To the best of our knowledge, this was the first study to present in detail the putative features as well as risk factors for developing high blood pressure in a healthy population of Angola. Also, this is the first study that presents the possible mean values of systolic and diastolic blood pressure in the healthy general population, residing in Luanda, the capital city of Angola. The global prevalence of hypertension worldwide is approximately 26% of the adult population,10 which was higher compared to the overall rate of 7.3% observed in this study. Epigenetic, sociodemographic, behavioral, and eating features of the Angolan or black population, could help to explain the low prevalence of about 3.6 times less hypertension in the Angolan population when comparing global prevalence. Our results suggest that about 7% of the healthy population in Luanda might have high blood pressure with a risk to develop cardiovascular disease,4 suggesting that further studies including epigenetic, sociodemographic, behavioral, clinical, and eating features should be carried out to help control blood pressure in the healthy population of Angola.
Previous studies showed that the blood donation candidate can only be approved when at least the maximum systolic blood pressure is below 140mmHg and the diastolic below 90mmHg.12,14 The mean values of systolic (131 ± 12.3 mmHg) and diastolic (80.1 ± 9.72 mmHg) pressure in our study correspond with the global blood pressure norms for the healthy population (Table 1). However, it is worth mentioning that the maximum systolic blood pressure was 160mmHg and the maximum diastolic blood pressure was 100mmHg, suggesting that some blood donors from Angola, a sub-Saharan African country, are experiencing high blood pressure and risk of developing stroke, myocardial infarction, heart and/or renal failure,2–4 even before donating at least 450mL of whole blood. Previous studies carried out independently contributed to a universal discovery in which reducing blood pressure levels in the general population remarkably reduces cardiovascular morbidity and mortality, as well as slowing the progression of kidney disease, retinopathy, and death from all causes.1,17 Therefore, further studies assessing changes in blood pressure levels pre- and post-blood donation should be carried out among the blood donors in Luanda, in order to help in the detection of possible cardiac complications pre- and post-blood donation, as well as identify and indicate early treatment for individuals with the predisposition of developing cardiovascular disease.
The sociodemographic description of this study, highlighting the predominance of young donors aged up to 40 years old, male, residents in non-urbanized regions, and with some employment is in agreement with population-based studies carried out in Angola,11 as well as other studies indicating that the Angolan population is young, living in undeveloped huts, and with limited sanitary access.18–20 Regarding the relationship between age and blood pressure, previous studies have shown that systolic blood pressure tends to increase with age, while diastolic blood pressure decrease.12,13 This physiological pattern was confirmed by our results in the blood donors population from Luanda, as we identified a significant increase in mean of systolic blood pressure (from 130 ± 15.2 mmHg to 134 ± 13.8mmHg, p = 0.059) and a slight decrease in mean of diastolic blood pressure (from 84.6 ± 6.97 mmHg to 84.1 ± 8.43 mmHg, p < 0.001) with increasing age from under 20 years to over 40 years, respectively (Table 1).
We expected that the highly educated population in our study would be more likely to develop high blood pressure since the increase in the level of education would also imply increasing age, which is an important risk factor for hypertension.12,13 Interestingly, blood donors with a high educational level had 0.76 times less likely to develop high blood pressure (Table 1), contradicting the results of a study carried out in Brazil in which 72% of the hypertensive population had a high educational level.5 In addition, this study carried out in Brazil reported a significant relationship (p = 0.004) between low income and the presence of hypertension, where the authors observed that 65.8% of the hypertensive population had the lowest salary.5 Our results are in agreement with these findings since we observed that the donors employed were 0.49 times less likely to develop high blood pressure, compared to the unemployed population. Indeed, the lack of employment, lack of resources with the high cost of living checked around the world are factors that could affect blood pressure levels in the population, mainly the population from LMICs, although a recent study reported that there are no significant differences in the prevalence of hypertension between developed and developing countries.9 On the other hand, despite the female population being predominant in Angola, representing about 52% of the general population,11 only 7% were part of this study (Table 1). Our results are in agreement with the findings reported by Sahu et al., in a study carried out with blood donors from India, where was reported also a low adherence of the female gender (1.4%) compared to the male gender (98.6%).21 Even without statistical significance (p > 0.05), our findings indicate that women are 1.87 times more likely to develop high blood pressure compared to men. Interestingly, the female gender has already been identified as a group prone to developing cardiovascular disease in Angola. In a recent study carried out by our research team, including patients with arterial hypertension and undergoing treatment in a tertiary unit in Luanda, we found that about 73% (72/99) of the hypertensive patients were female.22 Our results also correspond to a study carried out by Silva et al, with hypertensive patients in the state of São Paulo, Brazil, where 62.1% of the hypertensive population were female.5 Furthermore, a study comparing the hypertensive population of Portugal and immigrants from the PALOP, revealed that 51% and 67% of the population in both groups were hypertensive women from Portugal and immigrants, respectively.23 The physiological aspects such as hormonal climacteric changes, menopause, menstruation, pregnancy, and breastfeeding inherent to the female gender could explain the reduced participation of females in blood donation as well as being a factor for present high blood pressure.
