When it comes to surgery, people often focus on whether the procedure itself goes smoothly and the surgeon's skill. However, one easily overlooked detail is that many patients quietly feel cold during or after surgery. This isn't ordinary chills, but a medical condition known as mild perioperative hypothermia. While it might seem like just a slight drop in body temperature, it can trigger a cascade of complications.

I. First, Understanding: What is Mild Perioperative Hypothermia?

Our normal core body temperature (e.g., within the chest and abdominal cavities) is generally between 36.5°C and 37.5°C. Mild perioperative hypothermia refers to a drop in core temperature to between 34°C and 36°C around the time of surgery. While 34°C might not sound extremely low, for the body during surgery, this is considered a state of hypothermia requiring vigilance, and it is particularly common among surgical patients.

II. Why Do Patients Get Cold During Surgery?

1. Anesthesia "Weakens" the Body's Temperature Control

Anesthetic drugs cause the body's thermoregulatory system to become less effective. For instance, while the body would normally constrict blood vessels to conserve heat when cold, under anesthesia this "vasoconstriction threshold" is lowered by approximately 2-4°C. Simply put, the body waits until it's colder before activating its heat preservation mechanisms, making it easier for the temperature to drop.

2. Heat "Escapes" Too Quickly

During surgery, heat from the body's core areas (e.g., internal organs) is redistributed to peripheral areas like the limbs. Combined with the typically low ambient temperature of the operating room and exposed skin, heat loss accelerates. This is especially true for children, who have a higher body surface area to weight ratio than adults, leading to faster cooling and rewarming, necessitating greater attention.

III. The Significant Harms of Mild Hypothermia

Soaring Cardiac Risk: A drop in core temperature of just 1.3°C can triple the incidence of adverse cardiac events (such as arrhythmias, myocardial infarction). Cold stress can also increase norepinephrine levels in elderly patients, adding strain to the heart and potentially triggering dangerous events.

Distressing Postoperative Shivering: Shivering is the body's "self-rescue" response to generate heat. However, anesthesia lowers the shivering threshold (normally around 35.7°C, it can drop to about 35.4°C under epidural anesthesia). While shivering doesn't directly cause heart attacks, it causes significant patient discomfort. Notably, elderly patients shiver less frequently due to their weaker thermoregulatory responses. Medications like nefopam and clonidine can help prevent and alleviate shivering.

Impaired Coagulation: Hypothermia doesn't reduce platelet count but inhibits platelet function (possibly due to reduced release of pro-coagulant substances). Crucially, standard coagulation tests (like PT, APTT) are performed at 37°C and do not reflect this hypothermia-induced coagulopathy. At lower actual body temperatures, clotting slows down, potentially leading to increased intraoperative bleeding and higher transfusion requirements.

Increased Wound Infection Risk: Hypothermia causes subcutaneous vasoconstriction, reducing oxygen delivery to tissues. It also directly impairs the "bactericidal ability" of immune cells (like neutrophils and T-cells), making it easier for bacteria to thrive in wounds. Furthermore, hypothermia exacerbates postoperative protein loss, impairing wound healing, potentially prolonging hospital stays and increasing costs.

Slowed Drug Metabolism, Prolonged Recovery: Drug-metabolizing enzymes are temperature-sensitive. A 2°C drop in core temperature can double the duration of action of muscle relaxants. Hypothermia also slows the elimination of anesthetic agents, increasing the amount needing clearance and prolonging the patient's stay in the post-anesthesia care unit (PACU). For instance, patients receiving propofol whose temperature drops by 3°C can have plasma drug concentrations 30% higher than those maintaining normal temperature.

IV. Proactive Measures: Providing "Warm Protection" for Surgical Patients

1. Preoperative: Pre-warming

Many patients arrive in the operating room with a significant temperature difference already existing between their core and limbs. Active warming for 1-2 hours before anesthesia (e.g., using a warming blanket) not only improves patient comfort and vasodilation for IV access but also reduces the heat redistribution to peripherals that occurs after anesthesia induction. Additionally, premedication with midazolam can reduce heat loss, with moderate sedation (0.04 mg/kg) being more effective than deep sedation.

2. Intraoperative: Focus on Surface Warming

Approximately 90% of the body's heat loss occurs through the skin, making skin insulation crucial. Methods are divided into "passive insulation" and "active warming":

Passive Insulation: Essentially "covering with blankets." A single layer can reduce heat loss by 30%, but adding multiple layers has limited additional benefit as insulation primarily relies on the still air layer between the skin and the covering.

Active Warming: More effective than passive insulation. Key methods include:

Forced-Air Warming (FAW) Devices: Consist of an electric heater/blower and a disposable warming blanket. They can transfer 30-50W of heat via convection and also reduce radiant heat loss from the skin (e.g., from ~100W to ~70W). They are often more effective for warming limbs than the trunk.

Concerns about increased infection risk? Modern devices incorporate bacterial filters, and studies show they can actually reduce infection rates by about 30%, provided disposable warming blankets are not reused and air hoses are cleaned regularly.

Heating Mattresses/Pads: Can be as effective as FAW, are highly efficient (transferring most heat to the patient), and are particularly suitable for pre-hospital/field use (e.g., when battery-powered).

3. Intraoperative: Internal Warming as an Adjunct

Fluid Warming: Infusing 1 unit of refrigerated blood or 1 liter of room-temperature intravenous fluid can decrease the mean body temperature by approximately 0.25°C. Therefore, warming fluids is essential during substantial fluid administration. However, it cannot replace skin surface warming and is insufficient alone.

Amino Acid Infusion: Can increase metabolic heat production. It may also shorten hospital stay by potentially improving wound healing and gut function.

Conclusion

The "cold" experienced in the perioperative period might seem minor but can trigger significant issues like cardiac risks and wound infections. Today, healthcare teams actively work to maintain patient core temperature above 36°C using strategies like pre-warming and forced-air warming to reduce complications. If you or a family member is facing surgery, don't hesitate to discuss warming strategies with your medical team – this "warm protection" is a vital component of surgical safety.

References
[1] TANG Shuai, WANG Ling, HUANG Yuguang. Complications and Preventive Measures of Mild Perioperative Hypothermia [J]. Chinese Journal of Anesthesiology Online, 2007, 9(1): 1-4.