现在的位置: 首页时讯速递, 进展交流>正文
[MEDSCAPE]: 低体温与局部冷损伤(Part 3/5)
2018年02月02日 时讯速递, 进展交流 暂无评论

Hypothermia and Localized Cold Injury

Joseph U Becker, MD | January 3, 2018 | Contributor Information

The term hypothermia generally refers to inappropriate body temperature, usually resulting from deficient thermoregulation. It is commonly defined as a core body temperature lower than 35°C.[1,2] Hypothermia may be either accidental,[3] as in inadvertent overexposure to low temperatures, or intentional, as in therapeutic hypothermia after cardiac arrest. It may be either primary or secondary: Primary hypothermia typically results directly from exposure to external environmental conditions (eg, time spent outdoors in cold weather or immersion in cold water), whereas secondary hypothermia usually results from a medical illness characterized by impaired regulation of internal body heat (eg, sepsis, hypothyroidism). Hypothermia is also classified in terms of severity, as mild, moderate, or severe.

Shown are a group of medical students learning about hypothermia from a Coast Guard Aviation Survival Technician.

Image courtesy of Wikimedia Commons | Ed Yourdon.

Heat loss occurs via several different mechanisms. Under dry conditions, most heat loss (~50-60%) occurs via radiation.[4]Conduction and convection also contribute to heat loss, and respiration and evaporation account for the remainder of lost heat. In immersion situations, conduction causes the majority of heat loss because the thermal conductivity of water is far greater than that of air. Under normal physiologic conditions, the hypothalamus controls heat regulation both through conservation (vasoconstriction and stimulation of behavioral responses) and through production (elevation of the metabolic rate and activation of skeletal muscle via shivering). In hyperthermia, the hypothalamus also regulates temperature by stimulating perspiration and vasodilation.

Risk factors for hypothermia include alcohol use, homelessness, mental illness and older age.

Image courtesy of Dreamstime | James Steidl.

Measuring core temperature with appropriately calibrated, low temperature–reading thermometers is important for diagnosing hypothermia. Core body temperature is thought to be most accurately reflected by measurements from a distal esophageal thermometer probe (in an intubated patient). Rectal or bladder temperature measurements may provide a reasonable estimate of core temperature in patients with mild-moderate hypothermia.

Mild hypothermia is commonly defined as a core temperature between 32°C and 35°C. The physiologic effects of mild hypothermia are typically mild, though they may be influenced by the presence of comorbid conditions such as traumatic injury, intoxication, or diabetes. The body still maintains the ability to produce heat via shivering and vasoconstriction. In mild hypothermia, the hypothalamus attempts to reestablish thermal homeostasis by stimulating metabolic rate and shivering. This causes tachycardia and increased cardiac output. In moderate hypothermia (core temperature 28°-32°C), shivering ceases, and heat production falls. Moreover, metabolic processes slow, and the characteristic electrocardiographic (ECG) changes of hypothermia begin to be seen (see subsequent slide). Patients with moderate hypothermia will require active rewarming as they are not producing heat. Individuals with severe hypothermia (core temperature <28°C) are at risk for dysrhythmia, altered mental status, and eventually death.

Information in table from Durrer B et al.[6]

The preferred method for staging hypothermia is the Swiss Hypothermia Staging System (shown), which incorporates clinical condition in addition to core temperature measurements.[5]

However, it is likely that there exists significant overlap between the stages and core temperatures do not necessarily correlate with clinical findings at every stage.

Image courtesy of Wikimedia Commons | Brian McNeil / Dreamstime | Brianne Rankosky.

A homeless 63-year-old man is brought into the emergency department (ED) after he was observed mumbling to himself in the street. The paramedics report that the patient was found with several empty bottles of beer and that he smells of alcohol. The patient is awake but is not able to respond to the examiner or to contribute in any way to the history or physical examination. His core temperature is determined to be 30°C (rectal temperature).

Which of the following statements about this patient's condition is true?

  1. The patient's altered mental state should be assumed to be due to alcohol intoxication
  2. Alcohol increases the risk of hypothermia by dysregulating physiologic responses to cold and by impairing mental state
  3. Wet clothing can serve as an insulating layer and contribute to maintenance of body temperature
  4. Rewarming should be done slowly to prevent the so-called afterdrop phenomenon and the associated initial decrease in temperature

Image courtesy of Wikimedia Commons.

