Dogs resuscitated after 3 hours of cardiac arrest from exsanguination
Despite sensational news items about “zombie dogs,” biomedical researchers and clinicians have known for a long time that interruptions in consciousness and blood circulation can be reversed without neurological deficits, provided such events do not produce ischemic injury. There are even species who can enter a state of reversible metabolic arrest such as tardigrades (water bears). Naturally, researchers have recognized the opportunities that depressed metabolism holds for stabilizing trauma victims. Although the jury is still out on the question if therapeutic hypometabolism can be induced in large animals by chemical means, there are no doubts that lower temperatures reduce metabolism, allowing patients to tolerate longer periods of circulatory arrest.
The published record for reversible hypothermic circulatory arrest is 3 hours in a canine model. A recent study by Wu, Drabek, Tisherman et al. (2008) documented resuscitation from 3 hours of exsanguination cardiac arrest (2.5 hours of no flow) after rapid induction of profound hypothermia using cardiopulmonary bypass. These results are quite impressive in light of the fact that in the latter study cardiac arrest was induced at normal body temperature by exsanguination, and the organ preservation solution to replace the blood consisted of just chilled saline plus dissolved oxygen and/or glucose. The authors attribute their success in extending satisfactory neurological recovery from 2 to 3 hours of exsanguination cardiac arrest to the addition of energy substrates, and oxygen in particular, during induction of profound hypothermia.
Research of this nature benefits cryonics in a number of ways. If hypothermic circulatory arrest will become routine in emergency medicine and military medicine, the general public will get increased exposure to the fact that circulatory arrest does not equal death. Research of this nature also demonstrates that induction of ultra-profound hypothermic arrest in humans may be reversible and therefore the initial stages of cryonics stabilization procedures as well. The more practical application is that it offers the prospect of extending the period the brain can be kept viable after pronouncement of legal death during remote transport of cryonics patients. Last but not least, this specific study provides optimism that viability of the brain can be maintained if hypothermia is induced after circulatory arrest, provided metabolic support is given and cooling rates are fast enough to avoid irreversible injury to the brain.
Unfortunately, the technical capability to reverse 3 hours of asanguineous hypothermic arrest falls short of what is needed for cryonics patients who are stabilized in remote locations. Transport times between the location of cardiac arrest and the cryonics facility often exceed 24 hours. Although loss of viability of the brain does not constitute information-theoretic death, it would be desirable if cryonics organizations would be able to routinely secure viability of the brain between pronouncement of legal death and start of cryoprotectant perfusion.
Such advances will require substantial investments into the development and implementation of improved organ preservation solutions, perfusion techniques, and resuscitation protocols. Potential directions for such research include addition of effective neuroprotective compounds and “hibernation mimetics” to the organ preservation solution and low flow or intermittent perfusion during patient transport.
In 2005, when asked to comment on the prospects of using hypothermic circulatory arrest to treat trauma victims, Dr. Thomas Scalea, physician-in-chief at the R. Adams Cowley Shock Trauma Center at the University of Maryland Medical Center, was reported saying:
“As potentially crazy as this might sound, you’re comparing it against essentially certain death, so it’s hard to see how we can do any worse….all of us are incredibly energized by the thought of being able to do better.”
Such reasoning should equally apply to the practice of human cryopreservation, which employs even lower temperatures to protect people against “essentially certain death.”