Gyromagnetic ratio

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Gyromagnetic_ratio

In physics, the gyromagnetic ratio (also sometimes known as the magnetogyric ratio in other disciplines) of a particle or system is the ratio of its magnetic moment to its angular momentum, and it is often denoted by the symbol γ, gamma. Its SI unit is the radian per second per tesla (rad⋅s⁻¹⋅T⁻¹) or, equivalently, the coulomb per kilogram (C⋅kg⁻¹).

The term "gyromagnetic ratio" is often used as a synonym for a different but closely related quantity, the g-factor. The g-factor only differs from the gyromagnetic ratio in being dimensionless.

For a classical rotating body

Consider a nonconductive charged body rotating about an axis of symmetry. According to the laws of classical physics, it has both a magnetic dipole moment due to the movement of charge and an angular momentum due to the movement of mass arising from its rotation. It can be shown that as long as its charge and mass density and flow are distributed identically and rotationally symmetric, its gyromagnetic ratio is

${\displaystyle \gamma =\frac{q}{2m}}$

where ${\displaystyle q}$ is its charge and ${\displaystyle m}$ is its mass.

The derivation of this relation is as follows. It suffices to demonstrate this for an infinitesimally narrow circular ring within the body, as the general result then follows from an integration. Suppose the ring has radius r, area A = πr², mass m, charge q, and angular momentum L = mvr. Then the magnitude of the magnetic dipole moment is

${\displaystyle \mu =IA=\frac{qv}{2\pi r}\pi r^2=\frac{q}{2m}mvr=\frac{q}{2m}L.}$

For an isolated electron

An isolated electron has an angular momentum and a magnetic moment resulting from its spin. While an electron's spin is sometimes visualized as a literal rotation about an axis, it cannot be attributed to mass distributed identically to the charge. The above classical relation does not hold, giving the wrong result by the absolute value of the electron's g-factor, which is denoted g_e:

$${\displaystyle {\gamma }_{\mathrm{e}}=\frac{-e}{2m_{\mathrm{e}}}|g_{\mathrm{e}}|=\frac{g_{\mathrm{e}}{\mu }_{\mathrm{B}}}{\hslash },}$$

where μ_B is the Bohr magneton.

The gyromagnetic ratio due to electron spin is twice that due to the orbiting of an electron.

In the framework of relativistic quantum mechanics,

$${\displaystyle g_{\mathrm{e}}=-2\left(1+\frac{\alpha }{2\pi }+\cdots \right),}$$

where ${\displaystyle \alpha }$ is the fine-structure constant. Here the small corrections to the relativistic result g = 2 come from the quantum field theory calculations of the anomalous magnetic dipole moment. The electron g-factor is known to twelve decimal places by measuring the electron magnetic moment in a one-electron cyclotron:

$${\displaystyle g_{\mathrm{e}}=-2.00231930436118(27).}$$

The electron gyromagnetic ratio is

$${\displaystyle {\gamma }_{\mathrm{e}}=-1.76085963023(53)\times {10}^{11}\mathrm{r}\mathrm{a}\mathrm{d}\cdot {\mathrm{s}}^{-1}\cdot {\mathrm{T}}^{-1}}$$

$${\displaystyle \frac{{\gamma }_{\mathrm{e}}}{2\pi }=-28024.9514242(85)\mathrm{M}\mathrm{H}\mathrm{z}\cdot {\mathrm{T}}^{-1}.}$$

The electron g-factor and γ are in excellent agreement with theory; see Precision tests of QED for details.

Gyromagnetic factor not as a consequence of relativity

Since a gyromagnetic factor equal to 2 follows from the Dirac's equation it is a frequent misconception to think that a g-factor 2 is a consequence of relativity; it is not. The factor 2 can be obtained from the linearization of both the Schrödinger equation and the relativistic Klein–Gordon equation (which leads to Dirac's). In both cases a 4-spinor is obtained and for both linearizations the g-factor is found to be equal to 2; Therefore, the factor 2 is a consequence of the minimal coupling and of the fact of having the same order of derivatives for space and time.

Physical spin 1/2 particles which cannot be described by the linear gauged Dirac equation satisfy the gauged Klein–Gordon equation extended by the g e/4 σ^μν F_μν term according to,

${\displaystyle \left[\left({\mathrm{\partial }}^{\mu }u+ie{A}^{\mu }\right)\left({\mathrm{\partial }}_{\mu }+ie{A}_{\mu }\right)+g\frac{e}{4}{\sigma }^{\mu \nu }{F}_{\mu \nu }+m^2\right]\psi =0,g\ne 2.}$

Here, 1/2σ^μν and F^μν stand for the Lorentz group generators in the Dirac space, and the electromagnetic tensor respectively, while A^μ is the electromagnetic four-potential. An example for such a particle, is the spin 1/2 companion to spin 3/2 in the D^(½,1) ⊕ D^(1,½) representation space of the Lorentz group. This particle has been shown to be characterized by g = −+2/3 and consequently to behave as a truly quadratic fermion.

For a nucleus

The sign of the gyromagnetic ratio, γ, determines the sense of precession. While the magnetic moments (the black arrows) are oriented the same for both cases of γ, the precession is in opposite directions. Spin and magnetic moment are in the same direction for γ > 0 (as for protons).

Protons, neutrons, and many nuclei carry nuclear spin, which gives rise to a gyromagnetic ratio as above. The ratio is conventionally written in terms of the proton mass and charge, even for neutrons and for other nuclei, for the sake of simplicity and consistency. The formula is:

${\displaystyle {\gamma }_{\text{n}}=\frac{e}{2m_{\text{p}}}{g}_{\mathrm{n}}=g_{\mathrm{n}}\frac{{\mu }_{\mathrm{N}}}{\hslash },}$

where ${\displaystyle {\mu }_{\mathrm{N}}}$ is the nuclear magneton, and ${\displaystyle g_{\mathrm{n}}}$ is the g-factor of the nucleon or nucleus in question. The ratio ${\displaystyle \frac{{\gamma }_n}{2\pi g_{\mathrm{n}}},}$ equal to ${\displaystyle {\mu }_{\mathrm{N}}/h}$, is 7.622593285(47) MHz/T.

