Exovent: a study of a new negative-pressure ventilatory support device in healthy adults

Summary

Negative-pressure ventilation may have several advantages compared with positive-pressure ventilation. Negative-pressure ventilation simulates natural lung movements, does not require tracheal intubation and may reduce the incidence of barotrauma and adverse cardiovascular events. A group of engineers, doctors and nurses designed and bench-tested the Exovent, a new, lightweight, torso-only, negative-pressure ventilatory support system. We aimed to test the comfort, nursing acceptability and ventilatory support capabilities of the Exovent in healthy adult volunteers. We measured the effect of continuous negative extra-thoracic pressure on functional reserve capacity and the efficacy of ventilation produced by a combination of negative-pressure ventilation and negative end-expiratory pressure. Six members of the development team volunteered to test the device. The application of continuous negative extra-thoracic pressure did not change tidal volumes from baseline levels; however, functional reserve capacity increased by a mean (SD) of 1.1 (0.05) ml.kg^-1.cmH₂O^-1 (p = 0.0002). The combination of negative-pressure ventilation and negative end−expiratory pressure produced effective ventilation, with the resting tidal volume being exceeded by the application of -4 cmH₂O of extra-thoracic negative pressure. All the volunteers found the experience comfortable and none had ventilator dysynchrony. The Exovent allowed good nursing and monitoring access and was comfortable in both the semi-recumbent and prone positions. The Exovent delivered effective continuous negative extra-thoracic pressure and negative-pressure ventilation plus negative end-expiratory pressure to healthy adults. Further trials are needed to investigate the clinical utility of the device.

Introduction

Negative-pressure or ‘iron lung’ ventilators, where just the patient’s head remains outside the whole-body chamber, have saved the lives of many thousands of patients with poliomyelitis and pneumonia. Negative-pressure ventilation simulates natural lung movements, does not require tracheal intubation, and does not appear to cause barotrauma. In addition, as there is no increase in intrathoracic pressure, which occurs in positive-pressure ventilation, negative-pressure ventilation may have less effect on cardiac output [1].

Despite these benefits, negative-pressure ventilation was largely abandoned in the mid-20th century when much smaller and more convenient positive-pressure ventilators were introduced that allowed greater nursing access. Some centres have continued to use and modernise whole-body negative-pressure ventilation devices. Instead of using continuous positive airway pressure (CPAP) to increase functional reserve capacity (FRC), these devices work by generating continuous negative extra-thoracic pressure. In addition, negative end-expiratory pressure (NEEP) can be added which provides an equivalent effect to positive end-expiratory pressure (PEEP) which is used commonly during positive-pressure ventilation [2]. Attempts to make more convenient lightweight negative-pressure ventilatory support devices using ‘cuirass’ designs have been limited by the ineffective transmission of their negative pressures to generate lung movement. Wrap and poncho designs struggle to produce air-tight seals, while relatively rigid anterior ‘shells’ may restrict lateral movement of the ribs and splint the diaphragm by upper abdominal compression during inspiration [3].

We formed a development group of engineers, doctors and nurses to design and produce a convenient, ultra-lightweight device that encloses the torso from neck to hips. The device was designed to meet the following criteria: to be capable of delivering both continuous negative extrathoracic pressure (CNEP) and negative-pressure ventilation plus NEEP safely and efficiently; to have robust design that would be easy to manufacture; and to have relatively low maintenance costs. This project resulted in the creation of the Exovent negative-pressure ventilatory support system. The purpose of this study was to evaluate the comfort and usability of the Exovent and to measure its capacity to generate an adequate tidal volume and increase the FRC in healthy volunteers.

Methods

The Exovent development group collaborated with the Marshall Aerospace and Defence Group (Cambridge, UK) in designing, building and bench-testing a lightweight device before human testing. The chamber consists of a base fitted onto a standard hospital bed that contains its own section of mattress and a removable top that fits over the torso with neck and hip seals. A free-standing pump unit is connected to the base by flexible hoses and a control unit allows the following settings: background pressure up to -20 cmH₂O (for CNEP or NEEP); peak inspiratory pressure up to -25 cmH₂O; respiratory rate between 0 (for CNEP) and 20 breaths.min^-1; and inspiratory:expiratory (I:E) ratios of 1:1 to 1:5. The subject’s torso can be observed through a window and accessed through portholes that seal around the healthcare provider’s arms. The thin neoprene neck and hip seals are fitted loosely onto the person before the chamber being put in place, and then adjusted subsequently (the neck seal being of a hyperboloidal design) (Fig. 1).

Figure 1

Volunteer being ventilated in the Exovent in the semi-recumbent position.

Testing was performed on six healthy adult volunteer members of the development team after full ergonomic bench-testing had been completed and in the presence of three senior anaesthetists. The research and development department of Queen Elizabeth Hospital King’s Lynn NHS Foundation Trust confirmed that formal approval from an ethics committee was not necessary.

