I have been taking part in a phd project that involves walking on a treadmill at 10% gradient for 90 minutes at an altitude of 4000m. I can’t really say all that much more about exactly what was going on with the tests etc, suffice to say that I had done a few tests, and had done a few stints both of walking at that altitude, a VO2 max test at altitude and a 3km time trial under the same conditions and everything was hunky dory.
Then I went in for my first 90 minute session. I was fasted, like the other sessions, I didn’t have any coffee, like the other sessions. The difference this time was that I was to have a cannula. No great shakes. I had some blood taken on a previous session, and although I felt a little weird, I was fine after a couple of minutes.
We went up, I had a cannula put in. Then I went into shock.
Great.
Everything went purple in front of my eyes, I went light headed and the world was not a good place to be in. Not too much of a problem… just sit here for a moment and let it pass, I’ll be fine in a bit.
On previous occasions, when I’ve had that kind of feeling, a cup of sweet tea saw me right. However, I needed to stay fasted for the test. So I stayed fasted. After a few minutes I thought it was about time to go downstairs and into the altitude chamber (I needed to be in there for 30 mins anyway prior to having – or not having- breakfast, so I might as well do it now as later).
Standing up, the world went sideways, more purple spots – the only thing I can liken it too was when I broke my ankle 20 years ago… everything was distant, and as I walked down the stairs, the only thing I could think of was “don’t pass out”.
Got to the chamber, went in, lay down on a crash mat and waited for it all to pass.
After 30 mins, there was breakfast – porridge – which was most welcome… however, by now, the flashing lights and strange detachment from reality had not gone away… it had developed into a full blown migraine aura – I couldn’t see out of my left eye, and things generally were not looking great.
Another 30 mins went by as I digested the porridge… still woozy, but I put on the rucksack and started walking, with regular Spo2 (how much oxygen I was getting into me), blood samples and breathing samples being taken.
I don’t really remember too much about the hour I spent on the treadmill, apart from the fact I was listening to a podcast, I felt horrendous, and had no idea quite how I was going to muster the energy to do a 3km time trial at the end of it all.
After an hour – the test was meant to be 90 mins, the lead researcher came in and told me I had spend the last hour at less than 50%spo2 – in normal circumstances at sea level this is meant to be round about 98- 100%, so a large amount of time at 48% is not all that great. My head was still away with the faeries, my eyesight was still all over the place, and it was a total relief to hear him say he was pulling me from the trial on health grounds.
I stepped off the treadmill, took off the bag, at some point the cannula must have come out, but I don’t recall that, and then I sat on the floor for a while being plied with coffee and flapjack.
After about 20 mins I was able to get up, walk around, see and get out into the sunshine, about about half an hour after that I was back to normal and fine to drive home… but what the heck happened there? It couldn’t have been the altitude – on at least 3 occasions prior to then it had not been a problem at all… the main difference was the cannula.
Since this happened I have googled various things to workout what went slightly awry – and it was only after a friend reminded me of the term “vaso-vagal” that I hit upon the answer. Basically it was a vaso-vagal response from cannulation – essentially a huge drop in blood pressure caused by the dilation of the vagus nerve- brought about by the nervous system… something which is relatively common in people under 40 (check), people with a low resting heart rate (check), people who are fasted (check), and people who have had that kind of response before (check).
Talk about a perfect storm.
So the upshot is that I now know that I am pretty okish at altitude, as long as you don’t stab me with a cannula, and that low blood pressure seems to correspond to getting a migraine – at least in my case.
It was great fun while it lasted, but I might need to think about having needles plugged into my veins in other trials.