Should I Be Concerned About My Aneurysm And Is High Heart Rate Reducing After Magnesium Intake A Sign Of Improvement?

i am 61 years old my heart rate went to 200 bpm a month ago but recovered back in the 90s My heart doctor put me on 400 mg of magnesium without blood work.I went to my gp and my level was 2.8 I stopped taking it because of side effects i was having, i take bystolic 2.5mg and losartan 50mg. i walk 20/25 min a day lately my heart rate only gets to about a 100bpm and recovers in 3 min in 60s.It usually is 120 and takes longer to recover. Is my heart getting stronger or should i be concerned? My heart rate did go up three years ago to 200 for about an hour. my bp is under 120/80 and resting heart rate is upper 50s/lower60s. I also have a 4.0 aneurysm that we are watching, Why would they put me on magnesium without a blood test?