I went in for a second ultrasound at 8 weeks and was told the fetus was measuring 6 weeks with a low heart rate of 86. what chance of miscarriage?

A precise number ..: .... or chance of miscarriage can not be estimated from these reports, but you are right to be concerned about a low heart rate and small fetal size. Both of these ultrasound associated findings can be interpreted as risks for impending miscarriage. I understand waiting and hoping is difficult...I do hope this is just a temporary blip and all will be well. Stay in close touch with your provider.