Dr Eric Flam and Loretta Raab R.Ph, CCP
Introduction
The skin of a patient can be more resistant to pressure ulcers if its
surrounding environment is controlled. At the areas of contact with the
support surface, excess moisture and temperature weaken the skin and increase
the level of any friction or shear loads it encounters. Excluding wound
drainage and incontinence, the sources of this excess moisture are insensible
moisture loss and sweat. Both insensible moisture loss and sweat are produced
continuously and one of their functions is to constantly remove excess
body heat. Once they reach the surface of the skin this moisture and heat
must be removed to maintain the proper skin temperature and integrity.
Effective Low Air Loss systems supply a controlled flow of air to these
contact regions, coupled with a means of removing excess moisture and
heat. This process, based on the technical principles of fluid and thermal
dynamics, offers an excellent method of managing skin moisture and temperature.
The objective of this presentation is to explain, in a clinically useful
form, the nature of effective Low Air Loss therapy and how it functions.
Physiological
Rationale
A. Skin Unoccluded From Normal Room Environment
Skin unoccluded from normal patient environment retains:
• Proper moisture concentration
• Proper temperature
Figure 1 depicts this action.
Figure 1: Skin Unoccluded From Normal Room Environment
B. Skin Occlusion
by Moisture Vapour Impermeable Support Surface
Skin occluded by a moisture vapour impermeable support surface barrier
becomes over-hydrated and over-heated as shown in Figure 2.
Figure 2: Skin Occlusion by Moisture Vapour Impermeable Support Surface
Over-hydration and over-heating of the skin has some significant effects:
• The sweat rate doubles for every 1.5°C increase in skin temperature.1,2
• The potential for increased body temperature. This can occur if
the excess heat cannot be removed and the thermal regulation of the patient
is compromised.
• The shear and friction forces are doubled when the skin becomes
over-hydrated.3
• At a relative humidity of 100% the stratum corneum is 25 times
weaker than at 50% relative humidity.4
• At a temperature of 35°C the mechanical strength of the stratum
corneum is only 25% of the value at 30°C.5
• Over-hydration of the stratum corneum results in an increased
moisture vapour transmission rate.6
• Over-hydration of the stratum corneum causes the pH to rise from
its normally acid value of 4.9 to a neutral value of 7.2.7
• Over-hydration of the stratum corneum can lead to a 1000-fold
to 100,000-fold increase in the resident micro organism levels.7
• The normal stratum corneum has a high resistance to absorption
of external water. Over-hydration of the stratum corneum lowers this resistance
to 25% of the normal value.8
C. Effective
Low Air Loss Therapy Support Surface
Skin in contact with an effective low air loss therapy support surface
retains proper moisture concentration and proper temperature as depicted
in Figure 3
Figure 3: Effective Low Air Loss Therapy Support Surface
Components of an effective low air loss design
The major
components of an effective Low Air Loss design (Figure 4) are:
• Air supply for support surface cushions
• Cushions with Low Air Loss air escape feature
• Overlying moisture vapour permeable coverlet
In this system some of the air within the cushions is allowed to escape
into the space between the cushions and the moisture vapour permeable
coverlet. The moisture vapour produced by the patient passes through the
vapour permeable coverlet and is carried away by the air flow in the space
between the cushions and coverlet to the room environment.
Figure 4: The
major components of an effective Low Air Loss design
Principles
of Low Air Loss Therapy
Technical principles govern the operation of all processes. Low Air Loss
therapy works on the principles of Fluid Dynamics, Heat Transfer and Phase
Change to create a favourable microclimate for that part of a patient’s
skin in contact with and occluded by the support surface. This microclimate
has similar temperature and humidity characteristics to those to which
the unoccluded parts of the patient’s skin are exposed.
Creation of this micro-climate requires the management and removal of
the excess insensible moisture vapour, sweat, and heat that are produced
by the body at the parts of the patient’s skin which are occluded
by the support surface.
All these activities proceed simultaneously. However, for clarity, they
will be described individually in this section. Figure 5 shows the Low
Air Loss system with a cutaway section to clarify this description.
Figure
5: Low Air Loss system with a cutaway section
A. Fluid
Dynamics – Insensible Moisture Vapour
Two principles apply:
• Fluids flow from a high concentration to a low concentration
• The driving force for this flow is the difference between the
high and low concentrations
Moisture
vapour is produced continuously at 100 percent relative humidity (saturation)
and at a temperature of 37°C. This is where the highest moisture vapour
concentration occurs. The skin is partially permeable to moisture vapour.
