The population of Summit County, Colorado is projected to grow by 56% between 2010 and 2030. Along with adjacent Park and Lake Counties there are now over 40,000 people living above 2800 meters elevation. This is the largest high altitude population in North America. As opposed to native populations in South America, Africa and Asia who have been residing above 2800 m for centuries, the North American residents are acclimatized but not adapted. Symptoms related to hypobaric hypoxemia are notable above 2500 m. Recognized conditions associated with altitude include central sleep apnea leading to hypoxemia (abnormally low oxygen level in the blood) which activates the sympathetic nervous system. In susceptible persons this can cause systemic and pulmonary hypertension. The incidence of this potentially devastating side effect of mountain living is unknown. In order to better understand the potential side effects of nocturnal oxygen desaturation in healthy individuals, it is beneficial to investigate the normal physiological changes that occur during sleep, which leads to low oxygen levels in all individuals.
When the body enters the sleep state, many of the behavioral mechanisms that are active during wakefulness are blunted, and it’s been found that different sleep stages have varying effects as well. One of the major changes is a diminished response to hypercapnia (high carbon dioxide levels in the blood) and hypoxia. During sleep, the CO2 set point is elevated from 40 mmHg to 45 mmHg, which results in reduced alveolar ventilation. It’s also observed that minute ventilation is reduced, which is due to decreased tidal volumes that is normally compensated for with an increase in breathing frequency during wakefulness. Also, during sleep, there tends to be upper airway narrowing that is normal and there is reduced reflex muscle activation of the pharyngeal dilator muscle. All of the above factors contribute to decreased ventilation during sleep.
A lot of what is understood about the effects of nocturnal hypoxemia is due to extensive studies in individuals with underlying diseases, and these studies are not always conducted at higher altitudes. One such study investigated the effects of nocturnal desaturation (SaO2 < 90% occurring for > 30% of the sleep study) in chronic obstructive pulmonary disease (COPD) patients without a diagnosis of sleep apnea. The authors found higher rates of dyspnea, increasing rates of worsening COPD symptoms, poorer quality of sleep and health-related quality of life. Another such study found that some patients with COPD experience increased transient arterial hypoxemia (TAH) during rapid eye movement (REM) sleep. In this study, the authors observed that the study subjects experienced increased pulmonary vascular resistance (which can lead to pulmonary hypertension) and a few subjects experienced an increase in their cardiac output. The authors found that individuals could experience a decrease in this phenomena by using nighttime oxygen therapy.
Studies, such as above, do not assist in identifying healthy individuals that may need early intervention due to nocturnal hypoxemia at altitude. What about the healthy individuals without underlying diseases? In the study conducted by Gries and Brooks in 1996, the authors collected data from 350 patients. Their recorded average low saturation in the study of 350 subjects was a reported 90.4% lasting an average 2 seconds. This study was conducted at the Rainbow Babies and Children’s Hospital located in Cleveland Ohio, at an elevation of 653 feet (198 m). This is one of the largest studies done to assess normal oxygen levels observed during sleep, and the results, along with results from other studies are displayed in Table 1. As of right now, there is no equivolent study for subjects at elevations like that of Summit County, CO, which is at an average of 9110 feet (2777 m). Aside from the normal physiological changes noted above, the rates of developing underlying central sleep apnea leading to systemic and pulmonary hypertension is unknown. Further, there are no guidelines as to initiating treatment in patients that may be experiencing adverse effects of high altitude nocturnal hypoxemia, because there is a lack of data to establish baseline normal values observed at this elevation. This leads to unnecessary sleep studies, and further involvement of a myriad of healthcare professionals that have no specific guideline to reference when approached by one of these patients.
In order to further our understanding of the effects of high altitude and nocturnal hypoxemia in healthy individuals, like that of Summit County, there has to be preliminary and ongoing research in these individuals. Dr. Chris Ebert-Santos is currently conducting an overnight pulse oximetry study, which aims to recognize which symptoms they may or may not be experiencing, that are related to high altitude or sleep disorders, so that they may receive treatment, feel better, and remain active.
At this moment, initial study results reveal a decreased average low night oxygen saturation from that of the study conducted by Gries and Brooks. In a sample of just 14 individuals, the average low SpOs recorded overnight is at 81.3%, which is 9% lower than that recorded by Gries and Brooks (Graph 1). The study is also revealing a trend in lower night oxygen saturations in individuals that have lived at elevation for a longer period of time (Graph 2). These findings suggest the need to expand and build on the current study being conducted by Dr. Chris and her team at Ebert Family Clinic. If interested, you may apply in-person at Ebert Family Clinic, where you will be required to fill out a health questionnaire on your length of residence at altitude, medical history, and possible symptoms related to high altitude. Your basic vitals will be logged at the appointment. After the first study, you will then be rescheduled in 12 months for a follow-up overnight study to monitor for any changes. Overall, this study is designed to help with an understanding on the potential impact of high altitude on healthy individuals that are acclimated, but not necessarily adapted, to this environment.
Robert Clower is a second year physician assistant student at Red Rocks Community College in Arvada, CO. His undergraduate degree was in Biology, which incorporated both medical health science courses as well as independent research courses in general biology and ecology. While attending school at the University of North Georgia, Robert served in the Army National Guard for a cumulative time in service of 8 years. After completing his undergraduate degree, Robert gained medical experience as an operating room assistant, which included assisting support staff with surgical preparation and patient transport throughout the hospital for surgical appointments. Outside of his studies, Robert enjoys snowboarding, hiking, snowshoeing, exercising and spending time with family and friends.
Summit County Population Projections: Summit County, CO – Official Website. Summit County Population Projections | Summit County, CO – Official Website. http://www.co.summit.co.us/519/Population-Projections. Accessed March 3, 2020.
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