Underbody air warming systems have the potential to set a new standard of patient care in temperature management. This blog will expose you to the good, the bad and the delusional in hypothermia prevention. We will discuss the pathophysiology and the goals of temperature management as well as what is not and what is the clear answer to underbody air warming mastery.
The Pathophysiology of Hypothermia during General Anesthesia
Heat loss during anesthesia is best understood by dividing the patient body into the core and peripheral compartments. The core compartment is defined as the head, chest and abdomen which represents 50 to 60 percent of body mass. The temperature here remains relatively uniform. The peripheral compartment is comprised of the skin, legs and tail. The temperature here is 3.6 to 7.2 F degrees cooler than the core compartment creating a temperature gradient between the core and peripheral compartments. The peripheral compartment temperature can fluctuate depending on the ambient temperature.
The core to peripheral temperature gradient (3.6 to 7.2 F) is normal. This gradient is controlled by the hypothalamus to mitigate heat loss to the environment. The hypothalamus redirects blood flow from the peripheral to the core compartment by peripheral vasoconstriction and increases metabolic heat production to maintain stable core compartment temperature. Neurogenic thermoregulation by the hypothalamus exists to maintain stable core compartment temperature.
General anesthesia causes a loss of neurogenic thermoregulation by the hypothalamus thereby redistributing warm core blood to the cooler periphery. Virtually all anesthetic drugs are vasodilators which cause a loss of compensatory peripheral vasoconstriction. Consequently, warm core blood flows to the periphery where it is cooled by the four mechanisms of heat loss; conduction, convection, radiation and evaporation. The patient essentially becomes a heat radiator by losing body heat to the ambient environment. Additionally, there is a 20 to 40 percent decrease in metabolic rate and consequently a drastic drop in heat production. Unless preemptive heat loss preventative measures are taken, in the first 30 to 45 minutes after anesthesia induction the patient will lose 81 percent of the body heat that it will ultimately lose during the course of that anesthetic.
When considering hypothermia during general anesthesia, it is instructive to think of hypothermia in three Phases.
Phase 1
Phase 1 is the initial rapid decrease in body temperature which occurs during the first hour of anesthesia as seen in the accompanying graph. This is caused by the rapid redistribution of blood flow from the core to the cooler peripheral compartment due to peripheral vasodilation by anesthetic drugs. Body heat is then lost through the skin by the four mechanisms of heat loss. Redistribution of blood flow depends on the core to peripheral temperature gradient. A large temperature gradient causes a greater flow of warm core blood to the periphery. Phase 1 accounts for 81 percent of the heat loss that will potentially occur during the course of an anesthetic. Since Phase 1 is the time when interventions are most effective, active warming must start immediately upon anesthetic induction and sooner if possible.
Phase 2
Phase 2 is a slow linear decline in body temperature which occurs over 2 to 4 hours. During this phase heat loss exceeds metabolic heat production because the hypothalamus has been rendered inactive by general anesthesia. The decreased
metabolic rate causes a loss of heat production by 20 to 40 percent. The rate of temperature decrease depends on the difference between heat lost and the heat produced and the mass of the patient. A Saint Bernard will have a slower rate of heat loss than a Chihuahua.
Phase 3
Phase 3 is essentially a plateau in which there is core temperature stability. This occurs after 3 to 4 hours of anesthesia in which metabolic heat production equals heat loss.
The Goals of Temperature Management
In view of the pathophysiology we have just discussed what should be our temperature management goals?
Goal #1
Increase the temperature of the peripheral compartment to decrease the temperature gradient.
Goal #2
Prevent redistribution hypothermia by equalizing the temperature between the core and peripheral compartment. Some patients can become hypothermic upon anesthetic induction just by the massive shift of warm core blood to the cooler periphery by the vasodilatory effect of the anesthetic induction.
Goal #3
Expose as much body surface area to your active warming device as possible.
Goal #4
Retain as much heat coming from the patient and the warming device as possible.
Goal #5
Continue active warming to maintain normothermia until the patient emerges from anesthesia.
What is the Answer to meeting these Goals?
The above 5 goals are key to maintaining patient normothermia. Is there an underbody air warming system that is up to the task?
What is NOT the answer
The most common method of patient warming has been with a forced warm air blower attached to a disposable blanket with air channels and multiple pin sized air exit holes.
The disposable blanket is typically placed over the nonsurgical area of the patient thereby blowing warm air onto the surface of the patient.
