Live in Denver/Race at Coeur d'Alene
I live and train in Denver, 5280 feet.
Once I get my run heart rate zone's established for the IM pace how does one adjust the heart rate zones for racing at lower altitude - 2188 feet or so.
Assuming the bike would stay the same power ranges?
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I've spent a fair amount of time pondering variants of this question and gathering relevant data. I live 9 months of the year at sea level, 3 months at 8300' in CO, and have raced IM CDA 10x.
To start with, here's a link to a Friel blog post which contains a summary table of the impact of altitude on FTP. http://www.joefrielsblog.com/2010/09/altitude-and-aerobic-performance.html
Specific to your concern, you *might* expect a 3% or so increase in your FTP racing @ CDA compared to the Mile High city. If you were racing with power, you might consider a 2-5 watt increase in your race day target, but you are using HR, which is an output measure of work, not an input metric. In other words, you want to be using the same HR #s when racing @ 2000' compared to 5000'. The heart is a muscle, and you've been training it to handle a stroke volume and contraction frequency, which you should be able to sustain at any altitude, regardless of the oxygen content of the air.
Now here is an interesting thought, which I have not seen confirmed in any studies, but which makes intuitive sense to me. That extra power you will be able to generate will not make you any faster, I suspect. Remember, there will be more air resistance at the lower altitude. Pushing that air out of the way is the biggest source of friction you must overcome while cycling. The increased effort may just compensate for the increased resistance of the air.
Others who live in your area and have raced at lower altitude might want to chime in with their own experience. But keep in mind the difference between using power and using HR as a metric to follow on race day.
The advantage you may have from living at 5000' and racing @ 2000', IMO, will be seen towards the end of the run. You *should* be able to keep your motor going longer (NOT faster) than others who haven't been acclimatized over the long haul.
I found this nifty calculator which might also help:
http://www.cyclingpowerlab.com/EffectsOfAltitude.aspx
It combines the following impacts: rider/bike weight, altitude effect on both air resistance and FTP, bike type (TT vs Road) and position, road gradient. Important to note that even a moderate grade of 2-3% changes the results significantly. The bottom line is: when comparing one altitude to another, up to about 2500 meters, decreased air resistance has a greater impact on speed than decreased FTP, on level ground.