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Rewarming from immersion hypothermia: a comparison of three techniques
Resuscitation, 5, S-18
Rewarming from immersion hypothermia: a comparison of three techniques E. LL. LLOYD, B. MITCHELL and J. T. WILLIAMS Experimental Surgery Unit, Animal Disease Research Association, Moredun Institute, Edinburgh, Scotland
Summary Rewarming from immersion hypothermia has been assessed in sheep by the use of three techniques -hot bath, body insulation and airway warming. Though the hot bath was the fastest of the methods of rewarming studied, consideration of temperature gradients and therefore total body heat diminished its advantage in comparison with central body rewarming via the airway (CBRW), which in turn showed considerably advantage over body insulation alone. CBRW did not have any thermal advantage gained on assisting the ventilation as compared with spontaneous breathing. The results illustrate the importance of adequate insulation of the body to prevent further heat loss and this was found to be true whether or not airway warming was being used. The site of heat uptake with CBRW was determined and observations were made on the physical behaviour of temperature gradients.
Introduction Accidental hypothermia is a problem whose incidence is increasing (Freeman & Pugh, 1969) as more people leave the shelter of cities during work and leisure. While many sophisticated techniques are available for rewarming in hospital, most are not applicable in the wide open spaces where the majority of cases are found. Rescue teams have therefore to rely on passive spontaneous rewarming aided by insulation, or transport to a hot bath (Freeman & Pugh, 1969). Airway warming in addition to insulating the patient has shown encouraging results in man (Lloyd, 1973; Lloyd & Frankland, 1974) and a pilot study was undertaken to compare the effects of airway warming with two standard methods, i.e. passive rewarming and active rewarming in a bath at 45-50” C. Sheep were used as experimental animals because their weights approached those of man. While it is appreciated that there are physical and physiological differences between sheep and man, e.g. in their surface area/mass ratios, thermoneutral zone and critical temperatures, it was considered that there were sufficient similarities to justify the study. Method
Twenty-one mature black-face ewes whose body weights ranged between 40.5 and 57.2 kg were used. The animals were anaesthetized throughout the study by inhalation of nitrous oxide, oxygen and halothane; an endotracheal tube was passed and anaesthesia was maintained with a circle circuit with soda-lime absorber. During the next hour, temperature5
6
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS
monitoring probes were attached. Temperatures were measured by Ellab Thermistor probes (Sierex Ltd) adapted for D.C. potentiometric recording. The recording was on a Speedomax type W Cleertrend Multi-point Indicating Recorder (Leeds and Northrup Ltd). Temperatures were recorded from the sites indicated in Table 1, although not all sites were monitored in every case. The methods of insertion of the probes are also given in Table 1. Hypothermia was induced by immersing the sheep in cold water (7-15” C) with douching of exposed parts. The first four sheep in the study were shorn of their 2 inch fleece to aid the induction of hypothermia, but it was later found that even when the animals were unshorn their temperature dropped at a rate satisfactory for the experimental purpose. Once the core temperature, as recorded on the intra-arterial thermistor, had dropped to about 35”C, the bath was drained and a further period allowed for the completion of the ‘after-drop’ of core temperature before rewarming commenced. Comparison was made between three methods of rewarming. Spontaneous rewarming. These sheep were removed from the water, dried of excess water and wrapped in blankets with polythene sheeting between the sheep and the blankets. No additional heat was supplied. Central body rewarming via the airway (CBR W) with spontaneous ventilation. In addition to being wrapped in blankets and polythene, a water-bath humidifier (East Radcliffe) was added to the anaesthetic circuit. The inspired temperature in the endotracheal tube during inspiration was approximately 42-45” C. Hot bath. The sheep were rewarmed by immersion in a bath of water maintained at a temperature of 45-50” C. Control animals. There were three control groups. (a) Not-cooled: the sheep were prepared as above, but hypothermia was not induced; (b) deep hypothermia: no attempt was made to rewarm the sheep and readings were taken during the continuing drop in core temperature; (c) CBRW with assisted ventilation: ventilation was assisted throughout cool-
Table 1. Sites of measurement of body temperature. Probe
Desired site
Method of insertion
Aorta Central venous Jugular
Thoracic descending aorta Inferior vena cava Jugular bifurcation
Via a catheter in the femoral artery Via a catheter in the femoral vein Via a catheter inserted percutaneously the jugular vein
Oesophageal Rectal Nasopharyngeal Intra-tracheal Intramuscular Subcutaneous
Retrocardiac
Skin Rumen Pulmonary artery Air Water bath
15 cm above carina
into
Inserted via the external nares Lying in lumen of the endotracheal tube Needle probe in the thigh Needle probe in subcutaneous tissue of the thorax Loop probe on ear Via a catheter placed by needle puncture through the abdominal and rumen walls Recorded separately by the thermistor incorporated in a Devices cardiac output computer used as part of the study A probe exposed in the region of the sheep Placed in the water during cooling and hot bath rewarming
2 3
Unshorn
Unshorn
Deep hypothermia
Airway warming (CBRW), assisted ventilation
3
2
Unshorn
Unshorn
sheep
3
4
1
3
Shorn
Unshorn
Shorn
Unshorn
No. in each group
Non-cooled
Control
Hot bath
ventilation
(CBRW)
with covers
groups
warming
Spontaneous
Airway
Spontaneous
Rewarming
State of sheep
no. available
for interim
rewarming rewarming rewarming
Spontaneous Spontaneous Spontaneous
rewarming
rewarming
Spontaneous
Spontaneous
rewarming
Spontaneous covers
without
covers
without covers with covers without covers
without
covers applied immersion in bath immersion in bath
Before Before Before
3
1 1 2
Before
covers applied
Before
4
covers applied
and heating
and heating
and heating
covers
Before
1
applied
Before covers applied At conclusion of experiment At conclusion of experiment
Time of observation
All 21 sheep are included
2 1 1
No.
observations
numbers available for interim observations. and before hypothermia was induced.
Spontaneous rewarming without covers Airway warming spontaneous ventilation Airway warming assisted ventilation
Group
Additional
Table 2. Numbers of sheep allocated to treatment groups with additional analysis of the temperature rise in the period after the insertion of probes
started
started
started
for
8
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS
“i ; j, i ,
,
,
TIME 1HOURS) Aorta
Rectum
. .................
