Heat and Moisture Dissipation in Seating

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Many of the early Clinical Corner articles were written about seating and positioning. Some of the articles included:
With the summer months approaching, it is a good opportunity for us to re-visit wheelchair seating and to consider how seating can affect heat and moisture dissipation for individuals who use wheelchairs. While there are many factors that affect one’s risk for skin breakdown, moisture can contribute to the risk of skin breakdown for some individuals. Choice of materials, design and covers will influence the potential for heat and moisture build-up and/or dissipation in wheelchair cushions. The same can be said for back supports as well.

Materials can be considered either insulators or conductors of heat. An insulator helps to maintain heat, while a conductor helps to draw heat away from the body. It is important to remember that the various materials will have differing abilities to either conduct heat or insulate against heat loss. In general, the materials of foam, air and rubber are considered to be insulators. If we think about insulating pipes in our homes, often foam is wrapped around the pipes as a means of insulation. If we think about a wetsuit used for warmth and buoyancy in colder water, the neoprene rubber of the wetsuit contains millions of tiny gas bubbles within it, which are warmed by the body when the wetsuit is first put on and then act as insulation, whether for swimming or diving in cooler water to help keep the body warm.

Fluids and gels are conductors of heat, which mean that they draw heat away from the body. When we touch a gel cube or a fluid pad, they feel cooler than the ambient temperature of the room. Because these materials are conductors of heat, when an individual sits on fluid or gel, the initial feeling may be that it feels cool. If we think about the interaction between a person’s body heat and cushion material, eventually there will be an equilibrium of temperatures between the temperature of the buttocks and the temperature of the material. Because various materials conduct heat at different rates, the period of time for which the cooling effect takes place may vary.

When an individual sits on a foam cushion, because foam is an insulator and does not draw heat away from the buttocks, the individual may feel warm as there is no cooling effect. In fact, because foam is an insulator, the individual may feel warmer on the foam than on fluid or gel.

Unless there is impaired thermoregulation, when an individual begins to feel too warm, the body will naturally begin to sweat as sweating is the body’s method of relieving excess heat by producing moisture that evaporates to cool the skin. The evaporation of sweat occurs unless the environment, or microenvironment in the case of a cushion, has excess moisture to begin with.

Let’s think about what happens when we exercise on a warm, humid day. If we produce heat and sweat, we will get little relief from the heat through our sweat as the humid environment does not allow for the evaporation of our sweat, particularly if the air is still. If we think about a breeze on that same warm, humid day, the flow of air helps to evaporate some of our sweat to provide a cooling effect.

The materials used in, and the design of, a wheelchair cushion may enhance flow of air when an individual weight shifts. An effective weight shifting movement removes load either from an area of the cushion, or from the cushion entirely. If there is movement of the cushion material, such as decompression when load is removed, air flow is created with the weight shift off the cushion and with the return to the cushion. This flow of air may provide a cooling effect to an individual.

Of course, the materials used in the cushion cover must “breathe”, or allow air flow. If the material does not permit air flow, moisture can accumulate. While there are many different types of cushion covers, the most common ones are stretch, microclimatic and incontinent. An incontinent cover does not “breathe”. The material used in incontinent covers is impermeable, designed to keep moisture away from the cushion to protect the cushion material; however, this creates an environment that can lead to heat and moisture build-up, which offers little protection to an individual.

A stretch cover allows for movement of air through the material. A microclimatic cover facilitates the transfer of heat and moisture away from the body through a layer of spacer fabric. A microclimatic cover can be compared to technical shirts worn by runners that wick heat and moisture away from the body during strenuous activities.

We have talked about cushion materials and covers. We cannot forget about the materials that an individual wears when sitting on a cushion. For example, although cotton is a breathable material, it retains moisture when wet. An incontinent brief does not breathe; it creates a microclimate where heat and moisture can build-up, putting one at risk of skin breakdown. What an individual wears when sitting on a wheelchair cushion will affect what is happening at the skin surface.

If we think about back supports, similar concepts of heat and moisture dissipation related to the materials, design, and covers apply. A stretch cover will allow for air flow. A microclimatic cover will wick away heat and moisture. A spinal fluid pack will feel cool next to the skin, more so than foam, which acts as an insulator. Movement of a person’s back away from the back support will create air flow, particularly if there is movement or decompression/compression of the material with the movement. Manufacturers and researchers look for innovations that help improve heat and moisture dissipation in seating, particularly as some individuals do not have active movement, or enough active movement, to enable them to weight shift effectively or to move away from the back support. One example is the introduction of tiny fans embedded in a back support to enhance air flow to provide a cooling effect. This allows the back support to provide the air flow that an individual may be unable to create through their own movements.

With summer months approaching, heat and moisture dissipation may be at the forefront of our minds. We must keep this in mind, but also consider the other factors related to selecting appropriate seating for an individual, such as positioning/posture, stability/function, impact/vibration dampening, and skin integrity/pressure management.

As always, please provide your comments, questions and suggestions regarding Clinical Corner. I look forward to hearing from you!


Sheilagh Sherman, BA, BHScOT, MHM, OT Reg. (Ont.)
Clinical Educator
Sunrise Medical Canada

Follow me on Twitter @clinicalcorner

Note: The content of this article is not meant to be prescriptive; rather, it is meant as a general resource for clinicians to then use clinical reasoning skills to determine optimal seating and mobility solutions for individual clients.Sheilagh is unable to answer questions from members of the general public. Members of the general public are directed to their own therapists or other health care professionals to ask questions regarding seating and mobility needs.

This article is © Sunrise Medical, Inc., 2017 and cannot be copied, distributed, or otherwise reproduced in whole or in part without the express written permission of Sunrise Medical Canada.
 

Sheilagh Sherman,
BA, BHScOT, MHM, OT Reg. (Ont.)

Sheilagh joined Sunrise Medical Canada in 2010 as our full-time Clinical Educator. Prior to joining Sunrise, Sheilagh gained extensive clinical experience from working in a variety of settings, including in-patient rehabilitation, complex continuing care, and community rehabilitation. As Clinical Educator, Sheilagh is a clinical resource for therapists across Canada involved in seating and mobility. She teaches in-services and leads workshops and seminars on the clinical aspects of seating and mobility. In addition, Sheilagh hosts monthly webinars for therapists and vendors.

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