A special guest blog post by Dr Colin Dunlop.
Colin Dunlop is a Diplomate of the American College of Veterinary Anesthesiologists. His research interests include hypothermia and prevention of anesthesia morbidity and mortality. He consults in anesthesia and critical care for small and large animal practices, biomedical research, and provides Continuing Education programs for veterinarians and veterinary nurses world-wide.
In my spare time I try to run a company that manufactures anesthetic delivery equipment and devices for patient warming, so please understand that some of the information I have included references specific warming devices, some of which we manufacture. The information sheets linked to this post try to fairly assess the capabilities of the various technologies and summarize information we have from published research and data from our in-house testing. Written here is information I wrote in a hypothermia article some years ago. It is simply to highlight the practicality of warming a hypothermic patient using IV fluid. It is written in calories, and a Calorie is the heat required to raise 1 ml water by 1 degree Celsius.
Using the same information, you could also attempt to a very simplistic estimate of heat loss due to humidification of inspired air. Heat conservation mechanisms (the nose!) play a big part, and intubation completely alters them. Recent work shows that the loss of heat of warmed gas from the Y-piece to the distal end of the endotracheal tube is up to 10 degrees Celsius!
IV Fluid Warmer Comparison
Not all IV Fluid Warmers are the same, and some can even be dangerous. Test data from our evaluation of IV Fluid Warmers can be found here: IV Fluid Warmer Comparison. According to our tests, most are ineffective if their performance is tested 200mm downstream from the fluid warmer, which is equivalent to the distance to an anesthetized animal, draped for surgery. Their performance is also affected by IV fluid flow rate – the larger the volume of fluid administered rapidly the less effective they are at warming.
Hanging bags of warm IV fluids in a cold operating room will produce the same kind of result as using an IV Fluid Line Warmer, as illustrated in the evaluation of IV Fluid Warmers linked above.
The one-page guide linked here: Hypothermia Review summarizes information from various sources and includes an idealized graph that show heat loss occurs from the time of premedication, and that substantial heat loss occurs from induction to the time an animal is draped in surgery. Once draped, heat loss tends to stabilize, but warming hypothermic anesthetized animals during surgery is very difficult. In fact even the best forced warm air heating systems, which are the most effective way to deliver large volumes of heat, typically take 45minutes of “contact” before body temp begins to rise. So using forced warm air heating devices to “increase” the body temperature during anesthesia for short procedures is probably not very effective. The rationale for pre-warming and preventing heat loss prior to draping for surgery is where our efforts should be focused.
Forced Warm Air Heating Review
The white paper linked here: FWAH Review and Cage Warming shows the lag-time for warming and that not all these systems actually raise body temperature. Also this paper makes the case for warming animals after premedication, before induction. Typically, warming animals during recovery, who became hypothermic in surgery animals, takes 1 to 2 hours of technician time. Conversely, keeping animals at 37 degrees Celsius takes less than 45 min of “pre-warming”, and the blankets placed over animals in cages can be re-used.
Effective warming with fluids
Finally, use warm fluids effectively in severely hypothermic animals at the end of abdominal or thoracic surgery by pouring large volumes of warm (40 degrees Celsius) into the abdominal or thoracic cavity. Be patient. Wait several minutes before suctioning it out and then repeat this process three or four times, until the body temperature starts to rise. Then close the cavity. At the same time use forced warm air heating, which will further increase the body temperature.
Calculating Calories and Warming With IV Fluids
A calorie (cal) is the amount of heat required to raise 1 ml (or 1 gm) of H2O 1 oC.
The specific heat of animal tissue is 0.83 cal/gm. Therefore a 10 kg dog requires 8,300 cal (8.3 kcal) to raise its temperature 1 oC.
Warming IV fluid administered during surgery:
A 10 kg dog administered IV fluid at 10 ml/kg/hr = 100 ml/hr.
If the fluid is warmed to 44 oC and the dog is 34 oC, then we can deliver:
(44-34oC =) 10 oC x 100 ml/hr = 1000 cal/hr
To Warm the 10 kg at 34 oC dog to 37 oC using IV fluid at 100 ml/hr requires:
(37-34 =) 3oC x 8,300 cal = 25,000 cal (approx) / 1000 cal/hr (from the IV fluid) = 25 hours!
Warming IV fluid may help prevent cold fluid exacerbating heat loss but is not effective for warming severely hypothermic animals.
Respiratory heat loss due to humidification is significant
During inspiration the nose and pharyngeal mucosa transfer heat and moisture to the air which is largely recovered during expiration, thus conserving heat. Air has a low heat capacity (0.24 cal/gm) and a low weight (1.3 gm/l). Saturated air holds 44 mg H2O/L at 37 oC which requires 24 calories. A 10 kg dog taking 20 x 100ml breaths/min ventilates 120 L/hr so requires (24 cal/L x 120 L/hr) = 2880 cal/hr for humidification. Intubation inhibits heat/moisture conservation via the nose, resulting in body temperature loss of about 1/3 oC/hr.