Major drawbacks to this system are:
- The surface area that can be warmed is limited to a portion of the patient that is not part of the surgical field. For example, in an abdominal procedure only the top of the chest and head can be warmed which is woefully inadequate to maintain normothermia.
- The majority of patient blood flow when positioned for surgery is in the dependent portions of the body due to gravity. When only the superior surface of the body is accessible for the placement of a warming blanket, the warming of blood in the dependent areas of the body will be nonexistent.
- Placing the patient on a disposable warm air blanket or on a cloth bag tied to the end of a blower hose in the hope that underbody warming can be achieved is a delusional concept. When a patient is placed on a disposable blanket the air channels collapse and the pin sized air holes are obstructed by the weight of the patient making it useless in warming the patient.
Please see the thermographic image below with a 9 lb. block of wood on a disposable air warming blanket with a warm air blower. The picture speaks for itself. It demonstrates no warm air flow underneath the block (patient).
The obvious truth is that a forced air warming system with a disposable blanket or a cloth bag tied to the end of a blower hose cannot provide true underbody warming.
The Clear Answer – The HoverHeat
The HoverHeat patient warming system has raised the bar for the Standard of Care in temperature management. The HoverHeat is the only true underbody patient warming system that has been designed with the pathophysiologic principles and goals of temperature management in mind. The HoverHeat is designed with two proprietary, tissue friendly internal component layers which levitate the patient and provide for low resistance warm air flow underneath the patient. Simply insert the hose of your warm air blower into one of the inlets of the HoverHeat and you are ready to warm your patient. Warm air then flows circuitously to the air outlet and away from the surgical field.
The thermographic image below shows the same 9 lb. block of wood placed on a HoverHeat with the same warm air blower demonstrating the clear flow of warm air through the HoverHeat and underneath the block (patient). This demonstrates true underbody warming.
The HoverHeat patient warming system is the clear answer to the patient temperature management goals previously listed.
Goal #1
Start active warming in a cage 30 minutes or more before anesthetic induction by connecting the hose of your warm air blower to a HoverHeat Cage Warming Combo which facilitates the warming of the cage interior with your warm air blower. This warms the peripheral compartment (skin, legs, tail) before induction to prevent the rapid Phase 1 decline in body temperature.
Goal #2
Continue active warming during anesthetic induction and preoperative prep by placing the patient on a functioning HoverHeat. This prevents redistribution hypothermia by equalizing the temperature between the core and peripheral compartment. This limits the massive flow of warm core blood to the periphery because the temperature gradient has now been virtually eliminated. The HoverHeat is fluid proof and easily cleaned.
Goal #3
The key in patient warming is to expose as much surface area of the body to your active warming device as possible as well warming as much core and peripheral blood as possible in the dependent parts of the body. The HoverHeat does both. Since the HoverHeat provides true underbody warming, the entire undersurface of the body and the dependent blood flow can now be warmed no matter what the procedure which has never been previously possible with forced air warming. Additionally, since the air coming out of the outlet is approximately the same temperature as the air going into the HoverHeat inlet, a HoverHeat connector hose can be used to attach to another over body HoverHeat unit to provide under and over body warming with one warm air blower.
Goal #4
It is imperative that heat emitted from the patient and the warming device be retained during the perioperative period to maintain normothermia. The Conrad Thermal Blankets work synergistically with the HoverHeat to optimize heat retention.
Goal #5
The end of the procedure does not mean that active warming can be discontinued. Maintain active warming with the HoverHeat until the patient emerges from anesthesia. Postoperative return to normal core body temperature occurs when hypothalamic anesthetic concentration decreases sufficiently to again trigger normal thermoregulatory defenses; compensatory peripheral vasoconstriction and metabolic heat production. The HoverHeat can be used in any postoperative recovery area including cage recovery.
The HoverHeat is unmatched in providing clinical efficiency and effective temperature management. The HoverHeat accommodates any warm air blower that you currently have and will increase the warming capacity of your blower by 50 to 75 percent by increasing your warming surface area. Since the HoverHeat does not have multiple pin sized holes blowing contaminated air over the surgical field, the HoverHeat design is such that air is directed away from the surgical field. Additionally, the HoverHeat is a snap to clean and you will never need to purchase a disposable warming blanket again. The HoverHeat system is designed to handle any sized patient in your practice. Yes. Even a 300 lb. donkey.
The HoverHeat patient warming system is available as a Set or by individual units. Our VetPro warm air blower marries perfectly with the HoverHeat units and provides quiet efficient operation.
