physiotherapy, D ecem ber 1983, vol 39, no 4 85 HEAT STRESS IN YOUNG ADULTS' R. VAN ZYL SM IT, M.B. B.Ch (W itw atersrand) F.C .P. (SA) M .R .C .P. (UK).* SUMMARY The physiology of heat regulation is briefly d iscu ssed . Heat exhaustion and heat stroke are described an d finally heat stroke a s a medical em ergency is dealt with. I OPSOM MING I t)ie fisiologie van hitte regulasie word kortliks bespreek. Hitte-uitputting en hitteslag word beskryf en ten slotte word hitteslag as 'n mediese noodtoestand behandel. Man has little difficulty in com batting cold (huddling together, shivering, exercise, b etter insulation, clothes, etc.) — escape from heat unfortunately is not so easy. The only physiological devices available fo r cooling are sweating, panting and vaso-dilatation. The core tem perature o f the hum an body reflects the balance between heat production and heat loss. In tem perate climates: 50% of heat produced is lost through radiation 25% by convection in cool air 25% by evaporation o f insensible perspiration With increased heat production virtually all the extra heat has to be got rid o f by evaporation, i.e. sweating. The skin tem perature rises very little and it may even cool dow n due to evaporation from its surface, fu rth er reducing the tem perature gradient from skin to air. As the am bient tem perature approaches 35,5°C, all heat produced by the body m ust be lost by sweating. At higher tem peratures, the body gains heat from the environm ent ♦Head: Renal U nit, G roote Schuur H ospital. fP a p e r read at the Bloem fontein Congress Ju n e I983. which places an even greater burden on the sweating mechanism. Sweating The rate o f heat loss from sweating depends largely on two factors. The most im p o rtan t is the relative humidity, far greater cooling being produced in dry air. Secondly, evaporation aided by air currents which is maximized by wearing loose-fitting clothing allowing good ventilation, aids heat loss. It has been shown th at sweating may begin as soon as 1,5 seconds after com m encing heavy exercise; it reaches an equilibrium after 30 minutes, and thereafter is in direct relation to w orkload (heat production). At m axim um rates of w ork, up to 3,5 1 /h o u r o f sweat m ay b e lost, each 1 litre of which, if totally evaporated on the skin surface, w ould cool by 580 K /cal. All sweat which drips off the body is, o f course, lost for cooling purposes. H eat gain H eat gained from a hot environm ent is generally self- evident. W hat is less well appreciated is the trem endous heat produced from m uscular activity. Met aboli c r at e Muscul ar a c t i v i t y R a d i a t i on Con d uc t i o n SDA e f f e c t E n v i r o n m e n t L . BODY H E A T C o n v e c t i o n E v a p o r a t i o n -1 HEAT G A I N 3 6 ^ > H E A T ! L Q S S 37 °C Fig. 1. Heat Balance R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) 86 Fisioterapie, Desember 1983, deel 39, no 4 D u r a t i o n of e x e r c i s e ( m i n ) Fig. 2. B ody Temperature at D ifferent Work Loads Shivering alone can increase heat production 3 to 5 fold whilst vigorous exercise may increase m etabolic rate by 20- 25 times the basal level. This rate o f production can theoretically increase the body tem perature by 1°C every 5 minutes! Very heavy exercise is sufficient to m aintain an adequate body tem perature at am bient tem peratures of-30°C w ithout the need for heavy clothing. Regulation of body tem perature This is done by specialized neurones at the base o f the brain in the hypothalam us which act as a “ th erm o stat” . A re­ setting o f this regulator is responsible for the high tem perature associated with infections. Factors affecting heat tolerance Age: Studies o f heat tolerance in subjects as old as 85 years have failed to show any fall off in tolerance with age. Sex: It is well know n th at women start sweating at a higher skin and core tem perature and produce sm aller volumes, yet their heat tolerance is sim ilar to th at o f males. They probably rely more on circulatory adjustm ents and have the advantage o f less fluid loss an d hence better hydration. Obesity: Fat people