Previous research has found that marijuana can increase heart rate and blood pressure, which could tip a vulnerable individual over into heart attack territory
Links
[1] http://www.livescience.com/48246-odd-facts-marijuana.html
[2] http://www.rethinkpot.org/tags/heart
[3] http://www.rethinkpot.org/tags/animals
[4] http://www.rethinkpot.org/tags/hemp