Jim was 54 when he died suddenly, apparently of a heart attack.
Heart attacks in relatively young people are notoriously lethal. Sometimes the first symptom is death. A soft plaque sheers off, and a major vessel is suddenly obstructed. Smaller vessels may be fine, so there's been no slow development of backup "collateral" circulation. A healthy conduction system propagates bad signals. The heart fibrillates.
There's a good chance Jim had no warning symptoms, or at least nothing meaningful. If he'd had symptoms though, he'd have been worked up. Chances are the problem would have been fixed.
So, because it's the only meaningful thing I've thought of to mark this loss, this post is an informal review of the warning symptoms of heart disease.
I'll start with the set of symptoms that the American Heart Association describes as the Warning Signs of a Heart Attack ...
- Chest discomfort. Most heart attacks involve discomfort in the center of the chest that lasts more than a few minutes, or that goes away and comes back. It can feel like uncomfortable pressure, squeezing, fullness or pain.
- Discomfort in other areas of the upper body. Symptoms can include pain or discomfort in one or both arms, the back, neck, jaw or stomach.
- Shortness of breath with or without chest discomfort.
- Other signs may include breaking out in a cold sweat, nausea or lightheadedness.
The American Heart Association (AHA) recommends calling 911 -- "Don't wait more than five minutes"
I think you can see the problem here. I was very short of breath at the end of my swim sprint yesterday. I've been lightheaded or nauseous several times in my life (flu, etc) -- and I'm don't think any of them were heart disease. Taken out of context these symptoms aren't terribly specific. 
The AHA doesn't want to get more specific because they'd rather err on the side of over-diagnosis than under-diagnosis - and because they're writing for a wide audience. I think they can do a bit better for this blog's audience though . So here's a bit of context:
- If these things occur together it's more likely to be heart disease. So shortness of breath AND "cold sweat"  AND jaw discomfort all together means more than one of them by itself.
- It's one thing to feel short of breath when you're a healthy person running a 5 mile race, another if you are short of breath doing stuff that is normally easy (like watching TV).
- If symptoms like "arm discomfort" and " nausea" come on with exercise and get better with rest -- that's ominous. (Exercise means the heart needs more oxygen, so it can expose an underlying problem.)
- If your parents died of heart disease in their 40s and your LDL cholesterol is 250 and you smoke and you're male ... Ok. You get the point. Most of us have some heart disease by age 50, but some people have a lot. Weird pains at rest may not mean too much in a low risk 30 yo woman but in a "high risk" person they might be bad news.
In some cases, such as a man in his 50s with chest pains on exercise that get better with rest, the likelihood of serious heart disease is so high there's not much point in doing studies like an exercise stress test - nobody would be convinced by a negative result. 
Now you know some things to watch for. In memory of Jim.
- Cardiac testing: the classic application of Bayesian math to interpreting test results
- Remembering an Informatics Pioneer and Compassionate Friend: Memorial from UPMC.
 Technically this is the crummy way our body tells us that the heart is malfunctioning - most often due to lack of oxygen delivery with or without muscle damage (muscle damage = "heart attack"). It doesn't have proper pain receptors because there wasn't any point to it during the past billion years of cardiac evolution. There was no bypass surgery in the paleolithic.
The symptoms may also go completely away by themselves -- which doesn't mean the problem is gone.
 Autonomic nervous system freaking out.
 There are big debates about how doctors should investigate for ischemic heart disease, but that's way beyond the scope of this post.
 Really what we all want is a low cost highly predictive test we can do on everyone aged 50 that will tell us how bad their heart disease is. Or a set of cheap screening tests that we can put together with a risk factor profile to decide how should be imaged even if they have no symptoms at all.