have a definite reduction in heat tolerance as it has been shown th at they have a 3 ' / 2 times increased incidence o f fatal heat stroke com pared to lean individuals. This p o o r heat tolerance is due to the insulating properties o f fat coupled with a lower body surface area to weight ratio. Acclimatization A cclim atization in a hot environm ent is maximal during the first week and complete by a b o u t 10 d a y s. F e a tu re s are: (a) an increased sweating capacity; (b) production o f more dilute sweat; (c) less tachycardia with heat; (d) a more even distribution o f sweat over the whole body. Training Fitness prom otes heat tolerance by two mechanisms: (a) sweating begins at a lower body tem perature; (b) for the sam e w orkload a fit person will have a lower body tem perature. Presence of a pyrexial illness This is an extremely im p o rtan t issue. A person who has a mild cold o r “ flu ” an d is slightly feverish has had a “ re­ adju stm en t” o f the hypothalam ic tem perature regulating mechanism and is much more likely to develop hyperpyrexia s w e a t loss ( g / k g bwt ) 8 12 U A c c l i m a t i z a t ion 37-9 37-8 3 7-7 Rectal temperature A c c l i m a t i z a t i o n Fig. 3. Development, o f Heat Acclimatization D uring acclim atization heart rate changed most rapidly, then sweat rate and finally rectal tem perature. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) physiotherapy, D ecem ber 1983, vol 39, no 4 87 E X E R C I S E • U ntrained subjects □ T rained subjects Fig. 4. Relationship between and heat stroke than a “ n o rm al” person. These persons are at extreme risk when exercising in a hot environment. State of hydration D ehydration even of a m oderate degree (equivalent to a 1% drop in body weight) is associated with a significant increase in body tem perature com pared to the sam e exercise with normal hydration. W hen w ater loss reaches 4 to 5% of body weight, a definite im pairm ent in physical work capacity and physiologic function is noted. Ti me on m a r c h ( hr s ) Fig. 5. Relationship between Water Intake and Heat Tolerance 1. Water loss not replaced. 2. W ater d ran k ad lib J. Water loss replaced hourly 20 40 60 80 Tl ME ( m i n u t e s ) Training and Heat Tolerance H eat exhaustion This term encompasses a wide range o f heat related disorders ranging from “ fainting” whilst standing for a long period in the heat, to severe dehydration an d circulatory collapse. C ertain features, however, are com m on to these disorders:- (1) The body tem perature is norm al o r only slightly raised; (2) The cardinal feature is hypotension due either to m aldistribution of cardiac o u tp u t to skin and muscles and pooling o f blood in the legs resulting in a dro p in blood pressure and tachycardia o r associated with various , degrees o f salt and w ater deficit due to prolonged sweating w ithout adequate replacement. Clinically the p atient presents with headaches, dizziness, general weakness, a rapid pulse, reduced but not absent sweating and a body tem perature not dangerously high. Treatm ent includes stopping o f exercise, moving the person to a cool environm ent, and the adm inistration o f fluids. H eat stroke D uring W orld W ar 11 it is estim ated th at ab o u t 200 recruits died from heat stroke in training centres in the U.S.A. This figure probably underestim ates the problem as for every person who dies, there are many others who are severely incapacitated but eventually recover. With the increasing popularity o f running as a recreational activity, the correct assessm ent o f a patient suffering from heat stress is vitally im p o rtan t as cases of heat stroke will die rapidly if incorrectly handled. HEAT STROKE IS A M EDICAL EM ERGENCY H eat stroke results from failure o f the tem perature regulating mechanism s o f the body bro u g h t on by R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) 88 Fisioterapie, Desember 1983, deeI 39, no 4 excessively high tem peratures. Once this stage is reached, sweating becomes ineffective, the body tem perature rises rapidly and uncontrollably to levels in excess of 42°C, soon the patient