title: Systematic error in the determination of nocturnal blood pressure dipping status by ambulatory blood pressure monitoring.
abstract: BACKGROUND
  Vertical displacement of the arm relative to the heart causes inverse changes in blood pressure of approximately 0.8 mmHg for every centimetre change in arm position. Therefore a potential confounding issue in assessing diurnal variation in blood pressure during ambulatory blood pressure monitoring (ABPM) is arm position during sleep. An increase in the number of patients with 'excessive' nocturnal dipping (> 20% decrease in night/day blood pressure) was observed following the creation of an instructional videotape in which patients were advised to muffle the noise of the monitor with a pillow at night. This raised the possibility that patients were placing their arm on top of the pillow reducing nocturnal blood pressure readings.
  
  
  METHODS
  Ambulatory blood pressure monitoring data from 184 patients prior to and from 193 patients following specific instructions not to put their arm on top of the pillow was examined.
  
  
  RESULTS
  Following the instructions, the percentage of patients with 'excessive' nocturnal dipping in blood pressure decreased (excessive systolic dipping 17.4 versus 8.8%, P = 0.014; excessive diastolic dipping 37 versus 24.4%, P = 0.01). Consistent with an increase in the ratio of nocturnal/day pressures, there was an increase in the percentage of patients with inadequate nocturnal dipping (< 10% decrease in night/day blood pressure; systolic dipping 33.7 versus 45.6%, P = 0.02; diastolic dipping 13.0 versus 31.6%, P < 0.001)
  
  
  CONCLUSIONS
  Instructing patients to avoid resting their arm on a pillow at night has a substantial effect on the classification of nocturnal dipping status. Patients need clear instructions not to place their arm on a pillow at night during blood pressure monitoring.
authors.affiliation: Division of Nephrology, Faculty of Medicine, The University of Calgary, Calgary, Alberta, Canada.
authors.forename: Srinu
authors.initials: S
authors.lastname: Kammila
authors.forename: Norman R C
authors.initials: NR
authors.lastname: Campbell
authors.forename: Rollin
authors.initials: R
authors.lastname: Brant
authors.forename: Roberta
authors.initials: R
authors.lastname: deJong
authors.forename: Bruce
authors.initials: B
authors.lastname: Culleton
mesh_terms: D001132:Arm; D001794:Blood Pressure / Q000502:physiology*; D018660:Blood Pressure Monitoring, Ambulatory / Q000379:methods*; D002940:Circadian Rhythm / Q000502:physiology*; D003971:Diastole / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011187:Posture; D013599:Systole / Q000502:physiology