“How do we get more oxygen into our lungs at high altitude?" That's a question that many of us have tried to answer. Lots of approaches have been tried - from drugs and technology, to dietary changes and tweaks to a climber's ascent profile - but what if there was something simpler? Something that didn't cost a penny or force you to spent a moment longer on the mountain? Interested? Read on...
The amount of oxygen in our blood stream can be measured with a pulse oximeter. This handy little device measures the proportion of oxygen that is bound to haemoglobin molecules contained on the red blood cells. The measurement is given as a percentage. Typically, our oxygen saturation is 96% or more at sea level. However as we ascend to high altitude our oxygen saturation falls. High on an 8000m peak like Mt Everest this figure can fall as low as 34%! A team of researchers from the MEDEX 2015 studied the impact of something as simple as body position upon oxygen saturation.
Twenty eight healthy volunteers were tested at sea level and following a gradual (>2 week) ascent to 5150m. After 10 minutes of sitting at rest the volunteers were studied for 5 minutes in the sitting, supine and prone positions. On each occasion the order was randomly assigned. No differences could be seen between the 3 body positions at sea level. However at 5150m this all changed! Oxygen saturation was higher in the sitting position (81.3%) than in the supine (78.8%) or prone (79.7%) positions. To underline this finding it's worth saying that no fewer than 22 out of 28 (79%) volunteers had their highest measurements when in the sitting position.
The Medex 2015 results are consistent with another recently published study. In a group of 50 long term residents of Puno, Peru (3825m) oxygen saturations were 1% higher in volunteers when sat upright compared to supine. In all, 80% of the residents were found to have a higher oxygen saturation in this position
The reason why oxygen saturation should be higher when sat up at high altitude isn't entirely clear. Using ultrasound, echocardiography and lung function tests, the MEDEX team were able to rule out many of the potential causes. However what they weren't able to study was how blood flow (perfusion) and gas exchange (ventilation) changed with position. We know that when you lie on your back the weight of your heart and other organs presses down on your lungs and blood vessels, preventing the passage of oxygen into the blood stream. Since oxygen is so abundant at sea level this isn't noticeable in our day to day lives. However when the number of molecules fall as we climb to high altitude, the position we are in can matter.
The effect of the weight of our organs (and the fat that surrounds them!) upon gas exchange can be illustrated by looking at the oxygen saturations of volunteers at high altitude. In a study of 954 permanent residents of Puno in Peru (3825m), resting SaO2 fell from 91% (BMI 20-25) to 87% (BMI>30)
Irrespective of the mechanism, is a 2.5% improvement in oxygen saturation important at high altitude? The simple answer is "Yes"! There's a reasonable chance that 2.5% might be the difference between being well acclimatised at high altitude and developing AMS. Let me explain! Back in the late 1980's, Peter Bartsch and a team of Swiss researchers studied a large group of mountaineers who had ascended rapidly to the Margherita Hut (4559m) on Monte Rosa. On arrival they each had a blood sample of blood taken from their femoral artery. Amongst the many different measurements taken one was particularly important - the partial pressure of arterial oxygen. Using the oxygen dissociation curve it is a fairly straightforward task to estimate the arterial oxygen saturation (SaO2) from the partial pressure of arterial oxygen. Try this calculator! The difference in SaO2 between those who remained well and those who developed AMS was small - just 3% in "mild' and 9% in "moderate/severe" AMS. A few percentage points might just make the difference between staying healthy at altitude and developing AMS.
During the day time sit up as much as you can. Avoid lying down. It might just help!
If you would like to find out more about mountain medicine why not join the British Mountain Medicine Society? See this link for details.