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What is A Ventilator And Why Is It So Important?

Forget rebreathers. With the pandemic still spreading, ventilators have arguably become one of the most essential pieces of life-support kit on the planet. We asked anesthesiologist and director of Karst Underwater Research (KUR) Andy Pitkin to brief us on these life-saving devices. Take a deep breath.



by Andrew Pitkin MBBS MRCP FRCA
Header photo by Sungmin Cho from Pixabay

The COVID-19 pandemic has highlighted one piece of medical equipment above all: the ventilator. What are these life-support devices, and how do they work? 

Until the 20th century, with a few exceptions, any patient unable to breathe for themselves was doomed to die. The large polio outbreaks that swept through the world in the 1930s and 40s prompted the design and manufacture of the first reliable means of respiratory support, the iron lung. Properly termed a negative pressure ventilator, and commonly referred to as a ‘tank’ ventilator, the iron lung and some smaller derivatives enclosing just the chest (‘cuirasse’ ventilators) were developed by Drinker1 and Emerson into the only equipment able to keep patients alive when their respiratory muscles had been paralyzed by the poliovirus. Well-funded hospitals in the USA were able to set up large wards full of iron lungs (Fig 1), but it was across the Atlantic, in the northern European country of Denmark, that the modern intensive care unit with its ubiquitous positive-pressure ventilators was born. 

Fig 1. At the height of the 1948 poliomyelitis epidemic 82 patients required ventilation in iron lungs at the same time in Los Angeles. Photo courtesy from “Patenting the Sun” by Jane S. Smith.

In the mid-1950s, Copenhagen, Denmark’s capital city, was ravaged by a severe polio epidemic that completely overwhelmed its handful of iron lungs and forced the physicians there to adopt methods borrowed from the operating room just to keep people alive. In desperation, the director of the infectious diseases hospital, the Blegsdamhospitalet, asked his anesthesiologist colleague Dr Bjorn Ibsen for help. Under Ibsen’s direction,  patients received tracheostomies and intermittent positive-pressure ventilation using a rebreathing system called a Waters circuit (Fig 2). 

Fig 2. A Waters circuit used for hand ventilation of poliomyelitis victims in the 1952 outbreak in Copenhagen, Denmark. A simpler system without the soda lime canister was used later, with the disadvantage that a higher flow rate of oxygen/nitrogen mixture was required to prevent excessive rebreathing of carbon dioxide. Illustration by InDepth.
Bjorn Ibsen. Photo courtesy of P.G. Berthelson.

Rebreather divers will easily recognize this as a pendulum rebreather system, but in this case the gas moved in and out of the patient’s lungs not through their own efforts, but by a medical student squeezing and releasing the bag (counterlung) 20 to 25 times per minute, 24 hours a day, for periods of up to several months. In the absence of enough mechanical ventilators, hundreds of medical students risked exposure to the much-feared polio virus to keep these patients, many of them children, alive (Fig 3). The success of positive-pressure ventilation in dramatically reducing mortality in Denmark rapidly spread throughout the world and the modern intensive care unit was born.

Fig 3. A patient with poliomyelitis being hand ventilated with a Waters circuit through a tracheostomy. Medical students were paid 30 shillings (about $100 today) for an 8-hour shift (with a 10-minute break for a cigarette in the middle!). Photo by Lassen HCA.

Human muscle power was soon replaced with mechanical power using either motorized bellows or a pressure-regulated gas source controlled by an automatic valve (like pressing the purge valve on a regulator’s second stage). The first classification by Mapleson2 divided ventilators into flow generators and pressure generators; both could be further subclassified into time-cycled, pressure-cycled and volume-cycled units, depending on what parameter the machine used to switch from inspiration (breathing in) to expiration (breathing out) and vice versa. An example of a flow generator is a typical bellows ventilator such as the Oxford Penlon unit (Fig 4), still used in some hyperbaric chambers because of its mechanical simplicity, and many anesthesia machines in operating rooms have similar ventilators built in. 

Fig 4. The venerable Penlon Oxford ventilator is a simple bellows system driven by a 60 psi compressed gas supply. A smaller set of bellows can be substituted for use in children. Photo courtesy of A. Pitkin.

