Predicting Acute Mountain Sickness using Pulse Oximetry
Most trekking and expedition groups use pulse oximeters as a novelty item, and many groups carry a pocket pulse oximeter to periodically check the oxygen saturation (SpO2) as they as ascend the mountain. Pulse oximetry, though not 100% accurate, is useful in predicting acute mountain sickness (AMS). We all know that pulse oximeters are easy to use, noninvasive tools for the assessment of humans at high altitude. These instruments provide an estimate in percentage of arterial hemoglobin oxygen saturation, which is a function of arterial partial pressure of oxygen. The percentage denotes the hemoglobin binding sites that are occupied at any one time by oxygen. At sea level, healthy individuals will be on the “flat portion” of the oxyhemoglobin dissociation curve (see figure below), but at high altitude as the partial pressure of oxygen decreases with ascent, individuals will be at the “steep portion” of the curve (a slippery slope by comparison) where saturation changes significantly with respect to small changes in the partial pressure.
The higher the altitude, the higher the risk of symptoms like Acute Mountain Sickness (AMS) or High Altitude Pulmonary Edema (HAPE). Whether you are an athlete trying to increase your hemoglobin concentration or a mountain climber ascending Mount Everest, a pulse oximeter is important for several reasons while acclimatizing to altitude. Knowing that you are in a mild state of hypoxemia is probably most important. A pulse oximeter to monitor SpO2 is an important index to reflect your physical condition at high altitude. A recent study in Finland measured the effect of using a pulse oximeter to predict who would get AMS at high altitudes. Using a pulse oximeter may be able to tell you if it might be wise to take it easy for a day or two while acclimatizing. A pulse oximeter may also allow you know if you are dehydrated, which easily happens at altitude. Only pulse oximeters such as the Masimo Mighty Sat that have PVI will be able to detect if you are dehydrated, aside from the normal signs of elevated heart rate and feeling bad.
The Finish study measured both resting SpO2 (R-SpO2) and exercise SpO2 (Ex-SpO2) after moderate daily exercise (target heart rate 150 bpm) at altitudes of 7870 feet (2400 m) to 17000 feet (5300 m) during ascent. The point is that Ex-SpO2 was lower at all altitudes among those climbers suffering from AMS during the expeditions compared than among those climbers who did not get AMS at any altitude during the expeditions. It is not unreasonable that the Respiratory Rate (RR) function on the Mighty Sat could predict AMS as well since RR is the cornerstone of acclimatization. In other words, if hyperventilation is the cornerstone of acclimatization, AMS patients may have hypoventilation with resultant decreased SpO2.
Using medications to prevent AMS such as acetazolamide can be very effective in preventing AMS. It is possible to measure changes in SpO2 while taking acetazolamide. The changes are slight, but should show a positive trend nonetheless. This is not to advocate using pulse oximetry to determine who should take acetazolamide.
Indeed, there is no dearth of medical literature supporting the use of pulse oximetry to predict AMS at a higher altitude from a baseline reading. At least one possible pathophysiological rationale for the link between AMS and decreased SpO2 may be adequate ventilation.
Johnathan Edwards is a medical doctor with 30 years of sport medicine experience. A former professional athlete in the sport of motocross and a Category 1 bicycle racer, Johnathan understands human performance. He obtained a complete Physiology degree from UC Davis, medical school in Norfolk, VA, Internal medicine in Las Vegas, Sports medicine in Utah, and Anesthesiology in Florida. Today he is a performance coach, clinical instructor, team doctor for many cycling teams, UFC, motocross and Olympic athletes. Husband and father, he aspires to live part time in France and the United States, and riding his bike. Full Disclosure – Johnathan is a consultant for Masimo and the Mighty Sat.