You would think researchers would have already investigated how high altitude affects blood pressure, but this is not the case. In a new study published in the European Heart Journal, Italian scientists demonstrate for the first time that blood pressure monitored over a 24-hour period rises progressively as people climb to higher altitudes. Additionally, they discovered telmisartan, a drug used for lowering blood pressure, was effective in counteracting the effects of altitude up to 3,400 meters or about 2.1 miles — about 250 meters higher than the most elevated city in the United States, Alma, Colo. However, telmisartan was not found to be effective at 5,400 meters (3.4 miles) above sea level — which is the height of the Everest base camp and about 300 meters closer to the sky than the highest city in the world, La Rinconada in Peru.

Map of the Trip

To conduct the study, 13 of the total authors of the published paper joined an expedition of 47 hikers ascending to the Mount Everest south base camp. The team flew from Milan, Italy (120 meters) to Kathmandu, Nepal (1,355 meters) where they stayed for three days. From there, they trekked to Namche Bazaar (3,400 meters) where they stayed for another three days before spending the next five days climbing to the Everest base camp (5,400 meters) where they stayed for 12 days.

Of these 60 healthy “lowlanders,” half were randomly selected to take 80 mg of telmisartan, while the remaining half took a placebo. All along the way, the researchers measured conventional blood pressure, including after eight weeks at sea level, and under acute exposure to 3,400- and 5,400-meter altitudes, the latter upon arrival as well as after 12 days. Blood samples were also collected to check on catecholamines, renin, angiotensin, and aldosterone. Most importantly, though, participants wore a device that measured their blood pressure every 15-20 minutes throughout the day. This ambulatory blood pressure reading offered the researchers a window into nighttime pressure, which normally is 10 to 20 percent lower than daytime blood pressure. When the blood pressure does not "dip" at night despite the person being asleep, this may indicate a problem in the regulation of the heart and blood vessels.

The researchers found that exposure to the highest altitude was responsible for an increase of 14 mmHg in ambulatory systolic blood pressure and 10 mmHg in ambulatory diastolic blood pressure, averaged over a 24-hour period. They also found that telmisartan significantly reduced ambulatory blood pressure at sea level and at 3,400 meters, but no effects could be seen soon after arriving at 5,400 meters. In both groups, exposure to increasing altitude was associated with increased plasma noradrenaline — which contributes to alertness — and suppressed renin–angiotensin–aldosterone system — which regulates pressure and water balance in the body.

The fact that the blood pressure increases along with progressive oxygen deprivation “may have implications for the management of patients with chronic diseases, including chronic heart failure in which breathing is interrupted periodically, acute worsening of chronic obstructive pulmonary disease, obstructive sleep apnoea, and severe obesity,” said Professor Gianfranco Parati, professor of cardiovascular medicine at the University of Milano-Bicocca and leader of the study. “Together, these conditions affect more than 600 million people worldwide, making our results highly significant.”

Source: Parati G, Bilo G, Faini A, et al. Changes in 24 h ambulatory blood pressure and effects of angiotensin II receptor blockade during acute and prolonged high-altitude exposure: a randomized clinical trial. European Heart Journal. 2014.