Air
Endotracheal Subcut-thorax _.-.-._.-
-_--
Fig.1. Typical charts showing temperature changes and gradients during cooling and rewarming. (a) Spontaneous; (b) CBRW; (c) hot bath. Note that the intra-tracheal tracings show the temperature during inspiration at the lower part of the hatched zone with the temperature during expiration at the upper part. During use of the humidifier in CBRW the positions are reversed, the inspired temperature being higher than the expired.
ing and airway rewarming in an attempt to maintain a normal arterial carbon dioxide tension (Pa,C02). The number of sheep in each group and the numbers arising from overlap of groups at the beginning and end of the specified treatments are given in Table 2. Overlap of treatments arose because of three situations. Most animals had a period of ‘spontaneous rewarming without covers’ before the start of their allocated treatment. In the spontaneous
REWARMING FROMIMMERSION HYPOTHERMIA 9
32-
F
_,_,C._._ ._._. _.-.w ._._._,- ._._.___.
27-
z 5 +
22-
17-
5 x ,0 r
t E u ._ E I’
C ‘5 0
TIME I HOURS) Aorta Ret turn
.......... ...... Air
Endotracheal Subcut-thorax _._._._._.
a - -
-
-
F&l(b) rewarming experiment, when removal from the cold bath and insulation failed to stop the continuing drop in core temperature of two sheep both were treated with CBRW, one by spontaneous and one by assisted ventilation. Changes of core temperature were compared between the various methods of rewarming and, within the groups, between the sheep which had been shorn and those in which the fleece had been left unclipped. This resulted in the numbers of each group being small, but the results were more meaningful. Temperature gradients were also compared.
Results The initial temperatures of the sheep at the time of insertion of the probes are shown in
Table 3. Whilst the animals were being prepared the core temperature rose (Fig.1 and Fig.2) and the changes in the period before immersion are shown in Table 4. There was a fairly close correlation between the rise in body temperature and the rise in room temperature in the shorn sheep (the rise in body temperature was about one-third of that of the room), but in the unshorn sheep the rate of rise was greater and bore no relationship to the initial air temperature of the room or the rate of rise of room temperature. The
10
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS
TIME 1HOURS) Aorta Rectum Fig. l(c)
Endotracheal ................. S&cut-thorax Air _._._._. _
_ ._._
pre-immersion temperatures of the sheep are also shown in Table 3. After the sheep had been removed from the bath, the core temperature continued to drop rapidly until the core/shell temperature gradient had equilibrated (Fig. 1 and Fig.2); in every case except one the drop in temperature continued until treatment was started. The rates of rewarming achieved by the different methods used are shown in Table 5. The rate of rewarming bore no connection with any changes in room temperature and was unrelated to the weight of the sheep. During the study of CBRW the differential in the temperature of the respiratory gases between inspiration and expiration was recorded and, since the tidal volumes with assisted ventilation was markedly greater than during sponta-
REWARMING FROM IMMERSION HYPOTHERMIA
11
Table 3. Distribution of sheep in relation to core temperature on initial insertion of probes, and immediately before induction of hypothermia. Core temperature on initial Insertion of probes
<34”C
34-35°C
>35”C
Unshorn sheep Shorn sheep
2 _
12 3
3 1
Core temperature immediately before hypothermia
<36”C
36-31°C
>37”C
Unshorn sheep Shorn sheep
1 3
12 1
4 -
Table 4. Rate of rise in core temperature in the period before immersion.
No. of sheep
State of fleece
Mean rate of rise (and range) of core temperature (“C/h)
4 17
Shorn Unshorn
0.8 (0.5-1.0) 2.0 (1.5-3.5)
Rise in air temperature over the same period (“C/h) 2.6 (1.5-3.0) 2.9 (1.5-3.7)
Table 5. Rates of rewarming achieved by the methods used in the treatment groups.
Method Control, non-cooled Hot bath Airway assisted ventilation Airway spontaneous ventilation Spontaneous wrapped in polythene and blankets Spontaneous, no covers
State of sheep Unshorn Shorn Unshorn Unshorn Unshorn Shorn Unshorn Shorn Unshorn Shorn
No.
3 : 4 5 1 3 1 11 2
Mean rate (and range) of rewarming (“C/h)
Projected interval for return to initial temperature
0.5 (0.4-0.5) 2.8 (2.0-3.4) 2.0 (1.9-2.1) 0.9 (0.3-1.3) 0.7 (0.6-1.0) 0.1 0.2 (0.1-0.3) -0.7 -0.6 (-1.4-+0.6) -0.6 (-0.4--O.@
50 min 1 h 20 min 2 h 55 min 3h20min 31 h 30 mm 11 h 30min _
E. LL. LLOYD, B. MITCHELL AND .I. T. WILLIAMS
12
L2-
1 TIME (HOURS)
TIME(HOURS) .
Aorta
Skin- thorax ---------
Rumen ~~----
N-thigh
A comparison of core/shell temperature gradients, and the relation with the rumen temperatures. (a) CBRW; (b) hot bath.
Fii.2.
neous ventilation, the mean differentials recorded for each form of ventilation are compared in Table 6. After normothermia had been achieved in the hot bath and the water had been drained, the core temperature immediately started to drop at the rate of 1.3”C/h (range 0.272.25”C/h). During the study of spontaneous rewarming, disturbance of the blankets and polythene sheeting caused a drop in core temperature with a considerable delay in recovery (Fig.3). Pulmonary arterial and aortic temperatures were compared in the non-cooled control sheep (Fig.4) and during cooling and rewarming by the different methods (Fig.5). Other temperature gradients are shown for non-cooled sheep (Fig.4) and during cooling and rewarming (Fig.1 and Fig.2). The jugular venous temperature showed a small early rise in relation to the aortic temperature when the humidifier was introduced into the anaesthetic circuit, whereas there was no change with spontaneous rewarming. With hot water there was either no change or a slight fall. There was an initial drop in core temperature during rewarming in a hot bath though this was less than 1”C and not recorded from all the core probes in any individual case. This initial drop did not occur with the other rewarming methods. In these experiments the rectal temperature was found to be an inaccurate measure of absolute core temperature or change in core temperature.
