Clinical Science (1992) 83, 117426 (Printed in Great Britain)

I I7

Cutaneous heat loss shortly after burn injury in children Charmaine CHILDS',', H. B. STONER' and R. A. LITTLE' 'North Western Injury Research Centre, University of Manchester, Manchester, U.K., and lRegional Paediatric Burns Unit, Booth Hall Children's Hospital, Manchester, U.K. (Received 10 September 1991/19 February 1992; accepted 28 February 1992)

1. Total heat loss and its components have been studied in cool (2OOC) and warm (30°C) environments in 30 healthy children and 21 children who had been burned (10-17% body surface area) 0.5-29h previously. 2. In healthy naked children at 2WC, the partition of total heat loss was radiation, 64%; convection, 32%; evaporation, 4%. On transfer to the warm, total heat loss was reduced by approximately SO%, with disproportionate reductions in the contributions from radiation and convection being offset, to some extent, by an increase in evaporative heat loss. 3. In patients during the first 5.5h after injury, the magnitude and pattern of heat loss a t 20°C and 30°C were similar to those in control subjects and were unaffected by bandaging. 4. Ten to twenty-nine hours after injury, when the patients were bandaged and body temperature and heat content were significantly higher than in control subjects, radiant and convective heat losses were increased, but as evaporative heat loss tended to be reduced; total heat loss in the warm was unchanged. However, at this time at 20"C, total heat loss was reduced compared with healthy children at the same ambient temperature. 5. The findings of unchanged or reduced total heat loss and reduced evaporative heat loss in injured patients are interpreted as inappropriate responses to an increased body temperature and heat content in children after burn injury.

INTRODUCTION

Studies from this unit have shown that there is a characteristic disturbance in thermoregulation during the early phase of burn injury in children (for a review, see [l]). At the time of admission, deep body temperature measured in the rectum is either normal or slightly elevated and remains fairly constant for the first 6 h. Thereafter there is a rapid increase in rectal temperature (T), by as much as 3°C over a 2 h period in some patients [2]. Temperatures greater than 40°C can be reached within 2-4 h

of the start of the rise. Similar changes were seen in the heat content of the body, where an initial steady state was maintained for approximately 5 h after injury followed by a rapid increase irrespective of ambient temperature (T,). By 10-12h7 the heat content had reached a new steady state 5-7kJJkg above the initial level, which was maintained for at least a further 12-24 h, giving a second steady state [3]. Since the normal response to a rise in body temperature is the activation of heat loss mechanisms to restore euthermia, we now report the changes in heat loss after burn injury in the child, partitioning total heat loss into the pathways of wet (evaporative) and dry (radiant and convective) heat loss [4]. METH0DS Subjects

Patients. The clinical management of the patients and the conditions to which they were exposed have been described previously [3], so only a brief account will be given. Patients under the age of 5 years with burns covering 1&17% of total body surface area, admitted to the unit within 5 h of the accident, were studied. These were the only criteria for entry into the study. The patients were divided into groups on the basis of the environmental conditions in which they were nursed: those nursed, from admission, in a warm room at a (T,) of 30°C were designated as group W (n=14), and those nursed in a cool room at a T, of 23-25°C were designated as group C (n= 7) (Table 1). Although the ambient conditions differed, treatment followed the same protocol. Since all the patients had burns covering more than 10% of the body surface, they required intravenous resuscitation and intensive care. After the usual emergency nursing and medical procedures, management of the airway (when necessary), intravenous cannulation for fluid replacement, and pain relief and sedation, the wound was cleaned and covered with wet soaks of povidone-iodine (Betadine; Beecham Research Laboratories, Brentford, Middx., U.K.)

Key words: burns, children, heat loss, heat storage, thermoregulation. Abbrevlatlons: C, convective heat loss; e, evaporation rate; E , evaporative heat loss: R, radiant heat loss; T, temperature (subscripts a, ax, c, db, g, 0, r, sk and wb indicate ambient, ailla, core, dry bulb, globe, oral, rectal, skin and wet bulb, rerpenively). 7, mean temperature; W, weighting factor; subscript 'n', refers to body region; subscripts 'nor' and 'ban' refer to normal skin and bandaged surface; subscript 'exp' refers to exposed surface available for heat exchange. Correspondence: Dr Charmaine Childs, Burns Unit, Booth Hall Children's Hospital, Charlestown Road, Blackley, Manchester M9 ZAA, U.K.