Modern Ventilators

With advances in microelectronics in the 1970s it became feasible to integrate gas control valves with flow and pressure sensors to allow a single ventilator to work in a flow-generating mode as well as a pressure-generating mode, and many other modes became possible that could not be realized with a simple mechanical ventilator. Units such as the Siemens Servo 900 (Fig 5) and the Puritan Bennett 7200 were in the vanguard of this approach.

Fig 5. The Siemens Servo 900C, a ventilator I became intimately familiar with while training in intensive care in the early 1990s. Introduced in 1971, its use of electronically-controlled scissor valves to regulate gas flow based on input from pressure and flow sensors represented a major advance over earlier designs. Photo courtesy of Siemens Healthineers. Special thanks to Healthineers’ media team divers!

Modern intensive care ventilators have evolved into complex computer-controlled systems that operate with a degree of precision, flexibility and sophistication that would astonish Ibsen and his contemporaries (Fig 6). In the rare cases that adequate oxygenation and/or carbon dioxide clearance cannot be achieved with conventional ventilators, highly specialized options such as high frequency oscillatory ventilation (HFOV) and extracorporeal membrane oxygenation (ECMO) also exist.

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Fig 6. Simplified schematic of the pneumatic system of a modern intensive care ventilator. For simplicity, this diagram omits many important components, including non-return and safety valves, calibration and alarm systems. Diagram courtesy of A. Pitkin.

A patient with respiratory failure must be intubated with a breathing (endotracheal) tube in the windpipe (trachea) which connects to the ventilator, and to tolerate this they often need sedation and occasionally chemical paralysis to prevent them from “fighting” the ventilator, although most modern ventilators have useful support modes that synchronize the ventilator with the patient’s own respiratory efforts. An alternative to the endotracheal tube placed through the mouth, especially useful in the longer term, is to place a tube in the trachea through the front of the neck (tracheostomy), and this approach undoubtedly contributed to Ibsen’s success in Copenhagen as many polio patients had paralysis of the muscles of their throat and voicebox (larynx) and so were unable to protect their lungs from contamination by coughing. Both endotracheal tubes and tracheostomies bypass the normal humidification that occurs in the upper airways so humidification of the gas supplied to the patient is important to prevent drying out of the lining of the airways.

Current medical practice is to set tidal volumes at 6-8 ml/kg (ideal body weight) and adjust respiratory rate to achieve the desired carbon dioxide (CO2) clearance. Almost all modern ventilators can be time-cycled (most common), pressure- and volume-cycled. Patients are generally most comfortable when the partial pressure of CO2 in their blood (PaCO2) is just below normal but with severe respiratory failure it is frequently better to allow it to rise above normal as long as adequate oxygenation can be maintained rather than subjecting diseased lungs to a higher tidal volume or respiratory rate – this is called permissive hypercapnia.  Oxygenation is largely determined by the fraction of inspired oxygen (FiO2) and mean (average) airway pressure, the latter being influenced by a number of ventilator settings such as inspiratory/expiratory time ratio, positive end-expiratory pressure (PEEP) as well as patient factors such as lung compliance. For complex reasons it is common for other organ systems such as the heart and kidneys to fail in patients with respiratory failure, so for them to survive it requires a skilled team of doctors, nurses and other staff to monitor and intervene when problems occur. 

Bjorn Ibsen went on to establish the first real intensive care unit, which he called an ‘observation unit’, at the municipal hospital of Copenhagen where he was appointed chief of the anesthesiology department in 1954. He widened its scope to include any patient who needed respiratory or cardiovascular support for any reason, such as trauma, post-operative complications, sepsis and so on. One of the pioneering features was a multidisciplinary team approach that has stood the test of time and is still used today. Another was the idea that no matter what the original disease, the same principles of organ support apply. These ideas are so universally accepted now that it is hard for us to conceive of a time when they were not. 

