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Lower blood oxygen levels due to sleep apnea may explain the higher cardiovascular risk faced by people with this condition. Image credit: milorad kravic/Getty Images.
  • A new study finds that reduced blood oxygen levels may be a leading factor behind the link between obstructive sleep apnea and increased cardiovascular risk.
  • Researchers reviewed data from more than 4,500 middle-aged and older adults who completed medical check-ins and sleep assessments.
  • They believe that a high reduction in blood oxygen levels during sleep mostly due to severe obstruction of the airways may be the cause behind this link.

Obstructive sleep apnea occurs when the upper airway becomes blocked during sleep. This reduces or completely stops a person’s airflow.

Factors that increase an individual’s risk of developing obstructive sleep apnea include:

  • obesity
  • having large tonsils
  • experiencing changes in hormone levels.

Obstructive sleep apnea is the most common type of sleep-disordered breathing. A 2020 study reported that one-seventh of the world’s adult population is likely to have sleep apnea.

Obstructive sleep apnea is associated with increased cardiovascular risk, as noted in previous research.

“Sleep problems […] including sleep apnea contribute significantly to cardiovascular morbidity, as well as all-cause mortality,” Dr. Marishka Brown, director of the National Center on Sleep Disorder Research (NCSDR) told Medical News Today.

A new study now looks at the mechanism behind obstructive sleep apnea and increased cardiovascular risk. It suggests the link may be caused by reduced blood oxygen levels.

The study is published in the American Journal of Respiratory and Critical Care Medicine.

The Apnea Hypopnea Index (AHI) refers to the amount of apneas, when breathing stops, or hypopneas, when breathing is reduced, a person experiences per hour of sleep. It is a conventional measurement of obstructive sleep apnea severity.

“They use that for essentially everything as far as this disorder, but what the research has been finding — and really what this paper as well is strongly supporting — is that there are additional other measures […] besides the use of the AHI as the primary diagnostic or prognostic for people with apnea,” Dr. Brown explained for MNT.

Dr. Brown was not involved in the study. However, the NCSDR is located within the National Heart, Lung, and Blood Institute (NHLBI), which is part of the National Institutes of Health (NIH), which partially supported this study.

Researchers stress in their paper about the study that the AHI does not provide information on the intensity and duration of “ventilatory deficit, oxygen desaturation, and arousals.”

With this study, the researchers outlined different obstructive sleep apnea physiological features in order to show why some people with the condition are more likely than others to develop cardiovascular disease or die.

“Current research, particularly over the past several years, has shown that patients with obstructive sleep apnea are really quite heterogeneous […] meaning that not all people [who] suffer with sleep apnea have the disorder for the same reason,” Dr. Brown told MNT.

“And so, really, trying to identify the mechanisms underlying obstructive sleep apnea for an individual is really quite an imperative to helping in the space of personalized therapy,” she noted.

Physiological features of obstructive sleep apnea addressed in the study include:

  • hypoxic burden — a reduction in blood oxygen levels during sleep
  • ventilatory burden — interruptions in breathing due to airway obstruction
  • nighttime arousals — when a person suddenly wakes due to interrupted breathing.

“I think what they’re getting at here with these three different types of burdens […] from a conceptual standpoint, I can see how disruptions to sleep and in these forms might have different effects on your cardiovascular health,” Dr. Yu-Ming Ni, a cardiologist and lipidologist at MemorialCare Heart and Vascular Institute at Orange Coast Medical Center in Fountain Valley, CA, told Medical News Today. He was not involved in the study.

Researchers looked at data from more than 4,500 middle-aged and older adults who participated in the Osteoporotic Fractures in Men Study (MrOS) and the Multi-Ethnic Study of Atherosclerosis (MESA).

Sponsored by the NHLBI, the MESA was designed to study characteristics of subclinical cardiovascular disease. For this study about obstructive sleep apnea and cardiovascular risk, researchers used data from 1,973 men and women who participated in MESA. The average age was 67 years and participants were followed for about 7 years.

Researchers used data from 2,627 men from the MrOS study. The average age of participants was 76 years, and they were followed for about 9 to 12 years. Funded by the NIH, the MrOS study was designed to identify risk factors associated with osteoporosis and bone fracture in older men.

Participants in both studies completed medical check-ins and comprehensive sleep evaluations. Researchers followed participants through 2018. Approximately 110 participants in MESA and 382 in MrOS experienced a primary cardiovascular event.

A reduction in blood oxygen levels was associated with increased risk of having a heart attack or dying from cardiovascular disease, among participants in both cohorts.

The researchers stressed in their paper that hypoxic burden — reduction in blood oxygen levels — is a strong and consistent predictor of cardiovascular disease and mortality.

The researchers took care to note that a high hypoxic burden was mostly due to severe obstruction of the airway and not due to abdominal obesity or reduced lung function.

“We know that obesity is a real risk factor for sleep apnea, but again, this study is saying that that’s not the only thing […] because we know that there are people who are not obese who develop sleep apnea,” Dr. Brown said.

Among MESA participants, airway obstruction accounted for 38% increased associated risk for having a primary cardiovascular event.

Among MrOS participants, airway obstruction accounted for 12% increased associated risk for having a primary cardiovascular event. Similar findings for predicting premature death based on airway obstruction were also observed.

Among MESA participants, sudden awakenings were not associated with cardiovascular outcomes. However, among MrOS participants, sudden awakenings were linked with cardiovascular-related deaths.

Of the three measures of burden, arousal burden was the weakest predictor of cardiovascular disease and mortality.

Dr. Ni clarified for MNT:

“I guess what they’re trying to get at is: Even if you’re waking up in the middle of the night, that might not necessarily mean you’re getting poor quality sleep, if it’s still restful and you’re getting good oxygen levels and good air movement.”

The study’s findings may change how sleep apnea is assessed and how sleep apnea clinical trials are designed, according to Dr. Brown.

In their paper, the researchers noted that larger studies with longer follow-ups of participants are needed. They also pointed out a limitation of the study: It did not include younger individuals as well as individuals diagnosed with multiple illnesses.

If individuals experience excessive daytime sleepiness or have been told they snore, Dr. Brown stresses that they should speak with their doctors.

“We do know that sleep is required for overall health and well-being and poor sleep not only impacts your physical health, but your mental health as well,” she said.

Dr. Ni told MNT that sleep is a subject he frequently addresses with patients.

“Honestly, it’s not often talked about, and I think that we should be talking more, with our patients, about their sleep quality and seeing if there’s anything we can do to help help with that,” he said.