The portion that passes the skin is called the Insensible Moisture Vapour
(IMV). The coverlet is very permeable to moisture vapour and offers no
resistance to the passage of the IMV to the space containing the air flow
produced by the Low Air Loss system. This air is maintained at approximately
50 percent relative humidity and at a temperature between 22°C and
25°C. This is where the lowest moisture vapour concentration occurs.
The difference between the highest and lowest moisture vapour concentration
drives this IMV flow. The IMV then mixes with the air produced by the
Low Air Loss system and is carried out to the room environment.
Figure 6
Figure 7
B. Heat Transfer – Insensible Moisture Vapour (IMV)
Three principles apply:
• Heat flows from a high temperature to a low temperature
• The driving force for this flow is the difference between the
high and low temperatures
• Heat is absorbed, transferred and released by fluids in convection
heat flow
The IMV is a fluid produced at a temperature of 37°C and carried with
the Low Air Loss air flow to the room environment at 22°C to 25°C.
This transfers from 12 to 15 calories per gram of IMV from the body to
the room.
C. Fluid
Dynamics – Liquid Sweat
Two principles apply:
• Fluids flow from a high pressure to a low pressure
• The driving force for this flow is the difference between the
high and low pressures
Sweat is produced by the sweat glands in the skin, and is pumped as a
liquid to the skin’s surface. The high pressure is at the sweat
gland and the low pressure is at the skin surface. Since the coverlet
is impermeable to liquids, the liquid sweat will remain at the skin surface.
It must be evaporated to moisture vapour in order to pass through the
coverlet.
D. Phase
Change – Evaporation of Liquid Sweat
Three conditions apply for evaporation:
• Local temperature value at which both liquid and vapour co-exist
• Maintenance of an unsaturated local vapour concentration
• Sufficient energy to convert the low-energy liquid to the high-energy
vapour
The normal temperature of the skin surface is between 30°C and 32°C.
At this temperature liquid water and moisture vapour co-exist. The removal
of the IMV described in (A) above ensures that the local moisture vapour
concentration is unsaturated. The energy required to evaporate one gram
of liquid sweat to moisture vapour is 579 calories. In comparison this
is, on the average, 43 times the amount of heat transferred by one gram
of IMV from the body to the room environment.
E. Heat Transfer
– Sweat
Two principles apply:
• Heat transfer by phase change
• Heat absorbed, transferred and released by fluids in convection
heat flow
As discussed in section (D) the heat energy required to evaporate water
is quite high. This heat is drawn from the underlying skin and local surroundings
of the skin surface. Its removal by the evaporation is vital to maintaining
a constant skin temperature between 30°C and 32°C. If not removed,
this heat could raise the skin temperature to more than 35°C and approach
the body temperature of 37°C. In this worst case the rise in skin
temperature could approach 7°C. This would amount to a heat increase
in the skin of about 7 calories per gram of skin. The evaporation of sweat
requires 579 calories per gram of liquid. This is 83 times the amount
of heat that must be removed per gram of skin. Therefore, only 12 milligrams
of liquid sweat is required for this activity. The moisture vapour produced
by this evaporation is at the normal skin temperature of 30°C to 32°C.
This passes through the vapour permeable coverlet to the air flow from
the Low Air Loss system and out to the room environment.
The amount of heat transferred by convection heat flow equals the difference
between the temperature of the moisture vapour produced by evaporation
and the room environment, multiplied by the amount of this moisture vapour.
This is less than one-tenth of a calorie per gram of skin.
Comparison of Low Air Loss with Unoccluded Environment
A. Insensible
Moisture Vapour
Figure 6 (overleaf) compares the IMV flows.
The only performance difference is the direction of the air flow. In the
Low Air Loss system the air flow is parallel to the surfaces. The air
flow in the unoccluded environment is perpendicular; although circulating
air currents can cause more parallel flow. As it is interposed between
them, the moisture vapour permeable coverlet acts to physically separate
the skin and the air stream. Since this coverlet is permeable to the passage
of moisture vapour it does not directly affect the moisture transport.
B. Sweat
Figure 7 (overleaf) compares the minimum sleeping sweat rate (MSS) flows.
Again, one of the performance differences is the direction of the air
flow. In the Low Air Loss system the air flow is parallel to the surfaces.