Ventilators and the Covid-19 Virus

At the time of writing this, ventilators look like they could soon be in short supply and they cost tens of thousands of dollars. Why are they so complex and expensive? It is relatively easy to build a system of valves and sensors and an electronic control system that will function, but quite another to do so to the (justifiably) high standards of design quality, reliability, safety, ease of use and infection control that modern medicine demands. Getting any medical device approved by the US Food and Drug Administration (FDA) is very expensive and time-consuming, and similar regulatory processes exist in Europe and the rest of the world. Nevertheless, some would argue that medical device costs (like medications) are out of control, but essentially the price of sophisticated medical technology is a balance between its development costs and the price that the market will support. 

Dr. Andrew Pitkin at work.

The COVID-19 pandemic has spawned a number of projects to tackle the projected shortage of ventilators with homebuilt systems. Usefully, the Medicines and Healthcare products Regulatory Agency (MHRA) in the United Kingdom has published a set of specifications for a Rapidly Manufactured Ventilator System (RMVS), a minimum standard that such a ventilator has to meet to be useful, based on expert opinion3. These efforts are to be applauded, but use of such a ventilator would be close to a last resort, after all the ventilators in anesthesia machines in every operating room were in use. What is more, it doesn’t address the issue that patients with severe COVID-19 disease often have multiple organ involvement and it is the highly-skilled and cohesive medical and nursing teams to apply the principles that Ibsen and others pioneered that are the real limiting factor.


  1. Drinker P, McKhann CF. The use of a new apparatus for prolonged administration of artificial respiration. JAMA 1929; 92: 1658-61.
  2. Mapleson WW. The effect of lung characteristics on the functioning of artificial ventilators. Anaesthesia 1962;17:300-14.
  3. https://www.gov.uk/government/publications/specification-for-ventilators-to-be-used-in-uk-hospitals-during-the-coronavirus-covid-19-outbreak accessed April 17th, 2020.

Additional Resources

Karst Underwater Research

Andrew Pitkin learned to dive in 1992 in the cold murky waters of the United Kingdom and started cave and technical diving in 1994. His first exposure to exploration was in 1995 when he was one of a team of divers who were the first to reach the bottom of the Great Blue Hole of Belize at 408 fsw (123 msw). Subsequently he has been involved in numerous cave exploration projects in Belize, Mexico and Florida.
From 1996-2000 he was employed at the Royal Navy’s Institute of Naval Medicine, running a hyperbaric facility, treating decompression illness, participating in research into outcome after decompression illness, submarine escape and testing of new military underwater breathing systems. He is one of a handful of civilians to be trained by the Royal Navy as a diving medical officer.  
He moved to Florida in 2007 and is currently on the faculty of the College of Medicine at the University of Florida in Gainesville. With Karst Underwater Research he has participated in numerous underwater cave exploration and filming projects. Like many explorers, he spends much of his spare time developing and building innovative equipment for exploration purposes.


Out of the Depths: The Story of British Mine Diving

If sumps and solo cave diving are, well, a bit too Brit for you, you may want to consider diving into the perfusion of flooded serpentine chert, copper, limestone, silica, slate, and tin mines that honeycomb the length and breadth of the Kingdom. Fortunately, British tekkie and member of UK Mine/Cave Diving (UKMC) in good standing, Jon Glanfield, takes us for a guided tour.




By Jon Glanfield
Header image courtesy of Alan Ball.

When many think of the UK’s caves, with wet rocks and their penchant for darkness, often the images conjured are of tight, short, silty sumps, that can only be negotiated by intrepid explorers outfitted with diminutive cylinders, skinny harnesses, wetsuits and typically a beard. These are the domain and natural playground of the well-known, highly-respected, Cave Diving Group (CDG). 

In truth, much of our sceptered isle’s caves are of this ilk, but there is an alternative for the diver who favours a more conventional rig, extra room to manoeuvre, and perhaps a more team-orientated approach—one that is less than optimal in many of the true cave diving environments of the UK.

Holme Bank. Photo by Ian France.

