Permanently living at high altitude (HA) directly affects the cardiovascular system because of lower arterial blood oxygen content compared to sea-level and other associated physiological changes. It is uncertain if there are clear-cut benefits or risks to cardiovascular health from living at HA and whether these benefits or risks, if they exist, vary in different populations. In Nepal, a comprehensive cardiovascular risk assessment of a sample of individuals representing a HA population has not previously been performed. The main aims of this project were to estimate, in residents at HA, the prevalence of coronary heart disease (CHD) and cerebrovascular disease; to estimate the distribution of key cardiovascular risk factors; and to estimate any possible relationships between CHD or blood pressure with altitude.
The study design was a cross-sectional survey. The sampling technique was cluster sampling of study areas on the basis of altitude level, population density and logistical support to undertake the study, but the participants within the study areas were randomly selected. The sample consisted of 521 residents aged 30 years or over from the Nepal districts of Mustang and Humla, permanently living at altitudes of 2800 metre (m), 2890 m, 3270 m, or 3620 m. Data was collected by administering the WHO STEPS interview questionnaire for non-communicable disease risk factors, a questionnaire for verifying stroke-free status (QVSFS), bio-physical measurements (blood pressure, height, weight, waist, hip), biochemical measurements (lipid profile and glycated haemoglobin), and a resting 12 lead electrocardiogram (ECG). The prevalence of CHD was defined as the presence of pathological Q waves in the ECG or self-report of personal history of CHD (previous event of myocardial infarction (MI) or chest pain from heart disease (angina)). ECG recordings were categorized as definitely abnormal (e.g. showing evidence of previous MI, borderline (e.g. non-specific T-wave inversion) or normal after review by a cardiologist using standard widely accepted criteria. Blood pressure (systolic/diastolic) was classified as normal (<120/80 mmHg), pre-hypertension (HT) (120–39/80–89 mmHg), HT (≥140/90 mmHg), Stage I HT (140–159/90–99 mmHg), and Stage II HT (≥160/100 mmHg). Analysis of variance (ANOVA) and analysis of covariance (ANCOVA) models were used for the relationship between systolic blood pressure (SBP), diastolic blood pressure (DBP) and altitude. Logistic regression was used to estimate the association between an abnormal (or borderline abnormal) ECG and altitude in univariate and multivariate models.
None of the participants showed definite electrocardiographic evidence of CHD. Overall, 5.6% of the participants gave a self-reported history of CHD. Altogether 19.6% of the participants had an abnormal (or borderline abnormal) ECG. The main categories of abnormality were right axis deviation (5.4%) and left ventricular hypertrophy by voltage criteria (3.5%). Observed ECG abnormalities differed between ethnic populations: suggestive of left sided cardiac abnormalities in the Mustang district with a majority population of Tibetans; and right sided abnormalities in the Humla district with a majority population of Khas-Arya. There was a moderate association between the probability of abnormal (or borderline abnormal) ECG and altitude, adjusted for potential confounding variables, with an odds ratio for a greater probability of an abnormal ECG (95% CI) of 2.83 (1.07 to 7.45), P=0.03 per 1000 m elevation of altitude.
A history of stroke or of symptoms of stroke (by positive self-report of at least one criterion of the QVSFS) was identified in 6.7% of the participants.
A multivariate model adjusting for potential confounding variables showed that there was moderate evidence of an association between SBP and altitude; mean SBP (95% CI) increase by 11.3 mmHg (-0.1 to 22.7), P=0.05 for every 1000 m elevation.
The distribution and prevalence of key cardiovascular disease-related risk factors did not differ by altitude level. Rather, they differed by ethnicity, residential settings (urban or rural) and cultural practices. The prevalence of HT or being on treatment for HT was higher in the Mustang district with dominant Tibetan-related populations (between 41% and 54.5%) than in the Humla district, with dominant Khas-Arya (29.1%). Only 3.3% to 10.3% participants in Mustang self-reported being current smokers, whereas this rate was 38.6% in Humla. The prevalence of current drinker was high at all altitude levels ranging from 45.4% (3620 m) to 63.9% (3270 m). The prevalence of abnormal lipid components, diabetes or being on treatment for diabetes, and overweight or obesity, were all higher in urban (2800 m and 3620 m) compared to rural (3270 m and 2890 m) residential settings.
The HA populations sampled in this study had a lower prevalence of CHD and a higher prevalence of stroke than that of relevant comparator low altitude populations. None of the participants had ECG evidence of past CHD. Cardiovascular risk profiles of HA populations may depend on altitude, ethnicity, cultural lifestyle practices, and residential setting (urban or rural). Altitude per se could be an important additional risk factor because of its association with SBP and abnormal (or borderline abnormal) ECG. Different ancestry-related physiological responses to the low oxygen environment at HA may affect cardiovascular health consistent with the evidence of different patterns of ECG abnormality. The findings of the present study suggest that ethnicity and associated lifestyle or cultural practices (such as salt and alcohol intake, smoking habit) and residential settings (mainly differences in physical activity and fruit and vegetable consumption in urban and rural participants), are also likely to be important determinants of cardiovascular health for HA residents.||