Study links sleep duration and fragmentation to cardiovascular mortality in women

Insufficient sleep is associated with cardiovascular (CV) disease and fatal CV outcomes. But the length of sleep alone does not reflect critical neurophysiological aspects such as sleep quality, sleep continuity, and sleep depth. To assess the effects of sleep quality and architecture, most existing population-based studies have used self-reported sleep parameters which are not very accurate.

Shahrbabaki et al. measured the arousal burden (AB)- an objective parameter on overnight polysomnograms (PSGs) and have reported in this issue of the European Heart Journal that nocturnal AB is associated with long-term CV and all-cause mortality in women and to a lesser extent in men.

Brief intrusions of unconscious wakefulness, so-called cortical arousals, are a normal feature of sleep. Arousals, irrespective of the underlying mechanism, impact heart rate, blood pressure, and cardiac haemodynamics acutely, but, when frequent, may also disrupt the circadian rhythm of the CV system, which is associated with unfavourable metabolic profiles, such as higher blood pressure, dysregulated blood lipids, and insulin resistance. (Figure)

Clinically, the cause and rate of arousal occurrence are assessed using overnight PSG. The arousal index (AI), i.e. the number of arousals per hour of sleep, is often used to quantify the level of sleep fragmentation. Total sleep time (TST) was defined as the duration of scored sleep epochs, expressed in minutes. They defined the AB as the cumulative duration of all arousal events relative to TST.

The authors then analysed the prognostic value of sleep fragmentation in three large cohort studies (8001 participants in total, mean age 64–83 years, 38% female):1. Osteoporotic Fractures in Men Study (MrOS), 2. Study of Osteoporotic Fractures (SOF), and 3. Sleep Heart Health Study (SHHS).

Taken together, this constitutes a population at intermediate to high risk for CVD (40–50% of patients were overweight, 40–60% had hypertension, 8–17% had a history of coronary artery disease/myocardial infarction, and 5–16% had a history of prior stroke).

The nocturnal arousal burden (measured by at-home overnight polysomnography) was then calculated as the cumulative duration of all arousal events relative to the total sleeping time.

The authors found that female participants in the fourth quartile (Q4) of arousal burden (highest AB) presented consistently with significantly increased rates of CV as well as all-cause mortality over a 6.5- to 11-year follow-up. This association between nocturnal arousal burden and (overall and CV) mortality was less clear for men.

Among others, two very interesting findings from the study by Shahrbabaki et al. remain to be further explored from a mechanistic point of view: first, what is the reason for the varying impact of arousal burden on adverse events in men and women; and second, why do participants in the Q2 (and not Q1) of arousal burden consistently present lower mortality rates across the three cohorts.

Even though many knowledge gaps on the relationship between sleep and CVD remain to be studied in the coming years, this study provides solid evidence supporting the importance of sleep quality for a better CV health.

Arousal burden may represent a promising marker to identify patients at risk. The clinical implementation of AB assessment in routine risk stratification strategies will require easily scalable, widely accessible, and affordable techniques to estimate the duration and fragmentation of sleep and to detect arousals (e.g. wrist actigraphy or peripheral arterial tonometry). Arousal burden may also represent a modifiable risk factor for CV and all-cause mortality, which warrants future prospective intervention studies.

In summary, cumulative evidence shows that sleep quality (duration and fragmentation) is associated with adverse CV events. What remains to be determined is whether an intervention aiming at improving sleep quality is able to reduce the incidence of CV events and mortality.

Source: European Heart Journal: Shahrbabaki SS , Linz D , Hartmann S , Redline S , Baumert M. Sleep arousal burden is associated with long-term all-cause and cardiovascular mortality in 8001 community-dwelling older men and women. Eur Heart J 2021;doi:10.1093/eurheartj/ehab181e