Life is not predictable. Things can change in a heartbeat. Like most of the English expressions that are based upon the heart, this idiom makes sense with a little explanation. While being excited or in a state of deep mediation can change the length of a heartbeat, it can’t change it much. Heartbeats are quick, less than a second, and things that happen in that brief time happen quickly indeed.
These three little words can actually mean a couple of different things. The expression is often used to describe how quickly one state of being gives way to another. Typically, there is a sense of surprise, horror, or amazement. When two peoples’ eyes meet across a crowded room, they can fall in love in a heartbeat. A child who dashes after a runaway ball into the street and into the path of an oncoming car might find that his or her life changes in a heartbeat.
The expression is also frequently used as a term of strong agreement that suggests immediate action. It’s used to answer a question that is usually asking for a pretty big favor or commitment. A response that uses this phrase means the speaker doesn’t even need to think about it and that the relationship, be it friendship, familial, or passionate love, is so strong that doubt has no place to stand. A sister who asks a brother to house her during a messy divorce and is told, "I’ll do it in a heartbeat" can rest assured that the brother’s decision was made without a moment’s hesitation.
In English, as in most of the world’s languages, the heart is a major player in terms of idioms. Most cultures identify the heart as love’s first home. Many also see it as a source of all passionate feeling, good or bad. It is not surprising that so many common expressions take the heart or the heart’s functions as their material.
An infants finds a mother’s heartbeat a source of comfort as he or she is rocked to sleep. Probably the first sound heard is the heart's steady rhythm counting time in utero. Very young children are aware of their heartbeats, and many learn to count by listening to them. The beating heart is one of humankind’s first metaphors, and this identification extends into the language itself.
“No love lost between those two,” might be said about neighbors who can’t stand each other. “You warm my heart,” a teacher will tell her students when their work is exemplary. “I’m sick at heart,” someone might moan when they’ve lost a family heirloom. Even a change of heart can happen in a heartbeat.