18. Burns – Part 1: General Principles
- Biophysics of the burn wound: A burn wound is caused when living cells encounter large, abnormal quanta of energy which exceed their physiological capacity to adapt / cope with this energy transfer. These quanta might be thermal, chemical, electrical or radiational in nature. In mechanical wounds involving heavy friction, considerable heat may get generated also leading to a thermal burn of tissue which is being otherwise damaged. This chapter deals with thermal burns in three parts. The other forms of burn wounds will be dealt in separate smaller sections later.
- By a natural law, energy and movement go hand in hand. The greater the energy, the greater is the movement. When thermal energy impinges on living cells, it tends to move across tissue causing abnormal movements in the cells leading to molecular disruption and death of cells. The higher the temperature, the greater is the penetration. Boiling water has a smaller thermal quotient than steam because the change to steam involves incorporation of latent energy. A flame burn or a burn by melting metal or molten tar are also severely damaging. On the other hand very hot or even boiling water travelling over some distance will lose considerable amount of heat as it disperses through air. History of how and by what agent the burn occurs is therefore important. In India a sari catching fire is an ominous occurrence because removing the sari takes time and there is reluctance to remove the sari because of modesty.
- The slogan ‘pour water on burns’ has to do with substrate competition. Thermal energy trapped in tissues dissipates outside to spend itself on heating the water which when poured continuously reduces the effect the heat will have on tissues. Energy follows physical laws.
- The nature of cold injury and frost bite: The effect of very low temperatures on tissue is exactly the opposite. Molecular movement slows down as temperature drops and at very low temperatures water forms into ice that halts the molecular movement completely and formation of ice damages the cell. In fact at 4°C water expands though it is forming ice leading to physical damage to the cell. This whole sequence is in clinical terms called ‘frost-bite’. Lyophilisation is a process in which temperatures are lowered with simultaneous dehydration allowing a possibility of thawing and revitalization of tissue in a saline bath at a later date.
- Regional peculiarities in burn wounds: The effects of heat on tissue, particularly its penetration, will depend on the nature of the tissue. The soles of feet and palmar skin are quite thick so also the skin of the back. The chances that the dermis will be burnt in its entirety in these parts are therefore less. The eyelids and the ears are the opposite. Their skin is very thin and the damage is more even with lower temperatures. While burns around the eyes usually receive prompt attention for fear of losing vision, the external ears end up being poor cousins when it comes to treatment. In the case of the ear, the situation is complicated by the presence of cartilage under very thin skin and paucity of subcutaneous tissue. The cartilage being avascular contributes little to healing after it is exposed and the wounds have a protracted healing period. Chondritis when it occurs is very difficult to treat.
- Respiratory burn: One of the most sensitive areas when it comes to burn injury is the laryngo-tracheo-bronchial and alveolar mucosa. Inhalation of hot or even warm smoke can lead not only to local desquamation but has far reaching effects on the ventilatory mechanism which is crucial to recovery from any major burn. A selective burn of the respiratory system can be fatal even in cases of otherwise minor burns. History taking in a burns case must include a close enquiry into the possibility of injury to the pulmonary tree.
- The nature of dermis and the depth of burn: The dermis has three principal characteristics. It has a rich blood supply and consequently has the body’s immune system always on call. In addition its structure has its own reticulo-endothelial system and its regenerative potential is very good. The presence of hair follicles and other adnexal tissue in the dermis enables it to heal wounds rapidly.A full thickness loss of dermis, also called a ‘deep burn’, takes away both these attributes (regenerative and protective). A burn through half the depth of dermis will preserve some part of these functions. This is called an intermediate thickness burn. When the dermis is intact, and the burn extends only up to the dermo-epidermal junction, the burn is called superficial. Here dermis will be able to overcome the injury without any problems and healing occurs without any great effort from the treating physician as long as it can be ensured that the dermis is protected from trauma and infection by a proper cover, preferably a biological dressing (for e.g. amniotic membrane or some form of collagen preparation). It is worthwhile to note here that all burn wounds are rendered sterile by the very heat which causes the wound and remain so for several hours.The depth of the burn is judged by the appearance of the wound. Large and small blisters are usually synonymous with superficial burns. If the epidermis is peeled off, a white, leathery dermal surface usually indicates a deep burn. In intermediate thickness burns the dermis has various shades of red or pink. Intermediate thickness burns evolve over time in their appearance and it is easier to gauge the depth of burns after a few days.