Discussion The experiments reported here were carried out under anaesthesia. This not only de-
REWARMING FROM IMMERSION HYPOTHERMIA
subcut-
320
13
t hotax
31-
skln-cat
30 -
29-
28I
0 TIME
Fig.3. Effects on tissue temperatures
1 ‘12 (HOURS)
I
L
I
I ‘12
of disturbing the insulating covers during spontaneous rewarming.
Table 6. Differential in temperature of respiratory gases between inspiration and expiration with and without the use of the airway heat source. Mean temperature difference (and range)/ breath: inspired-expired (“C) Method of ventilation
No. of sheep
Without heat source
With heat source
Spontaneous Assisted
5 3
-1.26 (-0.8--1.8) -2.8 (-2.2--3.5)
+2.34 (+l.O-+4.5) +6.8 (+5.0-+8.1)
Total difference in temperature/ breath after inserting humidifier (“C)
creases heat production by affecting the hypothalamic heat centre, by inhibiting shivering, and by depressing metabolism to basal levels, but also increases heat loss by dilating the peripheral vasculature directly and by overcoming the vasoconstrictor response to cold (Burton & Edholm, 1955; Keatinge, 1969; Wylie & Churchill-Davidson, 1966). Anaesthesia therefore would tend to slow the rate of rewarming in the spontaneous and in the CBRW groups, i.e. the methods which rely on the intrinsic metabolic activity as the main heat source (Burton & Edholm, 1955; Lloyd, Conliffe, Orgel 8~Walker, 1972). It would, however, have minimal effect on rewarming in a hot bath since the main heat source is the
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS (b)
(a) LO+ /__I______
I
LO-_
I I TIME(HOURS)
I
I
I
I
I
I
Fi.4. Non-cooled sheep (a and b) showing the importance of considering the shell temperature as the core, i.e. total body heat.
as well
Aorta
1
TIME (HOURS) -
Subcut - thorax
Skin-ear -.- .-.
------
Pulmonary
artery
-
physical transfer of heat from the water to the body, In the sheep the respiratory tract is an important route for thermoregulation (Hales & Webster, 1967). Endotracheal intubation by curtailing the ventilation of the ethmoturbinate area and the use of the circle anaesthetic system by recycling moisture and warm gases would both tend to reduce the potential for heat loss from the respiratory tract. However, the demonstration that the temperature of the expired gas was higher than the inspired (Table 6) indicated that there still was some heat loss from the respiratory system. The net result of anaesthesia and of partial insulation of the respiratory thermoregulatory mechanism was a rise in the core temperature of all the sheep during the period before immersion. A number of factors could contribute to the consistently subnormal temperatures registered by the sheep at the time of initial placement of the thermistor probes (Table 3) and still present immediately before hypothermia was induced (Table 3) after 1 h of anaesthesia. Waterman (1975) found in dogs and cats that significant hypothermia could develop during general anaesthesia and that the greatest fall of temperature occurred in the first 20 ruin (0.088“ C/mm). In man the induction of anaesthesia causes a redistribution of body heat and a narrowing of the core/shell temperature gradient with a drop in core temperature balanced by a rise in the shell temperatures (E. Ll. Lloyd, unpublished observations; Vale, 1973). Anaesthetic agents also interfere with the fine control of ther. moregulation by removing the influence of the extra-hypothalamic temperature sensors on the responsiveness to changes in hypothalamic temperatures, while leaving the central sensor system and the coarse control of thermoregulation relatively intact (Bligh, 1973). A further factor, of metabolic origin, may have been the combined effect of cold overnight temperatures with restricted activity and the withdrawal of food as part of the preanaesthetic preparation to minimize the risk of rumen regurgitation. This withdrawal of food was the probable reason why the temperature changes in the rumen were those of any gas-filed space, lagging behind the temperature changes in the rest of the body (Fig.2) despite the fact that the rumen, as a site of continuous fermentation, might be considered to be a source of heat. The effect of fleece and subcutaneous fat was to make sheep pant like animals in an
REWARMING FROM IMMERSION HYPOTHERMIA 15
Spontaneous ur
LO firway
s/V
I:... ,
31”
...... PULMONARV
ARTERV
-
Cooling
Rewarming
AORTA
Fig.5. A comparison of sites of heat uptake during the various rewarming procedures. environmental temperature above the thermoneutral zone. Shearing alters the thermal balance and the effect of cold sensations on the skin reduces the respiratory rate from 90/min to 20/min in an environmental temperature of 20°C (B&h, 1973). The net result
was that during the period before immersion the unshorn sheep were warming at a rate of 2.5 times that of the shorn (Table 4). In the non-cooled sheep the insulating effect of combined fleece and circle anaesthetic circuit allowed the core temperature to rise continuously to values in the normal range for adult sheep, though the rate was slower than that observed during the pre-immersion period (Table 5). The intermediate (shell) temperature also rose, the largest rise in core temperature being accompanied by the greatest difference between the core and the subcutaneous shell temperature and vice versa (Fig.4). Therefore there may be no difference in total body heat. This relationship between rate of rise of core temperature and core/shell temperature differential was also noticed during spontaneous rewarming and CBRW and may account in part for the variation in rate of
16
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS
rewarming. With the hot-bath treatment, since a hot bath raises the body temperature by the physical transfer of surface heat without relying on any contribution from intrinsic metabolism, the core temperature of the shorn sheep rose faster than the temperature of the sheep with fleece, but the hot-bath method required vigorous movement of the victim and in man this movement has been known to precipitate ventricular fibrillation (Freeman &Pugh, 1969; Golden, 1973; Lloyd, 1973). During experiments with spontaneous rewarming with insulation of the body surface, the importance of good insulation was emphasized by the finding that the core temperature of the sheep continued to drop when the insulation was the clipped fleece, with or without blankets, or the intact fleece alone. (This drop was not the ‘after-drop’.) When, however, blankets were used in addition to the fleece, the core temperature rose. Disturbing the insulation once applied could delay the rewarming seriously (Fig.3). This fact is of