Hypothermia Flashcards
Hypothermia causes severe
Early and aggressive treatment can decrease morbidity and mortality in the critically ill patient.
____ is a common complication resulting from peripheral vasodilation when the periphery is warmed before the core
cardiovascular, respiratory, electrolyte, nervous system, acid-base, and coagulation abnormalities.
Rewarming shock
hypothermia <37 (98.6)- individual or combined effects of
excessive heat loss
decreased heat production
or a disruption of the normal thermoregulatory functions permit the core (vital organ) body temperature (CBT) to drop below species-specific physiologic parameters
Primary hypothermia, or “accidental” hypothermia
secondary hypothermia is a result of
subnormal temperature low environmental temperatures
disease, trauma, surgery, or drug-induced alteration in heat production and thermoregulation
Hypothermia traditionally has been classified as “mild,” “moderate,” or “severe” based purely on..
this classification is simple, it does not capture the functional changes that characterize the differing levels of symptoms not directly related to a specific CBT. Therefore some have proposed classifying the severity of hypothermia based on the clinical consequences at each stage, not strictly on the CBT
CBT
32°-37° C Mild Shivering, ataxia, vasoconstriction
28°-32° C Moderate Decreased level of consciousness, hypotension, ± shivering
<28° C Severe Loss of shivering, dysrhythmias, profound central nervous system deficits
severe hypothermia is marked by complete loss of the thermoregulatory system, an inability to _____ comatose states and susceptibility to ____
shiver (<28 - 82F) 32 = 89 +/- shiver
ventricular fibrillation
main thermostat of the body is the hypothalamus, with temperature changes sensed by the:
Secondary temperature sensors are located within the skin and
deep body tissues; namely, the:
preoptic and anterior hypothalamic nuclei
Temperature is sensed by the _____ family of ion channels, which are activated at distinct _____
peripheral input from the skin travels to the spinal cord or _____dorsal horn for passage to the _____ and thalamus
thermal information is then output to the ______
The behavioral and autonomic responses are linked to the ___ inputs in the brainstem
system is so precise that, in humans, changes in CBT of ____ degree Celsius result in ANS responses
transient receptor potential family of ion channels
temperature thresholds
trigeminal dorsal horn
midbrain
sensory cortex, producing the sensation of hot and cold
reticular
sweating to shivering within a span of 0.6° C
why is accurate CBT important
dramatic core/peripheral temperature gradients
Heat production is secondary to
most metabolically active systems:
depends on the metabolic rate of the body
As heat is produced, it distributes rapidly to core tissues but more slowly to the peripheral tissues via
chemical metabolism of energy substrates within cells.
brain, truncal organs, and active muscles
convective heat transfer through blood
conductive heat transfer through adjacent tissues
normal thermoregulatory response is to produce and retain heat, and thereby maintain the CBT. This is accomplished by
initial autonomic response causes specialized anastomoses, linking arterioles with veins, to open as the CBT nears 37° C, which prevents
Dogs have additional mechanism in their footpads, in which the veins are intimately associated with the arteries, so that
shivering, which is typically noted at a degree
behavioral -huddling and curling
physiologic - piloerection, peripheral vasoconstriction, and shivering
heat loss to the distal extremities
blood is heated before it is returned to the core
lower than the vasoconstrictive response
shivering, which is typically noted at a degree
important because it can be metabolically
inefficient and much of the heat can be lost to the environment
increase the metabolic rate by a factor of 4-10X
energy substrate for shivering is usually CHO
in glycogen-depleted patients, lipid and protein reserves need to be used.
.:.diminished in cachectic, old, and the very young
Convection
transfer of heat from the body surfaces to air
heat transfer is maximized when the air is circulated, as evidenced by the “wind chill factor” (perceived decrease in temperature with wind exposure).
Conduction
transfer of heat from body surfaces to objects that come into contact with the body, such as examination tables, and kennels.
Immersion hypothermia can cause a profound heat loss via this mechanism.
Radiation
loss of heat to surrounding structures that do not come into direct contact with the body, such as walls
via electromagnetic waves (photons) emitted from any object that has a temperature above absolute zero, and this energy transfers heat
This heat transfer occurs regardless of the temperature of the intermediary substance, such as air. Athletes commonly wear reflective blankets after extremely strenuous activities to limit this form of heat loss.
