title: Decreased nighttime heart rate variability is associated with increased stroke risk.
abstract: OBJECTIVE
  Prediction of stroke in healthy individuals is challenging and there is a diurnal variation of stroke onset. We hypothesized that heart rate variability with a focus on nighttime heart rate variability will predict the risk of stroke in apparently healthy middle-age and elderly subjects.
  
  
  METHODS
  The population-based cohort of the Copenhagen Holter Study, consisting of 678 healthy subjects between age 55 and 75 years with no history of cardiovascular disease or stroke, was evaluated. All underwent 48-hour ambulatory electrocardiogram monitoring. The SD of normal-to-normal RR intervals (SDNN) was selected as the method of measuring heart rate variability. Nighttime SDNN was measured between 02:00 and 02:15 AM and could be evaluated in 653 subjects. Median follow-up was 76 months.
  
  
  RESULTS
  Nighttime SDNN was lower in women than in men (P=0.0008), and in diabetics than nondiabetics (P=0.03). However, smoking, cholesterol, systolic blood pressure, and age were not associated with nighttime SDNN. The risk of stroke was significantly associated with nighttime SDNN in a univariate analysis (HR, 0.66; 95% CI, 0.50-0.88; P=0.004) and after adjustment for conventional risk factors (HR, 0.67; 95% CI, 0.51-0.89; P=0.005) per 10 ms increments of SDNN. Eighty-one percent of all strokes (21/26) occurred in 330 subjects with the lower half of nighttime SDNN (≤38 ms; HR, 4.31; 95% CI, 1.62-11.42; P=0.003).
  
  
  CONCLUSIONS
  Nocturnal heart rate variability is a strong marker for the development of stroke in apparently healthy subjects. The mechanism is unknown, but reduced parasympathetic activity may increase the risk of stroke by increasing the risk of arrhythmias.
authors.affiliation: Department of Cardiology, Copenhagen University Hospital of Amager, Denmark. zeynep.binici@dadlnet.dk
authors.forename: Zeynep
authors.initials: Z
authors.lastname: Binici
authors.forename: Mette Rauhe
authors.initials: MR
authors.lastname: Mouridsen
authors.forename: Lars
authors.initials: L
authors.lastname: Køber
authors.forename: Ahmad
authors.initials: A
authors.lastname: Sajadieh
mesh_terms: D000368:Aged; D002940:Circadian Rhythm / Q000502:physiology*; D015331:Cohort Studies; D015716:Electrocardiography, Ambulatory / Q000379:methods; D005260:Female; D005500:Follow-Up Studies; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D012307:Risk Factors; D012894:Sleep Stages / Q000502:physiology; D020521:Stroke / Q000209:etiology / Q000503:physiopathology*