rabbit heart question

Question
I am trying to find answers regarding the rabbit heart. Alot of sites, including yours, state they can have a heart attack easliy. On medirabit, there are examples of abnormalites of the rabbit heart, including the tendency towards ischemia - which would be the substrate for a heart attack and ventricular fibrillation. Also, on a few vetinarian sites, it states the rabbit WILL NOT have this heart attack if excersized regulary. ON YOUR site you state to one person a rabbit will have a heart attack/fibrillation (which kind by the way.. ventricular or atrial?) because there heart does not expand (which I am sure is true) if it goes too fast. Too another person, you state that this will happen if the heart rate DOES NOT go fast enough to keep up. So which is it? If it goes too fast or not fast enough? You also state fibrillation will cause heart failure.. which is not necessarily true. If it causes ventricular fibrillation then the heart stops. Anyways, I wish you would be more detailed in your answers. It seems to me that a Rabbit's heart is "weird" and lack of excersise makes it weaker (hutch rabbits would fall into this category). However, I am going to agree with the Vetinarian site and the University of Washington for Vet School, that they SHOULD NOT have a heart attack or be "scared to death" in the wild since they get plenty of excersize. Please state this in your answers so people will be better educated.Thanks

Answer
Dear Mandy,

I agree that there's little chance of a wild rabbit having a heart attack due to stress or fear of a predator.  But there are several possible reasons for this.

1.  As you say, wild rabbits get lots of exercise, and thus have greater cardiac fitness than domestic rabbits, even if said domestics have the run of a house.  I would not be surprised if a well-exercised domestic rabbit would have a much lower risk of suffering a stress-related heart attack than a flabby one.  But I am not aware of controlled studies that address this, to give you a reference.  (They could be out there!  Check Googlescholar.com)

2.  Wild rabbits have escape routes.  If they can run, then they are engaging in escape behavior.  This might be stressful, but not as stressful as being trapped.  A caged rabbit has nowhere to run, and so terror builds up without release.  Heart rate increases, and if the heart cannot expand adequately to handle the increased blood flow, there will be problems.

3.  Domestic rabbits are no longer under selective pressure to have hearts that do not suffer stress-related cardiac ischemia or ventricular fibrillation, which can--under the "right" circumstances--cause cardiac arrest.  Hence, there is probably a much greater variation in the levels of stress different domestic individuals can withstand before suffering cardiac arrest.  Wild rabbits would be under strong selective pressure to *not* keel over and die while being chased.  Those with such genetic tendencies would likely be rapidly removed from the gene pool.  So there could be an evolutionary reason, in addition to a fitness-related reason, for wild rabbits not to suffer stress-related heart attacks while being chased.

4. Risk of cardiac arrest under stressful circumstances might also have an age component.  I would be curious (but not curious enough to test it) to know whether young, healthy rabbits are less likely to die this way than older rabbits who may also have some degree of age-related cardiac deterioration or pathology of some kind.

Hope that helps clarify things, or at least gives us some points for discussion.

Dana