Mary B Hartley, RN
Hypothermia
Hypothermia is an artificial depression of body temperature from which man cannot recover without support. Early clinical use of hypothermia was recorded in 1797. Hypothermia is currently used in cardiac surgery. The ability to maintain body temperature within narrow limits despite changes in environmental temperature is a characteristic of homeotherms-man, other mammals, and birds. Man is a precise homeotherm who maintains a constant internal body temperature, plus or minus one degree, unless he develops a febrile illness.’ Mammals and birds are hibernators who conserve energy during cold months by lowering their body temperature a few degrees above the environmental temperature sometimes as much as 35 degrees.2 Hibernators Mary B Hartley, R N , is chief clinical perfusionist, department of cardiothoracic surgery, Naval Regional Medical Center, Sun Diego, Calif. She is a graduate of Western Pennsylvania Hospital School of Nursing and Shadyside School of Perfusion Technology, both in Pittsburgh, Pa. Hartley is editor of The Journal of Extra-Corporeal Technology.
764
can remain at low body temperatures for weeks or months and will spontaneously rewarm to a regulated homeothermic existance. Hibernation, therefore, is a naturally controlled physiological process, while hypothermia is a situation where normal physiological controls of temperature and metabolism have been altered.3 Interest in hypothermia began in 1797 when James Currie, a Scottish surgeon, advocated immersing patients into a cold tub of brine twice a day for physical illness. Interest in hypothermia continues to the twentieth century. For example, it was used by Temple Fay in 1938 in treating advanced malignant disease, and by Talbott in 1941 in treatment of psychotic disorders. These uses met with little success, but valuable information on cooling was obtained. In 1946, Alexander reported on the effects of experimental hypothermia on prisoners in German concentration camps. Wayburn, in 1947, observed airmen who suffered immersion in the North Sea during World War II.4 Today, the most common application of hypothermia is in cardiac surgery. Hypothermia’s use was suggested in 1950 when W G Bigelow demonstrated in dogs the possibility of performing cardiac surgical procedures on humans
AORN Journal, October 1976, Vol24, No 4
atients are cooled either by surface-induced hypothermia or bloodstream cooling.
P
by lowering their body temperature^.^ Prior to this, cardiac surgery was limited to surgery around the heart such as ligation of patent ductus arteriosus. Intracardiac procedures such as the repair of an atrial septa1 defect were considered heroic. In 1953, Swan reported a series of 15 patients who had undergone intracardiac surgery with their body temperatures ranging from 69.8 F (21 C) to 78.8 F (26 CL6 With the advent of cardiopulmonary bypass around 1955, more complex intracardiac procedures could be perf0rmed.I In the last 20 years, hypothermia has enjoyed several periods of enthusiasm in conjunction with cardiopulmonary bypass. Today, profound hypothermia with circulatory arrest is employed successfully in many medical centers to repair complex congenital cardiac defects in infants. Hypothermia is also used in routine cardiac surgical procedures worldwide. Hypothermia may be understood more clearly if the normal temperature regulation in man is briefly reviewed. Temperature control in man consists of positive and negative feedback. Heat is produced in the body as a by-product of metabolism at the cellular level. Factors influencing metabolism will therefore effect the amount of heat produced. Some factors increasing metabolism are the effect of thyroxine on cells, the effect of norepinephrine and sympathetic
766
stimulation on cells, and the effect of an increased temperature of the cells.8 The skeletal muscles are the greatest source of body heat and their activity can be controlled by voluntary or reflex actions such as shivering. Heat from the body is lost by radiation, conduction, and evaporation. The degree of heat loss by these methods is dependent on environmental conditions. Sweating is regulated by the autonomic nervous system and provides rapid cooling of the body. Also, the amount of blood flow to the skin acts as the radiator system and is the mechanism of heat transfer from the body core to the surface. Skin thermoreceptors relay stimuli to the hypothalmus. The anterior hypothalmus is responsible for heat loss, and the posterior hypothalmus is responsible for heat conservation. When the thermoreceptors are exposed to warm stimuli, the anterior hypothalmus initiates vasodilation flooding the periphery with blood. Respirations are increased, and sweating begins t o aid in heat loss. The heart rate slows and the skeletal muscles relax.g On the other hand, if thermoreceptors are exposed to a cold stimuli, the posterior hypothalmus is activated. Then vasoconstriction of the skin vessels occurs, sweating is abolished, vasodilatation of the muscle vessels occurs, and muscle metabolism hastens to increase the rate of heat