The gyromagnetic ratio of a nucleus plays a role in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). These procedures rely on the fact that bulk magnetization due to nuclear spins precess in a magnetic field at a rate called the Larmor frequency, which is simply the product of the gyromagnetic ratio with the magnetic field strength. With this phenomenon, the sign of γ determines the sense (clockwise vs counterclockwise) of precession.

Most common nuclei such as ¹H and ¹³C have positive gyromagnetic ratios. Approximate values for some common nuclei are given in the table below.

Nucleus	${\displaystyle {\gamma }_n}$ (106 rad⋅s−1⋅T−1)	${\displaystyle {\gamma }_n/(2\pi )}$ (MHz⋅T−1)
1H	267.52218744(11)	42.577478518(18)
1H (in H2O)	267.5153151(29)	42.57638474(46)
2H	41.065	6.536
3H	285.3508	45.415
3He	−203.7894569(24)	−32.43409942(38)
7Li	103.962	16.546
13C	67.2828	10.7084
14N	19.331	3.077
15N	−27.116	−4.316
17O	−36.264	−5.772
19F	251.815	40.078
23Na	70.761	11.262
27Al	69.763	11.103
29Si	−53.190	−8.465
31P	108.291	17.235
57Fe	8.681	1.382
63Cu	71.118	11.319
67Zn	16.767	2.669
129Xe	−73.997	−11.777

Larmor precession

Main article: Larmor precession

Any free system with a constant gyromagnetic ratio, such as a rigid system of charges, a nucleus, or an electron, when placed in an external magnetic field B (measured in teslas) that is not aligned with its magnetic moment, will precess at a frequency f (measured in hertz), that is proportional to the external field:

${\displaystyle f=\frac{\gamma }{2\pi }B.}$

For this reason, values of γ/ 2π , in units of hertz per tesla (Hz/T), are often quoted instead of γ.

Heuristic derivation

The derivation of this relation is as follows: First we must prove that the torque resulting from subjecting a magnetic moment ${\displaystyle \mathbf{m}}$ to a magnetic field ${\displaystyle \mathbf{B}}$ is ${\displaystyle \mathbf{T}=\mathbf{m}\times \mathbf{B}.}$ The identity of the functional form of the stationary electric and magnetic fields has led to defining the magnitude of the magnetic dipole moment equally well as ${\displaystyle m=I\pi r^2}$, or in the following way, imitating the moment p of an electric dipole: The magnetic dipole can be represented by a needle of a compass with fictitious magnetic charges ${\displaystyle \pm q_{\mathrm{m}}}$ on the two poles and vector distance between the poles ${\displaystyle \mathbf{d}}$ under the influence of the magnetic field of earth ${\displaystyle \mathbf{B}.}$ By classical mechanics the torque on this needle is ${\displaystyle \mathbf{T}=q_{\mathrm{m}}(\mathbf{d}\times \mathbf{B}).}$ But as previously stated ${\displaystyle q_{\mathrm{m}}\mathbf{d}=I\pi r^2\hat{\mathbf{d}}=\mathbf{m},}$ so the desired formula comes up. ${\displaystyle \hat{\mathbf{d}}}$ is the unit distance vector.

The model of the spinning electron we use in the derivation has an evident analogy with a gyroscope. For any rotating body the rate of change of the angular momentum ${\displaystyle \mathbf{J}}$ equals the applied torque ${\displaystyle \mathbf{T}}$:

${\displaystyle \frac{d\mathbf{J}}{dt}=\mathbf{T}.}$

Note as an example the precession of a gyroscope. The earth's gravitational attraction applies a force or torque to the gyroscope in the vertical direction, and the angular momentum vector along the axis of the gyroscope rotates slowly about a vertical line through the pivot. In the place of the gyroscope imagine a sphere spinning around the axis and with its center on the pivot of the gyroscope, and along the axis of the gyroscope two oppositely directed vectors both originated in the center of the sphere, upwards ${\displaystyle \mathbf{J}}$ and downwards ${\displaystyle \mathbf{m}.}$ Replace the gravity with a magnetic flux density ${\displaystyle \mathbf{B}.}$

${\displaystyle \frac{\mathrm{d}\mathbf{J}}{\mathrm{d}t}}$ represents the linear velocity of the pike of the arrow ${\displaystyle \mathbf{J}}$ along a circle whose radius is ${\displaystyle J\sin \varphi ,}$ where ${\displaystyle \varphi }$ is the angle between ${\displaystyle \mathbf{J}}$ and the vertical. Hence the angular velocity of the rotation of the spin is

${\displaystyle \omega =2\pi f=\frac{1}{J\sin \varphi }\left|\frac{\mathrm{d}\mathbf{J}}{\mathrm{d}\mathrm{t}}\right|=\frac{\left|\mathbf{T}\right|}{J\sin \varphi }=\frac{\left|\mathbf{m}\times \mathbf{B}\right|}{J\sin \varphi }=\frac{mB\sin \varphi }{J\sin \varphi }=\frac{mB}{J}=\gamma B.}$

Consequently, ${\displaystyle f=\frac{\gamma }{2\pi }B.\text{q.e.d.}}$

This relationship also explains an apparent contradiction between the two equivalent terms, gyromagnetic ratio versus magnetogyric ratio: whereas it is a ratio of a magnetic property (i.e. dipole moment) to a gyric (rotational, from Greek: γύρος, "turn") property (i.e. angular momentum), it is also, at the same time, a ratio between the angular precession frequency (another gyric property) ω = 2πf and the magnetic field.