The air flow in the unoccluded environment is perpendicular, although
circulating air currents can cause more parallel flow. The other performance
difference is the coverlet, which is impermeable to liquid moisture and
prevents direct contact and interaction of the liquid sweat with the air
flow in the Low Air Loss system. Only that portion of the sweat that is
evaporated can pass through the coverlet. Neither of these factors has
a negative bearing on the performance of the Low Air Loss system.
Example
The following example uses the principles in this article to relate the
excess moisture outputs of the patient to their management by effective
Low Air Loss therapy. The data used was derived from information presented
in previous publications3,9 and is presented in Table 1.
Table 2 presents the margins of safety afforded by the coverlet for the
free passage of the moisture vapour flows.
A. Insensible
Moisture Vapour Flow
In this example the insensible moisture vapour flow (IMV) is 1.2 milligrams
per hour over one square centimetre of skin area. The moisture vapour
transmission rate (MVTR) of the coverlet is 12.5 milligrams per hour over
a one square centimetre of skin area. The difference between the MVTR
of the coverlet and the IMV is 11.3 milligrams per hour over a one square
centimetre of skin area. As shown in Table 2, this provides a 950 percent
margin of safety for the free passage of the IMV.
B. Minimum
Sleeping Sweat Rate
In this example the minimum sleeping sweat rate (MSS) is 6.5 milligrams
per hour over a one square centimetre of skin area. The sweat is initially
produced as liquid and the coverlet is impermeable to this liquid. If
all this liquid sweat were evaporated it would produce 6.5 milligrams
of moisture vapour per hour over a one square centimetre of skin area.
The difference between the MVTR of the coverlet and the MSS is 6.0 milligrams
per hour over a one square centimetre of skin area. As shown in Table
2, this provides a 93% margin of safety for the free passage of the MSS.
C. Combined
Insensible Moisture Vapour and Minimum Sleeping Sweat Rate
The combination of the IMV and the fully evaporated MSS [IMV+ MSS] produces
a total of 7.7 milligrams of moisture vapour per hour over a one square
centimetre of skin area. The difference between the MVTR of the coverlet
and the [IMV+ MSS] is 4.8 milligrams per hour over one square centimetre
of skin area. As shown in Table 2, this provides a 63% margin of safety
for the free passage of this combined flow.
Summary
Effective Low Air Loss systems supply a controlled flow of air to the
occluded contact regions, coupled with a means of removing excess moisture
and heat. This process, based on the technical principles of fluid dynamics,
phase change, and heat transfer offers an excellent method of managing
skin moisture and temperature. The objective of this article is to explain,
in a clinically useful form, the nature of effective Low Air Loss therapy
and how it functions. In this article, the following topics are covered:
• Physiological rationale
• Components of an effective Low Air Loss design
• Principles of Low Air Loss therapy
• Comparison of Low Air Loss with unoccluded environment
An example is provided that compares the moisture vapour transmission
of the coverlet with the insensible moisture vapour and minimum sweat
flows. In this example the margins of safety for free passage of moisture
vapour were determined.
References
1. Robinson S: ‘Physiology of muscular exercise: temperature regulation
in exercise.’ Bard P (ed): Medical Physiology, The C.V. Mosby Company,
St. Louis, ed. Eleventh edition (1961), 521.
2. Bothorel B, Heller A, Grosshans E, Candas V: Thermal and sweating responses
in normal and atopic subjects under internal and moderate external heat
stress. Archives of Dermatological Research 1992; 284: 135–140
3. Flam E: Skin maintenance in the bed-ridden patient. Ostomy/Wound Management
1990; 28: 49–54.
4. Park A, Baddiel C: Rheology of stratum corneum-I. Journal of the Society
of Cosmetic Chemists 1972; 23: 3–12
5. Wildnauer R, Miller D, Humphries W: ‘A physicochemical approach
to the characterization of stratum corneum.’ R. Baier, Ed., Applied
Chemistry at Protein Interfaces, Advances in Chemistry Series (American
Chemical Society) 1975, vol. 145 (Chapter 4).
6. Rietschel RL: A method to evaluate skin moisturizers in vivo. Journal
of Investigative Dermatology 1978; 70: 152–155
7. Hartmann AA: Effect of occlusion on resident flora, skin moisture and
skin pH. Archives of Dermatological Research 1983; 275: 251–254
8. Blank I: Transport into and within the skin. British J of Dermatology
1969, 81 supplement 4: 4–10
9. Flam E: A new risk factor analysis: a comparison of cutaneous interface
environments of low air loss beds. Ostomy/Wound Management 1991; 33: 28–34