Evaporative
loss of heat from moisture on the body surfaces or through the respiratory tract to the environment
Although dogs and cats have minimal perspiration, this loss can be significant if the patient is wet, either incidentally or in preparation for surgery
primary detrimental cardiovascular changes found in hypothermia include:
initial response to hypothermia includes a mild
left shift in the oxygen-hemoglobin dissociation curve, may lead to peripheral tissue hypoxia or dysoxia
as the hypothermia progresses, vascular responsiveness to norepinephrine at the α1-receptor begins to:
bradycardia, hypotension, cardiac dysrhythmias, decreased cardiac output, and ultimately asystole
sinus tachycardia, vasoconstriction >CVP
decrease = loss of vasoconstriction and subsequent arterial vasodilation contributing to hypotension
bradycardia MoA:
atropine:
decrease in the rate of diastolic repolarization in the cells of the sinus node
makes the bradycardia nonresponsive to atropine administration
HR and contractility - temperature dependent changes:
should you elevate a bradycardic hypothermic patients HR
(a. non-reposive to atropine bc due to decreased rate of repolarization of SA node so stimulating will not increase it
b. mild-moderate low HR contractility is in fact increased)
Cardiac contractility is dependent on HR in hypothermic
if HR is allowed to remain low, the systolic function can actually increase, whereas the diastolic function decreases
when HR is artificially elevated, this protective benefit was lost
only applies to mild & moderate hypothermia
severe hypothermia = decreased myocardial contractility
dysarrhythmias
generally everything is just slower:
in humans, pathognomonic:
severe hypothermia initial dyarrhythmia:
As CBT approaches 23.5° C, 50% of dogs demonstrate ventricular fibrillation
cooling to 26° C profoundly increased the transmural dispersion of repolarization. This dispersion of repolarization is known to be a prerequisite for:
prolongation of the PR interval
widening of the QRS complex, and, in humans,
Osborn waves - J waves, are an acute ST-segment elevation at temperatures of 32° to 33° C
rarely been documented in small animals
atrial fibrillation, which can progress to vtach vfib
reentrant excitation and arrhythmogenesis and may be directly linked to ventricular fibrillation
respiratory:
decreased cellular metabolism = < CO2 =
at temperatures below 34° C, sensitivity to ppCO2 and carbon dioxide production decreases by 50% with an 8° C fall in body temperature
reduce the stimulus for respiration, < TV
- respiratory alkalosis may initially be present secondary to tachypnea, as the hypothermia progresses, the respiratory drive decreases and a respiratory acidosis occurs.
- The solubility of carbon dioxide is increased in cooled blood, which leads to further increases in blood carbon dioxide content.
loss of shivering, which in humans can occur at a wide range of temperatures:
(24° to 35° C) 75-95F
coagulation:
thrombocytopenia
platelet and coagulation factor activity dysfunction
disruption of fibrinolytic equilibrium
Primary hemostasis abnormalities MoA:
decreased platelet aggregation secondary to:
sequestration of plt - liver and spleen = quantitative platelet decrease
decreased production of thromboxane B2
decreased platelet granule secretion
attenuation of P selectin expression
diminished expression vWF receptor
Secondary coag.:
BIGGEST ISSUE WITH coag testing in hypothermic patients =
depressed enzymatic activity of activated clotting factors in hypothermia
kinetic coagulation tests are performed on warmed blood Therefore the standard clotting tests performed in the laboratory at 37° C will not reflect the effects of hypothermia on the patient’s clotting cascade
Thromboelastography (TEG) studies on hypothermic blood have resulted in mixed outcomes
coagulation testing should be performed with this in mind.
Alternatively, may cause hypercoagulability and DIC:
hematocrit increases by about __% for every 1° C decline in temperature
2 to circulatory collapse, thromboplastin release from cold tissues, or release of catecholamines and steroids.