The angular precession frequency has an important physical meaning: It is the angular cyclotron frequency, the resonance frequency of an ionized plasma being under the influence of a static finite magnetic field, when we superimpose a high frequency electromagnetic field.

Hypothermia

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Hypothermia

 

Hypothermia
During Napoleon Bonaparte's retreat from Russia in the winter of 1812, many troops died from hypothermia.
Specialty	Critical care medicine
Symptoms	Mild: Shivering, mental confusion Moderate: No shivering, increased confusion Severe: Paradoxical undressing, muscle rigidity, unconsciousness, cardiac arrest Profound: No obvious vital signs
Complications	Afterdrop
Duration	Until the body temperature is raised to near-normal levels
Types	Primary hypothermia: caused by exposure to a cold environment Secondary hypothermia: caused by an underlying pathology that prevents the body from generating enough core heat.
Causes	Mainly exposure to cold weather and cold water immersion
Risk factors	Alcohol intoxication, homelessness, low blood sugar, anorexia, advanced age, injuries and blood loss
Diagnostic method	Based on symptoms or body temperature below 35.0 °C (95.0 °F)
Prevention	Wearing adequate clothes for the weather, staying warm and dry
Treatment	Mild: Warm drinks, warm clothing, physical activity Moderate: Heating blankets, warmed intravenous fluid Severe: Extracorporeal membrane oxygenation, cardiopulmonary bypass
Medication	Sugar
Frequency	frequent in winter months, from November to March
Deaths	1,500 per year (US)

Hypothermia is defined as a body core temperature below 35.0 °C (95.0 °F) in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe hypothermia, there may be hallucinations and paradoxical undressing, in which a person removes their clothing, as well as an increased risk of the heart stopping.

Hypothermia has two main types of causes. It classically occurs from exposure to cold weather and cold water immersion. It may also occur from any condition that decreases heat production or increases heat loss. Commonly, this includes alcohol intoxication but may also include low blood sugar, anorexia and advanced age. Body temperature is usually maintained near a constant level of 36.5–37.5 °C (97.7–99.5 °F) through thermoregulation. Efforts to increase body temperature involve shivering, increased voluntary activity, and putting on warmer clothing. Hypothermia may be diagnosed based on either a person's symptoms in the presence of risk factors or by measuring a person's core temperature.

The treatment of mild hypothermia involves warm drinks, warm clothing, and voluntary physical activity. In those with moderate hypothermia, heating blankets and warmed intravenous fluids are recommended. People with moderate or severe hypothermia should be moved gently. In severe hypothermia, extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass may be useful. In those without a pulse, cardiopulmonary resuscitation (CPR) is indicated along with the above measures. Rewarming is typically continued until a person's temperature is greater than 32 °C (90 °F). If there is no improvement at this point or the blood potassium level is greater than 12 millimoles per litre at any time, resuscitation may be discontinued.

Hypothermia is the cause of at least 1,500 deaths a year in the United States. It is more common in older people and males. One of the lowest documented body temperatures from which someone with accidental hypothermia has survived is 12.7 °C (54.9 °F) in a 2-year-old boy from Poland named Adam. Survival after more than six hours of CPR has been described. In individuals for whom ECMO or bypass is used, survival is around 50%. Deaths due to hypothermia have played an important role in many wars.

The term is from Greek ῠ̔πο (ypo), meaning "under", and θέρμη (thérmē), meaning "heat". The opposite of hypothermia is hyperthermia, an increased body temperature due to failed thermoregulation.

Classification

Hypothermia classification

Swiss system	Symptoms	By degree	Temperature
Stage 1	Awake and shivering	Mild	32–35 °C (89.6–95.0 °F)
Stage 2	Drowsy and not shivering	Moderate	28–32 °C (82.4–89.6 °F)
Stage 3	Unconscious, not shivering	Severe	20–28 °C (68.0–82.4 °F)
Stage 4	No vital signs	Profound	<20 °C (68.0 °F)

Hypothermia is often defined as any body temperature below 35.0 °C (95.0 °F). With this method it is divided into degrees of severity based on the core temperature.

Another classification system, the Swiss staging system, divides hypothermia based on the presenting symptoms which is preferred when it is not possible to determine an accurate core temperature.

Other cold-related injuries that can be present either alone or in combination with hypothermia include:

• Chilblains: condition caused by repeated exposure of skin to temperatures just above freezing. The cold causes damage to small blood vessels in the skin. This damage is permanent and the redness and itching will return with additional exposure. The redness and itching typically occurs on cheeks, ears, fingers, and toes.
• Frostbite: the freezing and destruction of tissue, which happens below the freezing point of water
• Frostnip: a superficial cooling of tissues without cellular destruction
• Trench foot or immersion foot: a condition caused by repetitive exposure to water at non-freezing temperatures

The normal human body temperature is often stated as 36.5–37.5 °C (97.7–99.5 °F). Hyperthermia and fever, are defined as a temperature of greater than 37.5–38.3 °C (99.5–100.9 °F).

Signs and symptoms

Signs and symptoms vary depending on the degree of hypothermia, and may be divided by the three stages of severity. People with hypothermia may appear pale and feel cold to touch. Infants with hypothermia may feel cold when touched, with bright red skin and an unusual lack of energy.

Behavioural changes such as impaired judgement, impaired sense of time and place, unusual aggression and numbness can be observed in individuals with hypothermia, they can also deny their condition and refuse any help. A hypothermic person can be euphoric and hallucinating.

Cold stress refers to a near-normal body temperature with low skin temperature, signs include shivering. Cold stress is caused by cold exposure and it can lead to hypothermia and frostbite if not treated.