AORN Journal, October 1976, Vol24, No 4
production. Once muscle tone reaches a critical level, shivering ensues. The adrenal and thyroid glands also increase heat production by increasing cellular metabolism when norepinephrine, epinephrine, and thyroxine are secreted. An amazing system of checks and balances occurs to maintain the body temperature within one-half degree of normal. The primary concept of hypothermia is to decrease cellular metabolism, and, therefore, oxygen consumption of the heart and brain during a decrease or interruption of circulation. Patients are either cooled by surface-induced hypothermia or bloodstream cooling, or in some instances by a combination of both methods. To achieve hypothermia, heat must be lost a t a rate exceeding its production.1° This is one of the reasons surface cooling is reserved for children and infants generally weighing under 20 lbs (10 kg). They have a smaller surface area and can be adequately cooled to temperatures around 68 F (20 C ) . This is achieved by placing the infant in a tub of ice, or by application of ice bags and hypothermia blanket. A combination of surface cooling and bloodstream cooling by cardiopulmonary bypass is also employed in many pediatric centers. These methods are used in small children so circulatory arrest up to 60 minutes and hypothermia a t 68 F (20 C ) can safely be achieved." The ad-
vantage of profound hypothermia and circulatory arrest is obvious when a surgeon must repair a complicated congenital defect on a n infant who only weighs 6.6 lb (3 kg). Bloodstream or core cooling by cardiopulmonary bypass is commonly used in adults and larger children to temperatures as low as 78.8 F (26C ) . As stated earlier, hypothermia decreases the rate of cellular metabolism which lowers oxygen consumption. For example, the oxygen requirement for a n anesthetized man can be reduced to one-half of normal by lowering his temperature to 86 F (30 C ) , and by one-third if his temperature is lowered to 77 F (25 C ) . Temperatures a t 68 F (20 C ) will have decreased oxygen consumption by one-fifth of normal.12 Oxygen delivery to the tissues is also impaired as hypothermia becomes progressively deeper. The oxygen dissociation curve shifts to the left and could lead to tissue hypoxia, but this may not occur because oxygen requirements have been drastically reduced by hypothermia. Therefore, the A-V oxygen differences usually remain normal demonstrating sufficient oxyIt must be noted gen a~ai1ability.l~ that the oxygen consumption and metabolism of different organs vary. Uniform hypothermia, and thus uniform-decreased oxygen consumption, is not possible with methods employed today.
AORN Journal, October 1976, Vol24, No 4
767
Hypothermia in the homeotherm is not physiologic, and therefore many changes occur. For example, in the cardiovascular system, the heart rate slows to 20 to 30 beats per minute a t 64.4F (18 C).14 This is accompanied by a decrease in cardiac output, a lowering of blood pressure, and a decrease in resistance of the coronary arteries. Total vascular resistance is increased in the liver, kidneys, and especially in the pulmonary bed. Thus, hepatic blood flow and function are depressed. Other effects of hypothermia on the vascular system include venospasm and an alteration in the critical closing pressure of the capillary bed.I5 Arrhythmias such as atrial fibrillation and first degree heart block occur frequently a t temperatures around 86 F (30 C). More serious arrhythmias such as ventricular fibrillation have been linked with hypokalemia. Serum potassium and plasma insulin levels decrease during cooling. The administration of potassium during hypothermia not only increases potassium and insulin levels but also decreases the risk of ventricular arrhythmias.I6 Hypothermia also increases the viscosity of blood proportionately as the body temperature is decreasing. This is thought to occur when sodium moves into the cell and exchanges with potassium. This process results in hemoconcentration due to water from the intravascular space being shifted into the interstitual and intercellular compartment^.'^ Hemoconcentration can contribute to the capillary sludging and microcirculatory stasis believed to occur in hypothermia. The effects of hypothermia upon the other organs and systems are numerous. Although some of the effects of hypothermia are nonphysiologic, the advantages of decreased cellular metabolism and oxygen consumption
