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TECHNICAL CONSIDERATIONS FOR INTEGRATING VENTILATORS WITH MANUAL WHEELCHAIRS

Eric W.C. Tam Rehabilitation Engineering Center The Hong Kong Polytechnic University, Hong Kong

ABSTRACT

The quality of life of those individuals who are ventilator dependent will be greatly improved if their respiratory support equipment can be incorporated onto their wheelchairs. However, there exists limited space underneath the wheelchair which can be used, and the weight distribution of these equipment should not alter the stability of the wheelchair, as well as causing obstruction to the body. Depending on the requirement of each individual, the design considerations for integrating ventilators with wheelchairs will be slightly different. This paper describes the overall considerations involved in mounting respiratory support equipment onto manual wheelchairs, and discuss three different cases that were encountered in our recent practice.

BACKGROUND

Individuals suffering from progressive neuromuscular diseases, high level spinal cord injuries and other respiratory diseases will require tracheotomies and ventilators support to sustain their life. Traditionally, these patients are usually long term hospitalized and bedridden. Modern technology has enabled those ventilator-dependent patients to become ambulatory, so that they can access and reintegrate themselves back to the community. In Hong Kong, there seems to be an increasing demand on fitting portable ventilators onto wheelchairs for patients who are in need. Three separate cases have been encountered by our Center within the past 18 months. These included one congenital spinal muscular atrophy child, one teenager with cervical spinal fracture and one elderly suffering from respiratory disease.

PROBLEM

Ideally, the decision for integrating a mechanical ventilator with a wheelchair should be well considered before any equipment is purchased. This would help to ensure an optimal integration, and often, any necessary fittings and modifications can be installed by the wheelchair manufacturer. In reality, ventilators are often prescribed during the emergency phase, where patient's mobility is not a principal concern. However, as rehabilitation progresses, wheelchairs will be prescribed to improve the quality of life. It is then that rehabilitation engineers are being called upon to face the challenge of designing a mounting system that can safely be attached the complete ventilator unit, but without alternating the structure of the wheelchair. This often poses additional difficulties and creates unnecessary constraints on the design, particularly in cases where wheelchairs are purchased from a catalogue and very limited information is available.

DESIGN CONSIDERATIONS

The portable mechanical ventilator unit comprises of the ventilator, an external battery source and the necessary tubing. In addition, other accessories including oxygen cylinder, humidifier, suction device, etc., may be required depending on the need of individual patients. In designing the placement of the ventilator unit, safety of the mounting tray(s) and the overall stability of the wheelchair are primary concerns. In addition, the orientation of the ventilator must allow for adequate air intake, and the controls and displays at the front panel should not be obstructed by any fixtures. These additional weight of the ventilator components should be evenly distributed over the wheelchair frame.

Ventilator Placement

Ventilators are usually placed horizontally during normal operation and devices should not be mounted at an angle unless their performance in this position is guaranteed by the manufacturer. The space underneath the wheelchair seat is the obvious location for mounting the ventilator. Although chair frames can be retrofitted to create more space for the equipment, this implies major modification to the wheelchair structure, which can be rather expensive. Most ventilators can be fitted onto a 14 inches width manual wheelchair. To maximize use of the space underneath the seat of a common wheelchair, the ventilator and the battery are required to be mounted separately with trays on either sides of the cross brace. Self-locking drop-hooks can be used to secure the trays onto the chair. The equipment must be securely fastened onto the trays. If the chair has a rigid frame, the mounting trays can be designed to be permanently connected to the frame structure. The floor clearance for the trays should be at least 2 inches. When the ventilator is mounted at the rear which is the most common location, care should be taken to prevent it from being damage during the curb climbing maneuver. Sufficient clearance should be allowed between the seat upholstery and the top of the respiratory equipment, so that under occupancy, the body weight would not be transferred to the mounting fixture and the user would not be sitting on a hard surface.