..

C.Childs et al.

I I8

Table I. Characteristics of control subjects and patients. The ages of subjects in group 3 are individual values.

Age Median Range Burns size(%)* Ta("C)t Body coverings Average area bandaged (%)* Duration of study

Group I

Group 2

Group 3

Group W

Group C

(n=14)

(n=12)

(n =4)

(n=14)

@=7)

31 months 9months-4.3yean

I 9 months 7 months-lyean

25 months, 4.3yean 4.5years, 6.5yean

14months 8-26months 24.2 +I .O

-

-

-

21.3+ 1.9 30.1 k l . 4 Naked

22.9k1.8 31.4fl.9 Naked at Z l T , bandaged at 31°C

30.1 g . 3

10.5 months 5 months4.5yean 10-15 30.4 +I. I

Bandaged

Bandaged

Bandaged

-

29 60-9Omin

28 I2h

29 24h

24 24 h

60-90min

10-17

*Expressed as a percentage of total body surface area. t Expressed as means SD.

+

and dry gauze and crepe bandages. Apart from occasional repadding of the outer dressings, the bandages were left intact for about 3 days, whereupon they were removed and an assessment was made of the need for surgery for autograft. During the first 3 days, patients received clear fluids enterally; milk feeds were introduced towards the end of the resuscitation period. Heat loss in both patient groups was calculated on admission, when the wound was exposed, and at 1-2 hourly intervals throughout the first 24 h after the child was bandaged. When several measurements were made on the same patient during either thermal steady-state period, the values have been averaged to give one value for that period. Control subjects. The control subjects (Table l), who were divided into three groups, were the same healthy children described in our previous paper [3]. Measurements in group 1 (n= 14) and group 2 (n=12) were first made at a T, of 21.3-22.9"C after an equilibration period of approximately 60 min. At this stage the children were naked (except for nappies or underpants). Both groups were then moved to a warm room (T, 30.1-31.4"C). Group 1 remained naked, and group 2 had bandages applied to the trunk and upper arm (to provide a similar area of insulation to that of the bandaged patients). Measurements were then repeated after a period of 60-90min at the higher T, (30°C).Children in group 3 (n=4) were bandaged and studied for 12h at a T, of 30°C. Measurements were made regularly throughout and at the end of the 12h period.

Core temperature ( T,). Deep body temperature was measured in the rectum (TI) and axilla (Tax)in the patients, the latter so that comparisons could be made with control subjects. In healthy infants, Tax was used as a site for the measurement of T, and in older children oral temperature (To) was used. Although the usefulness of Taxmight be questioned as a true representation of T,, it was not appropriate to measure TI in the control group. In the patients, TI was measured using a thermistor inserted into the rectum, 4-6cm from the anus [5], and displayed continuously using a battery-operated monitor (Libra Medical, Reading, Berks, U.K.). Tax and To were measured using an 'Ivac' electronic thermometer (Imed, Abingdon, Oxon, U.K.) or a mercury-in-glass thermometer. Skin temperature (Tsk).Tskat the extremities (patients and control subjects) was measured by securing a thermistor (Libra Medical, Reading, Berks, U.K.) with Mefix tape (Molnlycke Co., Cork, Republic of Ireland) to the skin of the pulp of the hallux or (in children with burns to the feet) to the thumb. Tsk over the whole of the body surface of control subjects and patients was measured with an infra-red radiometer (KT 15; Heimann GmbH, Wiesbaden, Germany). Once bandaged the surface of the body of a burn patient is a mixture of normal, unburned skin and bandages covering the burns (and sometimes patches of exposed burn) and that of bandaged control subjects is a mixture of normal skin and the surface of the bandages. The temperatures of all these different surfaces were needed to calculate heat exchange by dry routes.

Temperature measurements

Calculation of surface temperature

Environment. An 'Edale Weather Station' (Edale Instruments, Cambridge Ltd, Cambridge, U.K.) was used to measure ambient conditions, i.e. dry bulb (Tdb),wet bulb (Twb) and globe (T,) temperature.

