From Longman Dictionary of Contemporary Englishin a heartbeatin a heartbeatAmerican English very quickly, without delay Things can change in a heartbeat. → heartbeat
Examples from the Corpus
in a heartbeat• If he asked me to marry him, I'd accept in a heartbeat.• Beyond Des Jardins' cooking, there are two things that would bring me back to Jardiniere in a heartbeat.• Don't think for a minute owner Jerry Jones wouldn't trade places with Bob McNair in a heartbeat.• For exam-ple, an emergency or operating room is not the best environment for monitoring subtle variations in heartbeats. 