The Effect of the Weather and Other Environmental Factors on the Performance of Surgical Dressings

  Abstract: Despite the fact that, globally, surgical dressings are used under very different environmental conditions, most wound care professionals are unaware of the effects of varying temperature and relative humidity on certain key aspects of dressing performance. In particular, both these parameters can have a marked effect upon the moisture vapor transmission rate of dressings which incorporate a semipermeable film or membrane within their structure. This paper describes the use of an online calculator, the development of which has been described previously, to illustrate for the first time how changes in the weather might affect the ability of a dressing to cope with the exudate produced by an exuding wound. The results suggest that, under certain conditions, dressing permeability may fall to about 30% of experimentally determined values with potentially important consequences for wear times and treatment costs.

Introduction

  Surgical dressings of various types are used throughout the world for a variety of indications. It is, therefore, inevitable that these will be stored, applied, and worn under very different environmental conditions, ranging from the intense dry desert heat found in parts of the United States, Australia, or equatorial Africa, through the hot moist conditions found in the tropics, to the subzero temperatures encountered in other parts of the globe. It is surprising that, other than some basic advice on storage (of adhesive products in particular), very little information is available on the effects of different environmental conditions on the clinical performance of many of these materials.   It has long been recognized that extremes of temperature can affect the viscosity of products impregnated with ointments or waxes, such as paraffin gauze dressings. This was reported as long ago as the 18th century by Tissot et al¹, who wrote the following instructions for the production of ‘Plaister No 65:’      “Melt four ounces of white wax; add to it, if made in Winter, two spoonfuls of oil; if in Summer none at all, or at most, not above a spoonful. Dip into this slips of linen cloth not worn too thin, and let them dry: or spread it thinly and evenly over them.”   This is the only published reference the author is aware of that suggests the formulation of a dressing should be modified to take account of seasonal changes in temperature.   Extremes of temperature can also adversely affect the performance of some self-adhesive bandages and dressings. For example, some hydrocolloid sheets are considerably less conformable and more difficult to apply when cold, although these materials appear to function acceptably in the heat of South Africa.²   Changes in humidity can, theoretically, affect the absorbency of dressings made from cellulose or other polysaccharide fibers because, when exposed to high humidity, the fibers absorb moisture from the air, which reduces their capacity to absorb exudate. In practice, however, under most conditions this effect is likely to be of limited clinical relevance.   Effect of temperature and humidity on semipermeable dressings. Far more significant are the effects of changing temperature and humidity upon the performance of products, such as foam sheets, that incorporate a semipermeable layer into their structure. Such materials rely on their permeability to water vapor to cope with the moisture transpired through normal skin and/or the fluid produced from exuding wounds.³ The permeability of a dressing combined with the absorbency of the product concerned, determines its total fluid handling capacity. Considerable emphasis is placed upon this parameter, which is often quoted in scientific papers and manufacturers’ literature, as it will influence wear times, and therefore, treatment costs.³   Moisture vapor transmission rate (MVTR), sometimes called water vapor transmission rate, is a measure of the passage of water vapor through a given area of material in a specific time, and is determined by the difference in partial pressure of water vapor across a membrane or semipermeable film dressing. Partial pressure is, in turn, determined by the relative humidity (RH) and the saturation vapor pressure of water, both of which are highly temperature dependent.⁴   In the United Kingdom, the MVTR of a dressing is normally determined in the laboratory by 1 or more standard methods, such as that contained in a European Standard, BS EN 13726-1. According to this method, a sample of dressing is applied to the open end of a Paddington Cup, a small chamber with an internal cross-sectional area of 10 cm², which is described in more detail below. A known humidity gradient is produced across a sample of film, and the passage of water is monitored, typically by a gravimetric method as previously described.⁴ For the purpose of this test it is assumed that within the Paddington Cup the RH will be 100%, but the conditions outside will be determined by an environmental chamber capable of maintaining 37º C ± 1º C and < 20% RH throughout the test, in accordance with the requirements of the standard.   All such tests have serious limitations, particularly when they are used to derive data used to predict the clinical performance of the item concerned. The principle problem is the temperature gradient within the test system does not reflect that which exists in a clinical situation, and thus, is likely to produce a misleading result.   Furthermore, in most laboratory tests, for practical reasons, the temperature on both sides of a dressing under test will be the same (ie, that of the environmental chamber); but in a clinical situation, a marked temperature gradient usually exists across the film. For example, the temperature at the surface of a wound covered with a semipermeable dressing is likely to be around 32º C - 35º C; but the temperature at the outer surface of the dressing will approach ambient, which can vary enormously.   In 2009, in London, the temperature ranged from a minimum of -10º C in January to a maximum of 38º C in August, with corresponding RH values of 86% to 62%. In Adelaide, Australia, the annual temperature varied from a minimum of 0º C to a maximum of 48º C, and the RH from 76% to 32%. In Jakarta, Indonesia, temperatures in excess of 30º C are common with a RH in excess of 90%.⁵   Indoors, variations in temperature and humidity are normally much less than this, but even if it is assumed that these are circa 19º C - 22º C, and 50% - 60% RH, these values are far removed from the standard laboratory test conditions. This means that in clinical practice the permeability, and therefore, the overall fluid handling properties, of a dressing may be significantly different from those predicted by the results of laboratory tests and quoted by manufacturers in clinical literature.   Although the affect of the weather on semipermeable dressings has been alluded to previously,^3,6 this is believed to be the first time an attempt has been made to quantify these effects or discuss their clinical significance.

