Depressed Metabolism

This is the third entry in a series about resuscitation of non-hibernating rodents from circulatory arrest at ultraprofound hypothermic and high subzero temperatures. Andjus and Smith were delighted that they had managed to modify Andjus’ chest-wall heating technique from using a hot metal spatula to using a focused beam of light in order to preferentially warm the heart before warming the whole body. This modification resulted in a substantially larger percentage of rats fully recovering from ultraprofound hypothermic temperatures as well as a significant reduction in the number of delayed deaths after partial recovery. However, some delayed deaths still occurred, and Andjus and Smith speculated that these were likely due to the inevitable burns caused by these techniques. So Andjus collaborated with J.E. Lovelock to further refine the protocol in an attempt to eliminate peripheral tissue damage during heating, as described in their 1955 article in the Journal of Physiology.

This was accomplished by using a microwave diathermy apparatus “powered by a 500 W continuous wave magnetron operating at a frequency of 3000 Mc/s feeding into an H01 mode waveguide.” An aperture was created in an extension of the waveguide and the animal was placed underneath for preferential heating of the heart. Rise in temperature varied across different parts of the body and was steepest in the left side of the chest.

Cooling was carried out in the same manner described in previous posts, and reanimation procedures began when the animal’s colonic temperature was between 0 and 1 degrees C. In a first series of experiments, warming was carried out using microwave diathermy and artificial respiration was given by means of a hand bulb and tubing inserted into the nostrils and discontinued after spontaneous breathing was reestablished. When the animal reached 15 degrees C it was then placed in a 40 degrees C water bath for whole body rewarming to 33 degrees C, whereupon it was placed in an incubator for 3 days before being transferred to long-term care in the animal facility.

This first series of experiments resulted in an 80% full recovery rate  — already a 5% improvement on the focused light beam method. However, the strong focus of microwaves through the aperture still resulted in occasional burns to the chest wall. With another slight modification to the protocol (the use of a horn radiator to produce a more even field distribution of microwaves), the recovery rate reached 100% and no burns were observed.

Since they appeared to have perfected the method, they performed an exploratory experiment on one rat, cooling it and reanimating it a total of 10 times (each experiment separated by 2-10 days) using the Series II microwave diathermy technique. This rat also recovered fully each time.

From an initial full recovery rate of 20% using the hot spatula method to a 100% recovery rate using microwave diathermy, Andjus demonstrated his grasp of the issues involved in resuscitating rats from ultraprofound hypothermia and a singular dedication to overcoming obstacles. Because of his primary interest in applying these resuscitation techniques to victims of accidental hypothermia and freezing, the road ahead was clear: now that he had a method for reanimating rats that had reached 0 – 2 degrees C, he next wanted to know the time limits to resuscitation (how long can a rat be held at these temperatures and still be successfully revived?), the effects of multiple coolings and reanimations, and — the ultimate question — whether a rat could be frozen and then resuscitated.

He tackled all of these issues in his next publication, which will be discussed in the next post.