Thermoregulation & Postoperative Care

Thermoregulation & Postoperative Care

• Learning Objectives:
  • List and describe the 4 mechanisms of heat loss and their contribution(s) to heat loss in veterinary patients.
  • List potential causes of hyperthermia or hypothermia in anesthetized patients.
  • Determine what body temperatures under anesthesia could cause concern for hypothermia or hyperthermia.
  • Describe harmful effects of hypothermia and hyperthermia on various body systems.
  • List ways to safely warm or cool patients as needed.
  • Detail the steps/goals in recovering a patient from general anesthesia.

What is Thermoregulation?

• Ability to maintain body temperature within a certain range (±0.2°C), even when surrounding temperatures vary.
• Normal range for dogs and cats is 100-102.5°F.
• Homeotherms: Most domestic mammals
• Poikilotherms: Reptiles, amphibians, some fish
• Core body temp is often 2-4°C higher than skin temp.
• Body systems involved in thermoregulation include CNS, CVS, respiratory, musculoskeletal, and integument.
• Abnormal body temperature in awake animals may indicate metabolic dysfunction.
• Abnormal body temperature in anesthetized animals can be due to fluid or drug-related factors or environmental factors.

Sensation of Temperature

• Sensory information travels from the periphery to the central nervous system (CNS).
• Primary afferent neurons carry sensory information from the periphery (skin) to the spinal cord via Aβ, Aδ, and C fibers.
• Sensory information (noxious and non-noxious) travels to higher centers (including the hypothalamus) via the spinothalamic tract.

Four Heat Loss Mechanisms

• Radiation: Heat loss via electromagnetic waves (e.g., sunshine, microwave).
• Convection: Heat loss through the movement of air or water molecules (e.g., wind, water current) across the skin.
• Conduction: Heat loss to objects in direct contact with the body (e.g., cold surgery table).
• Evaporation: Heat loss through conversion of water to gas (e.g., sweating, breathing).

Examples

• Radiation: Body warming by absorbing sunlight rays, losing heat by thermal radiation (e.g., wood stove warming a room), use of electromagnetic radiation (e.g., microwave to transfer heat to food).
• Convection: Heat exchange between the body and ambient air/water (e.g., fan blowing on you, swimming in a cold pool, wind chill factor).
• Conduction: Body cools by touching a cold surface (e.g., sitting on a cold surface, cold water bottle on head).
• Evaporation: Heat loss occurs through respiration (moisture in air exhaled) and sweating; body creating a layer of water to cool down (e.g., sweating).

Physics Review

• Radiation is the transfer of heat via electromagnetic waves through airspace.
• Radiation primarily occurs when ambient temperature is below 68°F.
• Most electromagnetic radiation coming from the sun is invisible to the naked eye; only a small portion is visible light.
• Light itself comprises waves of different frequencies.

Hypothermia

• Definition: Body temperature below the normal range.
• Common anesthetic complication, especially with prolonged anesthetic times.
• Related to rate of heat loss.
• Extent of hypothermia can vary (mild, moderate, severe).
• Physiological changes and clinical signs vary with severity. See the table in the document for details.

Strategies to Warm Anesthetized Patients

• Passive rewarming:
  • Blankets/towels
  • Wrapping extremities
• Active rewarming:
  • Forced-air blankets
  • Circulating warm water blankets
  • Resistive polymer electric heating blankets (e.g., HotDog).
  • Humidifiers placed in the inspiratory limb of the breathing system.
  • IV warm fluids to lavage abdominal cavity
  • Place in incubator to warm.

Strategies to Cool a Patient

• Aggressive cooling methods need caution since they can cause peripheral vasoconstriction leading to hypothermia.
• First-line strategies include using oxygen, room temperature, IV fluids, and tepid water applied to the skin, shaving fur, etc
• Second-line strategies include using peritoneal dialysis and cool IVF
• Avoid procedures that could damage the GI tract (cold water enema) and avoid gastric lavage.
• Monitor patient temperature every 5 minutes.
• Consider drug reversal(s) as indicated.

Hyperthermia

• Definition: Body temperature above normal.
• Common causes include: heatstroke, environmental heat/humidity, respiratory obstruction, exercise, histamine release, prolonged seizure activity, malignant hyperthermia.
• The first 5 hours post surgery are typically the time frame in which this is observed, so it should be monitored.
• Treatment involves supportive care and partial reversal of opioid-related hyperthermia (if applicable).

Postoperative Care (Day Patients)

• Release criteria for conscious, alert, and stable patients.
• Provide written discharge instructions and verbal instructions.
  • Expected behaviors
  • Activity restrictions
  • Bandaging checks
  • Instructions for food/water
  • Medication administration information

Postoperative Care (Hospitalized Patients)

• Treatment orders must include vitals, pain scoring, body weight, urine and feces output, hydration status, feeding instructions, or NPO status.

Treatment Orders (cont)

• IV fluids (~40-60 ml/kg/day)
• Fluid replacement needs and insensible losses (ongoing).
• Complete medication instructions (amount, route).
• Exercise/physical therapy.
• Other nursing care instructions (such as flush IVC, turn patient, eye lube, and clean perineum), etc.
• Clinician must sign off on orders.

Practice Problem

• Calculate IVF rate for a 20kg dog for maintenance after surgery.

Pain Scoring

• Method for evaluating patient pain. (Details vary by specific pain scale).

Prolonged Recovery

• Excessive time from inhalant anesthetic discontinuation (~30-45 minutes before extubation for discontinuation of anesthetic).
• Prolonged standing time in horses (>1 hour).
• Monitor patient vitals closely, including temperature.
• Check electrolytes and blood glucose.
• Reversal of specific drug types may be beneficial depending on the situation.
• IV fluids ± dextrose and/or calcium may be indicated.
• Rule out other potential diagnoses.

Resources

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