becomes confused, loses consciousness, suffers circulatory collapse and dies. P r e m o n ito r y signs in c lu d e in c o h e r e n t sp e e c h , disorientation, an altered gait and staggering. A nhydrosis, i.e. absence o f sweating, is classically described, BUT TH IS M UST NOT BE R ELIED ON TO D IST IN G U ISH HEAT STROKE FROM TH E LESS SE R IO U S HEAT INJURY, HEAT EXHAUSTION, as mild sweating may be present. The blood pressure is m oderately to severely reduced with a tachycardia (pulse 120-140/min) but it may be slower in highly trained athletes. Treatment The body tem perature must be lowered to below 40°C as soon as possible. Massage with wet towels packed with ice is very effective. Once below 40°C cooling may be more passive. Fluid therapy should include glucose and electrolyte solutions, but excessive adm inistration should be avoided to prevent circulatory overload and cerebral oedema. Serum levels o f glucose and electrolytes must be m easured early and thereafter patient closely m onitored in hospital for at least 36 hours. M arathon running D uring steady state exercise at 75% o f m axim um physical work capacity, heat production may reach 1200 K /c a l/h and corresponding rectal tem peratures o f 39 - 41°C. These tem peratures apparently cause no harm . The critical m axim um tem perature appears to be in the vicinity of 42°C M ost cases o f heat stroke occur in novice, overweight undertrained runners taking in too little fluid in long runs such as m arathons. It is, however, vital to realize th at heat stroke may also occur in well trained athletes running relatively short distances such as 5 km when dehydration and fluid replenishm ent play a minimal role. In fact, it is the elite ru n n er running at nearly m axim um energy expenditure who is at greatest risk as his rate o f heat production is so high. H eat stroke may occur w ithin minutes as has been reported in runners who, near the end o f a m arathon (which generally tends to be m id-m orning when the am bient tem perature has started rising), suddenly start speeding up for a fast finish and exceed the capacity o f their bodies to cope with the extra heat produced. Prevention G uidelines for conducting exercise in hot w eather such as m axim um tem peratures an d hum idity permissible, have been established and must be firm ly adhered to. As im portant should be the aw areness by all persons of all the factors involved in exercise in a warm environm ent and the prevention, diagnosis and m anagem ent o f heat injury. PHYSICAL ACTIVITY AND AGING* T. D. N OA K ES M.B. Ch.B. M.D. SUMMARY The effects of exercise on aging are discussed under 4 headings viz. whether there is evidence th a t e x e rc is e m ig h t d e la y a g in g , the physiological and biochemical factors that alter with age, how exercise training influences these factors and ends with advice on how to start exercising at an older age. The author concludes that regular exercise is of great benefit to preserve physical well-being into old age. M etropolitan Sport Science C entre, ! D epartm ent o f Physiology, University o f C ape Town Medical School O bservatory, 7935 •B ased on a lecture delivered at the U C T Summ er School Symposium on The aged in modern society, Jan u ary , 1982. OPSOM MING D ie u itw e fk in g s van o e fe n in g op d ie verouderingsproses word onder 4 hoofde bespreek nl. of oefening die verouderingsproses mag vertraag, die flsiologiese en biochemiese faktore wat met Ouderdom verander, hoe oefening hierdie faktore bei'nvloed en ten sJotte word daar raad gegee hoe om op 'n gevorderde oudenJom met oefening te begin. D ie skrywer kom tot die gevolgtrekkingdatgereelde oefening tot op 'n hoe ouderdom van groot baat sal wees. IN TRO D U C TIO N There are tw o ways to look at aging: T he C anadian hum orist, Stephen Leacock said “ A bout the only thing you can say ab o u t it (age) is, it’s better than being d ead ” . On turning 61, the A m erican Philosopher R alph Waldo Em erson wrote; “ W ithin, I d o not find wrinkles or a used heart, but unspent y o u th ” . R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. )