Mild

Symptoms of mild hypothermia may be vague, with sympathetic nervous system excitation (shivering, high blood pressure, fast heart rate, fast respiratory rate, and contraction of blood vessels). These are all physiological responses to preserve heat. Increased urine production due to cold, mental confusion, and liver dysfunction may also be present. Hyperglycemia may be present, as glucose consumption by cells and insulin secretion both decrease, and tissue sensitivity to insulin may be blunted. Sympathetic activation also releases glucose from the liver. In many cases, however, especially in people with alcoholic intoxication, hypoglycemia appears to be a more common cause. Hypoglycemia is also found in many people with hypothermia, as hypothermia may be a result of hypoglycemia.

Moderate

As hypothermia progresses, symptoms include: mental status changes such as amnesia, confusion, slurred speech, decreased reflexes, and loss of fine motor skills.

Severe

As the temperature decreases, further physiological systems falter and heart rate, respiratory rate, and blood pressure all decrease. This results in an expected heart rate in the 30s at a temperature of 28 °C (82 °F).

There is often cold, inflamed skin, hallucinations, lack of reflexes, fixed dilated pupils, low blood pressure, pulmonary edema, and shivering is often absent. Pulse and respiration rates decrease significantly, but fast heart rates (ventricular tachycardia, atrial fibrillation) can also occur. Atrial fibrillation is not typically a concern in and of itself.

Paradoxical undressing

Twenty to fifty percent of hypothermia deaths are associated with paradoxical undressing. This typically occurs during moderate and severe hypothermia, as the person becomes disoriented, confused, and combative. They may begin discarding their clothing, which, in turn, increases the rate of heat loss.

Rescuers who are trained in mountain survival techniques are taught to expect this; however, people who die from hypothermia in urban environments who are found in an undressed state are sometimes incorrectly assumed to have been subjected to sexual assault.

One explanation for the effect is a cold-induced malfunction of the hypothalamus, the part of the brain that regulates body temperature. Another explanation is that the muscles contracting peripheral blood vessels become exhausted (known as a loss of vasomotor tone) and relax, leading to a sudden surge of blood (and heat) to the extremities, causing the person to feel overheated.

Terminal burrowing

An apparent self-protective behaviour, known as "terminal burrowing", or "hide-and-die syndrome", occurs in the final stages of hypothermia. Those affected will enter small, enclosed spaces, such as underneath beds or behind wardrobes. It is often associated with paradoxical undressing. Researchers in Germany claim this is "obviously an autonomous process of the brain stem, which is triggered in the final state of hypothermia and produces a primitive and burrowing-like behavior of protection, as seen in hibernating mammals". This happens mostly in cases where temperature drops slowly.

Causes

The rate of death from hypothermia is strongly related to age in the United States

Hypothermia usually occurs from exposure to low temperatures, and is frequently complicated by alcohol consumption. Any condition that decreases heat production, increases heat loss, or impairs thermoregulation, however, may contribute. Thus, hypothermia risk factors include: substance use disorders (including alcohol use disorder), homelessness, any condition that affects judgment (such as hypoglycemia), the extremes of age, poor clothing, chronic medical conditions (such as hypothyroidism and sepsis), and living in a cold environment. Hypothermia occurs frequently in major trauma, and is also observed in severe cases of anorexia nervosa. Hypothermia is also associated with worse outcomes in people with sepsis. While most people with sepsis develop fevers (elevated body temperature), some develop hypothermia.

In urban areas, hypothermia frequently occurs with chronic cold exposure, such as in cases of homelessness, as well as with immersion accidents involving drugs, alcohol or mental illness. While studies have shown that people experiencing homelessness are at risk of premature death from hypothermia, the true incidence of hypothermia-related deaths in this population is difficult to determine. In more rural environments, the incidence of hypothermia is higher among people with significant comorbidities and less able to move independently. With rising interest in wilderness exploration, and outdoor and water sports, the incidence of hypothermia secondary to accidental exposure may become more frequent in the general population.

Alcohol

Alcohol consumption increases the risk of hypothermia in two ways: vasodilation and temperature controlling systems in the brain. Vasodilation increases blood flow to the skin, resulting in heat being lost to the environment. This produces the effect of feeling warm, when one is actually losing heat. Alcohol also affects the temperature-regulating system in the brain, decreasing the body's ability to shiver and use energy that would normally aid the body in generating heat. The overall effects of alcohol lead to a decrease in body temperature and a decreased ability to generate body heat in response to cold environments. Alcohol is a common risk factor for death due to hypothermia. Between 33% and 73% of hypothermia cases are complicated by alcohol.

Water immersion

Two American marines participating in an immersion hypothermia exercise

Hypothermia continues to be a major limitation to swimming or diving in cold water. The reduction in finger dexterity due to pain or numbness decreases general safety and work capacity, which consequently increases the risk of other injuries.

Other factors predisposing to immersion hypothermia include dehydration, inadequate rewarming between repetitive dives, starting a dive while wearing cold, wet dry suit undergarments, sweating with work, inadequate thermal insulation (for example, thin dry suit undergarment), and poor physical conditioning.

Heat is lost much more quickly in water than in air. Thus, water temperatures that would be quite reasonable as outdoor air temperatures can lead to hypothermia in survivors, although this is not usually the direct clinical cause of death for those who are not rescued. A water temperature of 10 °C (50 °F) can lead to death in as little as one hour, and water temperatures near freezing can cause death in as little as 15 minutes. During the sinking of the Titanic, most people who entered the −2 °C (28 °F) water died in 15–30 minutes.