Photographs courtesy: Shailesh Ranade, Mumbai.
- Biological consequences of a burn wound: The burn wound produces an inflammatory response which is proportionate to the size of the burn, whichis usually denoted by the term ‘body surface area (BSA)’. A burn wound of about 5% BSA will produce an inflammatory zone of about 10% BSA. Theoretically a 50% burn therefore involves the whole of the body. The inflammatory response is in the nature of hyperemia, capillary dilatation, increased permeability and the consequent exudate. This means the intravascular compartment loses fluids leading to hypovolemia. Equally sinister is the loss of proteins in the exudate which normally, because of their osmotic properties,are able to retain fluid in the intravascular compartment. By an agreed rough estimate the rate of loss of the inflammatory fluid from the intravascular compartment is highest in the first eight hours and equals the amount lost in the next sixteen hours. The rate of exudation then slows down though the quantity is the same over the next twenty four hours. These facts have a bearing on the rate and type of fluid that needs to be administered.
- Infection of the dead dermis: The burn wound, which is sterile to begin with, becomes susceptible to infection within hours. Here the infecting organisms are, more often than not, those that live on the skin or in the gut and far outnumber the normal living cells during normal life. Their normal co-existence with the body is disturbed as the immune mechanism breaks down. This breakdown of the immune system has several dimensions. The cellular component includes reduced phagocytosis, production of undesirable oxygen free radicals due to clogging of capillaries by a large number of polymorphonuclear cells (please see chapter 14 point 7, ‘How and Why of Flap Failure’), reduction of cytokines as well as cd+T helper cells and bone marrow depression. In a major burn the respiratory toilet is poor and redistribution of blood supply causes gut ischemia. In the event organisms in the skin appendages as well as the gut, which are normally controlled, multiply exponentially and a single organism can produce millions and then pour out to colonise the dead dermal tissue. Shock and loss of fluids as well as proteins are also responsible for this breakdown. Indiscriminate oral antibiotics are also responsible for emergence of virulent strains from the commensals such as E.coli from the gut. Exogenous infections can also occur and in a closed burn unit might travel from patient to patient. Burn wounds not cleaned properly on arrival at the treatment site can also be a source of such an infection.
- The dual objectives of the treatment of burns: In any major burn therefore, replacement of fluids to prevent shock and plans to protect the dead dermis from getting infected and later eliminating this dead dermis by early excision and effective skin cover, form the dual objectives of treatment. By a general consensus burn wounds, after they are cleaned with saline irrigation and debrided, are thought to be best covered with a dressing after application of topical anti-bacterial agents. Silver sulphadiazine, silver nitrate, sulphamylon and neosporin are used commonly and each has its adherents. Debrided tissue is sent for bacterial culture.