importance for the rescue teams and would be a factor against transporting a hypothermic patient and in favour of camping and rewarming on the spot. With CBRW, though the differential of temperature between the inspired and expired gases was three times greater with assisted ventilation compared with spontaneous ventilation (Table 6), there was no corresponding advantage in the rate of rise of core temperature (Table 5). If calculations of respiratory heat loss and gain are examined (Lloyd ef al., 1972) it becomes obvious that the main value of CBRWlies in its preventing the loss of respiratory heat and of moisture-heat and is not derived from the heat supplied, so that assisting the ventilation would result in negligible thermal benefit, Therefore, for practical resuscitation with CBRW, the main aim must be to supply a warm humid atmosphere and, although the temperature of the gas should obviously be above the patient’s core temperature, raising the gas temperature above 40-45°C would result in an increased risk of facial burning without any benefit to the patient. Assisting the ventilation would also fail to produce any thermal benefit and has the danger of altering the acid/base status rapidly. In these experiments a comparison of the techniques used for rewarming showed that the rate of rise in core temperature by CBRWwas three times faster than by spontaneous rewarming with efficient body insulation (Table 5), which was significant since the temperature gradients were similar with the two methods (Fig.1 and Fig.2). Warming in a hot bath was three to four times faster than CBRWbut the core/shell temperature gradients were greater than in the other two methods (Fig.1 and Fig.2) and therefore if total body heat is considered, the true advantage of the hot bath may be less than first appears. If the time needed for evacuation of a casualty and preparation of the bath is also considered and the fact that the rate of rewarming by CBRWshould be more rapid in nonanaesthetized subjects, the time-advantage of the hot bath becomes even less. A comparison of the change in aortic and pulmonary artery temperatures (Fig.5) shows that with spontaneous rewarming, and with CBRW with spontaneous ventilation, the heat gain occurred on the venous side of the pulmonary circulation, whereas in CBRW with assisted ventilation the heat gain was on the arterial side of the pulmonary circulation. The changes with warm water were intermediate. The early rise in the jugular venous temperature with CBRWwas probably a reflection of heat being absorbed through the nasopharynx, though there may have been some contribution from the heated aortic blood being selectively diverted into the cerebral circulation, before registering on the aortic probe which was situated in the descending aorta. During cooling whenever douching and agitation of the water stopped the skin and subcutaneous temperature rose and, after removal from the bath during the ‘after-drop’ of core temperature, all ‘shell’temperatures rose towards those of the core (Fig.1 and Fig.2).
REWARMING FROM IMMERSION HYPOTHERMIA
17
The living body obeys the laws of physics and the body in a hypothermia-inducing situation is attempting to reach heat equilibrium. The ‘after-drop’, far from being a mystical danger, is merely the achievement of thermal equilibrium, which, however, may be upset by a number of factors including voluntary or involuntary movement of the limbs. Man has large tissue masses in the limbs which not only act as great surface areas for heat loss but in hypothermia contribute significantly to the size of the cold shell. The limbs of the sheep are a smaller proportion of the body mass and therefore although the experimental design, in allowing the initial ‘after-drop’ of core temperature to level off before starting rewarming procedures, resulted in a minimum initial drop of core temperature with surface warming, this would not apply to man. Indeed, the initial drop of core temperature is one of the dangers of surface rewarming in man (Burton & Edholm, 1955; Freeman & Pugh, 1969; Keatinge, 1969) but the body is still obeying the laws of physics and adjusting to a new thermal equilibrium. This study therefore confirms the thermal advantage suggested by earlier work of CBRW over passive spontaneous rewarming (Lloyd, 1973; Lloyd & Frankland, 1974). In comparison with hot-bath rewarming, CBRWhas been shown toresultin the core temperature rising at a comparable rate in mildly hypothermic men (Hayward dz Steinman, 1975) and this present study suggests that the temperature gradients are more normal in CBRW. Further studies are required to analyse the effects of the different methods on total body heat rather than only measuring core temperatures.
Acknowledgments The work was assisted by a grant from the Scottish Home and Health Department. The help of Dr J. T. Stamp, Director of the A.D.R.A., Moredun Institute, in making facilities available is gratefully acknowledged. Thanks are also due to Mrs C. Cockburn for secretarial assistance.
References Bligh, J. (1973) Temperature Regulation in Mammalsand Other Vertebrates. North Holland Publishing Co., London. Burton, A. C. & Edholm, 0. G. (1955)Man in a Cold Environment, chapter 9,11. Hafner Publishing Co., London. Freeman, J. & Pugh, L. G. C. E. (1969) Hypothermia in Mountain Accidents. ht. Anaesthesiol. C7in. 7,997-1007. Golden, F. St C. (1973) Recognition and treatment of immersion hypothermia. Proc. Roy. Sot. Med. 66,1058-1061. Hales, J. R. S. & Webster, M. E. D. (1967) Respiratory function during thermal tachypnoea in sheep. J. Physiol. (London), 190,241-260. Hayward, J. S. & Steinman, A. M. (1975) Accidental hypothermia: an experimental study of inhalation rewarming. Aviation, Space and Environmental Medicine, 46, 1236-1240. Keatinge, W. R. (1969) Survival in Cold Water. Blackwell, Oxford. Lloyd, E. Ll. (1973) Accidental hypothermia treated by central rewarming through the airway. Brit. J. Anaes. 4541-48. Lloyd, E. LL, Conliffe, N. A., Orgel, H. & Walker, P. N. (1972) Accidental hypothermia: an apparatus for central rewarming as a first aid measure. Scot. Med. J. 17,83-91. Lloyd, E. Ll. & Frankland, J. C. (1974) Accidental hypothermia: Central rewarming in the field. Brif. Med. J. iv, 717.
18
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS
Vale, R. J. (1973) Normothermia: its place in operative and post-operative care. Anaesthesia. 28, 241-245. Waterman, A. (1975) Accidental hypothermia during anaesthesia in dogs and cats. Vet. Rec. 96, 308-313. Wylie, W. D. & Churchill-Davidson, H. C. (Ed) (1966) A Practice of Anaesthesia, chapter 18,43. pp.514,1122-1124. Lloyd-Luke, London.