Naked control subjects and patients. In naked control subjects the mean temperature (T,) of the exposed skin in each of seven body regions, head, trunk, arms, hands, thigh, legs and feet, was calcu-

II9

Heat loss in burned children

lated using appropriate weighting factors (W,) for each region [6], by: n=7

where h, is the radiant heat exchange coefficient and

n= 1

The weighting factors for the head, thigh and leg were adjusted according to the child’s age [7] as described previously [3]. The average temperature of exposed normal skin was obtained by summing the values in each of the seven regions, and was designated Trio,. The mean temperature of the exposed surface of naked patients was a mixture of normal and burned skin and was calculated in the same way as for naked control subjects. Bandaged control subjects and patients. Once bandages were applied, the surface of the body became a mixture of ‘normal’ skin and bandaged surface. To calculate heat exchange at the surface, the temperature of the skin and bandaged surface had to be considered separately. The mean temperature of the bandaged surface (Than) was calculated from the mean temperature of the surface of the bandages in each body region and the appropriate weighting factor representing the bandaged area of that region. Each patient therefore had a value for T,,, (temperature of normal skin after bandaging) and Than. An example of the method of calculating surface tem-‘ perature in a patient, naked and then bandaged, is given in the Appendix. Effective area for heat exchange

Not all of the body takes part equally in heat exchange [6]. Some parts of the body are not involved and must therefore be designated ‘hidden’ areas (i.e. those parts in contact with another surface). Posture must also be considered [6]. The effective area for heat exchange is therefore weighted. The proportion of the body ( Wexp) involved in heat exchange by radiation and convection in naked control subjects was reduced from 1 to 0.95 for a standing subject, to 0.90 for a sitting subject and to 0.80 for those lying on a bed [6]. Once bandages were applied, heat exchange from the body had to be considered in two parts, the exposed skin and the bandaged surface. The weighting factors had to be adjusted further. Using photographs and drawings, the area of the body available for radiant and convective heat exchange, via either the skin or the surface of the bandages, was calculated. Wexp represents the effective surface for dry heat loss from exposed skin and wba, the effective surface for dry heat loss from the bandages. Calculation of heat loss by dry routes

Radiant (R) and convective (C) heat losses in naked children were obtained from the standard equations:

where h, is the convective heat exchange coefficient. When part of the surface had been bandaged:

An example of the use of these equations is given in the Appendix. Heat exchange coefficients Radiation. Human skin, regardless of colour, is considered to be within 1-274 of being a ‘black body’ and radiates according to the StefanBoltzmann law; thus, radiant loss between skin and air can be expressed as:

R = a(T$ - T i )

where a=5.7x 10-8Wm-2K-4. This equation can be simplified to eqn. (1) and values of hR calculated from the tables devised by Kerslake [S]. For the range of skin and ambient temperatures in the study the value of hR was 6.5Wm-2K-’ [8]. Convection. The value for the convective heat exchange coefficient, h, in eqn. (2), depends on air movement in the room and posture [8a]. Since air movement was low, approximately 0.06 m/s, natural convection prevailed. Under these conditions hc is proportional to the Nusselt number, which can be calculated from constants described in detail by Kerslake [8a]. For the environmental conditions of the intensive care and study room, h,= 3.2Wm-2K-’. Evaporative water loss and calculation of evaporative heat loss

The rate of evaporation (e, in gh-’m-2) from skin, from the burn surface (in exposed patients) and from the surface of the bandages was measured with an Evaporimeter (EP1; Servomed, StockholmVallingby, Sweden). The evaporimeter was calibrated by placing the sensors into air-tight flasks containing saturated salt solutions (lithium chloride, magnesium nitrate and potassium sulphate) as standards. The flasks were themselves contained within an insulated box. The instrument was calibrated regularly before and after use on every patient or subject. Measurement of evaporative water loss using the

I20

C. Childs e t al. Table 2 Deep body and skin temperatures in control subjects and patients. Values are expressed as means+sD. There was a significant increase (P

Cutaneous heat loss shortly after burn injury in children.

1. Total heat loss and its components have been studied in cool (20 degrees C) and warm (30 degrees C) environments in 30 healthy children and 21 chil...
848KB Sizes 0 Downloads 0 Views