View my stats, in particular the heart rate graph to see how I kept my heart rate constant (but at an small increasing rate) from the start of the TT (around 40mins of riding time) until maximum of 200 (that might be a bit dubious) at completion (around 1hour 30mins).  Hover over red line on HR graph to get exact reading.  Use this link to access: http://connect.garmin.com/activity/62772492