2% bc more stiff and viscose
.:. increase in blood viscosity due to hemoconcentration and RBC stiffening and decreased deformability may also play a part
MoA cold diuresis (separate from dive reflex - face hits cold water and body via trigeminal nerve tries to save the animals life by shunting blood t core and causing essentially the cushing’s reflex - hypertension, baroreceptor mediated bradycardia - relevant for submersion injuries):
- moderate hypothermia, the glomerular filtration rate decreases secondary to decreases in:
- this reduction in tubular function causes a reduction in:
- humans, acute renal failure is seen in over ___% of patient accidental hypothermia
- observed in mild to moderate hypothermia
- an cause significant hypovolemia and hypotension
- initially sensed > in blood volume before a drop in CBT
- as the CBT drops, there is a decreased response to vasopressin (antidiuretic hormone) at the level of the distal tubule
CO
renal clearance of glucose and H+ ion secretion
-contributes to hyperglycemia (decreased insulin sensitivity and reduced insulin secretion from the pancreatic islet cells) and acidosis
40%
electrolyte abnormalities:
hypothermia-induced intracellular shift
= Hypophosp, hypoMg, and hypoK
Liver - metabolism:
prospective human study, postanesthetic recovery time was lengthened by ___ minutes for each ___° C decrease in CBT
immune function compromised
decreased metabolism = prolonged drug clearances
40 minutes for each 2° C decrease in CBT
impairment of chemotaxis and phagocytosis of granulocytes, leukocyte depletion, decreased mobility of macrophages, and impaired oxidative killing by neutrophils
CBT must be measured
PAC best, esophageal, rectal all okay
The technique used for rewarming depends on the degree of hypothermia and the stability of the patient’s condition
Lack of evidence-based treatment guidelines requires
therapy to be instituted based on the patient’s pathophysiology as well as the resources available
Passive vs Active rewarming
best for what patients:
Passive - simply augmentation of the patient’s own heat generation by minimizing loss
- insulation with cloth or reflective blankets
= to minimize the heat lost through conduction, convection, radiation, and evaporation
-mildly hypothermic patients, particularly those that are shivering, because the patients will be generating additional heat and can slowly rewarm themselves. If patients are moderately or severely hypothermic, their bodies will be unable to shiver or produce significant endogenous heat, and this technique is applied only to assist in diminishing further CBT drops.
Active rewarming, application of exogenous heat directly to the skin (active external warming) or to the vital organs (active core rewarming)
-required in moderate or severe hypothermia
bc body is no longer able to generate enough heat to rewarm effectively (loss of shivering) or underlying environment
Active external rewarming (AER):
complications to consider (3):
these complications are most likely to occur when:
effect on shivering:
application of exogenous heat to the skin through a variety of methods
- Surface rewarming causes peripheral vasodilation
=relative hypovolemia and hypotension
=REWARMING SHOCK - AFTERDROP core temperature afterdrop = colder peripheral blood is returned back to vital organs and thereby decreases the CBT further
- REWARMING ACIDOSIS returning colder blood and associated lactic acid are carried back to the core causing a rewarming acidosis
extremities are warmed before the core
.:.focuse on truncal regions of the body
AER is that heat applied to the skin diminishes shivering, an effective source of heat generation. However, a benefit of AER over vigorous shivering is decreased metabolic stress and less afterdrop
.:. clinical judgment is required to determine whether application of AER will provide more heat than the heat generation lost through cessation of shivering
3 types of active external rewarming:
1. and type of heat transfer:
- Forced-air surface rewarming (e.g., 3M Bair Hugger) provides forced heated air, circulated through a blanket with apertures
- convective t
+ve: minimizes the risk of thermal injury- vasoconstricted and unable to conduct heat away
-decrease the afterdrop effect - Resistive heating (e.g., Hot Dog warming blanket) transfers heat to the patient through low-voltage electricity embedded in a fabric. It has been shown to increase CBT in mildly to moderately hypothermic trauma victims. Blankets that circulate warm water can also be used with little risk of thermal injury to the patient.
- conductive - Radiant heat, hot water bottles and electric heating pads are other techniques for providing AER, but the risk of thermal injury is higher than with the other techniques listed.
Active core rewarming (ACR) is the
In veterinary medicine, this may be accomplished by
human medicine, additional methods include
previously described techniques for gastric, urinary bladder, and colonic lavage for rewarming are relatively
ACR is best suited for patients with moderate to severe hypothermia, particularly those patients in cardiovascular arrest.
