A simple method to resuscitate rats from cold circulatory arrest
This is the eighth entry in a series about resuscitation of non-hibernating rodents from circulatory arrest at ultraprofound hypothermic and high subzero temperatures. In 1982, P.D. Rogers and G.P. Webb published some of their observations (based on previous papers and a Ph.D thesis) after carrying out a classroom demonstration of suspended animation in which they cooled rats and then resuscitated them after 30 minutes at 0 degrees C. The demonstration was performed as a means to stimulate discussion among students regarding the characteristics and diagnosis of death, the effects of hypoxia during cooling, and the limitations of ECG measurements.
Because the “Giaja method” of cooling employed by Andjus and Smith induced hypoxia and hypercapnia, the authors were interested in comparing resuscitation rates in hypoxic vs. non-hypoxic animals. They did so by anesthetizing rats and immersing them (except for limbs, tail, and head) in crushed ice and water to induce ultra-profound hypothermia, as measured by rectal temperatures. During the cooling process, some animals were artificially ventilated until cardiac arrest (respired rats), while others were not (unrespired rats). After 30 minutes of cardiac arrest at temperatures near 0 degrees C, all rats were ventilated during rewarming in a 40 degree C water bath until heartbeat returned and reached 60 beats/min, at which point they were removed from the bath and warming was continued under a 100 W lamp. ECG was recorded throughout.
Rogers found that approximately 90% of respired rats began breathing spontaneously during rewarming and 100% regained heartbeat. On the other hand, less than 10% of unrespired rats recovered spontaneous respiration during rewarming, and when the heart did restart (it often did not), heartbeats were erratic and did not circulate blood due to severe vasodilation assumed to be caused by the combination of hypoxia and hypothermia. Rogers found that he was able to resuscitate 70-90% of unrespired rats by means of abdominal compression (i.e., “abdominal pumping”), but even this method was only successful when the heart restarted.
Though it is easy to assume that hypoxia is the cause of more difficult and less successful resuscitation of unrespired vs. respired rats, Rogers and Webb point out that respired rats may simply be benefiting from the protective effects of hypocapnia on pH changes during hypothermia. They discuss at length the question of “what is the optimal pH in the hypothermic animal,” which remains unanswered.
An interesting phenomenon known as “heart block” was also demonstrated by these experiments. ECG recordings obtained from unrespired rats often showed a QRS complex during rewarming, which most people would assume to indicate that the heart had restarted. However, because ECG is simply a record of electrical activity, this is not always the case:
The observation that a QRS complex occurs in the absence of cardiac output illustrates the limitations of ECG measurements. The ECG is a record of electrical activity within the heart, and any conclusions about mechanical events are extrapolation, though usually with sound theoretical and empirical foundation. In fact, when the chest cavity is opened in unrespired animals with temporarily restarted hearts, it is possible to record QRS complexes in the absence of any apparent heartbeat, i.e., dissociation between excitation and contraction.
Suggestions for further hypothermia experiments in rats include measuring blood pressure during cooling and rewarming, removing blood samples for pH and gas analysis during the experiment, and monitoring electroencephalogram (EEG). Having discovered in previous experiments that unrespired rats suffered from a collapse in blood pressure during cooling prior to cardiac arrest, while cardiac arrest and blood pressure collapse occurred simultaneously in respired rats, Rogers also wonders whether this pre-arrest collapse can be prevented with vasoactive medications and whether this would improve resuscitation rates in unrespired rats. Answering questions such as these would have far-reaching implications in the treatment of accidental hypothermia in humans.
The method that was used by Rogers et al. to resuscitate rats from ultra-profound hypothermia appears superior in terms of animal welfare and equipments needs. Because hypothermia is not induced by methods that induce hypoxia (as in the experiments of Andjus and Smith), the need for specific warming protocols are greatly lessened. The use of anesthetics and ventilation during cooling allows the researcher to exclusively focus on the mechanisms of cold circulatory arrest and investigate methods (such as administrations of medications or complete blood substitution) to prolong the period rats can tolerate ultra-profound circulatory arrest and even subzero temperatures.
