Polygraphy of sleep at altitudes between 5300 m and 7500 m during an expedition to Mt. Everest (MedEx 2006)

Sleep at extreme altitudes is characterized by the repetitive occurrence of central apneas that in some cases may lead to a marked decrease in arterial oxygen saturation. During the Ludwig Maximilians University Expedition to Mt. Everest (MedEx 2006), nocturnal polygraphic recordings were made at different altitudes and included the first recordings ever made at 7500 m, which were completed on 8 separate occasions during the expedition.

The study was performed on the author (K.M., 58 years, 181 cm, 75 kg), who is an experienced high-altitude mountaineer. The standard polygraphic parameters, such as nasal and oral airflow, thoracic and abdominal effort, oxygen saturation, heart rate, body position, movement, and snoring, were collected with a portable sleep recording device (AlphaScreen, SensorMedics, Germany, Hochberg) at different altitudes between 5300 m and 7500 m, and were compared with baseline assessments made in Munich, Germany (altitude 508 m). The daytime value of oxygen saturation at rest was measured at South Col (8000 m) and at the South Summit (8763 m) without breathing supplemental oxygen for at least 10 minutes.

The number of apneas and hypopneas of central origin increased up to a maximum of 148/h with a minimal blood oxygen saturation of 48% at 7500 m, compared with <5/h at Munich. After 11 days of acclimatization, data recorded at 5300 m showed a marked reduction of disturbed sleep. The Apnea-Hypopnea-Index dropped from 138/h to 51/h, and the minimal blood oxygen saturation rose from 57% to 67%. At South Col (8000 m), the daytime value of oxygen saturation at rest ranged between 53% and 55%, and on South Summit (8673 m) without breathing supplemental oxygen for at least 10 minutes, it fluctuated around 50%.

These data correlate well compared with those obtained in hypobaric chamber studies and show that regardless of physiologic adjustment to low oxygen conditions at Base Camp altitude, during the final summit attempt oxygen saturation drops further to 55% and even less. Thus recordings of nocturnal oxygen saturation at Camp 3 (7500 m) on Everest, where the night is spent before the summit attempt, may help to show the actual efficiency of hypoxic ventilatory response and to detect the individual hypoxic tolerance to altitudes above 8000 m.