
ICD-993.90 96.7
MeSHD012121
OPS-301 code8-71

Mechanical ventilation or assisted ventilation is the medical term for using a ventilator machine to fully or partially provide artificial ventilation. Mechanical ventilation helps move air into and out of the lungs, with the main goal of helping the delivery of oxygen and removal of carbon dioxide. Mechanical ventilation is used for many reasons, including to protect the airway due to mechanical or neurologic cause, to ensure adequate oxygenation, or to remove excess carbon dioxide from the lungs. Various healthcare providers are involved with the use of mechanical ventilation and people who require ventilators are typically monitored in an intensive care unit.

Mechanical ventilation is termed invasive if it involves an instrument to create an airway that is placed inside the

positive pressure ventilation where air is pushed into the lungs through the airways, and negative pressure ventilation where air is pulled into the lungs. There are many specific modes of mechanical ventilation
, and their nomenclature has been revised over the decades as the technology has continually developed.

History

Hospital staff examine a patient in an Iron lung tank respirator during the polio epidemic. The machine creates a negative pressure around the thoracic cavity, thereby causing air to rush into the lungs to equalize intrapulmonary pressure.

The Greek physician

asphyxiating
dogs and seemingly bringing them back to life. These experiments all demonstrate positive pressure ventilation.

To achieve negative pressure ventilation, there must be a sub-atmospheric pressure to draw air into the lungs. This was first achieved in the late 19th century when John Dalziel and Alfred Jones independently developed tank ventilators, in which ventilation was achieved by placing a patient inside a box that enclosed the body in a box with sub-atmospheric pressures.[3] This machine came to be known colloquially as the Iron lung, which went through many iterations of development. The use of the iron lung became widespread during the polio epidemic of the 1900s.

Early ventilators were control style with no support breaths integrated into them and were limited to an inspiration to expiration ratio of 1:1. In the 1970s, intermittent mandatory ventilation was introduced as well as synchronized intermittent mandatory ventilation. These styles of ventilation had control breaths that patients could breathe between.[4]

Uses

breathing
is inadequate to maintain life. It may be indicated in anticipation of imminent respiratory failure, acute respiratory failure, acute hypoxemia, or prophylactically. 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 liberate them from the ventilator.

Common specific medical indications for mechanical ventilation include:[5][6]

Mechanical ventilation is typically used as a short-term measure. It may, however, be used at home or in a nursing or rehabilitation institution for patients that have chronic illnesses that require long-term ventilatory assistance.

Risks and complications

Mechanical ventilation is often a life-saving intervention, but carries potential complications. A common complication of positive pressure ventilation stemming directly from the ventilator settings include volutrauma and

barotrauma.[11][12] Others include pneumothorax, subcutaneous emphysema, pneumomediastinum, and pneumoperitoneum.[12][13] Another well-documented complication is ventilator-associated lung injury which presents as acute respiratory distress syndrome.[14][15][16] Other complications include diaphragm atrophy,[17][18][19] decreased cardiac output,[20] 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.[21][22]

In many healthcare systems, prolonged ventilation as part of

intensive care is a limited resource. For this reason, decisions to commence and remove ventilation may raise ethical debate and often involve legal orders such as do-not-resuscitate orders.[23]

Mechanical ventilation is often associated with many painful procedures and the ventilation itself can be uncomfortable. For infants who require opioids for pain, the potential side effects of opioids include problems with feeding, gastric and

opioid dependence, and opioid tolerance.[24]

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.[25][5] There are several objective parameters to look for when considering withdrawal, but there are no specific criteria that generalizes to all patients.

Spontaneous breathing trials are conducted to assess the likelihood of a patient being able to maintain stability and breath on their own without the ventilator. This is done by changing the mode to one where patients trigger breaths and ventilatory support is only given to compensate for the added resistance of the endotracheal tube.[26]

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, though it is limited by moderate sensitivity and poor specificity.[27] 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.[28]

A cuff leak test is done to detect if there is airway edema to show the chances of post-extubation stridor. This is done by deflating to the cuff to check if air begins leaking around the endotracheal tube.[26]

Physiology

The function of the lungs is to provide gas exchange via oxygenation and ventilation. This phenomenon of

FiO2. Alveolar ventilation is the amount of gas per unit of time that reaches the alveoli and becomes involved in gas exchange.[30] PaCO2 is the partial pressure of carbon dioxide of arterial blood, which determines how well carbon dioxide is able to move out of the body.[31] Alveolar volume is the volume of air entering and leaving the alveoli per minute.[32]
Mechanical dead space is another important parameter in ventilator design and function, and is defined as the volume of gas breathed again as the result of use in a mechanical device.

Image of endotracheal tube placement required to connect a patient's physiologic airway to the ventilator.

Due to the anatomy of the human

airway adjunct
is not 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.