seem to outweigh the disadvantages. There continues to be much speculation in the field of cardiac surgery as to the efficacy of hypothermia. The present holds many unanswered questions regarding hypothermia, yet, one can envision many possibilities and applications of hypothermia for the future. Notes 1. Arthur Guyton, Textbook of Medical Physiology, 4th ed (Philadelphia: W B Saunders, 1971) 831. 2. Ewald Selkurt, Physiology, 3rd ed (Boston: Little, Brown & Co, 1971) 651. 3. Frank E South, et al, Hibernation and Hypothermia, Perspectives and Challenges (Amsterdam: Elsevier Publishing Co, 1972) VII. 4. Ernil Blair, Clinical Hypothermia, 1st ed (New York: McGraw-Hill Book Co, 1964) 1. 5. W G Bigelow, et al, "Hypothermia. Its posible role in cardiac surgery: An investigation of factors governing survival of dogs at low ternperatures," Annals of Surgery 132 (November 1950) 849-866. 6. Henry Swan, et al, "Surgery by direct vision in the open heart during hypotherrnia," Journal of the American Medical Association 153 (November 1953) 1081-1085. 7. Pierre M Galletti, A Brecher Gerhard, Heart Lung Bypass (New York: Grune & Stratton, 1962) 294. 8. Guyton, Textbook of Medical Physiology, 839. 9. Blair, Clinical Hypothermia, 33. 10. /bid, 15. 11. S Subrarnanian, et al, "Surface induced deep hypothermia in cardiac surgery," Journal of Pediatric Surgery 6 (October 1971) 612-617. 12. Yukihiko Nose, Manual on Artificial Organs: The Oxygenator Volume 11 (St Louis: C V Mosby Co, 1973) 256. 13. Blair, Clinical Hypothermia, 25. 14. David Baurn, et al, "Metabolic aspects of deep surgical hypothermia in infancy," Pediatrics 42 (July 1968) 93-105. 15. B Eiseman, F C Spencer, "Effect ot hypotherrnia on the flow characteristics of blood," Surgery 52 (September 1962) 532-544. 16. T C K Brown, et al, "Biochemical changes during surface cooling for deep hypothermia in open heart surgery," Journal of Thoracic and Car-. diovascular Surgery 65 (March 1973) 402-408. 17. Eisernan, Spencer, "Effect of hypothermia."
AORN Journal, October 1976, V o l 2 4 , No 4
769
Hypothermia
Hypothermia is an artificial depression of body temperature from which man cannot recover without support. Early clinical use of hypothermia was recorded in 1797. Hypothermia is currently used in cardiac surgery. The ability to maintain body temperature within narrow limits despite changes in environmental temperature is a characteristic of homeotherms-man, other mammals, and birds. Man is a precise homeotherm who maintains a constant internal body temperature, plus or minus one degree, unless he develops a febrile illness.’ Mammals and birds are hibernators who conserve energy during cold months by lowering their body temperature a few degrees above the environmental temperature sometimes as much as 35 degrees.2 Hibernators Mary B Hartley, R N , is chief clinical perfusionist, department of cardiothoracic surgery, Naval Regional Medical Center, Sun Diego, Calif. She is a graduate of Western Pennsylvania Hospital School of Nursing and Shadyside School of Perfusion Technology, both in Pittsburgh, Pa. Hartley is editor of The Journal of Extra-Corporeal Technology.
764
can remain at low body temperatures for weeks or months and will spontaneously rewarm to a regulated homeothermic existance. Hibernation, therefore, is a naturally controlled physiological process, while hypothermia is a situation where normal physiological controls of temperature and metabolism have been altered.3 Interest in hypothermia began in 1797 when James Currie, a Scottish surgeon, advocated immersing patients into a cold tub of brine twice a day for physical illness. Interest in hypothermia continues to the twentieth century. For example, it was used by Temple Fay in 1938 in treating advanced malignant disease, and by Talbott in 1941 in treatment of psychotic disorders. These uses met with little success, but valuable information on cooling was obtained. In 1946, Alexander reported on the effects of experimental hypothermia on prisoners in German concentration camps. Wayburn, in 1947, observed airmen who suffered immersion in the North Sea during World War II.4 Today, the most common application of hypothermia is in cardiac surgery. Hypothermia’s use was suggested in 1950 when W G Bigelow demonstrated in dogs the possibility of performing cardiac surgical procedures on humans
AORN Journal, October 1976, Vol24, No 4
atients are cooled either by surface-induced hypothermia or bloodstream cooling.
