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An iron lung, also known as a tank ventilator or Drinker tank, is a type of negative pressure ventilator, a mechanical respirator which encloses most of a person's body, and varies the air pressure in the enclosed space, to stimulate breathing. It assists breathing when muscle control is lost, or the work of breathing exceeds the person's ability. Need for this treatment may result from diseases including polio and botulism and certain poisons (for example, barbiturates, tubocurarine).

The use of iron lungs is largely obsolete in modern medicine, as superior breathing therapies have been developed, and due to the eradication of polio in most of the world. However, in 2020, the COVID-19 pandemic revived some interest in the device as a cheap, readily-producible substitute for positive-ventilation ventilators, which were feared to be outnumbered by potential victims temporarily needing artificially assisted respiration.

Design and function


The iron lung is typically a large horizontal cylinder, in which a person is laid, with their head protruding from a hole in the end of the cylinder, so that their head (particularly nose and mouth) is outside the cylinder, exposed to ambient air, and the rest of their body sealed inside the cylinder, where air pressure is continuously cycled up and down, to stimulate breathing.

To cause the patient to inhale, air is pumped out of the cylinder, causing a slight vacuum, which causes the patient's chest and abdomen to expand (drawing air from outside the cylinder, through the patient's exposed nose or mouth, into their lungs). Then, for the patient to exhale, the air inside the cylinder is compressed slightly (or allowed to equalize to ambient room pressure), causing the patient's chest and abdomen to partially collapse, forcing air out of the lungs, as the patient exhales the breath through their exposed mouth and nose, outside the cylinder.

Examples of the device include the Drinker respirator, the Emerson respirator, and the Both respirator. Iron lungs can be either manually or mechanically powered, but normally are powered by an electric motor linked to a flexible pumping diaphragm (commonly on opposite the end of the cylinder from the patient's head). Larger "room-sized" iron lungs were also developed, allowing for simultaneous ventilation of several patients (each with their heads protruding from sealed openings in the outer wall), with sufficient space inside for a nurse or a respiratory therapist to be inside the sealed room, attending the patients.

Smaller, single-patient versions of the iron lung include the so-called Cuirass ventilator (named for the Cuirass, a torso-covering body armor). The Cuirass ventilator encloses only the patient's torso, or chest and abdomen, but otherwise operates essentially the same as the original, full-sized iron lung. A modern version is the exovent.  A lightweight variation on the cuirass ventilator is the jacket ventilator or poncho or raincoat  ventilator, which uses a flexible, impermeable material (such as plastic or rubber) stretched over a metal or plastic frame over the patient's torso.

Normal breathing

 * Main section: breathing

Humans, like most mammals, breathe by negative pressure breathing: the rib cage expands and the diaphragm contracts, expanding the chest cavity. This causes the pressure in the chest cavity to decrease, and the lungs expand to fill the space. This, in turn, causes the pressure of the air inside the lungs to decrease (it becomes negative, relative to the atmosphere), and air flows from the atmosphere, through the patient's airway, into their lungs: inhalation. When the diaphragm relaxes, the reverse happens and the person exhales. If a person loses part or all of the ability to control the muscles involved, breathing becomes difficult or impossible.

Breathing with iron lung
The iron lung encloses the patient entirely, except for their head -- which includes their main airway orfices: nose and mouth, which remain exposed to outside ("ambient") air. A pump (typically a mechanically-powered diaphragm) momentarily reduces the air pressure inside the iron lung, to a pressure lower than outside (ambient) air.

This causes the air within the iron lung, but outside the chest wall, to be at a lower pressure than the air inside the patient's lungs (thus, the term "negative pressure"), so the patient's lungs and chest expand to equalize their air pressure to the lower pressure of the interior of the iron lung.

Now, the expanded lungs are at a lower pressure than the ambient air outside the iron lung. As a result, outside (ambient) air forces its way in through the patient's exposed nose and mouth, into their airway, to fill the lower-pressure space inside the patient's lungs -- and the patient has thus "taken a breath" ("inhaled").

