Depressed Metabolism

This is the second entry in a series about resuscitation of non-hibernating rodents from circulatory arrest at ultraprofound hypothermic and high subzero temperatures. As discussed previously, in 1951 Radoslav Andjus developed a simple method for resuscitating rats cooled to deep body temperatures between 0 and 2 degrees C which involved applying a hot spatula to the left anterior chest wall to warm the heart and re-initiate circulation before rewarming the rest of the body. While he experienced some success with this method, a high proportion of the rats which regained heart beat and respirations died during subsequent warming of the whole body or within a few hours or days of regaining normal body temperature and behavior.

When Andjus met Audrey Smith, they collaborated in 1955 to determine whether the reanimation failures and high mortality rate after initial success were the result of damage during cooling or due to imperfections of the reanimation protocol. They began by comparing Andjus initial cardiac heating method to an alternative method employing a high-intensity lamp to project focused light on the chest wall.

In both protocols, rats were cooled according to the previously described method of placing an animal in a jar and then putting the vessel in a refrigerator. Once rats were cool and lethargic, they were immersed in dishes of melting ice and buried under crushed ice. They remained under ice for exactly 1 hour from the time colonic temperature reached 15 degrees C and for approximately 40 minutes after it had reached 6 degrees C. In general, respirations ceased soon after ice immersion, the heart beat slowed suddenly between 10 and 13 degrees C, and final heart beats were observed at or above 8 degrees C. The rats were removed from ice with body temperatures between 0 and 1.8 degrees Celcius.

When attempting resuscitation using the spatula method, the spatula was warmed in a Bunsen flame and applied as frequently as 20 times per minute. When the first heart beat was observed, artificial respirations began using a small hand-bulb attached to tubing inserted into the nostrils. Local cardiac heating was performed less frequently as heart rhythm became regular, and was discontinued when heart rate increased spontaneously. At 10-11 degrees C the animal’s neck was heated under hot tap water and artificial respirations were continued until spontaneous breathing resumed.

When attempting resuscitation using a beam of light, the rats were placed on a platform under a duralite shield with an aperture that allowed for focusing of the light on the praecordium. The intensity of the light/heat could be controlled by changing the variac setting. The neck was warmed under the light beam when colonic temperature reached 10-12 degrees C and artificial respirations were given until spontaneous breathing resumed. Rewarming of the whole body in both protocols occurred by placing the rats in a 37 degrees C water bath until they could maintain normal posture, at which point they were transferred to an incubator for short-term recovery, then to an warm cupboard for long-term care.

Results obtained by the spatula method were similar to those obtained in Andjus’ initial experiments. Out of 25 rats, 4 exhibited irregular heart beats and then succumbed, 4 exhibited regular heart beats but no spontaneous breathing and were dead within an hour, 6 exhibited spontaneous breathing but no reflexes and died in the 37 degree C bath, 5 exhibited an apparently complete recovery and died within 24 hours, and only 5 survived for more than 66 days.

Results obtained by the beam of light method were better. Using the initial protocol, which involved a large number of changes in variac settings at lower intensities over the course of warming, 11 of 25 rats survived more than 66 days. Modifications to start at a higher intensity and reduce the number of changes in variac settings resulted in 17 of 25 rats surviving more than 66 days, representing a long-term survival rate of 75%. In addition, far fewer (2) delayed deaths occurred using this protocol.

Andjus and Smith speculate about the importance of proper cardiac warming for successful reanimation from ultraprofound hypothermia in their discussion of these landmark experiments:

It is likely that the method of reanimation is of great importance. When an animal with a deep body temperature of 0-2 degrees C is transferred to a hot bath at +45 degrees C as in the experiments of Lutz (1950) the skin and superficial tissues must rewarm rapidly and experience anoxia for many minutes before the heart is warm enough to beat and provide an adequate circulation. If, on the other hand, the heart is rewarmed first and a circulation established before the temperature of the bulk of the body rises, the degree and duration of tissue anoxia may be greatly reduced.

They go on to anticipate improvements in their method beyond those already achieved:

It was remarkable that the revival rate in our experiments was increased from 20 to 75% when local heating on the surface of the chest wall was superseded by heating with a beam of light. The amount of heat penetrating to the anterior surface of the heart was undoubtedly increased when the chest wall was irradiated, but the oesophageal thermocouple showed that the temperature of the posterior aspect of the heart lagged behind. These results suggest that a more efficient method for rewarming the heart rapidly should make it possible to revive all rats from body temperatures between 0 and 1 degree C.

And of course they went on to test this hypothesis, as well. But that’s a topic for the next post.