I have briefly passed out twice in the last 6 months because of a sudden slowing
of my heart rate (<50). I had all the heart tests; electrocardiogram, tilt-table, electrophysiology test, treadmill test, holter monitor for 24 hours, etc. The monitor never saw an abnormal heart rate. All the other tests were normal. It is something that happens to me though about every three months, only while I am at rest(lying, standing or sitting).