EUROPEAN TISSUE REPAIR SOCIETY
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EUROPEAN TISSUE REPAIR SOCIETY Clinical Warming |
Clinical Warming: Methods and Benefits - KS Satheesan1, A Melling1, D J Leaper21. University Hospital of North Tees, Stockton-on-Tees, UK. 2. Wound Healing Unit, Cardiff, UK.The negative impact of perioperative hypothermia on patient care is well documented. Its deleterious effects on human physiology have been well established through clinical and laboratory studies.1-10 The reduction of basal metabolic rate in response to hypothermia has been exploited to provide neuroprotective and cardioprotective effects in certain circumstances.11 Apart from these proven advantages, hypothermia causes significant morbidity and mortality especially in the perioperative environment. The application of heat therapy is a way of avoiding and treating hypothermia and its clinical consequences. The first documented heat treatment dates back to the Hippocrates era, about 400BC. Over the last few decades, a growing amount of evidence has highlighted the importance of avoiding hypothermia and the beneficial effects of warming. Warming strategies aim to transfer heat energy in order to supplement the body's net heat gain. Methods of patient warmingThermal manipulation to decrease cutaneous heat loss is the commonest strategy in hospitalised patients as approximately 90% of the metabolic heat energy is lost through the skin surface.12 Passive warming methods can be local (insulating only part of the body) or systemic where an insulating item such as a blanket covers the majority of the cutaneous area. This supports heat retention by providing insulation and thus preventing further heat loss. The best passive insulation can potentially reduce the cutaneous heat loss to near zero. However, in practice most insulating materials rarely prevent heat loss even by 50%.13-14 Although reflective wraps have been claimed to be superior to simple blankets, they may not alter the net heat gain significantly compared to simple blankets during general anaesthesia.15 Therefore passive warming strategies alone may not be an efficient way of treating hypothermic patients. In contrast to passive warming, methods of active systemic warming using external devices have been shown to be more effective in treating or avoiding hypothermia in 'at risk' patients.16-21 Active systemic warming may increase heat production by the body as well as transferring heat energy by conduction, convection or radiation. A fastgrowing trend in technological advancement means that the patient warming devices are user-friendly and have a high performance making them an attractive investment for many heath care providers. Circulating water mattresses are the classical active intraoperative warming systems and have been used for many years. Despite having advantages such as the cooling option and the provision of pressure relief, they can be bulky to handle with a high workload for routine maintenance. Forced air warming systems have a high performance and are fast to warm-up (e.g., the ‘Bair Hugger' by Arizant Health Care, Minneapolis, USA and ‘Thermacare' by Gaymar Industries, New York, USA). They may provide a more effective way of maintaining normothermia in surgical patients than the circulating water mattresses.22-23 A more recent technology involving conductive carbon polymer has aided the production of warming mattresses and blankets (Inditherm Plc, Rotherham, UK). A high performance coupled with their practical convenience, the carbon polymer based warming systems may offer a real alternative to the commonly used forced air systems. They do not affect the operating theatre's environmental temperature and their flexibility allows them to be used in a variety of clinical environments.24-27 Both the forced air and circulating water mattress systems rely on mainly conduction and convection for heat transfer. Radiant warmers use radiation as their main mode of heat transfer and have been used to treat intraoperative hypothermia (e.g., SunTouch by Fisher & Paykel Healthcare Ltd Auckland, NZ). They may be at least as effective as forced air warmers in maintaining patients' intraoperative core temperatures.28 The drawbacks are that they will inevitably heat the surroundings and may not be practical in operating theatres. An increasing number of active local warming devices have emerged in the last decade. Currently available methods include radiant heat dressings (by Augustine Medical), warm-up acute wound care (by Arizant Medical, Eden Prairie, MN, USA), carbon polymer pads (Inditherm Plc, Rotherham, UK), water and wax bath, microwavable gel pads, water bottle and hot fomentation. The latest products for example dressings with an incorporated warming technology / chemicals have shown encouraging results but methods such as water bath, hot wax treatment and water bottles are no longer considered as first line in routine clinical care. Active core warming is an invasive method of warming, which can be divided into two groups: endogenous and exogenous core warming. Core warming aims to transfer heat energy internally by warming viscus or blood. Amino acid infusion is the most established form of endogenous core warming. It increases the metabolic rate mostly in the extraspanchnic tissues and maintains patient core temperature 0.5°C warmer compared to a standard crystalloid infusion.29-30 Exogenous core warming methods have been used intraoperatively and in the resuscitation of trauma victims or patients who had been exposed to severe accidental hypothermia. The methods include warm infusion, warm inhalation, application of warm lavage to a viscus, haemodialysis, veno-venous and cardio-pulmonary bypass systems. These can be effective in complementing other rewarminng methods.31 Tissue repairWarming therapy may exert local and systemic physiological effects. The primary mechanism for its local effects may be based on the ability of the warming therapy to manipulate local tissue haemodynamics. Blood viscosity increases with decreasing temperature thus impairing blood perfusion.4 It has been demonstrated that tissue oxygen tension, a vital parameter for tissue repair, can be increased by local heat.32 Even in healthy adults, intermittent or systemic warming can increase subcutaneous oxygen tension.33-34 In vitro studies have demonstrated that intermittent radiant heating increased human fibroblast and microvascular dermal endothelial cell proliferation35-36 thus highlighting the need to investigate in-vivo wound healing outcomes. Clinical indications and outcomesIn the clinical environment, traditionally the purpose of giving warming therapy is to avoid or to treat hypothermia. The maintenance of normothermia by warming surgical patients has been shown to be beneficial by reducing the rate of postoperative wound infection37, adverse cardiac events,38-39 organ dysfunction and death.40 It may contribute towards the reduction of intraopartive blood loss.41-42 Used as an adjunct, warming has been shown to improve physiological parameters in patients with peritonitis.24 The postoperative morbidity and mortality rates following major abdominal surgery show an improvement in the warming group when compared to its predicted rates.25 The reduction in morbidity may account for the reduction of the length of hospital stay in normorthermic patients.37 Hypothermia associated complications may cost as much as $7,000 per surgical patient.43 Warming can also be beneficial when given to normothermic patients thus highlighting that hypothermia is not the sole indication when considering warming therapies. In acute surgical wounds following clean elective surgery, both local and systemic warming has been shown to reduce the rate of wound infection.27 In chronic ulcers, repeated ischaemic-reperfusion injury is thought to be part of the pathogenesis process and warming may influence the healing process by correction or reduction of tissue ischaemia. Local heat application shows better healing compared to standard treatment in spinal patients with chronic pressure ulcers.44 It may also play a role in the eradication of MRSA infection from pressure sores.45 During emergency transport, it may be difficult for a full clinical assessment of the patients and therefore may compromise their analgesic support. However warming has been shown to improve pain, anxiety and patient comfort during their transport to the hospital46-49 as well as during the initial hospital assessment.26 Nausea, an unpleasant symptom often associated with pain, may also be alleviated by warming.50 Warming of trauma victims may help with their oxygen monitoring.51 In addition local heat may help to reduce both the time and the number of attempts required when inserting peripheral venous cannulas.52 SummaryPerioperative hypothermia has deleterious physiological and clinical consequences. Warming has been shown to be beneficial by improving perfusion, tissue oxygenation and immunlogical responses. A wide range of clinical benefits has been shown from warming normothermic patients. Therefore the avoidance or the treatment of hypothermia should not be the only reason when considering clinical warming. Practical implications of using warming therapy include appropriate wound types and settings, obtaining product guidelines for administering therapy, patient teaching and cost benefit information. References
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