The prevalence of masked hypertension subtypes: it’s association with cardiovascular organ changes and renal dysfunction in a population of African descent

Nyundu, Franswell Thamsanqa
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Cardiovascular diseases are a leading cause of preventable mortality and morbidity worldwide and hypertension is the most prominent established risk factor for these diseases. In Africa, the burden of hypertension is increasing at a faster rate as compared to other parts of the world. Predictions based on the current data indicate that the burden of hypertension is still on the rise. In addition, the prevalence of hypertension in Africa is underreported and underestimated due to the use of conventional blood pressure monitoring. The introduction of home blood pressure monitoring and ambulatory blood pressure has made it easy to identify other forms of hypertension such as white coat hypertension and masked hypertension. White coat hypertension has less detrimental cardiovascular consequences whereas masked hypertension has adverse consequences that may be similar to sustained hypertension. Most of the studies conducted on masked hypertension have not taken into account the various subtypes of masked hypertension therefore very little is known about masked hypertension subtypes especially in African populations. Moreover, a number of studies have investigated the impact of hypertension on cardiovascular target organ damage but very little is known about the association between masked hypertension subtypes and cardiovascular target organ damage. Therefore, in this thesis I determined the prevalence of masked hypertension subtypes and the association between masked hypertension subtypes and cardiovascular target organ damage in a population of African ancestry. I also investigated if masked hypertension is associated with renal dysfunction independent of dietary salt intake in this population. Finally I investigated the relationship between masked hypertension subtypes and nocturnal blood pressure dipping. In a population sample of 1310 participants, I measured conventional and 24-hour ambulatory blood pressure, collected 24-hour and spot urine samples, and collected blood samples, determined left ventricular mass and arterial stiffness using echocardiography and applanation vi tonometry respectively. Masked hypertension was subdivided into three subtypes. These subtypes are 24-hour masked hypertension, night-time masked hypertension and daytime masked hypertension. Twenty-four-hour masked hypertension is whereby 24-hour ambulatory blood pressure is ≥130/80 mm Hg. Night-time masked hypertension is where night-time ambulatory blood pressure is ≥120/70 mm Hg. Daytime masked hypertension refers to an ambulatory blood pressure of ≥135/85 mm Hg. These ambulatory blood pressures are elevated in spite of a normal conventional blood pressure of < 140/90 mm Hg. The results show that the prevalence of the subtypes of masked hypertension in population of African descent was 7% for 24-hour masked hypertension, 15% for night-time masked hypertension and 9% for daytime masked hypertension. All subtypes of masked hypertension were significantly associated with smoking and alcohol consumption (24-hour masked hypertension p<0.0001; night-time masked hypertension p<0.0001; daytime MH p<0.0001). Pulse wave velocity, was significantly higher in all the mask hypertension subtypes compared to the normotensives; 24-hour masked hypertension (p= 0.0029), night-time masked hypertension (p=0.0061) daytime masked hypertension (p=0.0069). Left ventricular mass was also significantly increased in all masked hypertensive subtypes compared to normotensives; 24-hour MH (p= 0.0003), night-time MH (p<0.0001) daytime MH (p=0.0010). Dietary salt intake was not associated with any masked hypertension subtype. However, microalbuminuria was significantly higher in the night-time masked hypertensives (3.6±2.1 mg/mmol) compared to the 24-hour (1.4± 0.4 mg/mmol) and daytime (0.9±0.3 mg/mmol) masked hypertensives groups. In my assessment of the relationship between nocturnal blood pressure dipping and masked hypertension subtypes, the results show that nocturnal blood pressure non-dipping is associated with all masked hypertension subtypes; 24-hour masked hypertension (p=0.0021), daytime masked hypertension (p= 0.0002) and night-time masked hypertension (p< 0.0001). vii These results show that in a population of African descent, night-time masked hypertension is the most prevalent form of hypertension and all masked hypertension subtypes are associated with cigarette smoking and alcohol intake. Furthermore, all masked hypertension subtypes are related to arterial stiffness and left ventricular hypertrophy. When assessing kidney function and masked hypertension, the results indicate that only night-time masked hypertension is associated with renal dysfunction and this relationship is independent of dietary salt intake. The importance of these findings is that in African populations, smoking, alcohol intake and nocturnal blood pressure non-dipping predict masked hypertension above dietary salt intake. Therefore, strategies to reduce masked hypertension related cardiovascular target organ damage should focus on these parameters.
A thesis submitted to the Faculty of Health Sciences, University of the Witwatersrand, School of Physiology, in fulfilment of the requirements for the degree of Doctor of Philosophy, Johannesburg, 2020