The actual cause of death in cold water is usually the bodily reactions to heat loss and to freezing water, rather than hypothermia (loss of core temperature) itself. For example, plunged into freezing seas, around 20% of victims die within two minutes from cold shock (uncontrolled rapid breathing, and gasping, causing water inhalation, massive increase in blood pressure and cardiac strain leading to cardiac arrest, and panic); another 50% die within 15–30 minutes from cold incapacitation: inability to use or control limbs and hands for swimming or gripping, as the body "protectively" shuts down the peripheral muscles of the limbs to protect its core. Exhaustion and unconsciousness cause drowning, claiming the rest within a similar time.

Pathophysiology

Temperature classification

Core (rectal, esophageal, etc.) Hypothermia <35.0 °C (95.0 °F) Normal 36.5–37.5 °C (97.7–99.5 °F) Fever >37.5 or 38.3 °C (99.5 or 100.9 °F) Hyperthermia >37.5 or 38.3 °C (99.5 or 100.9 °F) Hyperpyrexia >40.0 or 41.0 °C (104.0 or 105.8 °F)

Note: The difference between fever and hyperthermia is the underlying mechanism. Different sources have different cut-offs for fever, hyperthermia and hyperpyrexia.

Heat is primarily generated in muscle tissue, including the heart, and in the liver, while it is lost through the skin (90%) and lungs (10%). Heat production may be increased two- to four-fold through muscle contractions (i.e. exercise and shivering). The rate of heat loss is determined, as with any object, by convection, conduction, and radiation. The rates of these can be affected by body mass index, body surface area to volume ratios, clothing and other environmental conditions.

Many changes to physiology occur as body temperatures decrease. These occur in the cardiovascular system leading to the Osborn J wave and other dysrhythmias, decreased central nervous system electrical activity, cold diuresis, and non-cardiogenic pulmonary edema.

Research has shown that glomerular filtration rates (GFR) decrease as a result of hypothermia. In essence, hypothermia increases preglomerular vasoconstriction, thus decreasing both renal blood flow (RBF) and GFR.

Diagnosis

Atrial fibrillation and Osborn J waves in a person with hypothermia. Note what could be mistaken for ST elevation.

Accurate determination of core temperature often requires a special low temperature thermometer, as most clinical thermometers do not measure accurately below 34.4 °C (93.9 °F). A low temperature thermometer can be placed in the rectum, esophagus or bladder. Esophageal measurements are the most accurate and are recommended once a person is intubated. Other methods of measurement such as in the mouth, under the arm, or using an infrared ear thermometer are often not accurate.

As a hypothermic person's heart rate may be very slow, prolonged feeling for a pulse could be required before detecting. In 2005, the American Heart Association recommended at least 30–45 seconds to verify the absence of a pulse before initiating CPR. Others recommend a 60-second check.

The classical ECG finding of hypothermia is the Osborn J wave. Also, ventricular fibrillation frequently occurs below 28 °C (82 °F) and asystole below 20 °C (68 °F). The Osborn J may look very similar to those of an acute ST elevation myocardial infarction. Thrombolysis as a reaction to the presence of Osborn J waves is not indicated, as it would only worsen the underlying coagulopathy caused by hypothermia.

Prevention

Staying dry and wearing proper clothing help to prevent hypothermia. Synthetic and wool fabrics are superior to cotton as they provide better insulation when wet and dry. Some synthetic fabrics, such as polypropylene and polyester, are used in clothing designed to wick perspiration away from the body, such as liner socks and moisture-wicking undergarments. Clothing should be loose fitting, as tight clothing reduces the circulation of warm blood. In planning outdoor activity, prepare appropriately for possible cold weather. Those who drink alcohol before or during outdoor activity should ensure at least one sober person is present responsible for safety.

Covering the head is effective, but no more effective than covering any other part of the body. While common folklore says that people lose most of their heat through their heads, heat loss from the head is no more significant than that from other uncovered parts of the body. However, heat loss from the head is significant in infants, whose head is larger relative to the rest of the body than in adults. Several studies have shown that for uncovered infants, lined hats significantly reduce heat loss and thermal stress. Children have a larger surface area per unit mass, and other things being equal should have one more layer of clothing than adults in similar conditions, and the time they spend in cold environments should be limited. However children are often more active than adults, and may generate more heat. In both adults and children, overexertion causes sweating and thus increases heat loss.

Building a shelter can aid survival where there is danger of death from exposure. Shelters can be of many different types, metal can conduct heat away from the occupants and is sometimes best avoided. The shelter should not be too big so body warmth stays near the occupants. Good ventilation is essential especially if a fire will be lit in the shelter. Fires should be put out before the occupants sleep to prevent carbon monoxide poisoning. People caught in very cold, snowy conditions can build an igloo or snow cave to shelter.

The United States Coast Guard promotes using life vests to protect against hypothermia through the 50/50/50 rule: If someone is in 50 °F (10 °C) water for 50 minutes, they have a 50 percent better chance of survival if they are wearing a life jacket. A heat escape lessening position can be used to increase survival in cold water.

Babies should sleep at 16–20 °C (61–68 °F) and housebound people should be checked regularly to make sure the temperature of the home is at least 18 °C (64 °F).

Management

Degree	Rewarming technique
Mild (stage 1)	Passive rewarming
Moderate (stage 2)	Active external rewarming
Severe (stage 3 and 4)	Active internal rewarming

Aggressiveness of treatment is matched to the degree of hypothermia. Treatment ranges from noninvasive, passive external warming to active external rewarming, to active core rewarming. In severe cases resuscitation begins with simultaneous removal from the cold environment and management of the airway, breathing, and circulation. Rapid rewarming is then commenced. Moving the person as little and as gently as possible is recommended as aggressive handling may increase risks of a dysrhythmia.

Hypoglycemia is a frequent complication and needs to be tested for and treated. Intravenous thiamine and glucose is often recommended, as many causes of hypothermia are complicated by Wernicke's encephalopathy.