Rewarming from immersion hypothermia: a comparison of three techniques E. LL. LLOYD, B. MITCHELL and J. T. WILLIAMS Experimental Surgery Unit, Animal Disease Research Association, Moredun Institute, Edinburgh, Scotland
Summary Rewarming from immersion hypothermia has been assessed in sheep by the use of three techniques -hot bath, body insulation and airway warming. Though the hot bath was the fastest of the methods of rewarming studied, consideration of temperature gradients and therefore total body heat diminished its advantage in comparison with central body rewarming via the airway (CBRW), which in turn showed considerably advantage over body insulation alone. CBRW did not have any thermal advantage gained on assisting the ventilation as compared with spontaneous breathing. The results illustrate the importance of adequate insulation of the body to prevent further heat loss and this was found to be true whether or not airway warming was being used. The site of heat uptake with CBRW was determined and observations were made on the physical behaviour of temperature gradients.
Introduction Accidental hypothermia is a problem whose incidence is increasing (Freeman & Pugh, 1969) as more people leave the shelter of cities during work and leisure. While many sophisticated techniques are available for rewarming in hospital, most are not applicable in the wide open spaces where the majority of cases are found. Rescue teams have therefore to rely on passive spontaneous rewarming aided by insulation, or transport to a hot bath (Freeman & Pugh, 1969). Airway warming in addition to insulating the patient has shown encouraging results in man (Lloyd, 1973; Lloyd & Frankland, 1974) and a pilot study was undertaken to compare the effects of airway warming with two standard methods, i.e. passive rewarming and active rewarming in a bath at 45-50” C. Sheep were used as experimental animals because their weights approached those of man. While it is appreciated that there are physical and physiological differences between sheep and man, e.g. in their surface area/mass ratios, thermoneutral zone and critical temperatures, it was considered that there were sufficient similarities to justify the study. Method
Twenty-one mature black-face ewes whose body weights ranged between 40.5 and 57.2 kg were used. The animals were anaesthetized throughout the study by inhalation of nitrous oxide, oxygen and halothane; an endotracheal tube was passed and anaesthesia was maintained with a circle circuit with soda-lime absorber. During the next hour, temperature5
6
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS
monitoring probes were attached. Temperatures were measured by Ellab Thermistor probes (Sierex Ltd) adapted for D.C. potentiometric recording. The recording was on a Speedomax type W Cleertrend Multi-point Indicating Recorder (Leeds and Northrup Ltd). Temperatures were recorded from the sites indicated in Table 1, although not all sites were monitored in every case. The methods of insertion of the probes are also given in Table 1. Hypothermia was induced by immersing the sheep in cold water (7-15” C) with douching of exposed parts. The first four sheep in the study were shorn of their 2 inch fleece to aid the induction of hypothermia, but it was later found that even when the animals were unshorn their temperature dropped at a rate satisfactory for the experimental purpose. Once the core temperature, as recorded on the intra-arterial thermistor, had dropped to about 35”C, the bath was drained and a further period allowed for the completion of the ‘after-drop’ of core temperature before rewarming commenced. Comparison was made between three methods of rewarming. Spontaneous rewarming. These sheep were removed from the water, dried of excess water and wrapped in blankets with polythene sheeting between the sheep and the blankets. No additional heat was supplied. Central body rewarming via the airway (CBR W) with spontaneous ventilation. In addition to being wrapped in blankets and polythene, a water-bath humidifier (East Radcliffe) was added to the anaesthetic circuit. The inspired temperature in the endotracheal tube during inspiration was approximately 42-45” C. Hot bath. The sheep were rewarmed by immersion in a bath of water maintained at a temperature of 45-50” C. Control animals. There were three control groups. (a) Not-cooled: the sheep were prepared as above, but hypothermia was not induced; (b) deep hypothermia: no attempt was made to rewarm the sheep and readings were taken during the continuing drop in core temperature; (c) CBRW with assisted ventilation: ventilation was assisted throughout cool-
Table 1. Sites of measurement of body temperature. Probe
Desired site
Method of insertion
Aorta Central venous Jugular
Thoracic descending aorta Inferior vena cava Jugular bifurcation
Via a catheter in the femoral artery Via a catheter in the femoral vein Via a catheter inserted percutaneously the jugular vein
Oesophageal Rectal Nasopharyngeal Intra-tracheal Intramuscular Subcutaneous
Retrocardiac
Skin Rumen Pulmonary artery Air Water bath
15 cm above carina
into
Inserted via the external nares Lying in lumen of the endotracheal tube Needle probe in the thigh Needle probe in subcutaneous tissue of the thorax Loop probe on ear Via a catheter placed by needle puncture through the abdominal and rumen walls Recorded separately by the thermistor incorporated in a Devices cardiac output computer used as part of the study A probe exposed in the region of the sheep Placed in the water during cooling and hot bath rewarming
2 3
Unshorn
Unshorn
Deep hypothermia
Airway warming (CBRW), assisted ventilation
3
2
Unshorn
Unshorn
sheep
3
4
1
3
Shorn
Unshorn
Shorn
Unshorn
No. in each group
Non-cooled
Control
Hot bath
ventilation
(CBRW)
with covers
groups
warming
Spontaneous
Airway
Spontaneous
Rewarming
State of sheep
no. available
for interim
rewarming rewarming rewarming
Spontaneous Spontaneous Spontaneous
rewarming
rewarming
Spontaneous
Spontaneous
rewarming
Spontaneous covers
without
covers
without covers with covers without covers
without
covers applied immersion in bath immersion in bath
Before Before Before
3
1 1 2
Before
covers applied
Before
4
covers applied
and heating
and heating
and heating
covers
Before
1
applied
Before covers applied At conclusion of experiment At conclusion of experiment
Time of observation
All 21 sheep are included
2 1 1
No.
observations
numbers available for interim observations. and before hypothermia was induced.
Spontaneous rewarming without covers Airway warming spontaneous ventilation Airway warming assisted ventilation
Group
Additional
Table 2. Numbers of sheep allocated to treatment groups with additional analysis of the temperature rise in the period after the insertion of probes
started
started
started
for
8
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS
“i ; j, i ,
,
,
TIME 1HOURS) Aorta
Rectum
. .................
