Thermodynamics Quiz

Body temperature < 36°C

Must document intervention

Hypothermia reduces metabolic O2 requirements

Cardiac arrhythmias, ischemia, increased peripheral vascular resistance

Reversible coagulopathy—platelet dysfunction

Increased post-op protein catabolism and stress response

Preoperative warming

Shivering postoperatively is caused by hypothermia, neurogenic after-effects of general anesthesia, and is associated with vasoconstriction. It increases oxygen consumption, carbon dioxide production, and cardiac output, and is more common in the PACU and postpartum.

Meperidine (12.5-25 mg) can be used to treat shivering in adults.

Newborns do not shiver due to the presence of brown adipose tissue, which generates heat.

Malignant Hyperthermia (MH) is a rare genetic hyper-metabolic disease triggered by certain anesthetics and succinylcholine.

The pathophysiology of MH involves uncontrolled increase of intracellular calcium in skeletal muscle, leading to sustained muscle contraction and hypermetabolic state.

Early clinical manifestations of MH include muscle rigidity, tachycardia, and unexplained hypercarbia.

Diagnosing MH involves ETCO2 levels, vital sign changes, and specific lab tests such as CK levels, hyperkalemia, hypermagnesemia, and reduced mixed venous O2 saturation.

Phase 1 involves a 1-2°C drop in the first hour of anesthesia, phase 2 lasts 3-4 hours with a gradual decline, and phase 3 represents the steady state of temperature.

Normal temperature regulation is maintained by the hypothalamus through mechanisms such as sweating and shivering.

Methods of heat transfer include conduction (heat movement through a substance), radiation (heat transferred between objects), convection (heat transfer through fluid movement), and evaporation (the loss of latent heat of vaporization from the skin's surface).

Heat loss from anesthetized patients is 40% from radiation, 32% from convection, and 28% from evaporation.

Anesthesia inhibits normal temperature regulation through vasodilation, vasoconstriction, and interference with the hypothalamic response.

Intraoperative considerations include cold OR temperature, prolonged exposure of open wounds, use of room-temperature IV fluids, and high flows of un-humidified gases.

Intraoperative management involves prewarming the patient with convective forced air warming blankets, warm water blankets, heated humidification of gases, and warming IV fluids.

Dantrolene interferes with muscle contraction by binding to the RyR1 receptor, inhibiting calcium release from the sarcoplasmic reticulum (SR).

The dose of Dantrolene for MH is 2.5 mg/kg IV every 5 minutes until the episode is terminated. It is available as a 20 mg hydrolyzed lyophilized powder, reconstituted with 60 ml sterile water (Revonto, Dantrium) or as a 250 mg powder, reconstituted with 5 ml (Ryanodex).

For a 70 kg patient, the recommended initial dose of Dantrolene for MH would be 175 mg.

Postoperative considerations for MH include confirming the diagnosis with a CK baseline and halothane-caffeine contracture test, as well as considering genetic testing and MH registries.

The differential diagnosis of MH includes CO2 insufflation, NMS, thyroid storm, pheochromocytoma, drug-induced hyperthermia, serotonin syndrome, iatrogenic hyperthermia, brainstem or hypothalamic injury, sepsis, and transfusion reaction. Conditions closely mimicking MH's clinical presentation include NMS, drug-induced hyperthermia, and serotonin syndrome.

NMS is characterized by hyperthermia, muscle rigidity with extrapyramidal signs, altered consciousness, and autonomic lability in patients receiving antidopaminergic drugs. It presents with mild hyperthermia, tachycardia, labile BP, diaphoresis, increased secretions, and urinary incontinence. Unlike MH, NMS does not involve the acute crisis seen in MH and may take hours to weeks to develop.

It is recommended that patients with a history of NMS do not receive succinylcholine or volatile agents.

Potential complications of Dantrolene use in MH include muscle weakness, including respiratory insufficiency and aspiration pneumonia. It may also cause phlebitis in peripheral veins, so central administration is preferred if possible.

Measures to cool a patient experiencing MH include surface cooling with ice packs, cold air convection, or cooling blankets, lavage with iced saline, and if necessary, cardiopulmonary bypass if other measures fail.