- First steps in the treatment of burns: On arrival, any patient with a major burn needs to be started on intravenous fluids through an adequate venous access. At extremes of age i.e. below 5 years and above 60, smaller burns have graver consequences. The depth of the burn is also a consideration in judging the severity of burns. By a rough rule any burn wound of an intermediate or deep nature above 10% body surface area is considered a major burn in these age groups. As opposed to this, in a healthy adult a burn wound of more than 25% body surface area is denoted as a major burn. It is to be noted that at extremes of age compliance in terms of cardiovascular and immunological responses is poorer. For adequate and sometimes rapid infusions a central line is necessary. Any venous access must be such that central venous pressure can be measured and monitored. At this time blood is collected for all the routine investigations, of which the hematocrit is the most important because it will tell you the state of the patients’ hydration. The patient’s respiratory status must be ascertained by way of examination of the upper respiratory tract. Any congestion or carbonaceous particles should indicate a possible deeper pulmonary injury. A baseline portable x-ray chest helps evaluation particularly when compared to successive x-rays during future treatment. A urinary catheter is equally important because urinary output is by far the best guide to hydration in addition to the hematocrit. At the time of the initial introduction of the catheter, the quantity of urine will give an indication of what has happened since the time of the burn. Of the intravenous fluids to be administered, Ringer lactate is almost universally accepted as a crystalloid of choice. It is also generally accepted that around 20% of the total intravenous ration should be colloids in the form of plasma expanders, which have constituents such as starch which hold on to water and do not allow it to leak through capillaries. The use of fresh plasma or albumin as colloids, though practiced in some units, are not a popular choice because they are expensive and even more so because they leak through dilated hyperpermeable capillaries and are wasted into the extravascular compartment. In the past various formulae were devised to calculate exactly the amount and type of intravenous fluids that needed to be administered and were adhered to. At the present time two cc’s per kg per percent of burn area is taken as a rough guide. A 70 kg patient with a 20% burn surface will therefore need 2800 cc’s of fluids in the first 24 hours half of which is given in the first 8 hours and the remaining in the next 16 (and the same in the second 24 hours)in addition to the daily requirement of between 1500-2000 cc’s over each 24 hours. The proportion of crystalloids to colloids is a matter of choice of each unit. What is important is to see the effects that the administration of fluids are producing by way of measuring the urine output, the hematocrit and the central venous pressure. It is customary at this time to start a first or second generation penicillin by an intravenous route to counteract respiratory infection because respiratory toilet is not normal because of the inhibitory effect on the movements of the villi due to burn trauma as well as inhalation of steam, smoke etc.Sunil Keswani of Mumbai adds that the use of albumin when colloids are transfused will help wound healing and also draws attention to paralysing effect on the pulmonary ciliary mechanism for several hours (with or without noxious fumes) which has a detrimental effect on normal respiratory toilet in cases of burns.
- Monitoring the state of hydration: Roughly speaking, a urine output of 1 cc / kg / hour and a hematocrit of between 30-40% indicates a properly hydrated patient. When possible, blood pressure should be measured by a Sphygmomanometer every half an hour if the burn wound allows such a measurement. If not, an indwelling arterial line may be needed. A cutaneous oxygen probe will tell you blood oxygen levels. The measurement of central venous pressure is ideally done via the internal jugular vein up to the beginning of the superior vena cava or through the sub-clavian vein up to the same point. A central venous pressure of 5 to 10 cms is normal and any rise indicates that the right heart is unable to cope with the fluid load. A low pressure on the other hand may or may not mean under-hydration and needs to be corroborated with the urine output.
- Metabolic changes and their diagnosis and management: The shock accompanying a major burn, the extensive death of tissue and later the infection of that dead tissue can allcontribute to produce metabolic, lactic acidosis. In any patient, with such an acidosis a SPO2, measured by a skin probe and showing the presence of 100% physical percentage of oxygen in the blood stream, may not reflect how much of that saturation is available to the tissues. The estimation of blood gases (PO2) is the correct guide to the availability of oxygen to the tissues. Any level below 60 is pathological and is usually accompanied by a lower pH and reduced levels of bicarbonates. Correction of this metabolic acidosis is done by replacement of bicarbonates and the proof of the success of such a treatment is the rising level of PO2. Tachypnoea induced by acidosis must be differentiated from tachypnoea due to respiratory acidosis and which classically occurs in a burn of the pulmonary tree and later because of acute respiratory distress syndrome. In this condition the SPO2 will be low and therefore also a lower PO2 with or without reduction in the level of bicarbonates. Respiratory acidosis can be corrected only by increasing the oxygen supply by way of assisted ventilation. A 50/50 oxygen and air combination should ideally suffice to maintain a SPO2 of 100%. The need for a higher percentage of oxygen, up to 100% to maintain the SPO2 is a bad prognostic sign though patients have been maintained for weeks together in this manner as local surgical treatment is undertaken.