Alongside our natural cave diving venues, we also sport a varied collection of flooded mines across the length and breadth of the Kingdom. In the south and southwest, miners have extracted metals such as tin and  copper, while in South Wales it was the mineral, silica. The Midlands Linley Caverns were a source of limestone before being converted to a subterranean munitions store in WWII. Sadly, access to these is no longer feasible. In the rolling hills of the Derbyshire Dales, flinty, hard chert strays close enough to the surface to be mined. In North Wales, the once-proud slate industry has left its Moria and Mithril redolent halls and tunnels beneath the landscape, while copper and slate underlay parts of Cumbria. Meanwhile, just over the border in Scotland, limestone was the resource that drove us to follow its veins into the earth.

Mike Greathead descending the stairway to heaven. Photo by Ian France.

Undeniably, here in the UK, mine diving has a much shorter documented history than that of its close cousin cave diving, but some of the luminaries of this dark world were, and are, active in both. Some of the initial dives in sites like the Cambrian slate mine were undertaken by the incomparable Martyn Farr, Geoff Ballard, and Helen Rider in 2006. But it wasn’t until 2014 that it was further explored and lined by the likes of Cristian Christea, Ian France, Michael Thomas, and Mark Vaughan amongst others. 

Both Rich Stevenson and Mark Ellyatt, who were part of the vanguard of the technical diving revolution in the UK, had personal dramas on trimix dives in the deep shaft of the Coniston Copper Mines, the depth of which runs to 310 m/1012 ft. Ellyatt made his dive at 170 m/555 ft in the early 2000s in a vertical 2 m/6.5 ft square shaft, dropping away into the 6º C/43º F frigid blackness.

Mines Over Matter

As was alluded to, the differences in cave and mine diving are significant. Conventional, redundant open and closed technical rigs can be employed in mine diving due to the predictably larger tunnels, passages, and chambers. Water movement is negligible, so often regular braided lines can be used, lines which would not endure the flow in many of the UK’s upland cave locations. Small teams can dive in safely. 

No Exit. Photo by Chris Elliot.

In general, it is not common to surface and explore the sumped sections of the mines, due to often dangerously contaminated or hypoxic air quality. Also, in some cases, oils and other contaminants have leached into the water. The ever-present risk of collapse—both in the submerged sections and in the dry access adits or portals—haunt divers’ thoughts and is far more common in mines than in the smooth, carved bore of a naturally-formed cave. Casevac (the evacuation of an injured diver) is complex, long-winded, and often dangerous for those involved, and in the event of an issue involving serious decompression illness (DCI), almost certainly helicopter transportation would be necessary given the remote locations.

Landowner access—or, more commonly, denial of access—is an ubiquitous spectre in the underground realm, dry or wet, and much effort is directed at maintaining relations with landowners to safeguard the resources. Some of the most frequented mines are accessible only via traverse of private property, which could be agricultural, arboreal, and in one case, bizarrely on the grounds of an architectural firm. Careful management of these routes into the mines is critical, as is demonstrating respect for the land owner and complying with their requirements when literally on their turf.

At the more prosaic level though, simply getting into some of the mines is a mission on its own, necessitating divers’ decent levels of fitness, the use of hand lines, and sometimes as much consideration of dry weight to gas volume as the dive planning itself. Careful thought and prior preparation are also required in terms of both accident response and post-dive decompression stress, given the exertion expenditure simply to clear the site.

A passageway in Aber Las. Photo by D’Arcy Foley.

Many of the mines are relatively shallow, mostly no more than 30 m/98 ft with exceptions in the notable and notorious Coniston, and the almost mythic levels in Croesor, extending beneath the current 40 m/130 ft galleries that are known and lined. Though, what the mines lack in depth, they make up for in distance and grandeur. 

Aber Las mine survey. Courtesy of UKMC.

Aber Las, or Lost, is more accurately a forgotten section of Cambrian that extends nearly 600 m/1961 ft from dive base at the 6 m/20 ft level, and a second level 300 m/984 ft long at 18 m/59 ft. The section features no less than 35 sculpted chambers hewn off the haulage ways with varying dimensions and exhibiting differing slate removal techniques. Cambrian’s chambers less than a mile away are larger still, and a lost line incident here could be a very bad day given the chambers’ cavernous aspect.

