How deadly is acute heart failure?

Could be very. Could lead to death if not treated but the problem is that acute heart failure, if recurrent, leads to to chronic heart failure which has a mortality of 50% at five years (if untreated).
Very depending. On severity...Immediate treatment is best way to survival & recovery.

Related Questions

Can acute heart failure turn into chronic heart failure, or are they completely different disorders?

Heart failure. Acute heart failure is usually an exacerbation of chronic heart failure. That said there's pretty much always a first presentation of heart failure and that can be acute. They are part of a continuum in general. Read more...