Thermoregulation Flashcards
(36 cards)
Basal metabolic rate
determines rate of metabolism of nutrients and rate of skeletal muscle contraction; both metabolism and muscle contraction generate heat
Heat retention mechaisms
Raising hairs (piloerection) – hairs trap warm air
▪ Vasoconstriction of blood vessels in skin – prevents loss from convection, conduction and radiation
▪ Behavioural responses (e.g., herding, huddling, nesting)
▪ Shivering
Cooling mechanism
o Decreasing metabolic rate
o Heat in deep tissues is transferred to the skin via flow of blood from the core to the body surface. Once at the body surface, heat is lost by conduction, convection and radiation
Normal temp of cat and dogs
38.5 +- 0.5(above 39.1 must be reported)
Normal temp for cow
38.5+- 0.5
Normal temp for horse
38.0+-0.5
when is temp fatal
43 and above-rapidly fatal
28-dying
Heat stress is
▪ 39.1 to 41.5 °C
▪ Lethargy, sweating, panting
▪ Decreased athletic performance
▪ Decreased production (milk, growth, breeding)
▪ Changes in hydration and electrolytes
o Increased temperature increases metabolic demand by muscle
Treatment for heat stress
▪ Passive surface cooling (soak, shade, drink):
* Shade
* Proper ventilation
* Decrease ambient temperature
* Fans, misting, decrease density
* Water
o Prognosis is good if it does not progress to heat stroke
Heat stroke is
o Core body temperature exceeds 41.5*C for a sustained period of time
o Any body temperature above 43 °C is critical and must respond NOW
o No accompanying signs of inflammation (in other words, this is not likely to be a true fever
Pathology of heat stroke
▪ Intracellular proteins denature and mitochondria stop functioning. This leads to cell damage that is followed rapidly by cell death
▪ Sloughing of the intestinal mucosa occurs → seen as vomit and diarrhea; can lead to sepsis from intestinal bacteria
▪ Dehydration (from sweating, panting and increased evaporative losses)
▪ Respiratory alkalosis (from panting)
▪ Excessive peripheral vasodilation can lead to decreased BP and ischemia
▪ Blood also moves away from core to the body surfaces for cooling; core tissues may not be properly perfused
▪ Cardiac arrhythmias, disseminated intravascular coagulation (DIC is described in vascular conditions), muscle injury, CNS injury, hepatic injury can all occur (due to protein denaturation)
▪ When 2 or more of the organs shut down, this is referred to multi-organ dysfunction syndrome (MODS)
Passive cooling
Cool room, shade
▪ Fans, ventilation, soak with tepid water
▪ Drink fluids (if patient is able to)
What is the goal when cooling an animal
Goal is to decrease to 39.5 C within 30-45 min. Stop cooling at 39.5 °C provided risk factors have been controlled
Active surface cooling
(If <43 °C)
▪ Apply cold packs to neck, foot pads, inguinal region and axilla’s - these are where the largest blood vessels are located
▪ Goal is cool blood as it moves through the largest blood vessels near the body surfaces
Active core cooling
Indicated for critical heat stroke
▪ Chilled IV fluids
▪ Cool water enemas
Mu-agonist induced hypothermia in cats
Uncommon to common
- Mu-agonists are the class of opioids that include morphine, hydromorphone, fentanyl
- Certain cats administered any of these mu-opioids may develop hyperthermia
- Diagnosis and Clinical Sign:
o Temperature does not drop as expected during general anesthetic
o Gradual increase in temperature
Treatment for mu-agonist induced hyperthermia in cats and prognosis
Treatment
o Intervene at 41°C (usually happens within 30-90 min of giving the drug)
o Treat by reversing the opioid
▪ Mu-reversal drugs include buprenorphine, butorphanol, naloxone
- Prognosis is good is caught in time and treated with reversal agent
- Can be fatal if not caugh
Malignant hyperthermia
Fatal hyperthermia triggered by exposure to inhalant general anesthetics (isoflurane and sevoflurane)
- Can happen in all species (including humans)
o Rare
o Most common in pigs (aka Porcine Stress Syndrome)
- Inherited defect in skeletal muscle metabolism; no effect unless triggered by exposure to gas anesthetic
o Mutation causes excessive Ca+ release by sarcoplasmic reticulum in muscle only when exposed to isoflurane or sevoflurane
o Excessive Ca+ causes muscles to contract and stay contracted
o Muscle activity generates heat
o Muscle also switch to anaerobic metabolism and this leads to metabolic acidosis
- Once triggered, temp increases 1-2 °C every 5 min (will rapidly exceed 43 °C)
- Treatment
o Stop anesthetic
o Start aggressive cooling (e.g., dump ice into open abdomen); muscle relaxants; aggressive CV support
Hypothermia due to increased heat loss risks
▪ Heat production is normal
▪ The problem is usually related to environmental conditions
▪ Risks include the following:
* Cold environmental temperature
* Wet
* Low BCS (lack of subcutaneous fat)
* Neonates – heat regulation is normal but less developed than in adults; increased body surface area for increased heat loss; low body fat. Example is kittens and foals born in snow
* Small animals - due to increased body surface area
Hypothermia due to lack of heat production
▪ Can occur with severe illness such as shock
▪ Brain trauma
▪ Consistent adverse effect of GENERAL ANESTHESIA
* #1 cause of hypothermia seen in a hospital setting; too frequent
* Highest risk: <10 kg, pediatrics, geriatrics, open abdominal surgery, trauma, too long a procedure, cold gasses; lack of warming
* All general anesthetic agents cause vasodilation (blood vessels dilate
and encourage cooling), decreased basal metabolic rate, stop shivering; depress hypothalamus function
Mild hypothermia
34-37*C
Lethargy, depression, shivering
Loss of suckle response
Moderate hypothermia
28-33*C
Loss of physiological warming response
Uncoordinated to unconscious
Decreased metabolism
Decreased HR and CO
Decreased muscle function (affects resp)
Severe hypothermia
<28*C
Collapse, uncoscious, coma
Death
Prevention of hypothermia
o Appropriate shelter for birthing and neonatal care
o Avoid wetness in cold temps
o Minimize anesthetic times, warm IV fluids during prolonged GA, warm ambient temperature; monitor temperature q15 min while under GA; provide proper patient support in the pre-/intra-/post-operative periods
