The most common cause of sudden death in horse athletes is a rupture of the aorta, which is the biggest artery in the body. The horse subsequently very quickly bleeds out into its chest cavity, much like Marfan's syndrome can cause in men. Some of these horses probably have an aneurysm of the aorta, or a prior weakening of the blood vessel wall, which predisposes to rupture when blood pressure increases during exercise. Research is underway to try to find the cause(s) of the underlying arterial wall weakness in certain horses.

The next most common cause is probably an arrhythmia, or abnormal heart rhythm during exercise. Arrhythmias are often triggered by adrenaline, and can be fatal because they can cause the heart to stop suddenly. Unlike aortic rupture, an autopsy performed on a horse that died from an arrhythmia would not necessarily show any abnormalities. This is because the arrhythmia is a transient electrical event which can't be seen. Other causes for sudden death in horses are less common, but include acute heart failure caused by abnormalities of the structure or function of the heart (either that the horse was born with or that the horse acquired), upper airway obstructions, or toxins.
 

https://www.paulickreport.com/horse-care-category/vet-topics/ask-vet-equine-heart-attacks-happen/

yes.....read that and we will wonder til the post mortem comes out. 

IN the meantime, we grieve another 3 yr old's death and a winner, too...

Mine too!! I was looking forward to his 4 year season

 I remain heartbroken- in disbelief.  Here is an article from the LA Times. It may be behind a paywall - Hope not.

https://www.latimes.com/sports/story/2021-12-06/medina-spirit-dies-six-things-to-know-bob-baffert-kentucky-derby

 I have read different newspapers about the death of this marvelous horse. OMG- if you read some of the comments, it is appalling, all blaming Baffert: the horse was doped, he was killed for insurance money, how horses are worked  and raced to death, blah, blah, blah, . People are so ignorant----- It just makes me sad to see such a champion essentially demeaned in this way- as  well as the demeaning of Bob Baffert.

 Medina Spirit's death was even reported in the Wall Street Journal with horrible comments about how horses are mistreated, raced to death,  doped up. It is truly awful to read these comments.

Yes.......gossip is Tragic, too.........wonder what the necropsy will show,as could be anything ........

This LA Times piece is good, but it flames the fire of anti racing that is already happening at the Santa Anita racetrack. 
This morning I still feel like I was gut punched, I still wish it was a sick joke that he is gone. 
I pray for peace for the entire Baffert crew who loved and cared for this grand animal.

There are 2 forces going here. You have the PETA types who hate all racing so they jump on any tragedy to try and end racing. Then you also have the Baffert haters, these people feel that he is only successful because he dopes his horses.

For my own sanity I do not read the comments. 

 Yesterday the shock was all encompassing and I cried quite a lot - Today, I am left with the immense sadness of the loss of this amazing horse. I too hope and pray for the Baffert family (personal and horse family), that they can find peace in these trying times.