The large adhesion of blood donors during January 2020 is not surprising, and the possible explanation is that once in this period, Luanda province like other Angola provinces, face heavy rainfall, and due to poor sanitation, there is an increase in stagnant water and thus increases also the circulation of vectors capable of increasing cases of malaria, dengue, or other vector-borne diseases in Luanda, as verified by other studies.20,24−27 However, the increase in malaria and dengue cases could be one of the reasons for the increase in blood donation since many patients with malaria or dengue need blood transfusions during hospital treatment.28 Indeed, this explanation could be supported by our results, since the risk of having high blood pressure in January 2020 was 2.5 times, compared to the other months of the same year. Also, we observed that the mean value of systolic (from 130 ± 11.7mmHg to 136 ± 11.1mmHg, p = 0.026) and diastolic (from 78.9 ± 9.13 for 83.3 ± 9.67, p < 0.001) blood pressure increased significantly from December 2019 to September 2020 (Table 1). Interestingly, the first cases of SARS-CoV-2 infection in Angola were identified in January 2020, by our research team.19 In the present study we showed that the blood pressure values as well as the frequency (from 4% in December 2019 to 28% in September 2020, p = 0.434) of the healthy population residing in Luanda, have increased with the spread time of an endemic form of the SARS-CoV-2 in Angola. We do not know whether this increase in blood pressure was influenced by previous exposure to SARS-CoV-2. Therefore, further studies evaluating the blood pressure pattern in an Angolan population exposed to SARS-CoV-2 infection should be carried out in the future. It is worth mentioning that other reasons, such as the pressure to donate blood due to a family member who needs a blood component transfusion, could also cause an increase in blood pressure levels. Indeed, our results showed that family donors (85.7%, 294/343) have been predominant in the blood donor population and this was also the group in which we find about 84% of individuals with high blood pressure. As we already expected, volunteer donors had 0.87 times less risk of developing abnormal blood pressure, possibly justified by the fact that this group not having any family, medical or social pressure to make a blood donation.
A previous study reported that the Angolan general population is mostly from the blood group O and Rh+,19,29,30 which is in agreement with the findings of our study (Table 2). Despite this, our results disagree with the results of a recent study carried out in 2020 by Sacomboio et al., where they identified a higher prevalence of blood group B (36.4%, n = 36/99) in a population from Luanda22, instead of the O group. We cannot rule out the fact that in recent years, there may be an increase in the Angolan population with group B while reducing the population with group O, especially in the population with a chronic diseases. From the point of view of evolution and trend of ABO blood groups in the general population between 1973 and 2020, there was an increase in blood group O (from 52–64%) and a decrease in blood groups A (from 21–18%) and B (24–16%), while the AB blood group remained around 3%.29 On the other hand, comparing the evolution within the hypertensive population in recent years (from 2019 to 2020) according to ABO blood group phenotypes, there was an increase in blood pressure among individuals of the groups O (from 33–64%) and A (from 17–28%) and a reduction among individuals of the group B (from 36–8%) and AB (from 13–0%).22 Despite the reduction of blood group B in the general and hypertensive population over the years, we identify that group B donors had up to 3 times more risk of developing high blood pressure, corresponding with the findings of Sacomboio et al., which revealed that blood group B individuals, seem to be more susceptible to hypertension.30 In this study, we did not explore the reasons why blood group B individuals present a higher risk of developing high blood pressure, despite the decline in frequency in the Angolan population. Therefore, further studies assessing the possible biological or non-biological factors that could influence systole or diastole blood pressure values in the general population, especially those belonging to blood group B, must be carried out.
This study has some potential limitations. Firstly, the small sample size of participants limited the significance of our results and may not be representative of the whole Angolan population. However, in order to obtain an updated and more representative picture of the hypertension situation in the healthy Angolan population, it is important to gather the most recent data and quantify the rate of high blood pressure in the healthy population from the different regions of Angola. Secondly, a family history of hypertension or chronic disease, especially in the donor population with high blood pressure, was not obtained or described in this studied population. Finally, a follow-up and assessment of blood pressure after blood donation or clinical outcome (recovered or deceased) whether from the blood donor after blood donation or the patient after receiving the blood component, was not performed on all blood donors included in this study, which limits the retention of the strong conclusion if these donors were hypertensive or the high blood pressure values were due to a clinical or psychosocial condition. Despite these weaknesses, our findings provide an important description of the mean values of systolic and diastolic blood pressure in the healthy population from Angola. Therefore, must be carried out further studies including epigenetic, sociodemographic, behavioral, clinical, and eating features, as well as, a description of the clinical reasons for carrying out the donation in the case of family donors. Also, studies assessing blood pressure post-blood donation over some time including laboratory tests to assess cardiac, hepatic, or renal function, should be considered for future investigation. These studies could provide a specific picture of which population should adhere more to the primary prevention measures of hypertension, involving actions at the community level such as reducing obesity, consumption of alcohol and salt as well as increased physical activity.14
In conclusion, we showed high levels of systolic and diastolic blood pressure in the healthy population of Luanda, the capital city of Angola. Individuals under 40 years of age, men, living in non-urbanized areas, with a high level of education, with some occupation, and family donors represented a risk group to develop hypertension. Blood group O was the most predominant, despite blood group B representing a high-risk group for developing hypertension and possibly cardiovascular disease. Further studies assessing biological and non-biological features related to blood pressure changes should be carried out in the Angolan population.