A recent television program told of a surgical procedure developed to repair an aneurism in the brain of a patient. In order to perform this delicate repair it was necessary to remove a large fraction of the blood from the patient to deflate the aneurism. However, the body needs oxygenated blood to keep cells, and particularly brain cells, alive. How is this to be done? Furthermore, the cells use energy through biochemical processes and these need to be slowed down. The program described the process by which the body temperature was lowered from 98.6F (37C) to 65F (18.3C) and made a big thing about the fact that the patient's heart beat stopped. It is this we wish to consider from the point of view of rhythms and chemistry.
A heart beat is a rhythm, or a chemical oscillation. That is, the nerve pulses (electrical impulses) that drive the heart muscle are the result of chemical reactions. This looks very much like the kind of thing we see in the Belusov-Zhabotinsky reaction where there are a number of chemical steps leading to oscillations. A lot is known about nerve signals and the heart beat, but lets find out what we can do with our knowledge of basic chemistry.
If the resulting heart beat of 72 beats per second were due to a single chemical step we could use the results of physical chemistry to gain an insight on its temperature dependence. Considering 72 beats/sec as a rate process we know that the rate constant depends on an Arrhenius equation for its temperature dependence.
The Arrhenius equation also depends on an activation energy, Ea, and these are usually the order of 100 kJ/mol. Put another way, activation energies must be in the neighborhood of 100 kJ/mol for natural chemical processes to be as they are observed! For the purpose of our story we will take this as a value. R = 8.314 J/mol K is the gas constant. The so called steric factor, A usually depends weakly on temperature and will be treated as a constant. What we want to know is: If the heart beat is 72 beats/sec at 98.6F what is it at 65F?
Solving for k(65F) = 6 beats/sec. But the program showed heart monitors with no heart electrical activity-no beat! This brings us to the rest of the story, or at least to the rest of the questions. The fundamental physical chemistry says the heart rate slows to 6 beats per second and that is not stopped. If we consider that in order to get chemical oscillatory behavior we must have a many step reaction sequence then perhaps this is an indication of such a sequence in the mechanism of forming the heart beat. Each of the rate constants in such a sequence of elementary reactions will be affected. If we look at the mechanism for the Belusov-Zhabotinsky reaction we can understand that perhaps a slowing down of one mechanistic step would stop all chemical oscillations. After all, this is a very non-linear system. In analogy, the slowing, but not stopping, of one heart beat mechanistic step could stop the heart.
A post script: If you go to the experiments of these oscillating reactions you can observe the change in oscillation with temperature. The temperatures rise from 22C to 74C and the oscillations do not stop, but grow faster. This would be in keeping with our understanding of simple reactions, or a reaction dominated by a single rate determining step, but is a little supprising in such a complicated system. It remains to be seen what happens to this model oscillating system if the temperature is lowered as in the medical case.