Exhaling is stimulated by allowing the pressure inside the iron lung to return to normal, matching the outside (ambient) air pressure. As a result, the patient's chest is no longer being expanded by negative pressure, and the natural elasticity of the patient's chest causes it to collapse, squeezing air from the patient's lungs, through the patient's airway, and out of their exposed mouth and nose -- and the patient has thus "exhaled."

The process continues cycling, repetitively, at a rate set by the person operating the iron lung's controls, to maintain continuous patient artificial respiration (artificial "breathing").

Normal application
The iron lung may assist normal patient respiration, or it may completely substitute for it, depending upon the patient's condition. It may be used temporarily, or it may be a permanent, lifetime source of respiration support for the patient (though only a handful of people still rely on it for daily life).

Advantages
Advantages over modern positive-pressure ventilators include:
 * Superior oxygenation
 * Reduced lung trauma (such as pneumothorax)
 * Reduced heart stress
 * No airway intubation required, so:
 * Can serve patient with little or no sedation required
 * Patient can speak
 * Patient can eat and drink
 * Patient can receive oral medication


 * Greater patient comfort
 * Low-tech simplicity, requiring less difficulty and expense in design, development, production, acquisition and maintenance.
 * Low-tech reliability (though dependent upon continuous external power, manual power is an option on some units)
 * Operable by minimally-trained healthcare workers.
 * Low-cost, readily producible, largely from parts not in competition with other ventilation options.

Disadvantages
Disadvantages, compared to modern positive-pressure ventilators include:
 * Risk of patient respiratory aspiration (inhalation of, and choking on, regurgitated or misdirected esophagus contents).
 * Lack of metered, pure-oxygen air supply, or oxygen-enriched air supply (although possible to be added on by use of normal supplemental-oxygen equipment or CPAP devices).
 * Patient inhales and exhales unfiltered air, to and from the surrounding ambient environment, exposing both the patient to others' germs, and vice-versa.
 * Lack of modern monitoring and signaling, typically, to alert healthcare providers of device malfunctions and other respiratory and health emergencies.
 * Size and weight -- and lack of portability.
 * Extreme restrictions to patient movement (patient typically must lay in supine position, continuously; limited excursions from the iron lung usually require external support, from both human aides and positive-pressure ventilation equipment).
 * Requirement that patient be continuously or frequently attended by healthcare worker or other support person (though, in a different way, this is generally also true of modern positive-pressure ventilators, as well)
 * Unfamiliarity, and traditional unacceptability, to the modern medical community -- and thus not readily integrated into the current practices of medicine.
 * Greater power requirements than positive-pressure ventilators. (Iron lungs must move larger quantities of air for every breath, because they must change the pressure of the air in the entire vessel, not just inflate the lungs).
 * Electrical power failure can result in prompt patient death. (Most iron lungs and similar devices lack backup power supplies. Manual power is an option on some units, but requires multiple attendants, working alternately, to provide direct, continuous physical pumping of the iron lung's pump or diaphragm.)
 * Physical hazard to people and things in the immediate vicinity, because of exposed moving parts (Not true of all iron lungs).
 * Low-tech design creates low barrier to entry -- along with low historical demand -- resulting in lack of support from manufacturers of competing complex ventilator systems, who currently dominate and shape the market for ventilation equipment.

Initial development
In 1670, English scientist John Mayow came up with the idea of external negative pressure ventilation. Mayow built a model consisting of bellows and a bladder to pull in and expel air. The first negative pressure ventilator was described by Scottish physician John Dalziel in 1832. Successful use of similar devices was described a few years later. Early prototypes included a hand-operated bellows-driven "Spirophore" designed by Dr Woillez of Paris (1876), and an airtight wooden box designed specifically for the treatment of polio by Dr Stueart of South Africa (1918). Stueart's box was sealed at the waist and shoulders with clay and powered by motor-driven bellows.

