Mechanical ventilation

Mechanical ventilation or assisted ventilation is the medical term for using a machine called a ventilator 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.

This article is about medical ventilation. For the use in architecture and climate control, see Ventilation (architecture).

Mechanical ventilation

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Mechanical ventilation is termed invasive if it involves an instrument to create an airway that is placed inside the trachea. This is done through an endotracheal tube or nasotracheal tube.^[1]

For non-invasive ventilation in people who are conscious, face or nasal masks are used.

The two main types of mechanical ventilation include 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.

Surgical procedures

Acute lung injury, including (ARDS),^[7] trauma, or COVID-19^[8]

acute respiratory distress syndrome

Pneumonia

Pulmonary hemorrhage

with respiratory arrest

Apnea

^[9]

Hypoxemia

requiring intubation

Acute severe asthma

Obstruction, such as a

tumor

Acid/base derangements such as

respiratory acidosis

Neurological diseases such as , amyotrophic lateral sclerosis (ALS), Guillain–Barré syndrome, myasthenia gravis, etc.

muscular dystrophy

Newborn premature infants with

neonatal respiratory distress syndrome

due to paralysis of the respiratory muscles caused by botulism^[10]

Respiratory failure

Mechanical ventilation is indicated when a patient's spontaneous 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.

alveolar collapse from major ^[32]

atelectasis

alveolar collection of material other than gas, such as pus from , water and protein from acute respiratory distress syndrome, water from congestive heart failure, or blood from hemorrhage^[32]

pneumonia

The function of the lungs is to provide gas exchange via oxygenation and ventilation. This phenomenon of respiration involves the physiologic concepts of air flow, tidal volume, compliance, resistance, and dead space.^[6]^[28] Other relevant concepts include alveolar ventilation, arterial PaCO2, alveolar volume, and FiO2. Alveolar ventilation is the amount of gas per unit of time that reaches the alveoli and becomes involved in gas exchange.^[29] 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.^[30] Alveolar volume is the volume of air entering and leaving the alveoli per minute.^[31] 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.

Due to the anatomy of the human pharynx, larynx, and esophagus and the circumstances for which ventilation is needed, additional measures are 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 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 maneuvers, an oropharyngeal airway or laryngeal mask airway may be employed. If non-invasive ventilation or negative-pressure ventilation is used, then an 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.^[32] The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation.^[32] 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.^[32] 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%.^[32] 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.^[32] Other such causes of a shunt include:

Monitoring[edit]

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%.^[47]

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.

Breath delivery mechanisms[edit]

Trigger[edit]

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[edit]

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[edit]

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

Breath exhalation[edit]

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.

— In resuscitation and for minor procedures under anaesthesia, 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, a problem for some patients. Face masks are also used for non-invasive ventilation in conscious patients. A full-face mask does not, however, provide protection against aspiration. Non-invasive ventilation can be considered for epidemics of COVID-19 where sufficient invasive ventilation capacity is not available (or in some milder cases),^[48] but pressurized protection suits for caregivers are recommended due to the risks of poorly fitting masks emitting contaminating aerosols.^[49]

Face mask

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 anaesthetized 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.

Tracheal intubation

— a supraglottic airway (SGA) is any airway device that is seated above and outside the trachea, as an alternative to endotracheal intubation. Most devices work via masks or cuffs that inflate to isolate the trachea for oxygen delivery. Newer devices feature esophageal ports for suctioning or ports for tube exchange to allow intubation. Supraglottic airways differ primarily from tracheal intubation in that they do not prevent aspiration. After the introduction of the laryngeal mask airway (LMA) in 1998, supraglottic airway devices have become mainstream in both elective and emergency anesthesia.^[50] There are many types of SGAs available including the esophageal-tracheal combitube (ETC), laryngeal tube (LT), and the obsolete esophageal obturator airway (EOA).

Supraglottic airway

— Patients requiring 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.^[51]

Cricothyrotomy

— When patients require mechanical ventilation for several weeks, a tracheostomy may provide the most suitable access to the 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 expected to be difficult to wean from mechanical ventilation, i.e., patients with little muscular reserve.

Tracheostomy

Mouthpiece — Less common interface, does not provide protection against aspiration. There are lipseal mouthpieces with flanges to help hold them in place if patient is unable.

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

Biotrauma

– French medical inventor, inventor of the pulmoventilateur

Charles Hederer

at eMedicine, article on mechanical ventilation along with technical information.

Mechanical Ventilation

Archived 2018-10-05 at the Wayback Machine, Resource of information for users of home mechanical ventilation.

International Ventilator Users Network (IVUN)

Archived 2020-07-27 at the Wayback Machine (detailed, illustrated slideshow presentation), by Amirali Nader, MD FCCP, Critical Care Medicine, Suburban Hospital, Johns Hopkins Medicine.

Mechanical ventilation