P
by lowering their body temperature^.^ Prior to this, cardiac surgery was limited to surgery around the heart such as ligation of patent ductus arteriosus. Intracardiac procedures such as the repair of an atrial septa1 defect were considered heroic. In 1953, Swan reported a series of 15 patients who had undergone intracardiac surgery with their body temperatures ranging from 69.8 F (21 C) to 78.8 F (26 CL6 With the advent of cardiopulmonary bypass around 1955, more complex intracardiac procedures could be perf0rmed.I In the last 20 years, hypothermia has enjoyed several periods of enthusiasm in conjunction with cardiopulmonary bypass. Today, profound hypothermia with circulatory arrest is employed successfully in many medical centers to repair complex congenital cardiac defects in infants. Hypothermia is also used in routine cardiac surgical procedures worldwide. Hypothermia may be understood more clearly if the normal temperature regulation in man is briefly reviewed. Temperature control in man consists of positive and negative feedback. Heat is produced in the body as a by-product of metabolism at the cellular level. Factors influencing metabolism will therefore effect the amount of heat produced. Some factors increasing metabolism are the effect of thyroxine on cells, the effect of norepinephrine and sympathetic
766
stimulation on cells, and the effect of an increased temperature of the cells.8 The skeletal muscles are the greatest source of body heat and their activity can be controlled by voluntary or reflex actions such as shivering. Heat from the body is lost by radiation, conduction, and evaporation. The degree of heat loss by these methods is dependent on environmental conditions. Sweating is regulated by the autonomic nervous system and provides rapid cooling of the body. Also, the amount of blood flow to the skin acts as the radiator system and is the mechanism of heat transfer from the body core to the surface. Skin thermoreceptors relay stimuli to the hypothalmus. The anterior hypothalmus is responsible for heat loss, and the posterior hypothalmus is responsible for heat conservation. When the thermoreceptors are exposed to warm stimuli, the anterior hypothalmus initiates vasodilation flooding the periphery with blood. Respirations are increased, and sweating begins t o aid in heat loss. The heart rate slows and the skeletal muscles relax.g On the other hand, if thermoreceptors are exposed to a cold stimuli, the posterior hypothalmus is activated. Then vasoconstriction of the skin vessels occurs, sweating is abolished, vasodilatation of the muscle vessels occurs, and muscle metabolism hastens to increase the rate of heat
AORN Journal, October 1976, Vol24, No 4
production. Once muscle tone reaches a critical level, shivering ensues. The adrenal and thyroid glands also increase heat production by increasing cellular metabolism when norepinephrine, epinephrine, and thyroxine are secreted. An amazing system of checks and balances occurs to maintain the body temperature within one-half degree of normal. The primary concept of hypothermia is to decrease cellular metabolism, and, therefore, oxygen consumption of the heart and brain during a decrease or interruption of circulation. Patients are either cooled by surface-induced hypothermia or bloodstream cooling, or in some instances by a combination of both methods. To achieve hypothermia, heat must be lost a t a rate exceeding its production.1° This is one of the reasons surface cooling is reserved for children and infants generally weighing under 20 lbs (10 kg). They have a smaller surface area and can be adequately cooled to temperatures around 68 F (20 C ) . This is achieved by placing the infant in a tub of ice, or by application of ice bags and hypothermia blanket. A combination of surface cooling and bloodstream cooling by cardiopulmonary bypass is also employed in many pediatric centers. These methods are used in small children so circulatory arrest up to 60 minutes and hypothermia a t 68 F (20 C ) can safely be achieved." The ad-
vantage of profound hypothermia and circulatory arrest is obvious when a surgeon must repair a complicated congenital defect on a n infant who only weighs 6.6 lb (3 kg). Bloodstream or core cooling by cardiopulmonary bypass is commonly used in adults and larger children to temperatures as low as 78.8 F (26C ) . As stated earlier, hypothermia decreases the rate of cellular metabolism which lowers oxygen consumption. For example, the oxygen requirement for a n anesthetized man can be reduced to one-half of normal by lowering his temperature to 86 F (30 C ) , and by one-third if his temperature is lowered to 77 F (25 C ) . Temperatures a t 68 F (20 C ) will have decreased oxygen consumption by one-fifth of normal.12 Oxygen delivery to the tissues is also impaired as hypothermia becomes progressively deeper. The oxygen dissociation curve shifts to the left and could lead to tissue hypoxia, but this may not occur because oxygen requirements have been drastically reduced by hypothermia. Therefore, the A-V oxygen differences usually remain normal demonstrating sufficient oxyIt must be noted gen a~ai1ability.l~ that the oxygen consumption and metabolism of different organs vary. Uniform hypothermia, and thus uniform-decreased oxygen consumption, is not possible with methods employed today.