The UK National Health Service advises against putting a person in a hot bath, massaging their arms and legs, using a heating pad, or giving them alcohol. These measures can cause a rapid fall in blood pressure and potential cardiac arrest.

Rewarming

Rewarming can be done with a number of methods including passive external rewarming, active external rewarming, and active internal rewarming. Passive external rewarming involves the use of a person's own ability to generate heat by providing properly insulated dry clothing and moving to a warm environment. Passive external rewarming is recommended for those with mild hypothermia.

Active external rewarming involves applying warming devices externally, such as a heating blanket. These may function by warmed forced air (Bair Hugger is a commonly used device), chemical reactions, or electricity. In wilderness environments, hypothermia may be helped by placing hot water bottles in both armpits and in the groin. Active external rewarming is recommended for moderate hypothermia. Active core rewarming involves the use of intravenous warmed fluids, irrigation of body cavities with warmed fluids (the chest or abdomen), use of warm humidified inhaled air, or use of extracorporeal rewarming such as via a heart lung machine or extracorporeal membrane oxygenation (ECMO). Extracorporeal rewarming is the fastest method for those with severe hypothermia. When severe hypothermia has led to cardiac arrest, effective extracorporeal warming results in survival with normal mental function about 50% of the time. Chest irrigation is recommended if bypass or ECMO is not possible.

Rewarming shock (or rewarming collapse) is a sudden drop in blood pressure in combination with a low cardiac output which may occur during active treatment of a severely hypothermic person. There was a theoretical concern that external rewarming rather than internal rewarming may increase the risk. These concerns were partly believed to be due to afterdrop, a situation detected during laboratory experiments where there is a continued decrease in core temperature after rewarming has been started. Recent studies have not supported these concerns, and problems are not found with active external rewarming.

Fluids

For people who are alert and able to swallow, drinking warm (not hot) sweetened liquids can help raise the temperature. General medical consensus advises against alcohol and caffeinated drinks. As most hypothermic people are moderately dehydrated due to cold-induced diuresis, warmed intravenous fluids to a temperature of 38–45 °C (100–113 °F) are often recommended.

Cardiac arrest

In those without signs of life, cardiopulmonary resuscitation (CPR) should be continued during active rewarming. For ventricular fibrillation or ventricular tachycardia, a single defibrillation should be attempted. However, people with severe hypothermia may not respond to pacing or defibrillation. It is not known if further defibrillation should be withheld until the core temperature reaches 30 °C (86 °F). In Europe, epinephrine is not recommended until the person's core temperature reaches 30 °C (86 °F), while the American Heart Association recommends up to three doses of epinephrine before a core temperature of 30 °C (86 °F) is reached. Once a temperature of 30 °C (86 °F) has been reached, normal ACLS protocols should be followed.

Prognosis

It is usually recommended not to declare a person dead until their body is warmed to a near normal body temperature of greater than 32 °C (90 °F), since extreme hypothermia can suppress heart and brain function. This is summarized in the common saying "You're not dead until you're warm and dead." Exceptions include if there are obvious fatal injuries or the chest is frozen so that it cannot be compressed. If a person was buried in an avalanche for more than 35 minutes and is found with a mouth packed full of snow without a pulse, stopping early may also be reasonable. This is also the case if a person's blood potassium is greater than 12 mmol/L.

Those who are stiff with pupils that do not move may survive if treated aggressively. Survival with good function also occasionally occurs even after the need for hours of CPR. Children who have near-drowning accidents in water near 0 °C (32 °F) can occasionally be revived, even over an hour after losing consciousness. The cold water lowers the metabolism, allowing the brain to withstand a much longer period of hypoxia. While survival is possible, mortality from severe or profound hypothermia remains high despite optimal treatment. Studies estimate mortality at between 38% and 75%.

In those who have hypothermia due to another underlying health problem, when death occurs it is frequently from that underlying health problem.

Epidemiology

Between 1995 and 2004 in the United States, an average of 1560 cold-related emergency department visits occurred per year and in the years 1999 to 2004, an average of 647 people died per year due to hypothermia. Of deaths reported between 1999 and 2002 in the US, 49% of those affected were 65 years or older and two-thirds were male. Most deaths were not work related (63%) and 23% of affected people were at home. Hypothermia was most common during the autumn and winter months of October through March. In the United Kingdom, an estimated 300 deaths per year are due to hypothermia, whereas the annual incidence of hypothermia-related deaths in Canada is 8000.

History

The armies of Napoleon retreat from Russia in 1812.

Snow-storm: Hannibal and His Army Crossing the Alps, J. M. W. Turner

Hypothermia has played a major role in the success or failure of many military campaigns, from Hannibal's loss of nearly half his men in the Second Punic War (218 B.C.) to the near destruction of Napoleon's armies in Russia in 1812. Men wandered around confused by hypothermia, some lost consciousness and died, others shivered, later developed torpor, and tended to sleep. Others too weak to walk fell on their knees; some stayed that way for some time resisting death. The pulse of some was weak and hard to detect; others groaned; yet others had eyes open and wild with quiet delirium. Deaths from hypothermia in Russian regions continued through the first and second world wars, especially in the Battle of Stalingrad.

Civilian examples of deaths caused by hypothermia occurred during the sinkings of the RMS Titanic and RMS Lusitania, and more recently of the MS Estonia.

Antarctic explorers developed hypothermia; Ernest Shackleton and his team measured body temperatures "below 94.2°, which spells death at home", though this probably referred to oral temperatures rather than core temperature and corresponded to mild hypothermia. One of Scott's team, Atkinson, became confused through hypothermia.

Nazi human experimentation during World War II amounting to medical torture included hypothermia experiments, which killed many victims. There were 360 to 400 experiments and 280 to 300 subjects, indicating some had more than one experiment performed on them. Various methods of rewarming were attempted: "One assistant later testified that some victims were thrown into boiling water for rewarming".