Answer: B. Alcohol increases the risk of hypothermia by dysregulating physiologic responses to cold and by impairing mental state.

Alcohol consumption, by impairing vasoregulation and preventing vasoconstriction, leads to an increased risk of hypothermia. Alcohol also impairs judgment and consciousness and thus can cause drinkers to act inappropriately and to fail to seek shelter or protective clothing. Answer A is incorrect. Whereas it is often true that patients are impaired as a result of alcohol consumption, assuming this to be the case, particularly early in management, can lead to costly errors in situations where there is another, potentially more sinister contributor to altered mental state. Answer C is incorrect as wet clothing actually serves to draw heat away from the body more rapidly. Answer D is incorrect because the afterdrop phenomenon, (see slide 14), should not limit efforts to rewarm hypothermic patients.

Image courtesy of Medscape.

Hypothermia has dramatic effects on many physiologic processes and organ systems including the cardiovascular system. Hypothermia decreases the depolarization of pacemaker cells of the heart, leading initially to bradycardia. Bradycardia caused by hypothermia will not respond to the usual pharmacologic interventions (eg, atropine), and rewarming is required in order to restore normal electrophysiologic function. As the effects of hypothermia progress, there are further decreases in mean arterial pressure (MAP) and cardiac output. These decreases lead to a sequence of characteristic ECG changes (shown). As hypothermia progresses sinus bradycardia may progress to atrial fibrillation (AF), possibly with a slow ventricular response, and eventually to ventricular fibrillation (VF) and ultimately asystole. Osborne J waves (sudden positive deflection after the QRS, at the J point) are often noted in hypothermic patients, with the amplitude of the Osborne wave proportional to the degree of hypothermia. Osborne waves are not specific to hypothermia and may be seen in other medical conditions.

Image courtesy of Flickr | Taco Witte (background) / Dreamstime (figure).

Central nervous system (CNS) metabolism decreases linearly with temperature. The consequence of this decrease is that at temperatures below 33°C, cerebral metabolism is markedly reduced. As such, patients may manifest confusion initially, which can progress to delirium and coma. The decrease in cerebral metabolism associated with moderate-to-severe hypothermia may be neuroprotective in some cases, and many case reports exist of patients with severe hypothermia and/or hypothermic cardiac arrest making complete or near complete neurologic recovery.[7]

Image courtesy of Flickr | Taco Witte (Background) / Dreamstime | Alila07 (Foreground).

Initially, the body's response to immersion in a cold environment is to vasoconstrict peripheral tissues, such as the extremities, shunting blood flow to the core. As a result, the kidneys experience a sudden increase in circulating volume. Hypothermia also impairs renal concentration of the urine. Both of these factors contribute to an early diuresis of dilute urine in hypothermia patients which may contribute to dehydration and electrolyte abnormalities. Another important physiologic effect of hypothermia is that it causes a shift in the oxyhemoglobin disassociation curve to the left, reducing oxygen release to the tissues. Despite reduced oxygen demand by tissues with lowered metabolic rates, overall tissue oxygen content may be low as a result.

Hypothermia may cause significant disturbances in blood chemistry. Hypothermic patients will commonly manifest metabolic acidosis (frequently from lactic acidosis). Further, hypothermia will typically cause the hematocrit to rise. Serum potassium has been noted to decline in hypothermic patients, but many patients may manifest hyperkalemia if comorbid conditions such as renal failure or rhabdomyolysis are present.[9] Coagulation times are typically elevated, due to hypothermic inhibition of normal coagulation pathways.[10]

The Arterial Blood Gas (ABG) as well changes with hypothermia with pH, PaCOand PaOall decline with hypothermia. ABG analyzers warm blood to 37°C for comparison to normal values, also calibrated at 37°C. Warmed ABGs from hypothermic patients will show a higher PaO2, higher PaCO2 and a lower pH than what is actually present in the patient's blood in vivo. The best approach to using the ABG in hypothermic patients is to use uncorrected values recognizing the effects of hypothermia on the sample and the acid/base and gas partial pressures in the patient bloodstream.[11]

Image courtesy of Wikimedia Commons.

A 39-year-old woman is brought to the ED, having been pulled from the bottom of a frozen lake after her fishing boat capsized. Her core temperature is 27°C (distal esophageal probe).

What combination of clinical factors might you expect to note on your initial physical examination?

