Chp. 25 Hypothermia

Question 1

What is significant Perioperative hypothermia in cats, dogs?

Answer 1

-Decrease in core body temp as little as 1C/~2F results in adverse challenges for many patients
-Healthy patients –> often compensate ok
-Compromised, critically ill –> increased morbidity, mortality secondary to avoidable perioperative hypothermia

Question 2

What effect does GA have on thermoregulation?

Answer 2

Alters one or more of the three components of heat balance in the body:
1) Afferent pathway
2) Central control mechanism
3) Efferent pathway

Question 3

What are the more specific effects of anesthetic agents on thermoregulation?

Answer 3

-Esp volatile anesthetics
-Reset threshold for thermoregulation so that broader range of body temps is tolerated without response
-Once response is triggered, magnitude of response may be near normal yet incapable of restoring normal body temp

Question 4

How does body temp decrease during GA?

Answer 4

Three phases:
1) initial, precipitous decrease in body temp DT vasodilation and redistribution of core body heat to the periphery
2) Progressive, linear decline in temperature
3) Vasoconstrictive response occurs to limit further decreases in body temp by reducing blood flow to peripheral tissues

Question 5

What physiologic mechanisms act to maintain, restore body temperature?

Answer 5

• Behavioral responses: seeking warm places, curling to minimize exposed surfaces
  -Piloerection
  -Shivering thermogenesis -> can happen under light planes of GA
  -Non-shivering thermogenesis
  -Vasoconstriction

Question 6

How does piloerection generate warmth/maintain body temp?

Answer 6

-Increases thermal barrier of warmed insulating air surrounding the body
-Barrier helps reduce further heat loss

Question 7

Non-shivering thermogenesis

Answer 7

-Generation of body heat through increased muscle tone
-Can also be blocked, perhaps to lesser extent, by GA

Question 8

Vasoconstriction to maintain body temp under GA

Answer 8

-Limits delivery of blood and heat from the core to the periphery
-Relatively well-maintained under GA

Question 9

Moderate Hypothermia

Answer 9

35C/95F

Question 10

Severe Hypothermia

Answer 10

30C/86F

Question 11

Profound Hypothermia

Answer 11

Btw 24-28C/75.2-82.4F

Question 12

Adverse circulatory effects of moderate hypothermia in anesthetized patients?

Answer 12

-Triggering of 2-7 fold increase in catecholamine release –> results in VC, tachycardia, hypertension
-Morbidity Assoc with these stress responses typically occurs during postoperative period vs ax/sx

Question 13

Adverse circulatory effects of severe hypothermia in anesthetized patients?

Answer 13

Increased risk of AFIB

Question 14

Adverse circulatory effects of profound hypothermia in anesthetized patients?

Answer 14

Refractory ventricular fibrillation, death

Question 15

Effects of hypothermia on coagulation, bleeding?

Answer 15

-Impairment of platelet function
-Decrease activity of coagulation pathways
-Increased fibrinolysis

Question 16

T/F: hypothermia has been shown to cause deficiencies in coagulation in many species, including dogs

Answer 16

True

Question 17

Does hypothermia increase risk of postoperative infection or delay wound healing?

Answer 17

• Retrospective study in dogs, cats with clean surgical wounds: mild Perioperative hypothermia not a significant risk factor for postoperative wound infection
  -Human patients undergoing colon surgery: incidence of IFX increased 3x (to 6%) if patient is normothermic to 19% if moderate hypothermia occurs
  -Similar results confirmed in animal models

Question 18

What are the proposed mechanisms of increased risk of POI/delayed wound healing in hypothermic patients?

Answer 18

-Impairment of macrophage function
-Reduced tissue oxygenation secondary to thermoregulatory VC
-Impaired wound healing attributed to decreased collagen deposition

Question 19

What are the metabolic consequences of hypothermia?

Answer 19

-Increased catabolic postoperative stress response –> muscle protein breakdown, negative nitrogen balance
-P: as little as 2*C loss of body temp –> doubling of BUN excretion vs normothermic patients
-Heat conservation w maintenance of normothermia shown to reduce muscle protein breakdown in geriatric p undergoing GI sx

Question 20

What changes in anesthetic requirement occur with hypothermia?

Answer 20

-increases solubility of volatile anesthetics in the body –> increasing effective dose delivered
-Also see decreased clearance of ax drugs
-Combined result = significant potential for anesthetic overdose in hypothermia patients

Question 21

Why important to monitor patient temp in the perioperative period?

Answer 21

• Significant potential for anesthetic overdose in hypothermic patient DT increased solubility of volatile anesthetics and decreased clearance
  -Temp monitoring –> recognition of hypothermia –> anesthetic doses/vaporizer settings appropriately reduced to minimize anesthetic complications

Question 22

Why does body temp decrease so rapidly under GA?

