Post-anesthesia shivering (PAS), experienced as uncontrollable shaking after a procedure, is a common and temporary side effect of surgery. This involuntary muscle activity is the body’s normal attempt to restore its internal temperature balance following the effects of anesthetic agents. It is a well-recognized phenomenon in the post-anesthesia care unit (PACU). Understanding the root causes helps explain why the body temporarily loses control of its temperature regulation.
Why Post-Anesthesia Shivering Occurs
Anesthesia, especially general anesthesia, significantly impairs the body’s natural ability to regulate its temperature, which is primarily managed by the hypothalamus. Anesthetic agents interfere with the hypothalamus, widening the normal temperature range, known as the interthreshold range, within which the body does not initiate warming or cooling responses. The normal interthreshold range is very narrow (around 0.2°C), but general anesthesia can increase this range up to 4°C, allowing the core temperature to drop without triggering a response.
A major factor in this temperature drop is peripheral vasodilation, where anesthetic drugs cause blood vessels near the skin surface to widen. This vasodilation leads to a rapid redistribution of heat from the body’s core to the cooler extremities and the environment. As the anesthesia wears off, the hypothalamic thermostat attempts to reset itself toward the normal temperature. It senses a core temperature that is below this new, higher threshold.
Shivering is the body’s last line of defense, a rapid, involuntary muscular contraction that generates heat through thermogenesis. This “rebound shivering” results from the body trying to quickly raise its core temperature back to normal after the anesthetic-induced thermal suppression ends. While hypothermia is the primary trigger, shivering can also be caused by factors such as postoperative pain or the residual effects of certain anesthetic drugs.
Practical Warming Methods Used for Recovery
The primary defense against post-anesthesia shaking involves external, non-pharmacological methods to actively rewarm the patient. Medical staff prioritize these interventions to increase the patient’s body heat content and improve comfort. Continuous monitoring of core temperature and vital signs is integral to tracking the effectiveness of the warming measures.
Forced-air warming (FAW) devices, often blanket-like coverings inflated with warm air, are effective and widely used. These systems transfer heated air to the patient, reducing heat loss through convection. FAW has been shown to be more effective than simple cotton blankets in achieving normothermia, helping the patient quickly pass the shivering temperature threshold.
Passive measures are also employed to provide insulation and prevent further heat loss. These include the use of warmed cotton blankets. Additionally, warming intravenous (IV) fluids before administration helps raise the patient’s core temperature internally. Providing warm drinks, once the patient is fully awake and able to swallow safely, can also contribute to the rewarming process.
When Medical Intervention is Necessary
While external warming is typically sufficient, severe or persistent shaking sometimes requires pharmacological intervention. Shivering increases metabolic demands, significantly raising the body’s oxygen consumption (potentially by 300% to 400%). This puts substantial stress on the cardiopulmonary system and is risky for patients with pre-existing heart conditions, raising concerns about myocardial ischemia.
The drugs used to stop severe shivering work by lowering the body’s shivering threshold temperature in the hypothalamus. The most common medication class used for this purpose is certain opioids, such as meperidine, which has demonstrated high effectiveness. Other agents, including tramadol and clonidine, may also be administered to manage intense shaking that does not respond to external warming alone.
Staff will administer additional oxygen via a mask to compensate for the increased oxygen consumption caused by the muscle activity. Shivering is usually short-lived, often stopping within 20 to 30 minutes. Immediate medical consultation is necessary if the shaking is accompanied by symptoms like severe pain, confusion, or difficulty breathing. The medical team weighs the benefits of quick pharmacological relief against the potential side effects of the medications used.