  1. Confusion, bradycardia, and shivering
  2. Coma, VF, and vasoconstriction
  3. Lethargy, AF, and shivering
  4. Coma, VF, and hypopnea

Image courtesy of Wikimedia Commons.

Answer: D: Coma, VF, and hypopnea.

This patient has severe hypothermia associated with cold-water immersion. At this temperature, physiologic responses to hypothermia have been overwhelmed, and the patient is no longer able to generate a shivering response. Furthermore, at this temperature, cerebral metabolism is impaired and reduced, leading to either delirium or coma. Cardiac arrhythmias in hypothermia progress from bradycardia to AF to VF and finally to asystole, one might expect VF in a patient with this degree of hypothermia. Finally, hypothermia reduces the overall metabolic rate and, thus, tissue carbon dioxide production which results in hypopnea.

Image courtesy of Wikimedia Commons | Anna Frodesiak.

Various rewarming methods may be considered, depending on the severity of hypothermia and the patient's clinical condition. Passive rewarming may be appropriate for patients with mild-to-moderate hypothermia who still are capable of generating their own heat; this involves using blankets, removing wet clothing, and taking the patient out of the cold environment. Patients with mild-to-moderate hypothermia who cannot generate their own heat (those who are not shivering and have deranged vasoregulation) may require active rewarming. Active rewarming involves returning heat to the patient. This may be accomplished via different approaches, including intravenous (IV) infusion of warmed (40°C) fluids and the use of forced-hot-air blankets (eg, Bair-Hugger). For patients with moderate-to-severe hypothermia, inhalation rewarming with warmed, humidified air delivered via a ventilator may be effective, as well as gastrointestinal (GI), pleural, or peritoneal lavage with warmed (40°C) fluids. For patients with severe hypothermia, hemodialysis (shown) or cardiac bypass is appropriate and has been shown to be effective.[12]

Image courtesy of Sawamoto K, Tanno K, Takeyama Y, et al. Int J Emerg Med. 2012 Feb 2;5(1):9. [Open access.] PMID: 22300441 PMCID: PMC3287102.

In the process of rewarming, the following effects may be noted and should be taken into account:

  • Afterdrop phenomenon – As the periphery is warmed (via forced air blankets or other methods that unselectively rewarm the core and the periphery), cold blood is returned to the core, contributing to a potential initial decrease in core body temperature. As such, in moderate to severe hypothermia, rewarming efforts should emphasize core rewarming over the periphery.
  • Volume depletion – Peripheral vasodilation from peripheral rewarming, as well as relative volume depletion from renal cold diuresis, may lead to initial hypovolemia and possibly shock
  • Rewarming acidosis – This effect, analogous to the afterdrop phenomenon, occurs as lactic acid is returned to the core from the periphery during peripheral rewarming

Shown is a chest CT (left) and a chest radiograph (right) from a 57-year-old male who was found in 12°C seawater with a core body temperature of 22.0°C (rectal). The images show suspected aspiration of a large amount of seawater.

Image courtesy of Medscape | Sam Shlomo Spaeth.

It is important to note that patients with cardiovascular instability should undergo rapid core rewarming. Current data suggest that improved outcomes are achieved when patients with severe hypothermia (or the clinical effects thereof) are brought to centers capable of cardiac bypass (shown). Hypothermic patients may not respond to normal resuscitation medications (eg, atropine or epinephrine); accordingly, resuscitation efforts should focus on high quality cardiopulmonary resuscitation (CPR) and the provision of rapid rewarming. Similarly, defibrillation is not thought to be effective at temperatures below 30°C.

Image courtesy of Wikimedia Commons | Stevage.

A 38-year-old man is brought to the ED, having been rescued from a mountaintop after becoming lost during a ski trip. His core temperature is 31°C.

Which of the following is the most appropriate next step in management?

  1. Avoid perturbing the patient with intubation, CPR, or IV lines so as to avoid precipitating VF
  2. Administer warmed fluids, provide the patient with blankets, and resuscitate as normal
  3. Administer warmed fluids, provide active rewarming with forced-air rewarming blankets, and provide resuscitation while attempting to limit unnecessary manipulation of the patient
  4. Do not resuscitate; a patient with a temperature in this range is not salvageable

给我留言

您必须 [ 登录 ] 才能发表留言!

×
腾讯微博