Mechanical or artificial ventilation: the different types and indications for use

By Cristiano Antonino On May 30, 2022

Mechanical ventilation (also called artificial ventilation or assisted ventilation) refers to breathing support for people who are partially or totally unable to breathe spontaneously; mechanical ventilation supplements or completely replaces the activity of the inspiratory muscles by providing the energy needed to ensure an adequate volume of gas to the lungs (oxygen therapy)

Mechanical ventilation is, in many cases, a life-saving treatment and is widely used, especially in intensive care units with the most critical patients; however, it is not entirely free of risks and complications.

Mechanical ventilation is indicated in the case of various conditions, including:

respiratory distress (ARDS)
apnoea associated with respiratory arrest;
severe and acute asthma;
acute or chronic respiratory acidosis;
severe hypotension;
moderate/severe hypoxaemia;
neurological diseases such as muscular dystrophy.

There are two main types of mechanical ventilation

negative pressure mechanical ventilation: this is the oldest type, it is permanent and is generally performed by means of a negative ventilation system, thanks to an air chamber surrounding the thorax, such as the so-called steel lung, which is rhythmically made to negative pressure to allow air to be sucked into the airways and lungs;
positive pressure mechanical ventilation: this is the most modern and most commonly used type of ventilation at present; it is temporary and relies on the use of positive pressure systems such as a ventilator or the rhythmic manual compression of an oxygen-enriched air reservoir such as the AMBU balloon or a so-called to-and-fro balloon, connected to the patient’s airway.

Given the anatomy of the airways, which share the first tract with the digestive system, and the circumstances in which assisted ventilation is used (the patient usually presents a decrease in vigilance or degree of consciousness), additional measures are necessary to ensure the smooth passage of air into the airways and to avoid the insufflation of gas into the stomach and the resulting vomiting reflex, which has as a dreaded complication the inhalation of solid or liquid material into the airways and an ab ingestis respiratory distress syndrome.

This type of ventilation is referred to as invasive

As a rule, airway isolation and direct connection to the positive pressure source is achieved by inserting a cannula into the larynx through the nose or mouth, or through a tracheotomy.

In other cases, it is possible to use simple airway manoeuvres or the laryngeal mask, which is a substitute for the endotracheal tube.

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If the patient does not require airway protection and there are no obstacles to the passage of air, non-invasive mechanical ventilation is possible

Artificial ventilation is often a life-saving intervention, but it is not without serious complications such as pneumothorax, injury to the airways or alveoli, and infectious pneumonia.

Being the cornerstone of intensive care, artificial ventilation in the critically ill patient and totally dependent on ventilatory support raises considerable ethical questions as to whether it should be used in very old patients, those with terminal illnesses or those so severe as to constitute a form of futile treatment.

Negative pressure machines

The steel lung, also known as the Drinker and Shaw tank, was developed in 1929 and was one of the first negative pressure machines for long-term artificial ventilation.

It was then perfected and used extensively in the 20th century for the polio epidemics that plagued the planet in the 1940s.

It is effectively a kind of cistern, in which the patient is literally enclosed up to the neck, where through a rubber sheath, the head protrudes and the airways are brought into direct contact with the ambient air.

By means of a bellows, a depression is generated inside the cistern, the rib cage expands and a depression is created inside the patient’s airway and ambient air, due to a pressure difference, enters the airway and lungs.

The interruption of the bellows function with the return to the starting position allows passive emptying of the lung.

The steel lung, therefore, does nothing more than reproduce the respiratory mechanics, which is observed under normal conditions and which a myopathy or neuropathy makes impossible due to the insufficient function of the muscles of the rib cage.

One of the big problems is that the abdomen is also located in the reservoir and consequently also expands during the action of the bellows and creates a sequestration of blood by reducing venous return to the right heart, a particularly dangerous situation in patients with hypovolaemia where a significant drop in blood pressure can occur.

Nowadays, negative pressure systems are still in use, mostly on patients with insufficient thoracic cage muscles, as in polio.

The machine in use is known as a respiratory armour, in case it is made of a metal shell, while it is called a Poncho Lung in case it is made of lighter materials and the airtightness is guaranteed by an outer jacket.

In both cases, only the thoracic area is involved, with the arms and legs being affected, leaving the patient free to move.

Positive pressure machines

Modern positive-pressure ventilators derive from devices used in World War II to assist the ventilation of military aircraft pilots at altitude.

The ventilator works by insufflating gas mixtures (usually air and oxygen) at positive pressure into the patient’s airway.

Exhalation is enabled by the ventilator’s pressure returning to the level of atmospheric pressure and the elastic return of the lungs and rib cage.

If breathing support is to be sustained for a long time, tracheotomy and insertion of a tube into the trachea through the neck are generally used.

Mechanical ventilation, indications for use

Artificial ventilation is indicated in surgical interventions involving the curarisation of the patient resulting in muscle paralysis and when the patient’s spontaneous breathing is unable to maintain vital functions.

The diseases that are treated with artificial ventilation are

acute lung damage (including ARDS, and trauma)
respiratory arrest apnoea, including cases of intoxication
flare-ups of chronic lung disease (COPD)
acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) > 50 mmHg and pH < 7.25
paralysis of the diaphragm by Guillain-Barré syndrome, Myasthenia Gravis, acute crises of muscular dystrophy or amyotrophic lateral sclerosis, spinal cord injury, or the effect of anaesthetics or muscle relaxant drugs
increased work of the respiratory muscles, evidenced by excessive tachypnoea, supraclavicular and intercostal re-entry and large movements of the abdominal wall
hypoxia with arterial partial pressure of oxygen (PaO2) < 55 mmHg despite oxygen supplementation (high FiO2 in insufflated air)
hypotension and shock, as in congestive heart failure or during sepsis.

Ventilation systems

Ventilation can be divided into two main types:

A) manual ventilation:

self-expanding balloon (AMBU)
back-and-forth balloon (or T-shaped device)

B) mechanical ventilator ventilation. Mechanical ventilators are classified into

transportable ventilators, which are small, rudimentary and powered pneumatically or by electricity from the mains or batteries.
intensive care ventilators. These ventilators are larger and only require a direct supply from the mains (although all have a battery to allow transport of the patient within the hospital or temporary power supply in the event of a blackout). These devices are also more complex and allow the control of multiple ventilation parameters. In addition, the latest models feature real-time graphics to visually assess the effect of the ventilators on airway flows and pressures.
Neonatal intensive care ventilators. These are designed for ventilation of preterm infants and have a higher resolution of control of ventilation parameters.
positive pressure ventilators. These instruments are designed for non-invasive ventilation, including at home as for the treatment of obstructive apnoea.

Risks and complications associated with mechanical ventilation

Mechanical ventilation is a safe treatment; however, it presents certain risks, including

damage to the pulmonary alveoli
pulmonary oedema;
loss of surfactant;
alveolar blood loss;
alveolar collapse;
atrophy of the diaphragm muscle;
pulmonary barotrauma (frequent): with pneumothorax, pneumomediastinum, pneumoperitoneum and/or subcutaneous emphysema;
reduced motility of airway cilia;
increased risk of pneumonia.