To the Editor:--In recent months, ANESTHESIOLOGY and other journals have carried advertisements from Level 1 Technologies, Inc. guaranteeing that 90% of patients in whom Hotline fluid warming is used will arrive in the recovery room with "final temperature greater or equal to 36 degrees C or net temperature decrease less or equal to 0.2 degree C." The claim that fluid warming alone will maintain normothermia does not agree with much of what is known about perioperative heat balance and seems inconsistent with the laws of thermodynamics.
Intraoperative hypothermia results from a combination of anesthetic-induced inhibition of thermoregulatory control and exposure to a cold environment. [1]Hypothermia during the first hour of surgery results largely from an internal core-to-peripheral redistribution of body heat. Redistribution typically reduces core temperature 0.5–1.5 degrees Celsius [2,3]and is difficult to treat even with active cutaneous warming. [4]Subsequently, mean body temperature decreases when heat loss (including that required to warm administered fluids) exceeds metabolic heat production. [4]During this phase, core temperature is most influenced by cutaneous insulation/heating, fluid warming, and other manipulations altering heat balance. Because radiative and evaporative losses associated with surgery can be enormous, [5]active warming usually is required to prevent hypothermia. [6,7].
One liter of crystalloid at ambient temperature or a unit of refrigerated blood reduces mean body temperature [nearly equal to] 0.25 degree C in adults. It is therefore easy to cool patients by administering unwarmed intravenous fluids. Conversely, it is not possible to significantly warm patients by administering heated intravenous fluids. The reason is that intravenous fluid temperature cannot much exceed 40 degrees C without harming blood cells.
Heat transferred by fluid warming therefore can be calculated as the difference between fluid and mean body temperature multiplied by the volume of fluid. For example, administration of 2 l of fluid at 40 degrees C to a patient with a mean body temperature of 36 degrees Celsius increases body heat content 8 kcal. Assuming a patient weight of 70 kg and a tissue specific heat of 0.86 cal *symbol* degrees C sup -1 *symbol* g sup -1, [8]8 kcal will increase mean body temperature only 0.1 degree C. This trivial amount of warming will rarely compensate for ongoing cutaneous and surgical losses. [4,5]Heat delivered by warmed fluids has the advantage of being directly inserted into the core thermal compartment; it is, of course, rapidly dissipated to peripheral tissues.
The Hotline warmer improves conventional designs by preventing fluid cooling within intravenous tubing. However, the amount of cooling at typical flow rates is of no consequence in adults. [9]In addition, this design compromises efficacy, and the device warms fluids only to 38 degrees C at a flow rate of 1 l/h. [10]The above calculation thus overestimates the benefits of Hotline warming.
To evaluate the Level 1 Technologies, Inc. claim, I conducted the following study. With Institutional Review Board approval, distal esophageal temperature (probes: RSP, Inc., Irvine, CA; monitor: SpaceLabs, Redmond, WA) was measured in all patients aged 18 yr or older to whom I administered general anesthesia during a 1-month period (excluding those undergoing cardiopulmonary bypass). Anesthesia was induced with fentanyl, methohexital, and rocuronium and maintained with desflurane in oxygen. Total fresh gas flow was 900 ml/min; ventilation was controlled to an end-tidal PCO2near 35 mmHg in others. Ambient temperature was 19.3 +/-1.3 degrees C (normal at NT Enloe Hospital), and the patients were covered with a single layer of surgical drape. [11]Fluids were administered at a rate of 10 ml *symbol* kg sup -1 *symbol* h sup -1, always using a Hotline warmer.
Twenty-two patients had surgery lasting at least 1 h (1.5 +/-0.7 h (mean+/-SD); among these, only seven underwent open abdominal procedures. Final core temperatures averaged 35.7 +/-0.4 degree C and equaled or exceeded 36 degrees C in only six. In addition, the decrease in core temperature exceeded 0.2 degree C in all but two cases. Consequently, 68% of the cases failed to meet the criteria "absolutely guaranteed" by Level 1 Technologies, Inc. To their credit, and as offered in the advertisement, a representative of Level 1 Technologies, Inc. immediately offered to refund the purchase price of the Hotline unit and replace the disposable materials. Nonetheless, my results suggest that the Level 1 Technologies, Inc. advertisement is based more on wishful thinking than usual clinical experience.
It would be relatively easy to design a study in which use of the Hotline would maintain temperature of patients greater or equal to 36 degrees C. Level 1 Technologies, Inc. may be able to cite such a study, although I am not aware that one has been published. Final core temperatures often will exceed 36 degrees C if the study population is restricted to well covered patients [12]undergoing relatively noninvasive procedures [13,14]in a warm environment. [15]Numerous studies demonstrate that fluid warming alone will not maintain normothermia in patients undergoing typical procedures in a typical ([nearly equal] 19–21 degrees C) environment. [4,6,7]My data are consistent with these peer-reviewed publications.
In summary, only [nearly equal to] 32% of patients given fluids warmed with a Hotline were normothermic at the end of surgery. That fluid warming alone would fail to maintain normothermia is expected from previous studies of intraoperative heat balance and the laws of thermodynamics.
Victor Werlhof, M.D., Attending Anesthesiologist, NT Enloe Hospital, Chico, California 95926.