Answer 22

• All anesthetics impair thermoregulation
• Ax induction: initial VD allows for redistribution of core heat to skin, extremities
• Loss of thermal units from core = irreversible

Question 23

T/F: subsequent VC as hypothermia proceeds will restore core temperature

Answer 23

FALSE
IT WILL NOT RESTORE CORE TEMP

Question 24

Poikilothermia

Answer 24

Inability to regulate body temp

Question 25

How much is body temperature decreased by GA?

Answer 25

- After initial decline DT VD, temp continues to fall until VC reduces blood flow and heat distribution to the periphery -Lower plateau: ~34*C/93*F -Decreases in body temp somewhat dose-dependent --> relatively linear overtime -Many factors influence extent of hypothermia in anesthetized patient

Question 26

Other than application of active warming, what can be done to reduce loss of body heat during GA?

Answer 26

-Covering/insulation to control radiant heat loss -Prevention of contact with cool surfaces and prep solutions -Use of lower flow gas circuits, techniques

Question 27

What is the relationship between temperature and MAC reduction?

Answer 27

5% decrease in MAC requirement for volatile anesthetics with each 1*C/1.8*F decrease in body temp

Question 28

Why does covering or wrapping the patient help maintain body temperature?

Answer 28

-Maintain crucial layer of warm air surrounding the body --> minimization of convective heat losses -Choice of material of little importance for maintained envelope of warm air -Exposure of large areas of skin for surgical procedures or particularly opening of large body cavities greatly increases cooling through both radiation, evaporation

Question 29

What active heating strategies can be employed during perioperative period to maintain/restore body temperature?

Answer 29

-Warmer ORs, recovery areas ambient air temperatures -Warm air blankets -Circulating warm water blankets -Radiant heat source above patient and surgical field -Warmed IV, surgical irrigation fluids +/- devices to warm, humidify inspired ax gases

Question 30

What is the most effective active heating strategy?

Answer 30

Convective warm air device Delivers controlled warm air through porous blankets to surround patient with envelope of warm air

Question 31

How are active heating devices optimized?

Answer 31

Applied early to prevent hypothermia ***Must also be used with monitoring of body temp***

Question 32

How safe are methods for warming patients?

Answer 32

-Risk of thermal injury with all active heating systems -Important to have broad distribution of heat to avoid localized hot spots, tissue drying, and burning

Question 33

Why are conventional electric heating patients not acceptable?

Answer 33

-Extreme risk of iatrogenic thermal injury DT unacceptably high temperatures of electric heating pads -Uneven distribution of delivered heat -Focal delivery of excess heat -Tissue burns frequently seen

Question 34

Problems with warmed IV fluids

Answer 34

-May cool to near room temp in the fluid delivery tubing -Only very high fluid flow rates significant in patient warming/cooling -Overheated IV or irrigation fluids can be very damaging

Question 35

Problems with radiant heat in surgical and recovery settings

Answer 35

Can overheat, dry tissues

Question 36

Other problems with heat regulation/active heating of the anesthetized patient

Answer 36

-Recumbent pressure may limit blood flow to skin areas in direct contact with warmed surfaces --> facilitates tissue burning -Ax, sedation interferes with ability of patient to sense overheating and to move away from heat sources to escaping injury

Question 37

Problems with HMEs

Answer 37

-heated/humidified anesthetic gases may burn or dry airway if too hot -May increase fluid load by delivery of excessive airway moisture

Question 38

Electric Heating Pads

Answer 38

-Designed for use on awake humans having both mental faculty and physical capability to recognize excess heat and make appropriate responses to remove device or alter heat

Question 39

Are hot water bottles reasonable for patient warming?

Answer 39

-Burns can result from same mechanisms as with electric heating pads -If warm or hot water bottles/gloves kept at acceptably moderate temperatures to avoid potential for burns, effectiveness of heating is lost -Once have cooled to temperature of patient and then continue to cool, contribute to loss rather than gain of body heat

Question 40

Does shivering occur under GA?

Answer 40

-P under light levels of GA may shiver --> very fine fibrillary ctx of facial m to more generalized, intense CTX of trunk or limbs -More common = gross shivering upon recovery from GA

Question 41

What are the consequences of shivering?

Answer 41

- Can range from subtle to severe - Can be effective in restoring body temp - Significantly increases metabolic oxygen consumption -Postoperative shivering can increase metabolic rate 400% --> more recent research suggests typically a doubling (40-200%)

Question 42

How to suppress shivering in the postop period?

Answer 42

-When necessary, postoperative opioid analgesics can be used to suppress shivering -Meperidine commonly used for this purpose in human med

Question 43

Why is shivering in the postoperative period problematic?

Answer 43

- Increased metabolic oxygen consumption --> oxygen debt DT residual reduction in ventilation, residual pulmonary atelectasis or other respiratory insufficiency - Even if 4x increase in minute ventilation, shivering patients can still have arterial oxygenation values well below baseline and inadequate oxygen delivery to tissues - Should be supplied with supplemental oxygen