Who, what & why

During my last cycle training camp in Tenerife I noticed that above 1700m altitude, I was starting to breathe more deeply and frequently for the same workload than I had been at sea level. It wasn‘t a pleasant feeling for the first couple of days, until I started to adapt.

The Mt Teide crater (2100m) is frequently chosen by top cycling teams as an altitude training destination. The riders will practice ‘sleep high – train low’, which allows them to train intensively during the day and then acclimatise to the lower oxygen content at night by producing more red blood cells (legally!). Different people experience effects such as shortness of breath and nausea at different altitudes. I decided to look a little deeper at where the individual differences might come from and whether heart rate variability (HRV) could tell us something about it.

I found a recent paper written by Czech and Australian sports scientists that looked at the relationships between aerobic fitness, blood oxygen levels, HRV and discomfort when oxygen was reduced by 50% compared to normal sea level.

What did they do

They recruited 28 physically active young male volunteers and firstly measured their peak oxygen capacity (VO2 peak) during a graded treadmill test. One week later, they were all subjected to a test where the amount of oxygen they breathed was halved compared to the normal amount. This was equivalent to an altitude of 6200m (just above the Uhuru peak of Mt Kilimanjaro, where a lot of people feel pretty unwell).

The volunteers had their HRV and blood oxygen (SpO2) measured whilst lying down; at the end of a 10 min hypoxic phase; and then finally after a 7 min recovery phase. They were also asked to rate their level of discomfort using a 4 point scale during the low oxygen phase.

What they found

The participants were grouped into a Sensitive group (▽), and a Resistant group (●) according to how much their blood oxygen saturation (SpO2) reduced during the low oxygen phase.

Reduced oxygen content, breathing and HRV

HRV (Ln rMSSD) reduced in almost all participants during the hypoxic stage, with a significant relationship between the reductions in HRV and SpO2 levels (right hand chart). To give an example, HRV reduced from an average of 80 on the ithlete (20xLnRMSSD) scale to 64 in the resistant group and 56 in the sensitive group.

What was also really interesting was that the participants with the highest aerobic capacity (VO2 max) showed the largest reductions in blood oxygen level. This relationship is shown in the left hand chart above.

What does it mean

Considering HRV as an index of stress, it is not surprising that the biggest reductions in blood oxygen saturation were accompanied by the largest reductions in HRV. However, although we often think about high HRV as indicating both higher aerobic fitness and a higher level of resilience to stress, that seems not to be the case here, because those participants with higher aerobic fitness were also those more sensitive to the effects of reduced oxygen in the air they were breathing.

From a practical perspective then, measuring your HRV at altitude and comparing to HRV at sea level can show how sensitive you are to the effects of altitude, and that if you are an elite level endurance athlete, you may be more sensitive than mere mortals!

It would be really interesting to hear from anyone that has measured their HRV throughout an altitude training camp to see whether the difference to sea level gets smaller as they spend more time and acclimatize.

by Simon Wegerif

Vagal activity and oxygen saturation response to hypoxia: Effects of aerobic fitness and rating of hypoxia tolerance. Tomáš Macoun, Michal Botek, Jakub Krejčí, and Andrew J. McKune