All subjects were studied while lying supine with the upper half of the bed and Exovent in a 30° head-up (semi-recumbent) position. In addition, three subjects were also studied while supine without elevation, and prone with a 10° head-up tilt. Tidal volume and vital capacity were measured at rest, then tidal volumes were measured after the application for 120 s of each of the following CNEP pressures: -5 cmH₂O; -10 cmH₂O; -15 cmH₂O; and -20 cmH₂O. These were delivered in a random order and with the subject blinded to the applied pressure. The effect of CNEP on the FRC was measured half-way through each 30-s trace by rapidly reducing the negative pressure back to atmospheric pressure by quickly opening the port holes (Fig. 2). We then measured the tidal volumes generated during ventilation at four combinations of negative-pressure ventilation and NEEP settings. Again these were applied in a random order, with the subject blinded to the applied pressure. All spirometry measurements were made in triplicate using a spirometer (Vitalograph Pneumotrac 6800, Vitalograph, Buckinghamshire, UK) which continuously recorded data during 30-s test periods with measurements expressed per kg of ideal body weight (estimated from volunteer’s height).

Figure 2

Method of measuring the change in functional residual capacity (FRC) induced by continuous negative extrathoracic pressure (CNEP). The figure shows three superimposed 30-s spirometry traces from a volunteer breathing with CNEP at -20 cmH₂0 for the first half of the recording period, and then breathing at atmospheric pressure after the Exovent portholes had been opened and the vacuum dissipated rapidly.

A structured questionnaire was used to assess the patient experience, specifically their comfort getting into and out of the Exovent and the sensations of receiving ventilatory support (see online Supporting Information Appendix S1).

The changes in tidal volume and FRC with increasing negative-pressure gradients were evaluated with one-way ANOVA testing. A value of p < 0.05 was considered to represent a significant difference.

Results

The characteristics of the six healthy adult volunteers are shown in Table 1. Nursing observations identified that the Exovent chamber could be positioned and removed quickly by two people, and that the window and portholes allowed ready observation of chest movements and comfortable, but incomplete, nursing access to the torso. When supine, the subjects’ heads could be positioned easily to allow view of the larynx for tracheal intubation without needing to remove or adjust the neck seal. The volunteers all found the chamber comfortable, and in particular reported that the neck and hip seals were soft and easy to adjust, and that they could voluntarily breach them to stretch their arms or touch their faces without this significantly affecting the stability of the chamber pressure. None of the participants felt claustrophobic inside the Exovent (despite two anticipating that they might) and all felt secure that they were ‘in control’. That is, the participants knew that they could move relatively freely, and immediately release the vacuum by opening a wide gap under one of the seals if they wished. They could also choose to put their hands out of the portholes at will. All three volunteers who were studied in the prone position felt comfortable; one more slender subject was able to turn from supine to prone unaided during ventilation, but the others needed to have the chamber top temporarily removed to do this. All were able to speak without difficulty (timed to coincide with expiration during negative-pressure ventilation), drink through a straw and eat with assistance.

Table 1. Characteristics of six volunteers who tested the Exovent device in the semi-recumbent position (supine at 30°).

	Volunteer number
	1^a	2^a	3	4	5	6^a
Sex	Female	Female	Female	Male	Male	Male
Height; m	1.63	1.60	1.57	1.83	1.81	1.80
Ideal body weight; kg	60	57	54	82	80	79
Respiratory rate; breaths.min^−1	12	11	13	14	12	20
Tidal volume; ml.kg^−1	11.9	12.8	15.6	7.9	12.1	8.2
Forced vital capacity; ml.kg^−1	50.5	60.1	57.3	49.8	57.0	66.9

• ^a Three volunteers were also studied in the prone position.

The application of CNEP did not change tidal volumes from baseline levels; however, FRC increased by a mean (SD) of 1.1 (0.05) ml.kg^-1.cmH₂O^-1 (p = 0.0002) (Fig 3a). The participants did not report any subjective awareness of CNEP until it reached -15 cmH₂O, by which time the FRC had increased by approximately 1.5 times more than their tidal volumes at baseline. At higher pressures, the feeling of having an increased chest expansion became noticeable to the subjects, but was not unpleasant.

Figure 3

Mean tidal volume and functional reserve capacity (FRC) values generated by using the Exovent in six healthy adults who had a mean (SD) forced vital capacity of 57 (6.4) ml.kg^-1. Plot (a) shows the effects of continuous negative extra-thoracic pressure (CNEP) and plot (b) shows the effects of combinations of negative-pressure ventilation (NPV) and negative end-expiratory pressure (NEEP). Error bars denote 1 SD.

When the ventilation mode was used, the subjects all allowed the Exovent to take over their respiratory effort without dysynchrony, and described the sensation as relaxing; one participant fell asleep within minutes. Another subject with a spontaneous respiratory rate of 20 breaths.min^-1 experienced a feeling of ‘waiting for the next breath’ when ventilated at 10 breaths.min^-1, but this was rectified easily by increasing the respiratory rate. Ventilation at the lowest negative-pressure gradient setting of -4 cmH₂O generated a tidal volume which exceeded the subjects’ mean spontaneous resting tidal volume; the tidal volume increased further at higher negative pressures (Fig. 3b).

