Its sound functioning is integral to a person's well being and healthy blood pumping and circulation.
Sun is the primary karaka of the human heart in a kundali and how strong or weak the Sun is(measured by various astrological technical data parameters) will decide how strong your heart is.
Moon controls the blood flow into the heart.
Mars controls the muscle element in the body and since Heart is the most vitally throbbing muscle in the body, this planet also has to be studied carefully.
Jupiter is also integral since it controls the arteries and the veins.
If Sun is well placed but Jupiter it might lead to artery blockage. Needs to be checked.
The HOUSE of heart is the 4th house.
Thus if there is Sun in 4th house but its debilitated, it means a heart problem of great intensity since its the hub area of this organ with nopower of its own.
So while looking at a heart ailment in chart, study the SUN and the 4TH HOUSE IN DETAIL. THESE ARE PRIMARY.
The No2 significators are MOON, MARS AND JUPITER.
Going a level deeper, the astrologer needs to see also the nakshatra in which these planets are placed. That gives you a good idea of whether a heart attack can occur and the timing of the event.