- After the first 48 hours: At the end of the first 48 hours, the exudative phase wanes and the intravascular shift of the interstitial fluid begins. The process is not uniform and fluctuations in serum electrolyte levels might be quite rapid during this period and need to be observed and corrected. The level of the potassium ion needs to be observed carefully. Potassium is an intracellular ion and in burns with widespread destruction gets released in the blood stream which is normally taken care of by normal renal tubular function. But in cases of burns the tubular function is vulnerable and fragile (toxaemia, infection, dehydration or over infusion) and a level of more than 5 milli-equivalents per litre may be suggestive of renal dysfunction. More importantly such levels are toxic to the heart and it becomes necessary to stop giving all substances which contain high levels of potassium (fruits). All intravenous infusions must be bereft of free potassium. Occasionally under these circumstances, dialysis might need to be done if creatinine levels rise. Creatinine is a specific indicator of kidney failure.
- Recognition and treatment of infection: This is also the time when first signs of infection may become apparent. Immediate post-burn ileus might not improve or an ileus might appear de novo. Both severe leucocytosis and abnormal sudden leucopenia might occur depending upon the organism, and the hematocrit and platelet counts may fall over a period of hours. The patient may show signs of disorientation and might start running a temperature or might become hypothermic. The choice of antibiotics at this stage is a problem because tissue debrided when the patient first arrived and sent for bacteriological diagnosis is usually reported as sterile. This is the time when a new swab or piece of tissue is sent for a fresh bacteriological examination. The choice of an antibiotic at this stage therefore is a matter of the unit’s protocol based on its past experience. This antibiotic now is in addition to the first or second generation penicillin which was administered at the time of admission to mainly prevent respiratory infection. Antibiotics which are effective against organisms grown in culture are the next step in the treatment.
- Preparation for early excision of burns: From here on, in a major burn the effort is to keep the patient as far as possible in a normal physiological state as preparations are made to start excising the burn wound. Hypoproteinemia is treated with intravenous albumin or plasma and anemia with blood. Ventilatory support and correction of acidosis have been mentioned earlier. Renal function needs to be watched closely and serum creatinine levels are a reliable guide to judge how the kidneys are behaving and this level needs to be checked frequently.
- Nutrition: Any significant burn wound as well as the process of healing imposes a heavy metabolic load on the individual, almost doubling his or her caloric requirement. Yet during the illness appetite is poor and bowels are sluggish. A safe but effective way to deliver calories when the patients’ voluntary intake is poor is to pass a nasogastric tube and use it at short intervals to instill high caloric, easily digestible liquid sometimes with the addition of enzymes. Several formulae are available for this purpose. There was at one time a trend to administer these calories by a central venous catheter. And because the number of calories administered was huge, up to 6,000 calories, the regimen was also called hyper-alimentation. The method occasionally had grave consequences due to infections around the long venous catheter particularly because they carried material suitable for bacterial growth. These blood borne infections were extremely difficult to treat and hyper-alimentation in burn cases is now less frequently used. The material that was administered was also quite expensive, certainly in India therefore hyper-alimentation has not been a popular choice.
- The advantages of a nasogastric tube are two-fold. The patient can be fed when he is resting or asleep or on a respirator. When not being used for feeding it can be left open without a spigot from time to time to judge the gastric content by way of a free flow or aspiration. Greenish yellow bile is a good sign and an hour or so after the patient has been fed the feed can be repeated. On the other hand a brown or rarely red aspirate should ring alarm bells. Gastro-intestinal hemorrhage is a life threatening complication of burns and is discussed in a separate section together with other miscellaneous subjects. In diabetics, when calories cannot be obtained from free sugar, the formula has to be modified to include greater amounts of fat and proteins. Patients particularly the elderly, with suspect renal function, require restriction of proteinsas well as fruit juices.
- Those who take responsibility of treatment of a major burn must remain vigilant, flexible and perseverant as they watch clinical parameters, a battery of investigations and evidence of infection. The description of early excision of burns follows in the next chapter.