Chp. 17: Perioperative Thermoregulation and Heat Balance Flashcards
Describe the difference between hyperthermia and pyrexia.
Both are increases in temperature. Hyperthermia is an increase in temperature that usually occurs in response to increases environmental temperature or altered thermoregulation. Pyrexia is due to an increase in the temperature set-point, often in response to a pathogen.
What ion channels are involved in body temperature sensing?
The transient receptor potential (TRP) family of cation channels.
How is the core-to-periphery temperature gradient maintained?
By the ANS through mechanisms that regulate peripheral blood flow.
How does the majority of heat transfer between core and periphery take place?
Via blood-borne convection (some due to tissue-to-tissue conduction).
What are the three phases of perianesthetic hypothermia?
An initial rapid hypothermic phase, the linear decrease, and the plateau phase
What are the four mechanisms of heat transfer?
Radiation: Transfer of infrared and near-infrared energy between surfaces.
Convection: Facilitated via an intermediary (moving air, flowing liquid).
Conduction: Direct transfer between two adjacent surfaces.
Evaporation: “Donation” of heat energy required to vaporize liquid.
What is the most important mechanism of heat loss under GA?
Radiation.
What is the second-most important mechanism of heat loss under GA?
Convection. Can become most important in environments with high airflow.
What is efficiency of heat transfer related to?
The proportion of body surface which can be exposed to the external heat source.
Where is heat loss (and gain) usually greatest?
In areas with large blood flow and low mass (e.g. limbs).
What is the most effective intra-operative warming device?
A forced warm air blanket
How does resistive polymer electric heating (e.g. HotDog) work?
As a conductive heat delivery device.
How are convective losses prevented?
Air-trapping with a sheet or blanket
How are conductive losses prevented?
Insulation
What are the most common causes of hyperthermia under GA?
Iatrogenic causes, heavy-coated animals undergoing procedures with little heat loss, hypermetabolic syndromes
What is mild hypothermia?
Between euthermic temperature and 96.8F
Why is the magnitude of hypothermia less in large animals?
Large body mass-to-surface ratio and high frequency of use of vasoconstricting alpha-2 agonists
What complications are associated with hypothermia?
Increased risk of anesthetic overdose, prolonged recovery, postoperative wound infection, impairment of coagulation, increased shivering and discomfort during recovery, increased blood viscosity, and cardiac complications (arrhythmia, arrest)
At what temperature does cardiac arrest occur?
Below approximately 70F
What are the cardiac complications associated with hypothermia?
Hypotension, myocardial ischemia, arrhythmia due to increase oxygen demands secondary to shivering in recovery
For what conditions is controlled hypothermia protective?
Neurologic disease, myocardial ischemia, tissue hypoxia
True or False: It is more difficult to raise body temperature in an animal with profound peripheral vasoconstriction.
True.
