Thermoregulation and Postoperative Care in Animals

Thermoregulation & Postoperative Care

• Thermoregulation is an organism's ability to maintain body temperature within a specific range, even with changing external temperatures.
• Normal body temperature in dogs and cats is around 100-102.5°F.
• Homeotherms (most mammals) maintain a constant core body temperature, while poikilotherms (reptiles, some fish) have variable core body temperatures.
• Body temperature fluctuations can be influenced by time of day, time of year, hormones, and activity level.
• Core body temperature is usually 2-4°C higher than skin temperature.
• Body systems involved in thermoregulation are the central nervous system (CNS), cardiovascular system (CV), respiratory system, musculoskeletal system, and integument.
• Abnormal body temperature in a conscious animal can point to metabolic dysfunction, while in an anesthetized animal, it suggests fluid or drug-related or environmental issues.
• The 4 mechanisms of heat loss are radiation, convection, conduction and evaporation, with radiation usually the largest contributing factor.

Learning Objectives

• Heat loss mechanisms: Students should list and describe four mechanisms of heat loss and identify those most crucial in veterinary patients.
• Potential causes of hyperthermia/hypothermia: Students must identify common causes of these conditions in anesthetized animals.
• Concerning body temperatures (under anesthesia): Students should be able to identify body temperature levels that necessitate concern regarding hypothermia or hyperthermia.
• Harmful effects of hypothermia/hyperthermia: Students should comprehend how these conditions affect various bodily systems.
• Patient warming/cooling techniques: Students should list different methods for safely warming or cooling surgical patients.
• Basic steps in post-anesthesia recovery: Students should explain the basic steps for recovering a patient from general anesthesia.

What is Thermoregulation?

• An organism's ability to maintain a consistent body temperature.
• Normally ranges from 100 to 102.5°F in dogs and cats.
• Most domestic mammals are homeothermic.
• Other groups such as reptiles and amphibians are poikilothermic.

Sensation of Temperature

• Temperature sensations are communicated from peripheral nerves to the spinal cord, then to the thalamus and finally to the hypothalamus.
• Temperature signals are transmitted from the periphery via sensory neurons (Aβ, Aδ, C fibers).
• The hypothalamus plays a central role in temperature regulation.
• The spinothalamic tract transmits temperature information to the CNS.
• Efferent pathways regulate responses like blood vessel tone, shivering, and sweating.

Four Heat Loss Mechanisms

• Radiation: Heat loss via electromagnetic waves.
• Convection: Heat loss through moving air or water molecules.
• Conduction: Heat loss to objects in direct contact.
• Evaporation: Heat loss through liquid-to-gas conversion (e.g., moisture loss from skin and lungs).

Examples

• Radiation: A body warmed by the sun loses heat through thermal radiation.
• Convection: If a patient is placed by a fan, convection occurs.
• Conduction: Patients lose or gain heat through contact with cold or hot objects.
• Evaporation: Sweating or panting helps the body cool down.
• Other examples in the PowerPoint include scenarios such as warming procedures performed during surgery.

Physics Review

• Radiation is the transfer of heat using electromagnetic waves in the air.
• It generally occurs when surrounding air temperature is below 68°F (20°C).
• Most of the sun's electromagnetic radiation is invisible to the human eye and is not heat radiation.

Hypothermia

• Body temperature below normal.
• Common in anesthesia, especially when it lasts long time.
• Size, extent of exposed skin, and other factors contribute to the rate of heat loss; factors influencing loss include patient size, environmental temperature and body surface area exposed.
• Categories of hypothermia include mild, moderate, and severe, each with different temperature ranges and physiological changes.

Strategies to Warm Anesthetized Patients

• Passive rewarming: Using blankets, towels, and wraps on limbs.
• Active rewarming: Using forced-air blankets, circulating warm water blankets, resistive polymer heating pads, humidifiers, and IV warmers.

The use of an electric heating pad/blanket is NOT safe for use in anesthetized or recovering patients

• Electric pads can lead to burns.

Hyperthermia

• Body temperature above normal.
• Causes include heat stroke (environmental), respiratory obstruction, exercise, histamine release, iatrogenic issues (medication-related), and prolonged seizure activity.
• Potential for malignant hyperthermia in certain individuals (possibly caused by genetics).

Strategies to Cool a Patient

• Careful use of cooling techniques (e.g. cooling blankets, ice packs, etc) as they may cause peripheral vasoconstriction and hypothermia.
• 1st line strategies: Oxygen, room temperature, IV fluids, and/or fans.
• 2nd line strategies: Peritoneal dialysis or cool IVF.
• Other cautions: Avoiding cold water enemas or gastric lavage.

Hyperthermia in Cats

• Post-anesthetic hyperthermia is commonly associated with hydromorphone use, but other drugs could also cause it.
• Monitor patients for hyperthermia in the first 5 hours post-surgery.
• Supportive care and partial opioid reversal can resolve the issue.

Postoperative Care (Day Patients)

• Release to owner: When the animal is alert, mobile, eating, and maintaining normal vital signs.
• Instructions: Provide written and verbal instructions for owners regarding activity restrictions, wound care, pain management, feeding, medications and when to contact the hospital.

Postoperative Care (Hospitalized patients)

• Treatment orders: Include vital signs, pain scoring, body weight, fluid intake and output, urine & fecal assessments, hydration status, and feeding/water instructions, as needed.

Treatment Orders (cont.)

• Include IV maintenance rate (approximately 40-60 mL/kg/day).
• Consider fluid replacement needs and insensible losses
• Instructions for medications, including amount, route, and timing.

Practice Problem

• Determine the appropriate IVF rate for a 20 kg dog using a solution of Normasol-M, assuming no fluid losses, ongoing or insensible, are occurring.

Pain Scoring

• System for evaluating pain by assessing various parameters such as body tension, reaction to palpation, posture, facial expression etc.

What is a Prolonged Recovery?

• Excessive time (30-45 minutes) from inhalant anesthesia discontinuation without extubation,
• Prolonged recovery times for standing in horses (approximately one hour),
• Important to monitor vital signs (especially heart rate, respiration rate, body temperature) and consider administering fluids, and/or glucose, and/or checking electrolytes, and reversing any medication given during surgery.

Resources

• Handbook of Veterinary Anesthesia: (5th ed., chapter 17)
• Lumb & Jones: Veterinary Anesthesia and Analgesia: (5th ed., chapter 19)

This lesson explores thermoregulation in animals, focusing on maintaining body temperature within a specific range. It covers normal body temperature in dogs and cats and factors influencing temperature fluctuations. It also discusses heat loss mechanisms and the body systems involved in thermoregulation.