What are Continuous Negative Extrathoracic Pressure (CNEP) and Negative Pressure Therapy (NPV)?

When we breathe in normally our diaphragm tightens and moves downwards and our rib cage moves outwards and upwards, increasing the space in the chest, which the lungs expand into. This creates a negative pressure in comparison to the air around us, as air is sucked into the lungs. The muscles between the ribs help expand the chest and we use them more as we breathe harder. When we breathe out the diaphragm and muscles just relax and the chest space becomes smaller, pushing air out. The Exovent works by using a pump to create a negative pressure, which can be increased gradually as needed, in the chamber around our chest and abdomen. This negative pressure can be applied constantly, Continuous Negative Extrathoracic Pressure (CNEP) to gently suck on the chest wall and improve the effort of breathing. If this does not provide enough support for the patient's breathing, the pressure can be changed to an increased cyclical  mode, Negative Pressure Ventilation (NPV) which helps more to take over the effort  improving the strength and efficiency of breathing.

Exovent devices are modern versions of tried & tested technology

There has been almost 100 years of research into the medical and engineering aspects of negative pressure ventilation (NPV). This type of non-invasive ventilation has saved many thousands of lives, particularly of polio victims in the 1950s and 60s, and has still been in use in recent times. In response to the COVID-19 pandemic, UK-wide team of doctors, nurses, engineers, academics and manufacturers have developed a prototype device that utilises NPV to provide breathing assistance.

The new device has been markedly changed and redesigned but uses the long established principles of negative pressure ventilators that have been used since the 1920s and by the NHS since its inception in 1948. It is a new, lightweight, reliable, torso-sized respiratory support device, based on proven principles. It is simple, safe and quick to set up, and requires little additional nursing or medical training. The Exovent team believe this will be the first of many devices, and are supporting an ever growing number of International Pioneer Groups to develop their own devices that employ negative pressure for CNEP and NPV.

Although some doctors are unaware that this type of device remains in use, others employed in specialist care of children and young adults still remain familiar with negative pressure ventilation.

Contemporary clinical research data has clearly shown that negative pressure respiratory support can be successful in treating acute respiratory failure and pneumonia under a variety of circumstances. The full details of this research are available in a medical review.

During the COVID-19 crisis (and in future treatment of respiratory failure) this negative pressure ventilatory support device may provide a rapidly deployable alternative for patients not able to gain access to CPAP or sophisticated positive pressure ventilators (PPV).

How do the therapies work?

Negative Pressure

Negative Pressure Ventilation works by helping natural breathing and is therefore more physiologically considerate than positive pressure ventilation.

Patient management

Ventilator designs which only deliver the negative pressure to the torso will provide optimal ventilatory and cardiac advantages and will also allow access to manage the patient’s bladder and bowels as well as access to their lower limbs for medical procedures.

Feedback

People report that when using negative pressure ventilation, they just relax and let the machine do the work, and that it feels like natural breathing.

Clinical Testing

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For Patients

Exovent assists patient oxygenation by supporting all the muscles of respiration – diaphragm and chest wall – in a way that mirrors how breathing happens naturally.

Highlights

  • No need to be intubated.
  • Use with or replace other forms of respiratory support.
  • Simple, reliable, graduated mechanism of operation.
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For Clinicians

There is substantial evidence that NPV can deliver treatment to patients with ARDS as well, or superior to, conventional PPV.

Highlights

  • Encompasses torso only.
  • Reduced pressure on the heart.
  • Similar to natural breathing.

Engineering
In response to the COVID-19 pandemic, a UK-wide team of doctors, nurses, engineers, academics and manufacturers have developed the exovent in conjunction with Marshall ADG. It is a new, ultra-lightweight, reliable, torso-sized respiratory support device, based on sound, well-tried principles. It is simple, safe and quick to set up, and requires little additional nursing and medical training. It could be used on an ordinary hospital bed.The engineering is relatively simple, well-understood, and requires parts which are cheap and easily available worldwide, even in medium and low-resource countries. A large number of similar devices could quickly be manufactured by engineering and medical teams locally. They will need to pass local regulatory requirements, but could be in use very quickly with straightforward, easy training and monitoring requirements. We will freely supply all our technical and clinical details and support. exovent runs on electricity and could be run from a battery or generator. It does not require pressurized air or oxygen to power it, reducing demand on this sometimes scarce resource.

Why will exovent suit COVID-19 patients?
Patients with COVID-19 pneumonia may do well with breathing in air with high oxygen concentration, but for some this may not be enough because the tiny air-sacs in their lungs (alveoli) collapse when they breathe out, and the energy they need to put into breathing gets tiring. This problem is commonly treated by blowing air and oxygen into the lungs under pressure by wearing a tightly-fitting face-mask – a technique called continuous positive airway pressure (CPAP). Opening the lungs with very high positive pressure can damage them. exovent can open up the alveoli just as effectively as CPAP, but we anticipate (in clinical trials) that moving the chest naturally with negative pressure will not damage the lungs, and the effect on the heart will be to improve its efficiency. We already know, from testing, it is possible to use on a person lying prone.

What else can exovent do for these patients?
As well as improving the way patients can breathe for themselves, exovent can also fully take over the work of breathing, and allow them to rest. It can do this non-invasively, whilst they are fully awake, and able to talk and drink. This is very different from the way that most patients with COVID-19 have their breathing taken over for them – conventional positive pressure ventilation needs them to have a tube placed in their windpipe, and that requires them to be sedated into a medically-induced coma. Increasing worldwide experience has led doctors to try to delay or avoid intubation and positive pressure ventilation, if at all possible for patients with COVID-19 pneumonia, and use less invasive treatment methods to deliver oxygen earlier in the course of the illness.

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Manufacturing Benefits

exovent has been designed specifically to meet the significant challenges presented by the COVID-19 pandemic, including shortages in materials, high demand, and exacting time constraints.

  • Can be rapidly mass produced.
  • Uses parts readily available in local markets.
  • Has a small number of moving parts.
  • Does not compete for the same resources required by the commissioned manufacturers of PPVs.
  • Less expensive than other forms of ventilation.
  • Can be assembled at speed.
  • Unlike PPVs or CPAPs, exovent does not require medical-grade compressed gases, which are at risk of shortage in the NHS due to heavy levels of demand for oxygen.

Summary

exovent is a modern version of tried & tested technology. Continuous negative extrathoracic pressure (CNEP) and negative pressure ventilation (NPV) are valuable methods of breathing support for the treatment of Covid-19.

"the use of negative pressure devices could transform the patient journey."

- Dr Jim Roberts