application of exogenous heat to core vital organs
warmed humidified air, heated infusions, and heated peritoneal or thoracic lavage
cardiopulmonary bypass
hemodialysis
extracorporeal warming
ineffective due to the limited surface area involved
Intravenous infusions of heated crystalloids (warmed to 40° to 42° C) can provide exogenous heat to patients when large volumes of fluids are required
limit:
common in-line veterinary fluid warmer was used to heat fluids delivered at low rates = cooled down by the time it got to patient
higher rates of fluid administration and in severely hypothermic patients, however, the results may improve
also32-40 is less of a jump than 42 to 5C so cooling = greater gradient
Airway rewarming can be accomplished through:
warmed (40° to 45° C) humidified air via a face mask or endotracheal tube
benefits of airway rewarming are that it is noninvasive, allows alveolar warming of the blood returning to the heart and conduction to contiguous structures in the mediastinum, prevents further respiratory heat loss, helps ensures adequate oxygenation, and reduces the amount and viscosity of secretions in cold-induced bronchorrhea
most in-hospital humidifiers do not exceed 41° C
minimal effect but help prevent evaporative losses
peritoneal lavage can be performed with heated _______, ________ infused via a peritoneal catheter at __________
dwell time of ____ minutes is allowed and then the fluid is aspirated and the procedure repeated
method allows transfer of heat to a large surface area, including great vessels and abdominal viscera. An additional benefit is
hypothermia model of dogs with cardiac arrest, the investigators found that peritoneal dialysis, with appropriate cardiopulmonary resuscitation (CPR), was as effective as _____
main disadvantages of PD=
40° to 45° C
isotonic dialysate (NalCl, LRS, 1.5% dextrose dialysate)
10 to 20 ml/kg
20 to 30 minutes
direct hepatic rewarming, which reactivates detoxification and conversion enzymes
partial cardiac bypass
invasive
can complicate ongoing coagulopathies
electrolyte shifts requiring careful monitoring
Thoracic lavage is described in the human literature as
Extracorporeal blood rewarming can be achieved through
an effective method of core rewarming in cardiac arrest. Heated (40° to 41° C) normal saline is infused through large-bore thoracostomy tubes into the hemithorax and then extracted after a 2-minute dwell time. This allows for closed-chest CPR and defibrillation as indicated
cardiopulmonary bypass
continuous arteriovenous warming
venovenous warming
hemodialysis
To date, the author knows of no application of these techniques in veterinary medicine for rewarming in hypothermia.
rewarming rate/technique
all about 1 C /hour 2 in second hour .:. they are slow
Rewarming strategy:
initial approach is aimed at stabilizing the patient’s condition and starting a slow rewarming process as dictated by the severity of signs.
Prevention of further decreases in CBT is indicated in all cases of hypothermia. This is best accomplished through:
A safe and steady rewarming rate should be established, typical rate:
passive rewarming techniques
-dry patient, avoid ambient T fluids
0.5° C/hr to 2.0° C/hr, through use of the appropriate active external and active core rewarming methods
Stability of the cardiopulmonary system:
most fluid shifts will be ____ by rewarming, and therefore in mild hypothermia ____ rates of fluids
-reversed by rewarming, and therefore in mild only modest rates of fluids
as the severity of hypothermia progresses, the complications of volume shifts and increased blood viscosity, increased vascular permeability, and low-flow states will dictate aggressive fluid resuscitation. It is important to note that, in severe hypothermia, it may be impossible to achieve complete stability without significant rewarming. Therefore the aggressiveness of rewarming will need to be altered appropriately.
Glucose:
in moderate to severe hypothermia, on the other hand, a profound hyperglycemia may develop secondary to ________
Electrolyte alterations may require supplementation
Acid-base alterations typically
Anticipation and prevention of known hypothermic complications should be part of the therapeutic plan
may need to be administered in mild to moderate hypothermia bc > increased catecholamine and cortisol production
decreased insulin sensitivity and secretion
–insulin therapy may be required
particularly of potassium
typically correct w rewarming
- Coagulopathies:
- hypovolemia: cold diuresis
- increased incidence of infection: Ab
- dysrhythmias associated with: aggressive rewarming
Cardiopulmonary Resuscitation
There are some unique challenges
in severe hypothermia, the patient may appear pulseless, have a nonauscultable heartbeat, and even have an apparent asystolic rhythm on electrocardiogram…are they dead?
extreme example is a human patient
these changes are due to severe bradycardia, peripheral vasoconstriction, and hypotension, which can lead to the erroneous assessment of cardiopulmonary arrest
lack of organized cardiac function can be confirmed with assessment of cardiac wall motion by echo
ROSC may not occur until severe hypothermia has been addressed, the duration of CPR may depend on the length of time it takes to warm the patient
6.5 hours of closed chest compressions
Therapeutic hypothermia:
large body of evidence supports use of mild therapeutic hypothermia (target temperature:_____) in PCA
to limit damage following ROSC
of importance for veterinarians, the initial data came from studies in dogs
neuroprotection afforded by hypothermia is thought to be due to:
32° to 34° C
prevention of apoptosis after cellular injury,
retardation of destructive enzymatic reactions
suppression of free radical reactions
reduction in cerebral oxygen demand
The Reassessment Campaign on Veterinary Resuscitation (RECOVER) initiative examined the evidence for the use of mild therapeutic hypothermia and concluded that the data suggest:
a beneficial effect on neurologically intact survival when the hypothermia is instituted as soon as possible after restoration of ROSC and maintained for longer 12 hrs
however, the practical aspects with regard to onset, level, and duration still need to be determined
