Nocturnal non-dipping blood pressure patterns and cardiovascular target organ damage
Bawa-Allah, Abdulraheem Babalola
The advent of ambulatory blood pressure monitoring has provided us with better understanding of the diurnal pattern of blood pressure and its role in physiology and pathology. One of the abnormalities of nocturnal blood pressure behaviour is the nocturnal blood pressure non-dipping pattern which is a failure of the night time blood pressure values to fall by at least 10% of daytime values. This blood pressure pattern is associated with several cardiovascular target organ pathologies. Although it is thought that black individuals have higher ambulatory blood pressure values when compared to their white counterparts, normal thresholds for ambulatory blood pressure have not been determined in people of African ancestry but in blacks who live in the developed world. Therefore, in this thesis, my first task was to determine normal thresholds for ambulatory blood pressure in people of African ancestry and to establish if their average ambulatory blood pressure values are comparable to those of other ethnic groups. Using 24-hour blood pressure monitors (Spacelabs model 90201), I measured ambulatory blood pressure in a total sample of 530 healthy participants of African ancestry. Results from the present thesis showed normal thresholds for 24-hour blood pressure to be 135/85mmHg, daytime blood pressure to be 140/90mmHg and night-time blood pressure to be 130/80mmHg. This is the first time that ambulatory blood pressure reference values have been determined in this population. The mean ambulatory blood pressure values in our study population are very similar to those measured in Caucasians. These findings contradict data from earlier studies which show that black individuals have higher ambulatory blood pressure values. The uniqueness of the findings of this study is that the study was conducted in people of African descent living in Africa. The determination of thresholds for ambulatory blood pressure is of immense clinical relevance as it provides reference values for the accurate diagnosis of hypertension as it can help expose masked hypertension and identify white-coat hypertension. Although previous studies have shown that nocturnal blood pressure non-dipping is more prevalent amongst black people, the actual prevalence and possible mechanisms behind nocturnal blood pressure non-dipping pattern in a people of African ancestry has not been determined. Moreover, the impact of dietary salt intake on dipping is currently unknown in this salt sensitive population. In this thesis I was able to show that in 1219 randomly recruited individuals of African ancestry there is a high prevalence of nocturnal blood pressure non-dipping pattern of 52%. This prevalence was higher in females (54%) compared to males (49%). Further analysis of our data indicates that the nocturnal blood pressure non-dipping pattern in this population might be caused by a high plasma aldosterone concentration which causes increased sodium retention and the subsequent increase in nocturnal blood pressure. This was further confirmed by the significant positive relationship between 24-hour urinary sodium excretion and dipping status amongst non-dippers but not amongst dippers indicating that 24-hour urinary sodium excretion is an independent predictor of the nocturnal non-dipping blood pressure pattern. Arterial stiffness is a known marker of arteriosclerosis and subclinical atherosclerosis and is an independent predictor of future adverse cardiovascular events. A non-dipping pattern of blood pressure is associated with a higher level of arterial stiffness and it is known that arterial stiffness increases with age. Because we reported a high prevalence of non-dipping in chapter 4, we investigated a possible involvement of non-dipping in modifying the relationship between arterial stiffness as measured by carotid femoral pulse wave velocity and its determinants in a population of African ancestry. In this thesis, I was able to show in 796 participants of African ancestry that age, hypertension and diabetes are independent determinants of arterial stiffness. I also show that non-dipping strengthened the relationship between age and arterial stiffness but blunted the relationship between arterial stiffness and diabetes. I show interactions of non-dipping in the relationship between pulse wave velocity and gender. The modification of the relationships between arterial stiffness and its determinants by non-dipping may have been due to masking effects or additive effects. Our results suggest that non-dipping might be an important phenomenon to consider in age related arterial stiffness as well as in gender differences in arterial stiffness. Gender differences have been reported in age related arterial stiffness and have been attributed to sex steroids. There is however controversy as to which gender exhibits a sharper age-related arterial stiffness. Because we reported a high prevalence of the nocturnal non-dipping blood pressure pattern, interaction of non-dipping with age related increase in pulse wave velocity and interaction of non-dipping with the relationship between pulse wave velocity and gender in chapters 4 and 5, we decided to investigate futher, the role of non-dipping in gender differences in arterial stiffness and age related arterial stiffnes. In this thesis, I have been able to demonstrate in 413 non-dippers (140 males and 273 females) of African ancestry that non-dipper females have lower levels of arterial stiffness and might be at a lower risk of developing adverse cardiovascular outcomes secondary to arterial stiffness as measured using carotid-femoral pulse wave velocity when compared to non-dipper males. Data from this thesis suggests that this observed difference might be mediated by higher serum levels of high-density lipoprotein cholesterol coulpled with lower serum levels of triglycerides in non-dipper females when compared with non-dipper males. High serum levels of high-density lipoprotein cholesterol and low levels of triglycerides are associated with improved arterial distensibility. The observed differences may also be due to a higher prevalence of diabetes in the non-dipper male group. These observations suggest that serum high density lipoprotein levels and serum triglycerides levels might be important targets for therapy in non-dipping related arterial stiffness. In conclusion, the data presented in this thesis have filled the gap in knowledge concerning normal thresholds for ambulatory blood pressure values in Africans living in Africa and have advanced our knowledge about the prevalence of the nocturnal non-dipping and possible mechanisms responsible for it in people of African ancestry. It has also advanced our knowledge on the possible mechanisms behind gender differences in non-dipping related arterial stiffness.
A thesis submitted to the Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, South Africa 2019
Bawa Allah, Abdulraheem Babalola (2019) Nocturnal non-dipping blood pressure patterns and cardiovascular target organ damage, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/29711>