“Why does anesthesia cause people to shake when coming out of it?”

Side effect of anesthesia sometimes related to disinhibition of excitatory pathways. Sometimes due to hypothermia.

Hello,

During general anesthesia, temperatures goes down (hypothermia). Shivering is the body's way to produce heat.

Shivering after surgery is indirectly caused by anesthesia. When a patient is put under general anesthesia, especially with inhalational anesthetics, their brain and it’s functions are temporarily “turned off”. This includes the area of the brain called the hypothalamus, which is responsible for many things. One of which is thermoregulation. Essentially our hypothalamus is our thermostat and it gets “turned off” during general anesthesia. By turning the thermostat off, the brain can’t respond to temperature changes. Operating rooms are typically kept cool, usually 65 degrees. So our body under general anesthesia begins to cool due to the ambient temperature being 30 degrees less. This is one of the responsibilities of your anesthesiologist. To prevent too much body temperature loss, the anesthesiologist often places a warming blanket over the parts of the body not being operated on. Most of these blankets have a large hose with hot air blowing into it (convection heating) Some have an electric heat source and heat by conduction. The anesthesiologist also often will warm IV fluids being administered by running the IV tubing through a warmed fluid tube.

Finally, the anesthesiologist also uses a heat and moisture exchange unit (HME) added to the patients breathing circuit so that warm, humid exhaled gases heat and humidify the patients oxygen source. The reason for shivering upon awakening is that the hypothalamus retains its function, detects a cooler temperature than it last detected and sends signals to the musculoskeletal system to shiver to generate heat. (Same reason we shiver when we go out into the cold). Some general anesthetics (Total IV Anesthesia) that don’t use inhalational techniques preserve hypothalamic function and thus the brain is able to attempt to maintain heat by methods such as vasodilation and shunting of blood from warmer core areas to cooler peripheral areas of the body.

I hope this answers your question.

There are several theories but no firm answers. There are a couple of treatments though that are quite effective.

Shivering is most commonly a result of a patient's feeling cold. The operating room is kept cool and anesthesiologists routinely give warmed IV fluids and apply forced warm-air heating blankets to patients during surgery to maintain normothermia. Warming blankets can also be applied in postanesthesia care units to warm patients and prevent shivering.

The temperature receptors get recalibrated. Also, the operating room is cold, so body temp may be low.

The anesthesia alters the temperature threshold at which the body shivers. It’s a temporary effect and is easily treated. 

Shivering to anesthesia is usually not related directly to anesthetics, but rather the IV fluids given and the cooler ambient room temperatures present in the operating room, and post-op area. The cool environment and cool IV fluids lower body temperature which is preferable surgical wise for a number of reasons. However, the natural response of the body is shivering
in order to activate the muscles to increase internal body temperature. This is usually temporary and subsides when body temperature returns to normal.