Method

  The relationship between temperature, relative humidity, and the vapor pressure of water is extremely complex. This has been discussed in detail in an earlier publication,⁴ which also described the development and validation of the Medetec MVTR Calculator© (Medetec, United Kingdom), a device used to predict the permeability of a film under different environmental conditions from standard laboratory data in order to help understand dressing performance.⁴   A series of graphs has been constructed, using data derived from this calculator, to illustrate the effect of temperature and RH on the clinical performance of a semipermeable dressing (Figure 1).   The MVTR of a dressing sample under standard laboratory conditions can be readily determined using a Paddington Cup. The apparatus essentially consists of a cylinder with an internal cross-sectional area of 10 cm² having a flange at each end. One end of the cylinder is fitted to an annular ring, the internal diameter of which is identical to that of the cylinder, and to the other a solid plate which can be clamped into position forming a water-tight seal.   According to the BS EN 13726-1 standard, a piece of the dressing under examination is cut to shape and clamped between the annular ring and one of the flanges. Approximately 20 mL of test fluid is added to the cup, and the plate is clamped in position. The cup is then weighed before being placed in a chamber capable of maintaining the internal temperature and humidity within specified limits, typically 37º C ± 1º C and < 20% RH throughout the period of the test.   After a predetermined period, usually 24 hours, the cylinder is reweighed and the amount of fluid lost through the back of the dressing by evaporation is calculated by difference. This test can be completed with the film in contact only with water vapor, or with the film downwards so it is in contact with liquid. The latter configuration is more relevant to the clinical situation, particularly for hydrophilic films whose permeability changes in contact with water or biological fluids.   For the purpose of this exercise it is assumed that a dressing, when tested in this fashion, has an MVTR of 10 g/10 cm²/24 h (10000g/m2/24h) under standard conditions (37º C ± 1º C and < 20% RH). It is further assumed that when applied to a wound the temperature beneath the film is 33º C and the RH beneath the dressing is 100%.

Discussion

  The graph in Figure 1 predicts that for normal ambient conditions indoors, 20º C and 65% RH, the MVTR of the film when applied to a wound will decrease to about 7 g/10 cm²/24 h, or 70% of the value determined experimentally.   In the case of a leg ulcer covered with several layers of bandage, it is possible that these additional layers will act as thermal insulators that will create a microclimate at the outer surface of the dressing. As a result, the outer surface of the film could become several degrees warmer than the surrounding air and significantly more humid. If, for example, the temperature beneath the bandages were to rise to about 30º C and 80% RH, the predicted MVTR will fall to about 33% of the measured value; but in even more extreme conditions, the MVTR of a film may become insignificant. The performance of semipermeable film dressings applied to pressure ulcers of patients in bed may similarly be affected by the insulating effects of bedclothes. Further clinical work is required to investigate both possibilities.   It follows, therefore, that although moisture vapor transmission clearly can (and does) make a significant contribution to the fluid handling properties of a dressing, the importance of this effect must not be over emphasized, particularly if the product is likely to be used under environmental conditions which may adversely affect this aspect of its performance. The results also raise another intriguing possibility—for some very permeable products, data derived from clinical trials in one geographic location may be much less relevant in another region where conditions of temperature and humidity are very different.   Another important indication for semipermeable film dressings is the retention of indwelling catheters of various types. It is known that accumulation of fluid beneath such materials can facilitate bacterial proliferation, which, in turn, may lead to local infection or sepsis. Once again, local environmental conditions will greatly affect the MVTR of these products, and thus, their ability to reduce these potentially life-threatening events. A recent review of the use of semipermeable films at catheter site dressings discussed these effects,⁷ and described numerous studies, some of which provided conflicting results. None of these publications considered the possible effects of environmental conditions on dressing performance.

Conclusion

  Local changes in the weather, more significant differences in the climate of various geographic locations, and the effect of bedding and bulky secondary dressings will modify the temperature and humidity gradient across the dressing, thereby having a greater effect upon the ability of a dressing to cope with exudate by evaporative loss than is generally recognized.   If experimentally determined values for MVTR are to be used to define the fluid-handling capacity of a dressing, the possibility of adjusting these to reflect clinical usage under particular conditions of use should be considered. This is especially important when financial comparisons are made between products to determine treatment costs based upon calculated wear times.

References

1. Tissot SAD, Kirkpatrick J. Advice to the people in general, with regard to their health: but particularly calculated for those, who are the most unlikely to be provided in time with the best assistance, in acute diseases, or upon any sudden inward or outward accident; with a table of the most cheap, yet effectual remedies, and the plainest directions for preparing them readily. Avis au peuple sur sa santé. Philadelphia: John Dunlap; 1771. 2. Groenewald JH. Comparative effects of HCD and conventional treatment on the healing of venous ulcers. In: Ryan TJ, ed. An Environment for Healing: The Role of Occlusion. London: Royal Society of Medicine; 1985:105-109. 3. Thomas S. Laboratory findings on the exudate-handling capabilities of cavity foam and foam-film dressings. J Wound Care. 2009;19(5):192,194-199. 4. Thomas S, Barry L, Fram P, Phillips PJ. The effect of temperature and humidity on the permeability of semipermeable film dressings. J Wound Care. 2010;20(10):484-489. 5. BBC. Weather Web site. https://news.bbc.co.uk/weather/hi/about/newsid_9390000/9390415.stm. Accessed November 14, 2012. 6. Thomas S, Young S. Exudate-handling mechanisms of two foam-film dressings. J Wound Care. 2008;17(7):309-315. 7. Thomas S. Surgical Dressings and Wound Management. Cardiff, South Wales: Medetec; 2010. The author is from Medetec, Cardiff, South Wales, United Kingdom. Address correspondence to: Stephen Thomas steve@medetec.co.uk Disclosure: Dr. Thomas discloses he is the owner and sole proprietor of Medetec.