self-imposed exhale
Where you are trying to hold in a deep breath but then unintentionally do something that obliges you to "pop" whether ya want to or not.
I was competing with a couple of friends to see who could hold his breath the longest, and I was reading the daily newspaper-comics to distract me from my respiratory discomfort. Then I read a cartoon that really struck me funny and so I hadda laugh --- talk about self-imposed exhale!