Thermoregulation, by definition, is a mechanism by which mammals maintain body temperature by tightly controlled self-regulation, no matter the temperature of their surroundings. Temperature regulation is a type of homeostasis, which is a process that biological systems use to preserve a stable internal state to survive. Ectotherms are animals that depend on their external environment for their body heat, and endotherms are animals that use thermoregulation to maintain a somewhat consistent internal body temperature to survive, even when their external environment changes. Humans and other mammals and birds are endotherms. Human beings have a normal core, or internal, temperature of around 37 degrees Celsius, which is equivalent to around 98.6 degrees Fahrenheit. Core temperature is most accurately measured via rectal probe thermometer. This is the temperature at which the human body’s systems work together at their optimum, which is the reason the body has such tightly regulated mechanisms. Thermoregulation is crucial to human life. Without thermoregulation, the human body would not be able to adequately function and, inevitably, will expire.
When the body’s ability to thermoregulate becomes hindered and is left untreated, organ failure is imminent. Blood flow will be reduced, leading to ischemia, and, ultimately, multiple organ failures.
Viral illness or another infectious disease can cause a person to develop a fever, and the body no longer has that same core temperature of 37 degrees Celsius. This is because when the body experiences an infection from invading pathogens, it tries to fight back by releasing pyrogens such as cytokines, prostaglandins, and thromboxane—all of which increase the body’s temperature. By releasing these pyrogens, the foreign pathogens are not able to breed. This allows for antibodies to develop and enzymes to be activated to fight the infection further.
The brain, or more specifically the hypothalamus, controls thermoregulation. If the hypothalamus senses external temperatures growing too hot or too cold, it will automatically send signals to the skin, glands, muscles, and organs. For example, when the body is in a very hot external environment, or simply undergoing high activity levels such as exercise, it’s temperature will rise, causing the hypothalamus to send signals to the cells of the skin that produce sweat. Sweating is the body's approach to cooling itself down. As the body’s temperature rises, sweat is expelled, the muscles relax, and body hair lies flat against the skin. These are all ways to release heat and therefore lower the temperature of the body. In contrast, when the body experiences a cold environment, the skeletal muscles tense up leading to the shivering reflex, and the arrector pili muscles, a type of smooth muscle, raise the bodily hair follicles where they are attached. These processes, in turn, create warmth and trap heat, respectively.
Multiple organs and body systems are affected when thermoregulation is not working correctly. During heat illness as a result of improper thermoregulation, the following organs and systems are impaired. Notice that many of these issues cause or are influenced by the other issues.
When body temperatures severely decrease in hypothermia, the body’s systems are negatively affected. The cardiovascular system experiences dysrhythmias such as the Osborn, or J, wave on EKG. The central nervous system's (CNS) electrical activity is noticeably diminished, noncardiogenic pulmonary edema will occur, as well as cold diuresis, and decreases in glomerular filtration rate (GFR) and renal blood flow (RBF). Basically, hypothermia causes pre-glomerular vasoconstriction, which leads to decreased GFR and RBF.
Slight changes in core body temperature occur every day, depending upon variables such as circadian rhythm and menses; but otherwise, the temperature is tightly controlled. When a person is unable to regulate his or her body temperature, various pathologies may occur. The human body has four different methods for keeping itself at its core temperature: vaporization, radiation, convection, and conduction. To keep the body functioning, it must be at its ideal temperature, and for this to happen, physical factors must be sufficient. This includes having enough intravascular volume and cardiovascular function; the body must be able to transport the rising internal heat to its surface for release. The reason that elderly people are at higher risk for disorders of thermoregulation is that they, as a whole, have less intravascular volume and decreased cardiac function.
Thermoregulation has three mechanisms: afferent sensing, central control, and efferent responses. There are receptors for both heat and cold throughout the human body. Afferent sensing works through these receptors to determine if the body is experiencing either too hot or too cold of a stimulus. Next, the hypothalamus is the central controller of thermoregulation. Lastly, efferent responses are carried out primarily by the body’s behavioral reactions to fluctuations in body temperature. For example, if a person is feeling too warm, the normal response is to remove an outer article of clothing. If a person is feeling too cold, they choose to wear more layers of clothing. Efferent responses also consist of automatic responses by the body to protect itself from extreme changes in temperature, such as sweating, vasodilation, vasoconstriction, and shivering.
The thermoregulatory sweat test (TST) is a specific clinical test that is used to diagnose certain conditions that cause abnormal temperature regulation and defects in sweat production in the body. It measures a patient’s ability to produce sweat in a controlled, heated and humidified environment and assesses the patient's central and autonomic nervous systems to determine if the thermoregulatory centers are working correctly.
To perform the thermoregulatory sweat test, the patient is placed in a chamber that slowly rises in temperature. Before the chamber is heated, the patient is coated with a special kind of indicator powder that will change in color when sweat is produced. This powder, when changing color, will be useful in visualizing which skin is sweating versus not sweating. Results of the patient’s sweat pattern will be documented by digital photography, and abnormal TST patterns can indicate if there is dysfunction in the autonomic nervous system. Certain differentials can be made depending on the type of sweat pattern found from the TST (along with history and clinical presentation) including hyperhidrosis, small fiber and autonomic neuropathies, multiple system atrophy, Parkinson disease with autonomic dysfunction, and pure autonomic failure.