In terms of power requirement, a 12VDC, 42AH battery is usually used. Modern design has reduced the overall dimension of these batteries, and they can be easily be fitted in front of the cross brace. As a safety measure, the two terminal connectors on top of the battery must be protected against any accidental contact with the wheelchair frame. Also, the position of the battery holder should not cause any obstruction to patient's legs, especially to those individuals with contractures. In addition, all respiratory equipment mounted onto reclining wheelchair has to be located below the level of the seat, so that the function of the recliner would not be limited.

Accessories

When oxygen is required for use, it is advisable to use the smaller cylinders. These must be properly secured but adequate provision should be made for their quick and easy interchange. The flow regulator should also be conveniently located. When the inhale air requires to be humidified, it is recommended that a humid vent device be used for the short period when mobility is essential. The jar type humidifier is excellent for longer term use but as it requires A/C power supply and uses a volume of hot water, it is not suitable for transport.

The length of the breathing tubing should be kept short to minimize the dead space. Also, the tubing should be routed and appropriately secured to prevent being disconnected or damaged during reclining and tilting of the seat as well as while the chair is in motion.

DISCUSSION

Three contrasting cases are used to illustrate the design considerations.

Child with Spinal Muscular Atrophy

This child requires a ventilator and a tilt-in-space chair with positioning support. The primary use of the unit is to allow him to attend school. Since his chair has a rigid frame, the ventilator and battery trays were directly attached using existing holes in the wheelchair frame. The breathing tube was routed from the back of the chair and, using a positioning clamp located on the side of the push handle, guided to the right side of his head. In order to prevent panel controls from being altered by other children, a quick release transparent protector was designed and installed. Since the child needs to be positioned at a particular angle for function, we decided to put a stopper to fix the tilt at an angle prescribed by the therapist. This would ensure that the caregiver can reproduce the desired tilting angle every time after transfer and also that, the seat frame will not accidentally touch the battery terminals.

Teenager with Cervical Spinal Fracture

This teenager has been hospitalized for a few years after he was injured during gymnastic exercise. As his condition stabilized, clinical professionals would like to improve the quality of his life by providing him with a manual recliner. Since the patient is very slim, the recliner is only 16 inches wide. Although the engineer was aware of the model prescribed, it was not known until the chair arrived that the only suitable location where the ventilator could be mounted was obstructed by the reclining lock mechanism. The solution was to swap the positions of the left and right lock mechanisms to create enough space, and to reposition the spreader bar further to the rear. Since the width of the wheelchair is maintained by the support structure of the mounting trays, the function of the spreader bar is actually enhanced despite of the minor modification. However, this modification has caused one problem. The lock mechanism failed to support the back post when the backrest angle exceeds 170o, and a stopper had to be installed to prevent shearing of the pivot pin when the backrest is in its full reclined position.

Elderly person with Emphysema

This lady has been hospitalized for over two years and now requires a ventilator with oxygen supply to sustain her life. Due to the body build, her manual recliner has to be tailor made. As oxygen was required, two small cylinders had to be incorporated onto the wheelchair. It was found that the best location for placing the two oxygen cylinders was on the top of the ventilator. Although we suggested that the client should be provided with a rigid seat base, we still had to ensure enough clearance between the seat and the equipment to avoid any direct load transfer.

REFERENCES

Wanger D., Lynott J., Kolar P. and Barnicle K. Functional Considerations for Portable Ventilator Users, Proceedings of RESNA International '92, 1992, pp. 98-99.

Laurence S., Integrating Ventilators with Power Mobility, TeamRehab, Oct. 1992, pp. 17-19.

ACKNOWLEDGMENTS

This work is supported by the Royal Hong Kong Jockey Club (Charitable) Ltd. and the Hong Kong Polytechnic University.

Eric W.C. Tam Rehabilitation Engineering Center The Hong Kong Polytechnic University Hunghom, Kowloon Hong Kong Email: RCERIC@PolyU.EDU.HK

Integrating Ventilator with Manual Wheelchairs