Air
Endotracheal Subcut-thorax _.-.-._.-
-_--
Fig.1. Typical charts showing temperature changes and gradients during cooling and rewarming. (a) Spontaneous; (b) CBRW; (c) hot bath. Note that the intra-tracheal tracings show the temperature during inspiration at the lower part of the hatched zone with the temperature during expiration at the upper part. During use of the humidifier in CBRW the positions are reversed, the inspired temperature being higher than the expired.
ing and airway rewarming in an attempt to maintain a normal arterial carbon dioxide tension (Pa,C02). The number of sheep in each group and the numbers arising from overlap of groups at the beginning and end of the specified treatments are given in Table 2. Overlap of treatments arose because of three situations. Most animals had a period of ‘spontaneous rewarming without covers’ before the start of their allocated treatment. In the spontaneous
REWARMING FROMIMMERSION HYPOTHERMIA 9
32-
F
_,_,C._._ ._._. _.-.w ._._._,- ._._.___.
27-
z 5 +
22-
17-
5 x ,0 r
t E u ._ E I’
C ‘5 0
TIME I HOURS) Aorta Ret turn
.......... ...... Air
Endotracheal Subcut-thorax _._._._._.
a - -
-
-
F&l(b) rewarming experiment, when removal from the cold bath and insulation failed to stop the continuing drop in core temperature of two sheep both were treated with CBRW, one by spontaneous and one by assisted ventilation. Changes of core temperature were compared between the various methods of rewarming and, within the groups, between the sheep which had been shorn and those in which the fleece had been left unclipped. This resulted in the numbers of each group being small, but the results were more meaningful. Temperature gradients were also compared.
Results The initial temperatures of the sheep at the time of insertion of the probes are shown in
Table 3. Whilst the animals were being prepared the core temperature rose (Fig.1 and Fig.2) and the changes in the period before immersion are shown in Table 4. There was a fairly close correlation between the rise in body temperature and the rise in room temperature in the shorn sheep (the rise in body temperature was about one-third of that of the room), but in the unshorn sheep the rate of rise was greater and bore no relationship to the initial air temperature of the room or the rate of rise of room temperature. The
10
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS
TIME 1HOURS) Aorta Rectum Fig. l(c)
Endotracheal ................. S&cut-thorax Air _._._._. _
_ ._._
pre-immersion temperatures of the sheep are also shown in Table 3. After the sheep had been removed from the bath, the core temperature continued to drop rapidly until the core/shell temperature gradient had equilibrated (Fig. 1 and Fig.2); in every case except one the drop in temperature continued until treatment was started. The rates of rewarming achieved by the different methods used are shown in Table 5. The rate of rewarming bore no connection with any changes in room temperature and was unrelated to the weight of the sheep. During the study of CBRW the differential in the temperature of the respiratory gases between inspiration and expiration was recorded and, since the tidal volumes with assisted ventilation was markedly greater than during sponta-
REWARMING FROM IMMERSION HYPOTHERMIA
11
Table 3. Distribution of sheep in relation to core temperature on initial insertion of probes, and immediately before induction of hypothermia. Core temperature on initial Insertion of probes
<34”C
34-35°C
>35”C
Unshorn sheep Shorn sheep
2 _
12 3
3 1
Core temperature immediately before hypothermia
<36”C
36-31°C
>37”C
Unshorn sheep Shorn sheep
1 3
12 1
4 -
Table 4. Rate of rise in core temperature in the period before immersion.
No. of sheep
State of fleece
Mean rate of rise (and range) of core temperature (“C/h)
4 17
Shorn Unshorn
0.8 (0.5-1.0) 2.0 (1.5-3.5)
Rise in air temperature over the same period (“C/h) 2.6 (1.5-3.0) 2.9 (1.5-3.7)
Table 5. Rates of rewarming achieved by the methods used in the treatment groups.
Method Control, non-cooled Hot bath Airway assisted ventilation Airway spontaneous ventilation Spontaneous wrapped in polythene and blankets Spontaneous, no covers
State of sheep Unshorn Shorn Unshorn Unshorn Unshorn Shorn Unshorn Shorn Unshorn Shorn
No.
3 : 4 5 1 3 1 11 2
Mean rate (and range) of rewarming (“C/h)
Projected interval for return to initial temperature
0.5 (0.4-0.5) 2.8 (2.0-3.4) 2.0 (1.9-2.1) 0.9 (0.3-1.3) 0.7 (0.6-1.0) 0.1 0.2 (0.1-0.3) -0.7 -0.6 (-1.4-+0.6) -0.6 (-0.4--O.@
50 min 1 h 20 min 2 h 55 min 3h20min 31 h 30 mm 11 h 30min _
E. LL. LLOYD, B. MITCHELL AND .I. T. WILLIAMS
12
L2-
1 TIME (HOURS)
TIME(HOURS) .
Aorta
Skin- thorax ---------
Rumen ~~----
N-thigh
A comparison of core/shell temperature gradients, and the relation with the rumen temperatures. (a) CBRW; (b) hot bath.
Fii.2.
neous ventilation, the mean differentials recorded for each form of ventilation are compared in Table 6. After normothermia had been achieved in the hot bath and the water had been drained, the core temperature immediately started to drop at the rate of 1.3”C/h (range 0.272.25”C/h). During the study of spontaneous rewarming, disturbance of the blankets and polythene sheeting caused a drop in core temperature with a considerable delay in recovery (Fig.3). Pulmonary arterial and aortic temperatures were compared in the non-cooled control sheep (Fig.4) and during cooling and rewarming by the different methods (Fig.5). Other temperature gradients are shown for non-cooled sheep (Fig.4) and during cooling and rewarming (Fig.1 and Fig.2). The jugular venous temperature showed a small early rise in relation to the aortic temperature when the humidifier was introduced into the anaesthetic circuit, whereas there was no change with spontaneous rewarming. With hot water there was either no change or a slight fall. There was an initial drop in core temperature during rewarming in a hot bath though this was less than 1”C and not recorded from all the core probes in any individual case. This initial drop did not occur with the other rewarming methods. In these experiments the rectal temperature was found to be an inaccurate measure of absolute core temperature or change in core temperature.
