7126m: Are you a summiter or not?

Did you know that your Total Vessel Density (TVD) predicts if you will reach the summit or not, and that the microcirculation can tell secrets the macrocirculation won't disclose?

In 2013 our resident mountain climber and Chief Technology Officer Dr Matthias Hilty was part of the Swiss High Altitude Medical Research Expedition. Reaching for the skies on Mount Himlung in Himalaya, Nepal.

As the most important part of the expedition were 41 volunteers. To our knowledge this was the largest cohort of mountaineers available for systematic examination of the sublingual microcirculation at such extreme altitudes - ever.

All 41 were aiming for the 7126 meter high peak.

Different but same.

If you met the volunteers on the street you wouldn't be able to find big differences between them. They were all fit adults. Their age span was about 40 to 50 years old. Within their gender their weight was approximately the same. Their Body Mass Index was approximately the same. They were fit, but not elite athletes, with their fitness level approximately the same.

Six weeks before the expedition traveled to Nepal they met up at the University Hospital in Bern, Switzerland. Here our Matthias analysed the microcirculation.

He wanted to see - even if the volunteers had approximately the same macrocirculation numbers (the systemic haemodynamic variables determined included cardiac output, systemic vascular hindrance and oxygen delivery) - if he could find important differences in their microcirculation. Alas: Could the microcirculation tell secrets the macrocirculation wouldn't disclose?

The macrocirculation is “easy.”

Depending on where you are from, the macrocircilation is the Autoroutes, the Autobahns, the Autostradas, the Highways. Then the macrocirculation branches off into regional roads, the arteries to arterioles, and the veins to venules. This is where it gets a bit more complicated. This is where macro ends, and micro begins.

It's where the blood vessels are thinner than a diameter of 100 micron, or 0,1 millimeter, that the threshold is situated between macro and micro in circulation.

And even the micro is differentiated.

From 100 micron down to 20 micron in diameter, are the arterioles and venules. Blood vessels below 20 micron, or 0,02 millimeter in diameter, are called capillaries. And it's in the thinnest capillaries, those of a mere 3 micron or 0,003 millimeter in diameter, that most of the oxygen delivery to the cells happen.

99 percent of the blood vessels in your body are capillaries.

Problem solved?

Basically all blood vessels (99%) in the body are part of the microcirculation. But their microscopic size makes them difficult to assess. Especially when you want to do it in real time.

Our founder, Professor Can Ince, invented the technology that makes it possible to see the microcirculation in real time. And his extended studies have concluded that - just as the best position for monitoring the blood pressure is on the overarm, and the best place to use a pulse oximeter is at the tip of the finger - the best place to monitor the microcirculation is sublingually - that's under the tongue. Professor Ince solved a huge problem.

And that's why our resident mountain climber not only carried clothes, and survival gear as he ventured above 7.000 meters. But also a handheld sublingual video microscope.

So, how would you know if you were a summiter or not?

TVD, or Total Vessel Density, says a lot about your total number of capillaries, or the density of your capillaries. And why is this important?

As a healthcare professional you know a lot about the body's compensatory mechanisms.

If you present some kind of elevated stress on your body, be it running up a flight of stairs, or by a disease, your body will start to compensate.

The “normal” way to measure the compensation is to use macrocirculation parameters. For example heart rate and breathing frequencies. Blood pressure. Etc.

All well and good, but also you know that these parameters are not very detailed in regards to the real goings-on in the human body. Exemplified by sepsis, and how difficult it is to detect using macrocirculation parameters.

So why is TVD important?

Your vessel density says something about your body's ability to get oxygen to the cells. And your body's ability to increase its ability if it needs to compensate.

Why altitude, and how to simulate it in a close to sea level hospital?

There is a long tradition, especially in China, to do high altitude research. The oxygen percentage in the air is the same at any atmospheric altitude, at 21 percent, but the barometric pressure decreases, so the air density and the availability of oxygen decreases. And so does the outside pressure on your body. This makes high altitude an excellent testbed for a multitude of diseases and conditions.

To simulate the 41 test subjects' ability to compensate, our Dr Hilty did not put the subjects in an altitude chamber, but he did give them a nitroglycerin challenge. "Simple" yet brilliant.

By doing this and monitoring the sublingual microcirculation, he was able to both analyse the subjects their normal TVD, but also their Total Vessel Density when the body worked hard to attract more capillaries because of the nitroglycerin challenge. Matthias named this parameter TVD NG (Total Vessel Density Nitro Glyserin.)

By assessing these numbers, our Matthias was able to predict who was going to reach the summit, and who was going to struggle with the altitude.

By monitoring the test subjects all the way up to the top of Mount Himlung, Dr Hilty was able to confirm his theories.

Pretty amazing isn't it?

Read his full study here:

Hilty 2019 J Physiol (4)

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