In The Eye of the Beholder

Beauty is—as they say—in the eye of the beholder, but it would be disingenuous to try to draw comparisons between the UK’s mines and the delicacy of the formations in the Mexican Karst, the light effects through the structures in the Bahamian sea caves, or the sinuous power tunnels of Florida. In mines, the compulsion to dive is due in part to the industrial detritus of man, encapsulated in time and water.

In mines, the compulsion to dive is due in part to the industrial detritus of man, encapsulated in time and water.

Parallels are frequently drawn between wreck diving and mine diving, but often the violence invoked at the demise of a vessel—the massive, hydraulic inrush of fluid and the subsequent impact on the seabed—wreaks untold damage and destruction upon its final resting place. In contrast, nature reclaims her heartlands in the mines by stealth: a slow, incremental and inexorable seep of ground water, no longer repulsed by the engines from the ages of men, gradually rising through the levels to find its table. The result is often preserved tableaus of a former heritage with a rich diversity of artefacts left where last they served.

A leftover crate in the Croesor mine. Photo by Alan Ball.

Spades, picks, lanterns, rail infrastructure, boots, slowly decomposing explosive boxes, battery packs, architectural joinery, scratched tally marks, and, even in some cases, the very footprints of the long-past workers in the paste that was cloying, coiling dust clouding the passages and stairways, can be picked out in the beam of a prying LED.

Spades, picks, lanterns, rail infrastructure, boots, slowly decomposing explosive boxes, battery packs, architectural joinery, scratched tally marks, and, even in some cases, the very footprints of the long-past workers in the paste that was cloying, coiling dust clouding the passages and stairways, can be picked out in the beam of a prying LED.

Underpinning, protecting, preserving, and improving these gems of the realm is the UK Mine and Cave Diving Club (UKMC), which formed as mine diving intensified in the mid 2000s. So it was that Will Smith, D’Arcy Foley, Sasha London, Jon Carter, Mark Vaughan, and Ian France, all of whom are respected and experienced cave divers in their own right, forged the club to foster and engage with a community of like-minded divers. 

Sadly, in 2014, Will Smith fell victim to the insidious risks of contaminated air in the Aber Las mine system, which he had been lucky enough to re-discover and in which he conducted early exploratory dives as the club gained traction and direction.

As new members filter into the ranks, new ideas, new agendas, and new skill sets re-shape the club’s direction. At present, we are rebooting the club with a remastered website, focusing on new objectives and seeking opportunities to improve, catalogue, and document the resources we husband.

Lines laid in the Cambrian slate mine. Photo by Mike Greathead.

Exploration continues: the club is laying new line in some areas. What’s more, through our demonstrable respect and care for existing sites, the club is facilitating exploration in previously inaccessible sites, and lost and forgotten sites will resurface. Meanwhile, we’re improving the locations we frequent weekly for the benefit of trainees, recreational (in the technical sense) divers, and survey divers alike. Archaeological projects are rising from the ennui of lockdown; we’re establishing wider links with mine diving communities elsewhere to share techniques, data, and ultimately hospitality.

In Welsh folklore, a white rabbit sighted by miners en route to their shifts was believed to be a harbinger of ill fortune, but for Alice, following the rabbit into its hole led her to a whimsical and magical place. Be like Alice, and come visit the Wunderland!

Additional Resources:

Jon Glanfield was lucky enough to get his first puff of compressed air at the tender age of five, paddling about on a “tiddler tank,” while his dad was taught how to dive properly somewhere else in the swimming pool. A deep-seated passion for the sport has stayed within him since then, despite a sequence of neurological bends in the late 90s, a subsequent diagnosis of a PFO, and a long lay-off to do other life stuff like kids, starting a business, and missing diving. Thankfully, it was nothing that a bit of titanium and a tube couldn’t fix. He faithfully promised his long-suffering wife (who has, at various anti-social times, taken him to and collected him from recompression facilities) that “this time it would be different” and that he was just in it to look at “pretty fishes.” So far, only one fish has (allegedly) been spotted in the mines. The ones Jon has encountered in the North Sea while wreck diving just obscured the more interesting, twisted metal.

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