The word heart has come quite a few times in Ramana Maharshi’s, “Who Am I?” as mentioned in the quotes below:

“If one inquires as to where in the body the thought ‘I’ rises first, one would discover that it rises in the heart.”

“When the mind that is subtle goes out through the brain and the senseorgans, the gross names and forms appear; when it stays in the heart, the names and forms disappear. Not letting the mind go out, but retaining it in the Heart is what is called “inwardness” (antarmukha). Letting the mind go out of the Heart is known as “externalisation” (bahir-mukha). Thus, when the mind stays in the Heart, the ‘I’ which is the source of all thoughts will go, and the Self which ever exists will shine.”

Heart here does not mean the physical heart – a body organ. Heart here means centre, the source i.e. the field of consciousness/awareness or Self. When you say I, try to notice its source. It originates from pure consciousness. This is the basic idea behind the importance of I thought emphasised by Ramana Maharshi and Nisargadatta Maharaj. When you take breath and notice its origin, you tend to reach the very same source, i.e. pure consciousness which is like a field.