When external environments are exceedingly warm, or a person is engaging in an excessive amount of physical activity, the heat is produced inside his or her body is typically transported to the blood. The blood then carries the heat through numerous capillaries that are located directly under the skin. Because the blood is near the surface, it can cool the person down. This cooled blood can then be transported back through the body to prevent the body temperature from becoming too high. Sweat is also a means by which the body cools itself down; it is created by glands to carry out evaporation at the topmost skin layer, the epidermis, to release heat. This describes vaporization, one of the four mechanisms used to maintain core body temperature. Radiation is when the heat that is released from the body’s surface is moved into the surrounding air; convection occurs when cooler air surrounds the body’s surface, and conduction comes into play when a person is either immersed in cold water or uses an ice pack—their internally generated heat is transferred to the cold water or the ice pack. This is another reason why it is very important to stay hydrated in the heat or during physical activity—not only to maintain adequate intravascular fluid volume, but also to aid in conduction processes that cool the body down. When cold fluids are ingested, the heat is released into the fluid and excreted out of the body as sweat or urine.
While infection can cause the body’s temperature to rise internally, a number of mechanisms can cause body temperature to rise externally. As previously discussed, a number of diseases with dysfunctional thermoregulatory mechanisms prove to be clinically significant, including small fiber and autonomic neuropathies, radiculopathies, and central autonomic disorders such as multiple system atrophy, Parkinson disease with autonomic dysfunction, and pure autonomic failure. Decreased cardiac function is also a notable risk factor for abnormal thermoregulatory function. Patients who are in externally hot environments do not have the ability to use their heart as efficiently to pump blood to their bodies’ surface. With the loss of contractility, the blood is unable to be cooled, and therefore they are at risk of having heat-related illnesses. Even patients with the diagnosis of hypertension have an increased risk of these types of illnesses because some medications used to treat hypertension decrease the pumping ability of the heart, thereby decreasing the heated blood’s means of getting cool at the body surface.
When patients are dehydrated, they contain a decreased amount of intravascular volume. This in itself is another risk factor for dysfunctional thermoregulation. If there are not enough fluids in the body, there is not enough blood. When there is not enough blood, the body cannot transport the internally made heat to the body’s surface to be cooled. Heat will remain retained, blood will become thicker, and the heart becomes strained.
In contrast, hypothermia is defined as low internal body temperature, or a temperature less than 35 degrees Celsius (95 degrees Fahrenheit). It is usually caused by too much heat loss from cold weather exposure or cold water immersion. During cold water immersion, there is a reflex known as the diving reflex, which causes vasoconstriction in the visceral muscles. The body does this as a protective mechanism to keep a person’s essential organs, like their heart and brain, supplied with blood. The body also does this to prevent the brain from hypoxia, since metabolic demand will have decreased.
There are two different types of hypothermia: primary and secondary. Primary hypothermia is when the cold environment is the direct pathology, and secondary hypothermia is when a patient’s illness is what causes his hypothermia. Conduction, convection, and radiation also come into play with hypothermia; this is how the rate of heat loss is determined. Hypothermia decelerates all physiologic roles include metabolic rate, mental awareness, nerve conduction, neuromuscular reaction times, and both the cardiovascular and respiratory systems. As previously mentioned, the vasoconstriction caused by hypothermia induces renal dysfunction and, eventually, cold diuresis due to the decreased levels of ADH. These decreased levels of antidiuretic hormone mean that the patient’s urine will be very dilute, the definition of cold diuresis and another outcome of hypothermia. Because of the vasoconstriction, the patient may not know he is hypovolemic, which may lead to cardiac arrest or abrupt shock as the patient’s vasculature is dilated while being rewarmed, a condition referred to as the rewarming collapse.
When a person who cannot sustain his or her core body temperature presents with heat illness, multiple organ support is indicated. It is imperative to be comprehensive in their care and utilize any and every cooling method available to sustain their body. Support must be provided to their respiratory, hepatic, renal, and circulatory systems. Giving fresh frozen plasma and blood platelets as needed is also recommended. Previous animal studies have also shown that administration of thrombomodulin can be helpful because it demonstrates anti-DIC and anti-inflammatory mechanisms.
Malignant hyperthermia is a rare type of event, but one that is life-threatening. It can occur when a susceptible patient is given certain anesthetic agents, notably halogenated ones or ones that are agents that depolarize neuromuscularly. Predisposed patients who are given such anesthetics either have abnormal ryanodine receptors (RYR-1) or have other myopathies such as Duchenne muscular dystrophy, central core disease, neuroleptic malignant syndrome, and King-Denborough syndrome. Ryanodine receptors are located in skeletal muscles, which typically contain calcium channels and sarcoplasmic reticulum; these are what control the release of calcium. However, in malignant hyperthermia, calcium release is not regulated, and this leads to a continued release, which leads to an increased frequency of muscle metabolism. The clinical features first would present as muscle rigidity and an overwhelmingly high heart rate and end-tidal carbon dioxide. The person’s temperature will increasingly elevate, and soon after, muscle tightening and shortening will occur as well as metabolic acidosis.