When 100% oxygen (1.00 FiO
2) is used initially for an adult, it is easy to calculate the next FiO
2 to be used, and easy to estimate the shunt fraction.[33] The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation.[33] In normal physiology, gas exchange of oxygen and 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 de-oxygenated blood.[33] When using 100% oxygen, the degree of shunting is estimated as 700 mmHg - measured PaO
2. For each difference of 100 mmHg, the shunt is 5%.[33] 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.[33] Other such causes of a shunt include:

  • alveolar collapse from major atelectasis[33]
  • alveolar collection of material other than gas, such as pus from
    congestive heart failure, or blood from hemorrhage[33]

Technique

Modes

Main article: 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 conventional positive pressure ventilators modes fall into one of two categories:volume-cycled or pressure-cycled.[34][25] A relatively new ventilation mode is flow-controlled ventilation (FCV).[35] FCV is a fully dynamic mode without significant periods of 'no flow'. It is based on creating a stable gas flow into or out of the patient's lungs to generate an inspiration or expiration, respectively. This results in linear increases and decreases in intratracheal pressure. In contrast the conventional modes of ventilation, there are no abrupt drop intrathoracic pressure drops, because of the controlled expiration.[36] Further, this mode allows to use thin endotracheal tubes (~2 – 10 mm inner diameter) to ventilate a patient as expiration is actively supported.[37] 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.[38]

Further information: Prone ventilation

Types of Ventilation

Carl Gunnar Engström invented in 1950 one of the first intermittent positive pressure ventilator, which delivers air straight into the lungs using an endotracheal tube placed into the windpipe.

Positive pressure

Main article:
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.[2]

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

Main article: Negative pressure ventilator

Negative pressure mechanical ventilators are produced in small, field-type and larger formats.

biphasic cuirass ventilation.[42] Its main use has been in patients with neuromuscular disorders that have some residual muscular function.[43] 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.[2]

The larger units have their origin in 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.[42] 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 recoil 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. 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.[3]

Some of the problems with the full body design were such as being unable to control the inspiratory to expiratory ratio and the flow rate. This design also caused blood pooling in the legs.[4]

Intermittent abdominal pressure ventilator

Another type is the intermittent abdominal pressure ventilator that applies pressure externally via an inflated bladder, forcing exhalation, sometimes termed

generic name for the type.[45][46]

Oscillator

3100A Oscillator

The most commonly used high frequency ventilator and only one approved in the United States is the 3100A from Vyaire Medical. It works by using very small tidal volumes by setting amplitude and a high rate set in hertz. This type of ventilation is primarily used in neonates and pediatric patients who are failing conventional ventilation.[47]

High Frequency Jet Ventilation

The first type of high frequency ventilator made for neonates and the only jet type is made by Bunnell Incorporated. It works in conjunction with a separate CMV ventilator to add pulses of air to the control breaths and PEEP.[47]

Neonatal Jet ventilator

Monitoring

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 assess trends.

In ventilated patients, pulse oximetry is commonly used when titrating FIO2. A reliable target of Spo2 is greater than 95%.[48]

The total PEEP in the patient can be determined by doing an expiratory hold on the ventilator. If this is higher than the set PEEP, this indicates air trapping.

The plateau pressure can be found by doing an inspiratory hold. This shows the actual pressure the patient's lungs are experiencing.

Loops can be used to see what is occurring in the patient's lungs. These include flow-volume and pressure-volume loops. They can show changes in compliance and resistance.

Functional Residual Capacity can be determined when using the GE Carestation.

Modern ventilators have advanced 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 Tracheal tube test.

Types of ventilators

SMART BAG MO Bag-Valve-Mask Resuscitator

Ventilators come in many different styles and method of giving a breath to sustain life.

artificial airway and maintain breaths with their hands. Mechanical ventilators are ventilators not requiring operator effort and are typically computer-controlled or pneumatic-controlled.[25]
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:

  1. Transport ventilators—These ventilators are small and more rugged, and can be powered pneumatically or via AC or DC power sources.
  2. Intensive-care 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.
  3. Neonatal ventilators (bubble CPAP, HFJV, HFOV[clarification needed])—Designed with the preterm neonate in mind, these are a specialized subset of ICU ventilators that are designed to deliver smaller volumes and pressures to these patients. These may be conventional or high frequency types.[47]
  4. COPD
    and in the ICU setting.

Breath delivery mechanisms

Trigger

The trigger, either flow or pressure, 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 based on the set respiratory rate.

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.

Limit

Limit is how the breath is controlled. Breaths may be limited to a set maximum pressure or volume.

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

Main article:
Artificial airway

There are various procedures and mechanical devices that provide protection against airway collapse, air leakage, and aspiration:

See also
  • Biotrauma – Severe inflammatory response in mechanical ventilator patients
  • Charles Hederer – French medical inventor, inventor of the pulmoventilateur
  • Mean airway pressure

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