Altitude medicine – getting the message across 

Providing advice to your patients about travel to altitude can feel incredibly daunting, yet as with any area of practice, confidence improves with knowledge and understanding.  

In reality, it’s not surprising this subject appears a little complex, particularly when there are high-altitude medical societies, books and journals full of complicated articles, charts, talk of partial pressures, disassociation curves and complex stats.  

However, altitude medicine can also be kept to a level that’s understandable by every clinician, enabling us to give the best information to our patients.  

To be confident in providing high-altitude medical advice I would recommend clinicians revise and develop their understanding of the following areas: 

1. The high-altitude environment 

2. Basic respiratory physiology 

3. The interaction between the high altitude environment and human respiratory physiology 

4. Acute Mountain illnesses 

5. Mountain illness prevention strategies 

I will cover points one to three in this post and points four & five in Part II.

There is no short-cut to clinical expertise, and like any area of travel medicine, improving one’s knowledge gives patients confidence in their clinician, and is highly likely to positively impact their health. So with this in mind, let’s start simple and work our way up a mountain of high-altitude mountain medicine knowledge.

What is high altitude? 

High altitude is generally been considered as anywhere between 2,400m – 3,500m above sea level. Very high altitude is considered as between 3,500 – 5,800m, extreme altitude over 5800m and anywhere above 7500m as ‘the death zone’, where humans cannot survive for any significant length of time without supplementary oxygen.  

Why does altitude matter? 

The easiest way to explain the effects of air pressure is to go in the opposite direction and use a diving analogy. We all know that the deeper we dive in the ocean, the greater the water pressure, despite actual volume of water around the body remaining the same. It is the weight of the water between you and the surface that causes that pressure. At 4,000m (the depth of the Titanic), the immense pressure is about 310,289mmHg (6,000psi). 

Stand out of the water on the shoreline and, whilst there is no water pressure, you are now standing at the bottom of an ocean of air, the surface of which is about 60 miles above you. Although we don’t feel it, the weight of this air exerts about 760mmHg (14.7psi) at sea level. As we ascend up through this ocean of air (by climbing a mountain), the air pressure decreases. The percentage of oxygen in the air molecules around you remains the same, but its availability decreases because of the loss of pressure and the air molecules being spread out.  

So now we have an understanding of air pressure and how this relates to altitude, let’s build on this knowledge and review some respiratory physiology.  

Respiratory function 

Gas molecules always move from high to low pressure areas as they seek to establish an equilibrium.  

During respiratory pulmonary gas exchange, the higher partial pressure of oxygen (PO₂) in the air naturally moves from our alveoli into the lower PO₂ of our blood, whilst at the same time the higher partial pressure of carbon dioxide (PCO₂) in our blood moves across into the lower PCO₂ in the alveoli.  

These changes in alveoli air pressure are caused by inspiratory and expiratory muscles. As the diaphragm flattens, and the chest wall expands, alveoli air pressure drops and air rushes in. Conversely, as the diaphragm domes, alveoli air pressure increases, and air is forced out.  

The stimulus for breathing (our respiratory control), is driven by three main factors: 

• PCO₂ – Primary stimulus 
• PO₂ – Primary stimulus when PO₂ falls below 60mmHg 
• pH – Changes caused by metabolic stimulus. Acidosis causes an increased respiratory rate 

Changes in PCO₂ are detected by chemoreceptors in the respiratory centre of the medulla oblongata found in the brainstem, and peripheral chemoreceptors around the heart and carotid bodies. These receptors then stimulate the intercostal muscles.  

To summarise this process: 

• Gas exchange is driven by pressure differences 
• Pressure differences are driven by respiration 
• Respiration is driven by our respiratory centres 

Why does air pressure matter at altitude? 

As we have already seen, the higher the altitude, the lower the air pressure and the lower the partial pressure of oxygen (PO₂₎. This drop in atmospheric PO₂ affects the efficacy of gas exchange, along with the body’s stimulus for breathing.  

A low PO₂ will stimulate the peripheral chemoreceptors, which in turn stimulates breathing (hyperventilation), causing the PO₂ to increase. This mechanism is referred to as the Hypoxic Ventilatory Response, or in simple terms, the body’s mechanical response to becoming short of oxygen. 

Reality check 

So, what does this look like in our patients climbing a mountain? Imagine a 45-year-old who has decided to climb Mt. Kilimanjaro (5,895m). As they start their ascent, rising slowly above 3,000m (we’ll come back to ascent rates in a while), they are likely to find it increasingly hard to breath as the air pressure drops. They might also become a little dizzy, particularly when standing quickly, and may even have the start of a slight headache. In an attempt to maintain organ oxygenation, the body increases both respiratory and heart rates. However, at altitude, other strange physiological things can be observed, particularly at night, and it is these we shall now consider. 

Negative effects of hyperventilation 

The desired effect of hyperventilation is to increase our oxygen intake, returning our PO₂ to normal levels. A secondary effect of hyperventilation is that we expel much more CO₂. As you now know, it’s the CO₂ in our blood that is the primary respiratory driver. Therefore, a drop in CO₂ has the effect of decreasing our respiratory rate. During the day, this is not noticed as we are hiking around. However, at night it is a different story. If you observe our patient sleeping at altitude you are likely to see the following breathing pattern: the individual breaths normally, but then as they blow off their CO₂ their respiratory drive isn’t stimulated and their breathing eventually stops. After a short while, the CO₂ builds up in their blood, the individual takes a deep gasp and respiration continues. They may even become disturbed or wake. They then go back to sleep and this cycle continues throughout the night.

By the morning, that individual has had cumulatively much less oxygen (through repetitive periods of sleep apnoea), and a rubbish night’s sleep. For most individuals, this process persists for the duration of the trek, and the higher up the mountain you trek, the worse is becomes.  

Can you acclimatise? 

In short, yes you can, but only to a point. Above about 5,000m acclimatisation stops, which is why the highest permanent habitation in the world is at 5,000m. 

Short-term acclimatisation occurs thanks to our renal system. Our kidneys excrete bicarbonate (HCO₂) into the urine and hydrogen (H⁺) into the blood. this reduces blood pH and the consequent slight metabolic acidosis increases our respiratory rate. 

Long-term acclimatisation (occurring if you spend weeks at altitude) occurs, once again, thanks to our renal system. The proximal convoluting tubules secrete erythropoietin which stimulates bone marrow to increase red blood cell (RBC) production. Increases in RBC, coupled with an increase in ventilation equals a more efficient gas exchange. 

Very long-term acclimatisation occurs over months as, amazingly, our bodies grown extra blood vessels. More capillaries plus more blood plus increased ventilation equals an even better gas exchange.  

Recap 

So, we now understand the high-altitude environment and the associated differences in air pressure. We have revised our understanding of respiratory physiology and lastly, we understand how the high-altitude environment interacts with our human physiology and know what to expect if observing humans trekking at altitude.  

Next week we’ll go deeper into Acute Mountain illnesses and how to prevent them. Spoiler alert – it involves descending.