Drinker and Shaw tank
The first of these devices to be widely used however was developed in 1928 by Drinker and Shaw of the United States. The iron lung, often referred to in the early days as the "Drinker respirator", was invented by Philip Drinker (1894–1972) and Louis Agassiz Shaw, Jr., professors of industrial hygiene at the Harvard School of Public Health. The machine was powered by an electric motor with air pumps from two vacuum cleaners. The air pumps changed the pressure inside a rectangular, airtight metal box, pulling air in and out of the lungs.

The first clinical use of the Drinker respirator on a human was on 12 October 1928, at the Boston Children's Hospital in the US. The subject was an eight-year-old girl who was nearly dead as a result of respiratory failure due to polio. Her dramatic recovery, within less than a minute of being placed in the chamber, helped popularize the new device.

Variations
Boston manufacturer Warren E. Collins began production of the iron lung that year. Although it was initially developed for the treatment of victims of coal gas poisoning, it was most famously used in the mid-20th century for the treatment of respiratory failure caused by poliomyelitis.

Danish physiologist August Krogh, upon returning to Copenhagen in 1931 from a visit to New York where he saw the Drinker machine in use, constructed the first Danish respirator designed for clinical purposes. Krogh's device differed from Drinker's in that its motor was powered by water from the city pipelines. Krogh also made an infant respirator version.

In 1931, John Haven Emerson (5 February 1906 – 4 February 1997) introduced an improved and less expensive iron lung. The Emerson iron lung had a bed that could slide in and out of the cylinder as needed, and the tank had portal windows which allowed attendants to reach in and adjust limbs, sheets, or hot packs. Drinker and Harvard University sued Emerson, claiming he had infringed on patent rights. Emerson defended himself by making the case that such lifesaving devices should be freely available to all. Emerson also demonstrated that every aspect of Drinker's patents had been published or used by others at earlier times. Since an invention must be novel to be patentable, prior publication/use of the invention meant it was not novel and therefore unpatentable. Emerson won the case, and Drinker's patents were declared invalid.

The United Kingdom's first iron lung was designed in 1934 by Robert Henderson, an Aberdeen doctor. Henderson had seen a demonstration of the Drinker respirator in the early 1930s, and built a device of his own upon his return to Scotland. Four weeks after its construction, the Henderson respirator was used to save the life of a 10-year-old boy from New Deer, Aberdeenshire, who was suffering from poliomyelitis. Despite this success, Henderson was reprimanded for secretly using hospital facilities to build the machine.

Both respirator


The Both respirator, a negative pressure ventilator, was invented in 1937 when Australia's epidemic of poliomyelitis created an immediate need for more ventilating machines to compensate for respiratory paralysis. Although the Drinker model was effective and saved lives, its widespread use was hindered by the fact that the machines were very large, heavy (about 750 lbs), bulky, and expensive. In the US, an adult machine cost about $2000 in 1930, and £2000 delivered to Melbourne in 1936. The cost in Europe in the mid-1950s was around £1500. Consequently, there were few of the Drinker devices in Australia and Europe.

The South Australia Health Department asked Adelaide brothers Edward and Don Both to create an inexpensive "iron lung". Biomedical engineer Edward Both designed and developed a cabinet respirator made of plywood that worked similarly to the Drinker device, with the addition of a bi-valved design which allowed temporary access to the patient's body. Far cheaper to make (only £100) than the Drinker machine, the Both Respirator also weighed less and could be constructed and transported more quickly. Such was the demand for the machines that they were often used by patients within an hour of production.

Visiting London in 1938 during another polio epidemic, Both produced additional respirators there which attracted the attention of William Morris (Lord Nuffield), a British motor manufacturer and philanthropist. Nuffield, intrigued by the design, financed the production of approximately 1700 machines at his car factory in Cowley, and donated them to hospitals throughout all parts of Britain and the British Empire. Soon, the Both-Nuffield respirators were able to be produced by the thousand at about one-thirteenth the cost of the American design. By the early 1950s, there were over 700 Both-Nuffield iron lungs in the United Kingdom, but only 50 Drinker devices.