Medical use

Main article: Targeted temperature management

Various degrees of hypothermia may be deliberately induced in medicine for purposes of treatment of brain injury, or lowering metabolism so that total brain ischemia can be tolerated for a short time. Deep hypothermic circulatory arrest is a medical technique in which the brain is cooled as low as 10 °C, which allows the heart to be stopped and blood pressure to be lowered to zero, for the treatment of aneurysms and other circulatory problems that do not tolerate arterial pressure or blood flow. The time limit for this technique, as also for accidental arrest in ice water (which internal temperatures may drop to as low as 15 °C), is about one hour.

Other animals

Hypothermia can happen in most mammals in cold weather and can be fatal. Baby mammals such as kittens are unable to regulate their body temperatures and have a risk of hypothermia if they are not kept warm by their mothers.

Many animals other than humans often induce hypothermia during hibernation or torpor.

Water bears (Tardigrade), microscopic multicellular organisms, can survive freezing at low temperatures by replacing most of their internal water with the sugar trehalose, preventing the crystallization that otherwise damages cell membranes.

Cryotherapy

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Cryotherapy 

Cryotherapy, sometimes known as cold therapy, is the local or general use of low temperatures in medical therapy. Cryotherapy may be used to treat a variety of tissue lesions. The most prominent use of the term refers to the surgical treatment, specifically known as cryosurgery or cryoablation. Cryosurgery is the application of extremely low temperatures to destroy abnormal or diseased tissue and is used most commonly to treat skin conditions.

Cryotherapy is used in an effort to relieve muscle pain, sprains and swelling after soft tissue damage or surgery. For decades, it has been commonly used to accelerate recovery in athletes after exercise. Cryotherapy decreases the temperature of tissue surface to minimize hypoxic cell death, edema accumulation, and muscle spasms, all of which ultimately alleviate discomfort and inflammation. It can be a range of treatments from the application of ice packs or immersion in ice baths (generally known as cold therapy), to the use of cold chambers.

Cryotherapy chamber

Partial-Body Cryotherapy chamber by Vacuactivus

There are different types of cryochambers, each with different mechanisms of action and uses. The Partial-Body Cryotherapy (PBC) makes use of nitrogen to decrease the temperature. This cryochamber is an individual, tube-shaped enclosure that covers a person's body with an open-top to keep the head at room temperature.

The second cryochamber is called the whole body cryotherapy (WBC) and makes use of electricity to reduce the temperature inside the chamber. In contrast to the first, the user fully enters the electrically operated chamber.

This is a specific type of low-temperature treatment used to reduce inflammation and painful effects.

Cryotherapy was developed in the 1970s by Japanese rheumatologist Toshima Yamaguchi and introduced to Europe, US and Australia in the 1980s and 1990s. Both cryochambers decrease the skin temperature, but WBC reaches lower temperatures than PBC and might be considered more effective.

Mechanism of action

When the body is vulnerable to extreme cooling, the blood vessels are narrowed and make less blood flow to the areas of swelling. Once outside the cryogenic chamber, the vessels expand, and an increased presence of anti-inflammatory proteins (IL-10) is established in the blood. Cryotherapy chamber involves exposing individuals to freezing dry air (below −100 °C) for 2 to 4 minutes.

Main uses

Proponents say that cryotherapy may reduce pain and inflammation, help with mental disorders, support exercise recovery performance and improves joint function. Cryotherapy chambers belong to the group of equipment associated with sports rehabilitation and wellness.

• Reducing the symptoms of eczema

Cryosurgery

Medical cryotherapy gun

Main article: Cryosurgery

Cryosurgery is the application of extreme cold to destroy abnormal or diseased tissue. The application of ultra-cold liquid causes damage to the treated tissue due to intracellular ice formation. The degree of damage depends upon the minimum temperature achieved and the rate of cooling. Cryosurgery is used to treat a number of diseases and disorders, most especially skin conditions like warts, moles, skin tags and solar keratoses. Liquid nitrogen is usually used to freeze the tissues at the cellular level. The procedure is used often as it is relatively easy and quick, can be done in the doctors surgery, and is deemed quite low risk. If a cancerous lesion is suspected then excision rather than cryosurgery may be deemed more appropriate. Contraindications to the use of cryosurgery include but are not limited to; using it over a neoplasm, someone with conditions that are worsened by exposure to cold (i.e. Raynaud’s, urticaria), and poor circulation or no sensation in the area. There are some precautions to using cryosurgery. They include someone with collagen vascular disease, dark-skinned individuals (due to high risk of hypopigmentation), and impaired sensation at the area being treated.

Ice pack therapy

Ice pack therapy is a treatment of cold temperatures to an injured area of the body. Though the therapy is extensively used, and it is agreed that it alleviates symptoms, testing has produced conflicting results about its efficacy and possibility of producing undesirable results.

An ice pack is placed over an injured area and is intended to absorb heat of a closed traumatic or Edematous injury by using conduction to transfer thermal energy. The physiologic effects of cold application include immediate vasoconstriction with reflexive vasodilation, decreased local metabolism and enzymatic activity, and decreased oxygen demand. Cold decreases muscle spindle fiber activity and slows nerve conduction velocity; therefore, it is often used to decrease spasticity and muscle guarding. It is commonly used to alleviate the pain of minor injuries, as well as decrease muscle soreness. The use of ice packs in treatment decreases the blood flow most rapidly at the beginning of the cooling period, this occurs as a result of vasoconstriction, the initial reflex sympathetic activity. Although the use of cryotherapy has been shown to aid in muscle recovery, some studies have highlighted that the degree of muscle cooling in humans is not significant enough to produce a considerable effect on muscle recovery. Based on previous research comparing human and animal models, the insufficient degree of cooling is due to larger limb size, more adipose tissue, and a higher muscle diameter in humans.

