When Altitude Gets Aggressive: HAPE and HACE

In my time working as a doctor I am not left with much mental capacity to perform anything that calls upon true concentration when I get home. So, I've taken to listening to audiobooks. Amongst the gems that I've discovered in my exploration of these, I recently started listening to Jon Krakauer's account of the 1996 Everest disaster in which 8 people died on Everest in a single night.

I thoroughly enjoy Krakauer's style of writing and so dived into the (audio)book enthusiastically. In the pages, Krakauer talks about the concept of two conditions called HAPE and HACE. Respectively, these stand for high altitude pulmonary (HAPE) and cerebral oedema (HACE). Both conditions, though rare, are potentially life-threatening and so a poignant talking point to kick off this new blog.

In the medical field, 'oedema' (pronounced "ee-dee-mah") refers to what one would call 'swelling' in layman's terms. For example, have you ever noticed how your legs swell after a long day standing or sitting for a lengthy period of time on a protracted flight? That's oedema!

Oedema occurs as a result of fluid accumulating either within or around the cells in the body. Vasogenic oedema occurs when fluid leaks out of your vessels and occupies the minuscule spaces between your cells. Cytotoxic oedema occurs when the cells themselves accumulate fluid and swell much like a balloon would if you were to attach it to a water tap.

So why is oedema so dangerous? Well, it usually isn't. But in the brain, in particular, swelling is exceptionally dangerous. Imagine you have an object fitting snugly into a tight space... the minute you start to increase the size of that object, it will start to squeeze out of the space. This is exactly what happens with the brain inside of the skull. Cerebral oedema (brain swelling) ultimately leads to what is called 'herniation' where the brain is squeezed downwards and out of the skull through the hole in the bottom of the skull called the foramen magnum (latin for 'big hole'). This is a particular problem because the movement of the brain out of the skull places pressure on your respiratory and cardiac centres and ultimately leads to the cessation of your heartbeat and breathing i.e. death.

The problem that occurs in the lungs is somewhat more sophisticated than in the brain. The issue arises within the alveoli, the tiny grape-like sacs within the lungs in which the body swaps poisonous carbon dioxide for life-sustaining oxygen. This exchange occurs across an exceptionally thin membrane that is highly-specialised and most efficient at performing its function. But when fluid is allowed to leak out of the vessels and into these alveolar spaces, it impedes the movement of gases across the membrane and essentially traps oxygen in your lungs and carbon dioxide in your blood, much like raising a fence at a border crossing. In turn, these changes lead to hypoxia (low oxygen levels in the blood) which has obviously poor consequences.

Interestingly, Garmin's new Fenix 5 Plus devices now include a pulse oximeter that can show you the concentration of oxygen in your blood. Previously, the closest gadget to a hospital pulse oximeter was a portable oximeter. This is a development from Garmin that really excites me. Exactly whether or not it is useful or accurate enough to be used properly in the process of acclimatisation is a subject for another talk. But it's great to see wearable sports tech heading in such a sophisticated direction.

Perhaps the most interesting thing about HAPE and HACE is that nobody really knows yet exactly how they occur! There is logical evidence that the low oxygen levels in the blood are a root cause. In the lungs, 'hypoxic pulmonary vasoconstriction' occurs. It's a mouthful, I know, and what it means is that the blood supply to the areas of the lungs that receive the least amount of oxygen shuts down. In turn, this shunts the blood towards the areas still receiving adequate oxygen and subsequently raises the pressure in those vessels which forces the fluid out of them and into the alveolar spaces. But, puzzingly, there also appear to be inflammatory-like changes that occur but without actual inflammation taking place! This explains why 'dex' (a steroid properly named dexamethasone and efficient at resolving inflammatory reactions) can bring about wonderous changes in people suffering from HAPE and HACE.

HAPE and HACE are conditions that require careful attention and a high index of suspicion. Their management is likely to receive more and more attention in the future and will hopefully lead to improvements in the safety of high altitude mountaineering. In the meantime, remember, just breathe!

Everest