My normal heart rate is between 75 to 85. today it was 52. what could cause it to drop so much? i am 58 not athletic

Vagal Tone vs Meds: Most likely in your case is something called increased vagal tone. The vegus nerve is our rest and digest nerve, and if it gets stimulated -- by a large meal, changing temperatures, and sometimes positions, rapidly, and often following urination or defecation -- your heart rate may lower. Certain medications like verapamil can also result in transiently lower heart rate.