I just read this article (which is well worth reading for anyone interested in cryonics). One of the important things that the article points out is that, while it takes some time for the memory structures of the brain to degrade due to ischemia, one of the more rapid effects is blood clotting in the fine capillaries of the brain after fairly brief ischemia. This reduces the flow of cryoprotectant, and causes large swathes of neural tissue to be frozen, instead of vitrified, which would be catastrophic for personal identity. While this is not a problem for best-case 'standby' cryonics, it is a problem for those who cannot afford a standby team, or are simply hit by cars.
Being an engineer, my first thought is that this is ridiculous, and there has to be a better solution to the problem. It may be possible to build a device, maybe the size of a shoe box, which can be deployed in the field by a minimally-trained amateur (like a defibrillator), and perfuses the brain with cold saline and anti-coagulants -- or even a synthetic oxygen carrier). I'm picturing a cylinder of fluid, large needles with sterilizing caps for tapping the jugular and carotid arteries, and a gas cylinder to provide pressure. You'd simply break a chemical cold pack, put a plastic neck brace in place and insert the needles, and press a button.
Such a device could even be useful to non-cryonicists, as a way to prevent ischemic injury in people found medically dead at the scene of an accident, during transport to the hospital.
Does anyone with more of a medical background know if such a machine would be at all feasible? I can't imagine it would be expensive to construct.