Ethnic disparities in the morning surge: Which utility for typifying the hypertensive patient?

In their study, Gavin et al1 examined whether differences occur in the blood pressure (BP) morning surge (MS) between 199 black and 207 white participants of the Sympathetic Activity and ambulatory Blood Pressure in Africans (SABPA) Study. The authors documented a higher ambulatory BP and a lower rate of BP rise at awakening in black vs white Africans. An increased MS, ascertained using ambulatory blood pressure monitoring (ABPM), has been found to be associated with an increased risk of stroke and other cardiovascular events.2 However, the strength of the association between the MS and cardiovascular events is largely dependent on the definition and threshold used to define the MS and of the patients' characteristics, with one study also reporting a significant association for a blunted MS.3 Few studies have also reported substantial ethnic differences in the degree of MS, a finding which can help understand the role of ethnic factors in cardiovascular risk assessment and identify the most effective measures to be implemented for preventing BP-related cardiovascular events (Table 1).4-6 According to these studies, including both normotensive and hypertensive subjects, the amplitude of BP rise at awakening is highest in Asians and lowest in blacks. One longitudinal study, based on mixed populations from Europe, Asia, and South America, anticipated some potential differences in the risk of cardiovascular events according to the ethnic group.7 At variance from the majority of previous studies, in the study of Gavin et al the MS determination was based on the pattern of the changes in BP independent of waking, following a methodology which previously proved to be more reliable for the prediction of risk than the traditional sleep-trough approach.8 This methodology relies upon a line of best fit based on the entire recording and can independently estimate the power of the MS and the rate of the morning rise during the transition period, making no assumption about when the BP is rising. This is relevant since the calculation of MS bears several unsolved and critical issues, particularly related to the methodology used for its calculation, as summarized in Table 2.9-11 Although the major strength of the study by Gavin et al stands in the rigorous methodology employed to assess the MS, nonetheless the study is not free of some critical issues. First, the ABPM was performed in the Research Unit facility. Although the authors observed no difference in the proportion of participants reporting insomnia, they did not address the quality of sleep, which may have differed from that reported when sleeping in a cozier environment as that of their home. A worse quality of sleep may have contributed to rise the average level of nighttime BP and to reduce the amplitude of the MS. Second, although the authors did not find any association between antihypertensive medication use and the rate and power of the MS, they did not specifically evaluate the number of drugs used and, most importantly, the timing of their administration (morning vs evening). Since treated black patients had higher BP levels and a higher chance of uncontrolled BP, adjustment for these confounding factors would have helped to remove their deleterious effects on the determination of the actual MS. Third, a previous study comparing the sleep-trough MS in black and white South Africans has shown that ethnic differences in MS are abolished after accounting for the nighttime dipping.5 Using a dynamic approach, MS remains higher in white individuals even after adjusting for the day-night change in BP (Table 1). As a matter of fact, in the results presented by Gavin et al, blacks display higher BP levels during night sleep and a reduced day-night dip, as in previous studies. Although the authors discussed this aspect, they did not adjust their results for the extent of day-night dipping and thus we do not know whether the extent of the diurnal BP variation may have an impact on the determination of the MS. Fourth, the authors ran a regression analysis in order to determine the association of the MS, evaluated by mean BP, with various demographic and clinical characteristics of the subjects. However, when comparing the median MS between the two ethnic groups, they did not adjust results for variables such as age or body mass index, which significantly differed between the two groups. In a previous study comparing the MS between whites and African Americans, such adjustments resulted in the loss of a statistically significant difference of the MS between the two ethnic groups (Table 1).4 Fifth, in this study a relatively low frequency of BP measurements was set (2 measurements per hour during the day and 1 measurement per hour during the night) with a median number of readings over the 24 hours of 36 in white and 30 in black participants. The number of readings obtained during the day and the night significantly differed between the two ethnic groups, with a larger number available in white participants. Although the calculated rate of rise in BP during awakening and the power of the MS in BP were not associated with the number of BP readings, one cannot exclude that the low sampling rate may have led to an inaccurate estimation of the MS. A higher and more homogeneous BP sampling rate during the ambulatory recording may be critical for the accuracy of the calculations, particularly when a curve fitting method is applied. In conclusion, the study by Gavin et al introduces a novel concept for the objective assessment of the MS from ABPMs which may bring some advantage in the evaluation of the individual patients' risk, compared to the traditional sleep-trough approach. It also brings to the reader's attention the existence of a difference in the extent of the MS between black and white subjects, whose underlying mechanisms need to be further investigated. Since it is well known that blacks are more prone to develop arterial hypertension, diabetes, and cardiovascular complications than white individuals, whether the blunted MS observed in the blacks of the SABPA study brings or not an additional cardiovascular risk must be elucidated in a longitudinal phase of the study. Future studies should also evaluate whether ethnic disparities in the MS bear any clinical relevance, as to target specific antihypertensive treatment or allow personalized patient management. The author reports no conflicts of interest to disclose.