The tidal volumes of three volunteers were similar when CNEP was applied in the semi-recumbent, supine and prone positions (Fig. 4).

Figure 4

Tidal volume measurements in three healthy adult volunteers at various levels of continuous negative extrathoracic pressure (CNEP) while supine at 0° (Δ), supine at 30° head-up (Ο) and prone with 10° head-up (●).

Discussion

We found the Exovent to be a comfortable, practical, efficient and lightweight negative-pressure device that can deliver CNEP and increase the FRC in healthy volunteers in a manner that parallels CPAP. The Exovent was also able to deliver effective ventilation with NEEP to conscious healthy adults who remain able to cough, talk, eat and drink throughout the process.

When the subjects were supine, they were more comfortable in the semi-recumbent (30° head-up) position than flat; lying prone with a slight head-up tilt was also comfortable, and had no impact on the subjects’ tidal volumes during CNEP. These findings contrast with wrap or poncho devices where air leaks limit their effectiveness, or with anterior chest and upper abdominal shells which may be unable provide full ventilation [4-6]. The volunteers’ reports of feeling relaxed during both CNEP and negative-pressure ventilation plus NEEP, and their lack of respiratory dysynchrony, was consistent with historic clinical experience of using whole-body tank negative-pressure ventilators. It is also consistent with the reported suppression of diaphragmatic activity among naïve volunteers receiving negative-pressure ventilation once they had been given reassurance about what to expect, and advice to relax [7].

This device has been developed over 6 months by a team of engineers, doctors and nurses who have created the Exovent charity to promote the investigation and development of negative-pressure ventilation for international benefit. The initial build was undertaken by Marshall Aerospace and Defence Group, and a complete technical file has been prepared for submission to the Medicines and Healthcare products Regulatory Agency (MHRA) with a view to obtaining a CE mark. It is likely that the UK version of the Exovent will cost approximately £8000 (US$10,496, €8856), which is considerably cheaper than existing positive-pressure devices. It is anticipated that a low-cost global version of the Exovent could be produced for less than £500 (US$652, €550). Anticipated advantages of the Exovent over positive-pressure ventilators, which may be especially relevant in low- and middle-income countries, include the lower resources required to ventilate conscious patients and the potential for greater oxygen conservation (as this will only need to be supplied to the patients directly via a facemask or nasal cannulae).

This study has a number of important limitations. This is the first study of a novel device and the data are from a small number of healthy volunteers and cannot be extrapolated to patients with lung pathology. In addition, our study only exposed individuals to up to 2 h of treatment, whereas its anticipated use in the clinical setting would be for far longer periods.

In summary, we have described a novel negative-pressure ventilatory support device that effectively supports spontaneous ventilation in conscious, healthy adult volunteers. We plan to undertake further testing in the near future to investigate the clinical utility of this device.

Acknowledgements

We thank R. Atkinson, J. Burnip, R. Fryer, C. Goodwin, A. Hughes, M. Johnston, D. Saul, D. Wichard and P. Wood (all from Marshall Aerospace and Defence Group), A. Finberg (Vitalograph) and K. Karaoglanis for their generous support in realising this project. No external funding or competing interests declared.

Appendix 1: Members of the Exovent Development Group

MG Coulthard, Associate Clinical Lecturer, Translational and Clinical Research Institute, Newcastle University, UK and Honorary Consultant, Department of Paediatric Nephrology, Great North Children’s Hospital, Newcastle, UK; D. Ackerley, General Practitioner (retired), Dapdune House Surgery, Guildford, Surrey, UK; N.A. Downie, Emeritus Professor, Royal Academy of Engineering, London, UK; A. J. Fielding, Managing Director of AJ Fielding Consulting Ltd, London, UK; J. Harris, Chief Engineer, Costa Express Ltd, High Wycombe, UK; E. Hodges, Surgical Risk and Governance Manager, Queen Elizabeth Hospital, King’s Lynn, Norfolk, UK; D. Howard, Professor, Department of Head and Neck Oncology, Imperial and UCLH Trust Hospitals, London, UK; I.R. Joesbury. Chief Executive Officer, Exovent, London, UK; H.J. Lambert, Consultant, Department of Paediatric Nephrology, Great North Children’s Hospital, Newcastle, UK; V. Lund, Emeritus Professor of Rhinology, Royal National Ear, Nose and Throat Hospital, London, UK; D.S. McKeown, Senior Engineer, Institute of Civil Engineers, London, UK; A. Patel, Consultant, Department of Anaesthesia, Royal National ENT and Eastman Dental Hospitals, UCLH, London, UK; J. Roberts, Consultant, Department of Anaesthesia, Royal National ENT and Eastman Dental Hospitals, UCLH, London, UK; N. Ryan, Head of Technology, Steer Energy Solutions Ltd, Miserden, Stroud, UK; C. Speight, General Practitioner, Covid-19 Assessment Centre and Lothian Unscheduled Care Service, Western General Hospital, Edinburgh, UK; P. Young, Consultant, Department of Critical Care Medicine, Queen Elizabeth Hospital King’s Lynn, Norfolk, UK.