How doctor determine the minimum rate on my pacemaker? at the beginning, my base rate is 60 bpm, but then changed to 70 bpm. why?

Pace rate: We often start programmed at 60 to see if the patient's own rhythm will overdrive the 60bpm. If the patient's own rate stays below there so that the pacer is always pacing at 60, we often increase the pace rate to 70 to give the patient more cardiac output than paced at 60

Depends: In general the rate of the pacemaker is set depends on the condition that requires the need for the device .So if the heart is slow causing symptoms then it is set at a higher rate than the baseline ,and can be adjusted as needed. Or if the slow heart predisposes to frequent premature beats or attacks of rapid heart beat then it is set even higher.The rate it is set is arbitrary and changeable