AORN Journal, October 1976, Vol24, No 4
767
Hypothermia in the homeotherm is not physiologic, and therefore many changes occur. For example, in the cardiovascular system, the heart rate slows to 20 to 30 beats per minute a t 64.4F (18 C).14 This is accompanied by a decrease in cardiac output, a lowering of blood pressure, and a decrease in resistance of the coronary arteries. Total vascular resistance is increased in the liver, kidneys, and especially in the pulmonary bed. Thus, hepatic blood flow and function are depressed. Other effects of hypothermia on the vascular system include venospasm and an alteration in the critical closing pressure of the capillary bed.I5 Arrhythmias such as atrial fibrillation and first degree heart block occur frequently a t temperatures around 86 F (30 C). More serious arrhythmias such as ventricular fibrillation have been linked with hypokalemia. Serum potassium and plasma insulin levels decrease during cooling. The administration of potassium during hypothermia not only increases potassium and insulin levels but also decreases the risk of ventricular arrhythmias.I6 Hypothermia also increases the viscosity of blood proportionately as the body temperature is decreasing. This is thought to occur when sodium moves into the cell and exchanges with potassium. This process results in hemoconcentration due to water from the intravascular space being shifted into the interstitual and intercellular compartment^.'^ Hemoconcentration can contribute to the capillary sludging and microcirculatory stasis believed to occur in hypothermia. The effects of hypothermia upon the other organs and systems are numerous. Although some of the effects of hypothermia are nonphysiologic, the advantages of decreased cellular metabolism and oxygen consumption
seem to outweigh the disadvantages. There continues to be much speculation in the field of cardiac surgery as to the efficacy of hypothermia. The present holds many unanswered questions regarding hypothermia, yet, one can envision many possibilities and applications of hypothermia for the future. Notes 1. Arthur Guyton, Textbook of Medical Physiology, 4th ed (Philadelphia: W B Saunders, 1971) 831. 2. Ewald Selkurt, Physiology, 3rd ed (Boston: Little, Brown & Co, 1971) 651. 3. Frank E South, et al, Hibernation and Hypothermia, Perspectives and Challenges (Amsterdam: Elsevier Publishing Co, 1972) VII. 4. Ernil Blair, Clinical Hypothermia, 1st ed (New York: McGraw-Hill Book Co, 1964) 1. 5. W G Bigelow, et al, "Hypothermia. Its posible role in cardiac surgery: An investigation of factors governing survival of dogs at low ternperatures," Annals of Surgery 132 (November 1950) 849-866. 6. Henry Swan, et al, "Surgery by direct vision in the open heart during hypotherrnia," Journal of the American Medical Association 153 (November 1953) 1081-1085. 7. Pierre M Galletti, A Brecher Gerhard, Heart Lung Bypass (New York: Grune & Stratton, 1962) 294. 8. Guyton, Textbook of Medical Physiology, 839. 9. Blair, Clinical Hypothermia, 33. 10. /bid, 15. 11. S Subrarnanian, et al, "Surface induced deep hypothermia in cardiac surgery," Journal of Pediatric Surgery 6 (October 1971) 612-617. 12. Yukihiko Nose, Manual on Artificial Organs: The Oxygenator Volume 11 (St Louis: C V Mosby Co, 1973) 256. 13. Blair, Clinical Hypothermia, 25. 14. David Baurn, et al, "Metabolic aspects of deep surgical hypothermia in infancy," Pediatrics 42 (July 1968) 93-105. 15. B Eiseman, F C Spencer, "Effect ot hypotherrnia on the flow characteristics of blood," Surgery 52 (September 1962) 532-544. 16. T C K Brown, et al, "Biochemical changes during surface cooling for deep hypothermia in open heart surgery," Journal of Thoracic and Car-. diovascular Surgery 65 (March 1973) 402-408. 17. Eisernan, Spencer, "Effect of hypothermia."
AORN Journal, October 1976, V o l 2 4 , No 4
769
