Comparing effect of definition of diurnal periods by diary, fixed periods, and actigraphy on ambulatory blood pressure parameters in a Chinese population
Authors: Lee Eric Kam-Pui , Zhu Mengting , Chan Dicken Cheong-Chun , Yip Benjamin Hon-Kei , Mihailidou Anastasia Susie , Wong Samuel Yeung-Shan 
Published: July 2022

Background:
Reliable measurement of daytime and night-time blood pressure (BP), and degree of BP dipping during sleep during ambulatory blood pressure monitoring (ABPM) requires an accurate definition of sleep time (diurnal definition). However, superiority of any diurnal definition on ABPM remains unclear. The present study compared mean daytime and night-time SBP and DBP using different methods for diurnal definition: patient's diary, wide-defined and narrow-defined fixed periods, and actigraphy, in a Chinese population with diagnosed essential hypertension. We hypothesize that BP values from actigraphy are different from BP obtained by other methods and associated with end-organ damage (i.e. impaired renal function, proteinuria, left ventricular hypertrophy).

Methods:
From April 2017 to October 2019, 203 Chinese patients diagnosed with hypertension were recruited prospectively from Lek Yuen Clinic and 179 completed a 48-h ABPM study, wearing a validated actigraph and completed a sleep diary. Presence of end-organ damage was retrieved from the computerized clinical management system. The differences in the mean BP values provided by different diurnal definition were compared using paired t tests and Bland–Altman plots. The prevalence of elevated BP, dipping status categories, overall percentage agreement and the Kappa statistic were calculated by pairwise comparisons between different diurnal definitions. The reproducibility was also estimated and logistic regression was used to examine the relationship between BP values from different diurnal definitions and end-organ damage.

Results:
Mean daytime and night-time BP values were similar regardless of the definition used (mean difference <2 mmHg). Kappa statistics and overall percentage agreement found excellent agreement between different definitions to diagnose elevated daytime BP (Kappa ranged from 0.80 to 0.91) and night-time BP (Kappa ranged from 0.74 to 0.89). Good agreement to diagnose nondipping was also detected (Kappa ranged from 0.65 to 0.78). Furthermore, ABPM values were most reproducible when diurnal periods were defined by patient's diary (intra-class correlation coefficient = 0.82–0.93). Daytime and night-time BP values obtained using different diurnal definitions did not differ in their association to end-organ damage.

Conclusion:
Differing definitions of diurnal periods provide similar mean BP values among a Chinese hypertensive population and have good agreement for diagnosis of elevated BP and dipping status. In individual patients, clinicians should be aware that different definitions of diurnal periods can lead to a 3–5 mmHg difference in patient's BP values and may affect the diagnosis of elevated BP in patients with BP close to diagnostic thresholds. The current study supports using the patient's diary to define diurnal periods, which provided the best reproducibility.