Discussion The experiments reported here were carried out under anaesthesia. This not only de-
REWARMING FROM IMMERSION HYPOTHERMIA
subcut-
320
13
t hotax
31-
skln-cat
30 -
29-
28I
0 TIME
Fig.3. Effects on tissue temperatures
1 ‘12 (HOURS)
I
L
I
I ‘12
of disturbing the insulating covers during spontaneous rewarming.
Table 6. Differential in temperature of respiratory gases between inspiration and expiration with and without the use of the airway heat source. Mean temperature difference (and range)/ breath: inspired-expired (“C) Method of ventilation
No. of sheep
Without heat source
With heat source
Spontaneous Assisted
5 3
-1.26 (-0.8--1.8) -2.8 (-2.2--3.5)
+2.34 (+l.O-+4.5) +6.8 (+5.0-+8.1)
Total difference in temperature/ breath after inserting humidifier (“C)
creases heat production by affecting the hypothalamic heat centre, by inhibiting shivering, and by depressing metabolism to basal levels, but also increases heat loss by dilating the peripheral vasculature directly and by overcoming the vasoconstrictor response to cold (Burton & Edholm, 1955; Keatinge, 1969; Wylie & Churchill-Davidson, 1966). Anaesthesia therefore would tend to slow the rate of rewarming in the spontaneous and in the CBRW groups, i.e. the methods which rely on the intrinsic metabolic activity as the main heat source (Burton & Edholm, 1955; Lloyd, Conliffe, Orgel 8~Walker, 1972). It would, however, have minimal effect on rewarming in a hot bath since the main heat source is the
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS (b)
(a) LO+ /__I______
I
LO-_
I I TIME(HOURS)
I
I
I
I
I
I
Fi.4. Non-cooled sheep (a and b) showing the importance of considering the shell temperature as the core, i.e. total body heat.
as well
Aorta
1
TIME (HOURS) -
Subcut - thorax
Skin-ear -.- .-.
------
Pulmonary
artery
-
physical transfer of heat from the water to the body, In the sheep the respiratory tract is an important route for thermoregulation (Hales & Webster, 1967). Endotracheal intubation by curtailing the ventilation of the ethmoturbinate area and the use of the circle anaesthetic system by recycling moisture and warm gases would both tend to reduce the potential for heat loss from the respiratory tract. However, the demonstration that the temperature of the expired gas was higher than the inspired (Table 6) indicated that there still was some heat loss from the respiratory system. The net result of anaesthesia and of partial insulation of the respiratory thermoregulatory mechanism was a rise in the core temperature of all the sheep during the period before immersion. A number of factors could contribute to the consistently subnormal temperatures registered by the sheep at the time of initial placement of the thermistor probes (Table 3) and still present immediately before hypothermia was induced (Table 3) after 1 h of anaesthesia. Waterman (1975) found in dogs and cats that significant hypothermia could develop during general anaesthesia and that the greatest fall of temperature occurred in the first 20 ruin (0.088“ C/mm). In man the induction of anaesthesia causes a redistribution of body heat and a narrowing of the core/shell temperature gradient with a drop in core temperature balanced by a rise in the shell temperatures (E. Ll. Lloyd, unpublished observations; Vale, 1973). Anaesthetic agents also interfere with the fine control of ther. moregulation by removing the influence of the extra-hypothalamic temperature sensors on the responsiveness to changes in hypothalamic temperatures, while leaving the central sensor system and the coarse control of thermoregulation relatively intact (Bligh, 1973). A further factor, of metabolic origin, may have been the combined effect of cold overnight temperatures with restricted activity and the withdrawal of food as part of the preanaesthetic preparation to minimize the risk of rumen regurgitation. This withdrawal of food was the probable reason why the temperature changes in the rumen were those of any gas-filed space, lagging behind the temperature changes in the rest of the body (Fig.2) despite the fact that the rumen, as a site of continuous fermentation, might be considered to be a source of heat. The effect of fleece and subcutaneous fat was to make sheep pant like animals in an
REWARMING FROM IMMERSION HYPOTHERMIA 15
Spontaneous ur
LO firway
s/V
I:... ,
31”
...... PULMONARV
ARTERV
-
Cooling
Rewarming
AORTA
Fig.5. A comparison of sites of heat uptake during the various rewarming procedures. environmental temperature above the thermoneutral zone. Shearing alters the thermal balance and the effect of cold sensations on the skin reduces the respiratory rate from 90/min to 20/min in an environmental temperature of 20°C (B&h, 1973). The net result
was that during the period before immersion the unshorn sheep were warming at a rate of 2.5 times that of the shorn (Table 4). In the non-cooled sheep the insulating effect of combined fleece and circle anaesthetic circuit allowed the core temperature to rise continuously to values in the normal range for adult sheep, though the rate was slower than that observed during the pre-immersion period (Table 5). The intermediate (shell) temperature also rose, the largest rise in core temperature being accompanied by the greatest difference between the core and the subcutaneous shell temperature and vice versa (Fig.4). Therefore there may be no difference in total body heat. This relationship between rate of rise of core temperature and core/shell temperature differential was also noticed during spontaneous rewarming and CBRW and may account in part for the variation in rate of
16
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS
rewarming. With the hot-bath treatment, since a hot bath raises the body temperature by the physical transfer of surface heat without relying on any contribution from intrinsic metabolism, the core temperature of the shorn sheep rose faster than the temperature of the sheep with fleece, but the hot-bath method required vigorous movement of the victim and in man this movement has been known to precipitate ventricular fibrillation (Freeman &Pugh, 1969; Golden, 1973; Lloyd, 1973). During experiments with spontaneous rewarming with insulation of the body surface, the importance of good insulation was emphasized by the finding that the core temperature of the sheep continued to drop when the insulation was the clipped fleece, with or without blankets, or the intact fleece alone. (This drop was not the ‘after-drop’.) When, however, blankets were used in addition to the fleece, the core temperature rose. Disturbing the insulation once applied could delay the rewarming seriously (Fig.3). This fact