"Hawthorn is a heart tonic-period. In the past it has been confused in value with such drug plants as Digitalis and Strophanthus, but it is slow, gentle (even feeble), acts to strengthen weak functions or to decrease excessive functions, and has no part in overt heart disease. First of all, it is a mild coronary vasodilator, increasing the blood supply to the heart muscles and lessening the potential for spasms, angina, and shortness of breath in the middle-aged or older individual with stage l impairment (symptom-free at rest) or stage 2 impairment (difficulty with moderate effort). Further, I have seen it help the middle-aged mesomorph, with moderate essential hypertension, whose pulse and pressure are slow to return to normal after moderate exertion, and whose long, tiring days leave the pulse rapid in the evening. It will gradually help to lower the diastolic pressure and quiet the pulse; it combines well with Passion Flower for such individuals. As with all uses of Hawthorn, the benefits take weeks or even months to be felt, but are well maintained, not temporary.
It is useful for arrhythmias or extrasystoles of a functional nature and not from overt heart disease, and for disturbing but not dangerous episodes of rapid heartbeat (tachycardia). The German Ministry of Health recommends Hawthorn for mild bradycardia (slow pulse) and for bradycardiac arrhythmias of a functional nature; but frankly, the only ill effect I ever observed was with an older woman with bradycardia and occasional palpitations who found Hawthorn made her faint and dizzy within several days of beginning the herb. The symptoms ceased after she stopped the tea. Since then, I don't like recommending Hawthorn for a slow pulse but instead for the above types of early cardiac imbalance. Use it to prevent degenerative disorders, as it is slow, safe (except as mentioned), and gradual. It is not useful in real heart disease- period."
from "The Male Herbal" by James Green, The Crossing Press, 1991