the first cell to produce the action potential in the sa node isn’t always the same, this is known as pacemaker shift. in certain species of animals, for example, in dogs, a superior shift (i.e. the cell that produces the fastest action potential in the sa node is higher than previously) usually produces an increased heart rate whereas an inferior shift (i.e. the cell producing the fastest action potential within the sa node is further down than previously) produces a decreased heart rate.