is of importance for the rescue teams and would be a factor against transporting a hypothermic patient and in favour of camping and rewarming on the spot. With CBRW, though the differential of temperature between the inspired and expired gases was three times greater with assisted ventilation compared with spontaneous ventilation (Table 6), there was no corresponding advantage in the rate of rise of core temperature (Table 5). If calculations of respiratory heat loss and gain are examined (Lloyd ef al., 1972) it becomes obvious that the main value of CBRWlies in its preventing the loss of respiratory heat and of moisture-heat and is not derived from the heat supplied, so that assisting the ventilation would result in negligible thermal benefit, Therefore, for practical resuscitation with CBRW, the main aim must be to supply a warm humid atmosphere and, although the temperature of the gas should obviously be above the patient’s core temperature, raising the gas temperature above 40-45°C would result in an increased risk of facial burning without any benefit to the patient. Assisting the ventilation would also fail to produce any thermal benefit and has the danger of altering the acid/base status rapidly. In these experiments a comparison of the techniques used for rewarming showed that the rate of rise in core temperature by CBRWwas three times faster than by spontaneous rewarming with efficient body insulation (Table 5), which was significant since the temperature gradients were similar with the two methods (Fig.1 and Fig.2). Warming in a hot bath was three to four times faster than CBRWbut the core/shell temperature gradients were greater than in the other two methods (Fig.1 and Fig.2) and therefore if total body heat is considered, the true advantage of the hot bath may be less than first appears. If the time needed for evacuation of a casualty and preparation of the bath is also considered and the fact that the rate of rewarming by CBRWshould be more rapid in nonanaesthetized subjects, the time-advantage of the hot bath becomes even less. A comparison of the change in aortic and pulmonary artery temperatures (Fig.5) shows that with spontaneous rewarming, and with CBRW with spontaneous ventilation, the heat gain occurred on the venous side of the pulmonary circulation, whereas in CBRW with assisted ventilation the heat gain was on the arterial side of the pulmonary circulation. The changes with warm water were intermediate. The early rise in the jugular venous temperature with CBRWwas probably a reflection of heat being absorbed through the nasopharynx, though there may have been some contribution from the heated aortic blood being selectively diverted into the cerebral circulation, before registering on the aortic probe which was situated in the descending aorta. During cooling whenever douching and agitation of the water stopped the skin and subcutaneous temperature rose and, after removal from the bath during the ‘after-drop’ of core temperature, all ‘shell’temperatures rose towards those of the core (Fig.1 and Fig.2).
REWARMING FROM IMMERSION HYPOTHERMIA
17
The living body obeys the laws of physics and the body in a hypothermia-inducing situation is attempting to reach heat equilibrium. The ‘after-drop’, far from being a mystical danger, is merely the achievement of thermal equilibrium, which, however, may be upset by a number of factors including voluntary or involuntary movement of the limbs. Man has large tissue masses in the limbs which not only act as great surface areas for heat loss but in hypothermia contribute significantly to the size of the cold shell. The limbs of the sheep are a smaller proportion of the body mass and therefore although the experimental design, in allowing the initial ‘after-drop’ of core temperature to level off before starting rewarming procedures, resulted in a minimum initial drop of core temperature with surface warming, this would not apply to man. Indeed, the initial drop of core temperature is one of the dangers of surface rewarming in man (Burton & Edholm, 1955; Freeman & Pugh, 1969; Keatinge, 1969) but the body is still obeying the laws of physics and adjusting to a new thermal equilibrium. This study therefore confirms the thermal advantage suggested by earlier work of CBRW over passive spontaneous rewarming (Lloyd, 1973; Lloyd & Frankland, 1974). In comparison with hot-bath rewarming, CBRWhas been shown toresultin the core temperature rising at a comparable rate in mildly hypothermic men (Hayward dz Steinman, 1975) and this present study suggests that the temperature gradients are more normal in CBRW. Further studies are required to analyse the effects of the different methods on total body heat rather than only measuring core temperatures.
Acknowledgments The work was assisted by a grant from the Scottish Home and Health Department. The help of Dr J. T. Stamp, Director of the A.D.R.A., Moredun Institute, in making facilities available is gratefully acknowledged. Thanks are also due to Mrs C. Cockburn for secretarial assistance.
References Bligh, J. (1973) Temperature Regulation in Mammalsand Other Vertebrates. North Holland Publishing Co., London. Burton, A. C. & Edholm, 0. G. (1955)Man in a Cold Environment, chapter 9,11. Hafner Publishing Co., London. Freeman, J. & Pugh, L. G. C. E. (1969) Hypothermia in Mountain Accidents. ht. Anaesthesiol. C7in. 7,997-1007. Golden, F. St C. (1973) Recognition and treatment of immersion hypothermia. Proc. Roy. Sot. Med. 66,1058-1061. Hales, J. R. S. & Webster, M. E. D. (1967) Respiratory function during thermal tachypnoea in sheep. J. Physiol. (London), 190,241-260. Hayward, J. S. & Steinman, A. M. (1975) Accidental hypothermia: an experimental study of inhalation rewarming. Aviation, Space and Environmental Medicine, 46, 1236-1240. Keatinge, W. R. (1969) Survival in Cold Water. Blackwell, Oxford. Lloyd, E. Ll. (1973) Accidental hypothermia treated by central rewarming through the airway. Brit. J. Anaes. 4541-48. Lloyd, E. LL, Conliffe, N. A., Orgel, H. & Walker, P. N. (1972) Accidental hypothermia: an apparatus for central rewarming as a first aid measure. Scot. Med. J. 17,83-91. Lloyd, E. Ll. & Frankland, J. C. (1974) Accidental hypothermia: Central rewarming in the field. Brif. Med. J. iv, 717.
18
E. LL. LLOYD, B. MITCHELL AND J. T. WILLIAMS
Vale, R. J. (1973) Normothermia: its place in operative and post-operative care. Anaesthesia. 28, 241-245. Waterman, A. (1975) Accidental hypothermia during anaesthesia in dogs and cats. Vet. Rec. 96, 308-313. Wylie, W. D. & Churchill-Davidson, H. C. (Ed) (1966) A Practice of Anaesthesia, chapter 18,43. pp.514,1122-1124. Lloyd-Luke, London.