Pulse Oximeters at Altitude: What the Readings Mean

A pulse oximeter reports the share of your blood's haemoglobin that is carrying oxygen (SpO2), and that share always falls as you climb, so a lower number at 4,000 m than at 1,400 m is expected and is not by itself a sign of illness. What matters on a Nepal trek is the trend against your own earlier readings, how you feel, and whether you are getting worse. Nobody in the guidelines we cite gives a pass mark, and you should be suspicious of anyone who does.

This guide explains what the device measures, why trekkers like it, where it misleads, and how to use it without letting it overrule your body. It is general information and not medical advice; see a doctor before you go if you have a heart, lung or blood condition. For the full picture of altitude illness, start with altitude sickness basics and the acclimatisation schedule.

What a pulse oximeter actually measures

A finger oximeter shines two wavelengths of light through the fingertip and works out, from how much of each is absorbed with every heartbeat, the proportion of haemoglobin that is oxygenated. Wikipedia's summary of the method puts typical accuracy at within 2 per cent of the invasive arterial reading, and within 4 per cent in 95 per cent of cases, under good conditions. Those good conditions are the catch. The same article notes that fingertips work because they are well supplied with blood, and that the method struggles when circulation is poor, which is exactly what a cold Khumbu morning does to your hands.

The device also reports pulse rate. Resting pulse rises as you gain height, and many guides watch it alongside the saturation figure. Neither number is a diagnosis.

Why the number falls as you go up

Air pressure drops with altitude, so each breath delivers less oxygen to your lungs. Your body responds by breathing faster and, over days, by making other adjustments, which is what acclimatisation means. Saturation therefore falls as you climb and then partly recovers as you adapt. It also dips at night, when breathing slows, which is one reason sleep at altitude is poor for many people; see what normal looks like day by day at altitude.

Because of this, a reading of, say, the 80s at a high lodge is not the same event as the same number at sea level. The sensible comparison is you against yourself: your reading at Namche on the evening you arrive, your reading after a rest day, your reading next to a healthy partner who has made the same climb. The CDC Yellow Book, the Wilderness Medical Society guidelines and the NHS all rest their advice on symptoms and the speed of ascent, not on a saturation threshold.

What the readings can tell you

Used carefully, an oximeter gives three modest benefits.

• A trend. If your resting reading improves over two nights at the same height, you are probably acclimatising. If it keeps falling while you rest and you feel worse, that supports what your symptoms are already telling you.
• A comparison. Reading a healthy companion and yourself in the same room, hands warmed, on the same device, shows whether you are an outlier.
• A prompt to look closer. An unusually low number is a reason to ask about headache, appetite, nausea, balance and sleep, and to ask the guide or a clinic to take a proper look.

What the readings cannot tell you

An oximeter cannot diagnose acute mountain sickness (AMS), high-altitude pulmonary oedema (HAPE) or high-altitude cerebral oedema (HACE). Those are recognised by symptoms and signs: headache with nausea or fatigue for AMS; breathlessness at rest, cough and a feeling of drowning for HAPE; confusion and loss of balance for HACE. Read HAPE and HACE warning signs and response before you trek.

A good-looking number can also be wrong. A cold hand can show a falsely low figure and a poorly fitted sensor can show nonsense. A fine reading offers no protection if you have a worsening headache and are vomiting. Equally, a poor reading in a healthy, rested person with cold fingers may simply be a bad reading.

How to take a reading you can trust

1. Warm your hands first, ideally inside your jacket for several minutes, because cold fingers distort the signal.
2. Sit still and relaxed for at least five minutes before measuring, since walking or talking changes the figure.
3. Remove nail polish and rest your hand at chest height, palm down.
4. Wait for the number to settle, then note it and the pulse rate.
5. Repeat two or three times and record the middle value.
6. Measure at the same time each day, such as before dinner, so readings are comparable.

Write the reading, the time, the altitude of your lodge and how you feel in a notebook or on your phone. A seven-line log across a trek tells you far more than any single measurement.

Choosing a device

Cheap fingertip oximeters are light and common in Kathmandu and Pokhara pharmacies and online. Quality varies, and some low-cost units are not tested against any medical standard. If you carry one, test it at home first, compare it against a second device if you can, keep it warm in an inner pocket, and carry spare batteries because cold drains them quickly. A wrist or finger model that stores readings is more useful than one that does not.

Do not treat the device as a safety certificate for a group. If one trekker carries the only oximeter, make sure the habit of daily checks does not replace the daily conversation about symptoms.

Guides, clinics and the oximeter

On the Annapurna Circuit and the Everest Base Camp trail, many guides check clients with an oximeter each evening and keep their own notes. The Himalayan Rescue Association runs seasonal clinics at Manang and Pheriche that give talks and assess trekkers; see our guide to the HRA clinics at Manang and Pheriche. Whatever your device says, a guide's decision to stop, rest or descend is a safety decision, and you should treat it as binding. Our guide versus porter roles post explains who is responsible for what.

What to do with a worrying result

The NHS advice is the simplest to follow. If you have symptoms of altitude sickness, tell someone you are with, rest at the same altitude and do not go higher. Mild symptoms usually improve in one to three days. If symptoms get worse or do not improve after a day, go down, around 300 to 1,000 m if you can. Get medical help immediately for confusion, problems with balance, breathlessness at rest, a frothy or bloody cough, blue or grey lips or being very sleepy; descend straight away. None of that depends on a number on a screen.

Medication such as acetazolamide is a separate decision to make with a doctor before the trek; read Diamox on Nepal treks for the background.

Planning with a partner

A pair or small group can use the habit well: agree before you start that the evening check is routine, that anyone can ask for another reading, and that nobody is teased for sitting out a climb. Agree too that if one person descends, the group has a plan; our guides on what to do if your partner cannot continue and trekking partner safety roles set out how to do that. If you are still looking for someone to walk with, see how to find a trekking partner safely.

Routes where this matters most

The risk is highest where you gain height quickly or spend several nights above 4,000 m. On the Everest Base Camp trek, Gorak Shep sits at 5,164 m and the high point, Kala Patthar, at about 5,545 m, so the evening check at Dingboche (4,410 m) and Lobuche (4,940 m) is when a log is most useful. On the Annapurna Circuit, the climb from Manang (3,540 m) through Yak Kharka (4,050 m) to Thorong Phedi (4,540 m) before the 5,416 m pass is where the trend matters. If you want a trekking companion for either, see partners for Everest Base Camp and partners for the Annapurna Circuit.

The short version

Carry one if you like gadgets, use it consistently, and never let it argue with your symptoms. Rest, hydrate, climb slowly and descend when in doubt. For the wider set of health topics, see the altitude and health category.