In reply to Removed UserBradleywigginscyclingshorts: to expand a bit, your body produces more red blood cells as you gain height so as to carry more oxygen around your blood when there is less in the atmosphere. That's basically what acclimatisation is. Whilst your body does that your heart will beat faster to compensate for the lower oxygen level. Once you have acclimatised to that level your HR should return to normal. Obviously you can't acclimatise forever to every new altitude, but you get the idea. You may or may not get AMS, you won't know until you try to acclimatise to new altitudes. Keeping hydrated helps a lot, getting dehydrated can be very bad. But an elevated HR On its own is not a sign of AMS, with other symptoms it may be.
