Exploring the link between nocturnal heart rate, sleep apnea and cardiovascular function in African and Caucasian men : the SABPA study
Van Rooyen, Yolandi
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Motivation: There is a rapid escalation in urbanization amongst South Africans and it is known that urbanized South Africans are subjected to lifestyle factors conducive to an increase in the risk for cardiovascular disease (CVD). Obstructive sleep apnea (OSA) has been described as an independent risk factor for CVD, especially hypertension. OSA has also been associated with insomnia, and plays a contributory role in the co-morbidity of this disorder. The mechanisms employed by OSA, which promote the development of CVD are not fully understood but it has been described that OSA and insomnia both act through an increased sympathetic nervous system activity, which may lead to changes in cardiovascular variability, such as an increase in the nocturnal heart rate and blood pressure. These physiological changes may have adverse cardiovascular outcomes which may be evident in early markers of target organ damage, such as left ventricular hypertrophy. It has recently been shown that ethnicity might contribute to the susceptibility to OSA and that African Americans are more susceptible to OSA when compared to Caucasians. Notwithstanding, studies comparing the prevalence and cardiovascular effects of OSA between African and Caucasian men are limited especially where the effect of the nocturnal heart rate is also taken into consideration. There are also a limited amount of studies investigating the prevalence and co-morbidity of insomnia among African and Caucasian men with OSA. Objectives: The general aim of this study was to determine whether the risk of sleep apnea and self-reported insomnia are independently associated with nocturnal blood pressure and ECG Cornell product in African and Caucasian men with an elevated nocturnal heart rate. Subsequent objectives included determining the risk for sleep apnea and the prevalence of self-reported insomnia by respectively implementing the Berlin Questionnaire and the ambulatory diary card. We further aimed to divide the African and Caucasian men separately into two groups based on the nocturnal heart rate and to draw a parallel between the effects of OSA and insomnia on the nocturnal blood pressure and ECG Cornell product within these groups. Methodology: This study is a subsection of the SABPA (Sympathetic activity and Ambulatory Blood Pressure in Africans) study which is a multidisciplinary population comparative study. The study was executed during 2008/2009 on 200 urbanized African and 209 Caucasian school teachers. Our study focused on African and Caucasian men and for the purpose of this study, HIV positive participants were excluded. The final study sample consisted of 88 African men and 101 Caucasian men. The median of the nocturnal heart rate of the 189 men was calculated and the African and Caucasian men were divided into two separate groups, namely those with a nocturnal heart rate ≥ 67 bpm and those with a nocturnal heart rate < 67 bpm. Ambulatory blood pressure monitoring (ABPM) was performed and the blood pressure measurements took place at 30 minute intervals during the day and 60 minute intervals during the night. Participants completed the ambulatory diary card where they reported on insomnia (hours awake per night). The Berlin Questionnaire for sleep apnea risk was also completed by each participant and anthropometric data was collected by registered biokineticists. A standard 12-lead ECG was recorded during rest and the ECG left ventricular hypertrophy was determined by using the ECG Cornell product. Registered nurses collected fasting venous blood samples and the medical history of each participant. The plasma and serum samples were stored at - 80°C prior to the analysis of the biochemical markers. Fasting serum samples for total cholesterol, as well as high-density lipoprotein cholesterol, triglycerides, gammaglutamyltransferase, cotinine and ultrahigh-sensitivity C-reactive protein were analyzed using two sequential multiple analyzers. Statistical analyses were performed using Statistica version 10.0. Results: The high risk of sleep apnea based on the Berlin Questionnaire, did not indicate a significant difference between African and Caucasian men but the occurrence of insomnia, based on self-reported hours of wakefulness at night, was significantly higher in African men when compared to Caucasian men. African men showed a more unfavourable cardiovascular profile when compared to Caucasian men with significantly higher values for the nocturnal systolic blood pressure (SBP), nocturnal diastolic blood pressure (DBP) and ECG Cornell product. There was no significant association between the Berlin Questionnaire sleep apnea risk and any cardiovascular variables in any of the groups, but selfreported occurrences of insomnia predicted an increase in both nocturnal systolic blood pressure (r = 0.469, p = 0.001) and nocturnal diastolic blood pressure (r = 0.499, p ≤ 0.001) in African men with a nocturnal heart rate ≥ 67 bpm. This association was absent in African men with a nocturnal heart rate < 67 bpm and in the case of both Caucasian groups. Conclusions: The risk for sleep apnea between African and Caucasian men based on the Berlin Questionnaire does not differ and further validation of the Berlin Questionnaire, especially in the African population might be necessary to determine the sensitivity and specificity when predicting sleep apnea in this population group. Self-reported insomnia is associated with an increase in nocturnal blood pressure in African men with an increased nocturnal heart rate. The contributory physiological role of insomnia and the accompanying cardiovascular effects, especially the augmentation in nocturnal blood pressure should be considered when investigating OSA.
- Health Sciences