Polio epidemic
Rows of iron lungs filled hospital wards at the height of the polio outbreaks of the 1940s and 1950s -- helping children, and some adults, with bulbar polio and bulbospinal polio. A polio patient with a paralyzed diaphragm would typically spend two weeks inside an iron lung while recovering.

Modern development and usage
Polio vaccination programs have virtually eradicated new cases of poliomyelitis in the developed world. Because of this, and the development of modern ventilators, and widespread use of tracheal intubation and tracheotomy, the iron lung has mostly disappeared from modern medicine. In 1959, there were 1,200 people using tank respirators in the United States, but by 2004 there were only 39. By 2014, there were only 10 people left with an iron lung.

Replacement
Positive pressure ventilation systems are now more common than negative pressure systems. Positive pressure ventilators work by blowing air into the patient's lungs via intubation through the airway; they were used for the first time in Blegdams Hospital, Copenhagen, Denmark, during a polio outbreak in 1952. It proved a success and soon superseded the iron lung throughout Europe.

The iron lung now has a marginal place in modern respiratory therapy. Nowadays, respiratory therapists and doctors help most patients with paralysis of the breathing muscles use modern mechanical ventilators that push air into the airway with positive pressure. These are generally efficacious and have the advantage of not restricting patients' movements or caregivers' ability to examine the patients as significantly as an iron lung does.

Continued use
Despite the advantages of positive ventilation systems, negative pressure ventilation is a truer approximation of normal physiological breathing, and results in more normal distribution of air in the lungs. It may also be preferable in certain rare conditions, such as central hypoventilation syndrome, in which failure of the medullary respiratory centers at the base of the brain results in patients having no autonomic control of breathing. At least one reported polio patient, Dianne Odell, had a spinal deformity that caused the use of mechanical ventilators to be contraindicated.

There are patients who today still use the older machines, often in their homes, despite the occasional difficulty of finding the various replacement parts. Joan Headley of Post-Polio Health International said that as of May 28, 2008, there were about 30 patients in the U.S. still using an iron lung. That figure may be inaccurately low; Houston alone had 19 iron lung patients living at home in 2008. Martha Mason of Lattimore, North Carolina, died on May 4, 2009, after spending 60 of her 72 years in an iron lung.

On 30 October 2009, June Middleton of Melbourne, Australia, who had been entered in the Guinness Book of Records as the person who spent the longest time in an iron lung, died aged 83, having spent more than 60 years in her iron lung.

In 2013, the Post-Polio Health International (PHI) organizations estimated that there were only six to eight iron lung users in the United States; as of 2017 its executive director knew of none. Press reports then emerged, however, of at least three (perhaps the last three) users of such devices, sparking interest amongst those in the makerspace community such as Naomi Wu in the manufacture of the obsolete components, particularly the gaskets. Another is retired lawyer Paul Alexander, 72, of Dallas.

COVID-19 pandemic
In early 2020, reacting to the COVID-19 pandemic, to address the urgent global shortage of modern ventilators (needed for patients with advanced, severe COVID-19 disease), some enterprises developed prototypes of new, readily-producible versions of the iron lung. These developments included:
 * a compact, torso-sized "exovent" developed by a team in the United Kingdom, which included the University of Warwick, the Royal National Throat Nose and Ear Hospital, the Marshall Aerospace and Defence Group, the Imperial College Healthcare NHS Trust, along with teams of medical clinicians, academics, manufacturers, engineers and citizen scientists;
 * a variation on the lightweight, torso-covering "poncho" ventilator was developed in Canada by researchers and experimenters from the University of Calgary and University of Alberta, made from very inexpensive, relatively common products.
 * a full-size iron lung developed in the United States by a team led by Hess Services, Inc., of Hays, Kansas.