Altitude change affecting heart rate monitor accuracy?

[Kathleenie]

I've noticed that when I hike, my Charge3 rarely refects my HR accurately, particularly the higher I go. Now, I'm not scaling Mt. Everest here. I'm talking up to 4000ish feet at the highest spot, with maybe 1500 feet in elevation gain throughout the hike (so, starting a hike at 2500 feet and ending around 3500 or 4000. I've noticed on steeper trails, I'm really breathing hard and feeling a good cardio heart rate.. but the Charge3 says it's like 97. I've actually stopped to manually check it and it's more like 140. I've tried wearing the watch differently, tightening, loosening, different position, no difference. It thinks I'm strolling across a parking lot or something. I know the HR monitor isn't really accurate, but that seems excessively inaccurate. Is it just the change in altitude? Something else? Is anyone else noticing this?

[Lisa272]

Same here! It’s really frustrating. I first noticed it with Nordic skiing workouts and chalked it up to more wrist movement than with other workouts. But then I noticed it with hiking at higher altitudes. My heat would be pounding out of my chest and my HR according to Fitbit would be about 95. I wonder if it’s something related to lower circulation to extremities at altitude - same thing that causes fingers to be a bit pale and puffy.