The rapid treatment of malignant hyperthermia is of the utmost importance to prevent fatality. Once malignant hyperthermia is suspected, dantrolene must be given intravenously. This will prevent that uncontrolled release of calcium as described above. It is also important to rapidly cool the patient, give 100% oxygen, and regulate metabolic acidosis.
Serotonin syndrome and neuroleptic malignant syndrome are other disorders of temperature regulation. They both are due to adverse drug reactions, and they both present with clinical hyperthermia. It is important to distinguish them from one another. Neuroleptic malignant syndrome (NMS) continues over many days and is depicted by slow movements or the complete loss of voluntary movements. Serotonin syndrome is much more rapid, happening over a timespan of hours and presenting with fast movements such as tremor, muscular spasms, and hyperactive reflexes. These two reactions must be managed rapidly to avoid organ failure, especially if the patient’s temperature goes past 40.5 degrees Celsius. There are both physical and pharmaceutical ways to cool the affected patients. Physical methods include providing cool towels, ice packs, cooling blanket systems, and intravenous infusion of fluids. Pharmaceuticals include sedatives and antipyretics such as paracetamol and NSAIDs. (Note that in critically ill patients, it is important to consider individual patient’s renal function and gastric function before administering NSAIDs.)
Hyperthyroidism is another disorder of thermoregulation. In this endocrinological disease, the core temperature is raised in the body because the basal metabolic rate is raised. Essentially, with this disorder, all of the body’s metabolic pathways are accelerated. Heat production is increased as a result, and both oxygen consumption and ATP turnover are increased. The thyroid is overactive and causes a person to feel too warm.
Certain patients who have endured traumatic brain injuries (TBI) or suffered from cardiac arrests may do well with therapeutic hypothermia. In these patients with TBI and post-cardiac arrest, it is very dangerous for them to have an elevation of their core temperatures. High temperatures in these patients are associated with higher mortality rates and slower recovery, especially neurologically. Thus, hypothermia in post-cardiac arrest patients and perinatal hypoxic patients is used to prevent neuronal injury. This is because therapeutic hypothermia (TH) lowers the body’s demand for metabolic oxygen. With this mechanism, neurons can be protected before they are injured.
When TH is recommended, it should proceed very rapidly. Ice packs and intravenous administration of cold fluids are imperative to keep the patient cool. Cooling blanket systems or cooling suits may be utilized as well as cooling helmets and caps. For post-cardiac arrest patients, intranasal cooling using nasal probes also may be utilized.
In contrast, some diseases that can cause decreased heat production, or hypothermia include endocrinological diseases such as diabetes, hypothyroidism, hypoadrenalism, and hypopituitarism. Those who are most at risk for hypothermia are elderly patients, trauma patients, those who are mentally ill, those who are abusing alcohol, drugs, or on other types of medication, and, lastly, those with low socio-economic status. Ordinarily, people who get hypothermia have an underlying issue—either from a disease or surgery.
When a patient with one of these diseases presents with hypothermia, it is imperative to treat the underlying disease to effectively treat the hypothermia. This includes treatments with pharmaceuticals including triiodothyronine and steroids. Other causes of hypothermia from decreased metabolic rates are people who are very malnourished, those who suffer from burns, and also those with hypoglycemia.
Hypothermia, like hyperthermia, affects all of the human body’s systems. When the core temperature drops below 30 degrees Celsius, the heart responds with arrhythmias. The body may be hypovolemic, hypokalemic, and hypomagnesemic as a result of their hypothermia; therefore, it is important to keep the patient hydrated and to manage their electrolyte disturbances.
The reason that patients with traumatic brain injury are likely to have impaired thermoregulation is that the hypothalamus regulates the core body temperature. When this essential body part is injured, the body is unable to control how it regulates the body’s heat. Other problems in the CNS that affect thermoregulation by the same mechanism can include tumors in the CNS, spinal cord injuries, intracranial hemorrhage, and diseases such as Parkinson, Wernicke encephalopathy, and multiple sclerosis.
Patients who are on the extreme spectrums of age (such as infants and elderly persons) are at higher risk for disorders of thermoregulation and exhibit these features more readily when sick. The very young and the very old cannot increase their metabolic rates, and this can provoke them into hypothermia since they do not have the shivering reflux or much muscle mass. Changes that occur as one ages include those affecting vasomotor sweating function, skeletal muscle response, and temperature perception. Elderly persons have lower than normal internal body temperatures and decreased immunity, so when they have an infection, they may not get the normal pyretic response. Instead, they may present with hypothermia caused by a septic infection. In fact, studies have shown that the core temperatures of elderly patients with sepsis within their first 24 hours of presentation are a huge predictor of their mortality. This is the reason that elderly patients who are septic have a higher mortality rate than younger patients who are septic.
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