This chapter reviews specific anesthetic considerations concerning thermoregulation, regional anesthesia, and acute pain management. The role of thermoregulation is of concern for all patients undergoing surgical procedures, given that the operating room is typically extremely cold, and the patient is exposed and at risk for hypothermic consequences. Ultrasound technology developments have revolutionized the practice of regional anesthesia. As well, newer drugs with long duration have become commonplace for anesthesia providers. In this regard, enhanced recovery after surgery protocols relies strongly on ultrasound-guided nerve blocks that result in lower opioid consumption postoperatively and earlier hospital discharge. Adjuvant agents are currently being studied to provide additive and/or synergist effects with local anesthetics in evolving acute pain management strategies.
CLINICAL ANESTHESIA CONSIDERATIONS OF THERMOREGULATION
Thermoregulation is of paramount importance in the perioperative setting related to the potential complications associated with hyperthermia or hypothermia. While hypothermia is the most common thermal disturbance, there are many additional concerns that must be considered by the anesthesia provider. Skin is the primary route of heat loss via radiation. Conduction, or kinetic energy loss via motion of skin tissue molecules to the air, is another route of heat loss. Heat loss may also occur through evaporation, but this is rare in the clinical setting.
While skin temperatures may rise and fall with the environment, the core temperature remains constant (98.0–98.6°F). This thermoregulation of the core temperature occurs through afferent sensing, central control, and efferent responses.1
Thermal-sensitive receptor cells are located throughout the body. While most cold receptors are A-delta nerve fibers, heat receptors signal impulses through unmyelinated C fibers. These C fibers also conduct pain. The signal is sent through the spinal cord to the hypothalamus thermoregulatory center. The hypothalamus controls thermoregulation by coordinating afferent and efferent signals to maintain normothermia. Common efferent responses involve behavior modification, including dress, body positioning, and involuntary movement such as shivering.2
Anesthesia Effects on Thermoregulation
Anesthetics such as propofol, certain opioids, and volatile anesthetics cause vasodilation which promotes heat loss. Many anesthetics also impair hypothalamic thermoregulation. Opioids cause sympathetic suppression. Nitrous oxide causes less thermoregulatory depression than volatile anesthetics. Most benzodiazepines have minimal thermoregulatory influence.
After induction of general anesthesia, body temperature declines in three parts. In the first part, vasodilation and a decreased-threshold hypothalamus cause body heat redistribution from core to skin, leading to radiation loss. Phase 2 occurs via heat loss exceeding body heat production (after 1 hour). Phase 3 is from 3 to 5 hours as heat loss matches heat production, leading to thermoregulated vasoconstriction.
Regional anesthesia causes hypothermia, especially after spinal and/or epidural anesthesia. Large regional blockade prevents cold signals back up to the hypothalamus. The hypothalamus misinterprets the temperature at the skin, making patients feel warm despite the core temperature decreasing.3