Ice is not commonly used prior to rehabilitation or performance because of its known adverse effects to performance such as decreased myotatic reflex and force production, as well as a decrease in balance immediately following ice pack therapy for 20 minutes. However, if ice pack therapy is applied for less than 10 minutes, performance can occur without detrimental effects. If the ice pack is removed at this time, athletes are sent back to training or competition directly with no decrease in performance. Ice has also been shown to possibly slow and impair muscle protein synthesis and repair in recreational athletes. This is especially true for cold water immersion, but equivalent controlled studies have not been done to see if the same effects hold true for ice packs. Regardless, ice has been shown in studies to inhibit the uptake of dietary protein post-muscle conditioning exercise.

Although there are many positive effects of cryotherapy in athletes' short-term recovery, in recent years, there has been much controversy regarding whether cryotherapy is actually beneficial or may be causing the opposite effect. While inflammation that occurs post-injury or from a damaging exercise may be detrimental to secondary tissue, it is beneficial for the structural and functional repair of the damaged tissue. Therefore, some researchers are now recommending that ice not be used so as not to delay the natural healing process following an injury. The original RICE (rest, ice, compression, elevation) method was rescinded because the inflammatory response is necessary for the healing process, and this practice may delay healing instead of facilitating it. Animal studies also show that a disrupted inflammatory stage of healing may lead to impaired tissue repair and redundant collagen synthesis.

There is a study that concludes that cryotherapy has a positive impact on the short-term recovery of athletes. Cryotherapy helped manage muscle soreness and facilitate recovery within the first 24 hours following a sport-related activity. Athletes who use cryotherapy within the first 24 hours to alleviate pain recovered at a faster rate than athletes who did not use cryotherapy after their sport-related activity.

Cryotherapy following total knee replacement

Post-surgical management following total knee replacement surgery may include cryotherapy with the goal of helping with pain management and blood loss following surgery. Cryotherapy is applied using ice, cold water, or gel packs, sometimes in specialized devices that surround the skin and surgical site (but keeps the surgical site clean). Evidence from clinical trials regarding the effectiveness of cryotherapy is weak and because of this, the use of cryotherapy may not be justified. Weak evidence indicates that cryotherapy used postoperatively may be associated with a small decrease in blood loss and pain following the surgery. No clinically significant improvements in range of motion have been reported. There are not many side effects or adverse effects reported with this intervention.

Cold spray anesthetics

Main article: Freeze spray

In addition to their use in cryosurgery, several types of cold aerosol sprays are used for short-term pain relief. Unlike other cold modalities, it doesn’t produce similar physiological effects due to the fact it decreases skin temperature, not muscle temperature. It reflexively inhibits underling muscle by using evaporation to cool the area. Ordinary spray cans containing tetrafluoroethane, dimethyl ether, or similar substances, are used to numb the skin prior to or possibly in place of local anesthetic injections, and prior to other needles, small incisions, sutures, and so on. Other products containing chloroethane are used to ease sports injuries, similar to ice pack therapy. Cold aerosol spray could also be used to relieve trigger points and improve range of motion. After applying the cold spray, one can stretch the muscle and will then have improved mobility and a decrease in pain immediately. However, this is only a short-term effect as the pain relief and improved range of motion can wear off within a minute.

Whole body cryotherapy

Cryotherapy patients during preparation of treatment of c. 3 minutes

An increasing amount of research is done on the effects of whole-body cryotherapy (WBC) on exercise, beauty, and health. Research is often inconsistent because the usage of the different types of cryo-chambers, and different treatment periods. However, it becomes increasingly clear that WBC has a positive effect on muscle soreness and decreases the recovery time after exercise. Some older papers show inconsistencies in the effects.

Cryotherapy is also increasingly used as a non-drug treatment against rheumatoid arthritis, stress, anxiety, or chronic pain, multiple sclerosis and fibromyalgia. Studies for these, and other diseases (Alzheimer's, migraines), are ongoing although more evidence becomes available on the positive effects of Whole Body Cryotherapy. The FDA points out that the effects of Whole Body Cryotherapy lacks evidence and should be researched more.

Cryotherapy treatment involves exposing individuals to extremely cold dry air (below −100 °C) for two to four minutes. Yet, three to four minute exposure to WBC is different from a one to two minute exposure. It is more beneficial to expose for a shorter amount of time to increase therapeutic benefits. Longer durations have negative effects on thermal sensation, tissue oxygenation, and blood volume. Also, the amount of sessions is an important part of the healing process. Just one session will not exhibit significant effects. A minimum of twenty sessions is required. Thirty sessions is recommended for optimal effects though. To achieve the subzero temperatures required for WBC, two methods are typically used: liquid nitrogen and refrigerated cold air. During these exposures, individuals wear minimal clothing, which usually consists of shorts for males, and shorts and a crop top for females. Gloves, a woollen headband covering the ears, and a nose and mouth mask, in addition to dry shoes and socks, are commonly worn to reduce the risk of cold-related injury. The first WBC chamber was built in Japan in the late 1970s, introduced to Europe in the 1980s, and has been used in the US and Australia in the past decade.

Adverse effects

Reviews of whole-body cryotherapy have called for research studies to implement active surveillance of adverse events, which are suspected of being underreported. If the cold temperatures are produced by evaporating liquid nitrogen, there is the risk of inert gas asphyxiation as well as frostbite. However, these risks are irrelevant in the electronically operated chambers.

Contraindications

Contraindications include patients with cardiovascular disease, arterial hypertension, acute infectious diseases, seizures, cold allergy, and some psychiatric disorders.

Partial body

Partial body cryotherapy (PBC) devices also exist. If the cold temperatures are produced by evaporating liquid nitrogen, there is the risk of inert gas asphyxiation as well as frostbite.