400k doesn't sound so rare. The entire population of Tulsa OK, Wichita KS, or New Orleans, LA just drops dead every year?

ROC – Resuscitation Outcomes Consortium datum were gathered between June 1, 2014 and May 31, 2015 and includes EMS-assessed and EMS-treated out-of-hospital cardiac arrest from multiple regions of the US.

According to the ROC:

Approximately 356,461 people in the US experienced an out-of-hospital cardiac arrest (347,922 adults, 7,037 children under age 18)

22,520 of those 356,461 were witnessed by bystanders

12.4% survived to hospital discharge

Of the EMS patients who experienced non-traumatic cardiac arrest outside a hospital, and did not have bystander intervention, 10.8% survived until hospital discharge

[999] - https://www.aedsuperstore.com/resources/sudden-cardiac-arres...

It's rare in young healthy-acting people.

For example, http://www.sca-aware.org/sca-news/aha-releases-2015-heart-an... says that the incidence is 0.24 per 100k annually for high school athletes in Minnesota. Extrapolating that rate to the whole US population would give ~800 deaths per year. The observed 400k number includes quite a lot of people who suffer cardiac arrest after a history of heart disease.

If this is a topic you'd like to know more about or donate to, consider: https://viaheartproject.org