Nocturnal dipping is reproducible in the long term

Abstract

To examine the long-term reproducibility of nocturnal dipping as a dichotomous and continuous variable. Retrospective review of an ambulatory blood pressure monitor (ABPM) database of approximately 15 000 patients. Reproducibility of ABPM was assessed by repeatability coefficient. Kappa (kappa) statistic and intraclass correlation coefficient were used to quantify dipping as a dichotomous and continuous variable, respectively. There were 512 never-treated patients with two ABPM assessments, a mean of 29 (+/-19) months apart. Blood pressure was 2/0 mmHg (awake) and 2/1 mmHg (asleep) higher on the second ABPM. The repeatability coefficient for awake systolic and awake diastolic pressure was 5.9 and 6.2%, respectively. When the dipper/non-dipper (dichotomous) classification was applied to both ABPMs, dipping status was unchanged in 76% of patients, kappa = 0.29. When nocturnal dip was expressed as a continuous variable, the mean nocturnal dip was 14% and the median absolute change between assessments was 3.8%, intraclass correlation coefficient = 0.60. The time interval between assessments was subdivided into (i) within 6 months, (ii) 6 months to 1 year, (iii) 1-2 years, (iv) 2-3 years and (v) more than 3 years. There was remarkable consistency independent of time interval for dipping expressed as a continuous variable but not as dipper/non-dipper. These findings show that in the long term, nocturnal dip is more stable when expressed as a continuous variable. As recent evidence show cardiovascular risk to be inversely related to nocturnal blood pressure in a continuous manner, surely it makes more sense to express nocturnal dip in a similar way to aid stratification of overall cardiovascular risk.