Study design

The methods have been previously reported [18]. Briefly, the PERU MIGRANT (PEru's Rural to Urban MIGRANTs) study, conducted in 2007, was designed to investigate the magnitude of differences between rural-to-urban migrant and non-migrant groups in specific cardiovascular disease risk factors. For this, three groups were selected: people who had always lived in a rural environment, those who had migrated from rural to urban areas and those who had always lived in an urban environment. Mass-migration seen in Peru from 1980s onwards was largely driven by politically-motivated violence rather than economic reasons, epidemiologically resulting in less 'healthy migrant' selection bias [18]. For the period 1988-1993, 50.7% of the total emigrants from the Ayacucho region moved to Lima, the capital of Peru, making Ayacucho the largest source of migrants to Lima. The village of San Jose de Secce, located in the Santillana district, Huanta province in Ayacucho was selected as the rural study site. Both urban and rural-urban migrant participants were selected from Las Pampas de San Juan de Miraflores, a peri-urban shantytown in the south of Lima.

Participants

A single-stage random sampling method was used in all groups. For all study groups, individuals from both sexes aged 30 years-old and over, permanently living in their residence, were considered eligible. Pregnant women and those with a formal psychiatric health disorder that would impair survey completion were excluded. Participant selection was stratified by age-groups (30-39; 40-49; 50-59; >60 years old) and sex to ensure sufficient numbers of people in each stratum. Response rate at enrolment was 73.2%, and a detailed flowchart with response rates in each specific group has been previously reported [18].

Data collection

A team of community health workers with previous fieldwork experience of household visitation enrolled participants. They were trained to conduct the specific questionnaires of this study and all assessments were made at the study coordinating centre. A demographic and socioeconomic survey collected age, sex and socioeconomic information. Systolic and diastolic blood pressures were measured using appropriately-sized cuffs for arm circumference in the sitting position using the right arm, supported at chest level. Three measurements at least five minutes apart using an oscillometric device (Omron M5-i, Omron, Japan) were made. The mean of the second and third systolic (SBP) and diastolic blood pressure (DBP) measurements were used for the analysis. All laboratory assessments were performed by trained personnel on venous blood samples taken in the morning after a minimum of eight hours fasting. The total cholesterol, triglycerides and HDL levels were measured in serum samples. In individuals with triglycerides below 400 mg/dL, LDL was calculated using the Friedewald equation in mg/dL (LDL = total cholesterol - HDL - (0.2 × triglycerides)) [19]. In individuals with triglycerides greater than 400 mg/dL, LDL was measured from the serum samples. Fasting glucose, fasting insulin and glycated haemoglobin were measured in plasma, serum and whole blood samples respectively. Waist circumference, measured in triplicate at the midpoint between the lower rib and the iliac crest, and hip circumference, measured in triplicate at the point yielding the maximum circumference over the buttocks, were also included. In addition, the 12-item General Health Questionnaire (GHQ-12) for mental health previously validated in Latin America [20, 21] and a social capital instrument previously validated in Peru (SASCAT) [22] were also applied.

Study variables

Hypertension was defined as SBP ≥ 140 mm Hg or DBP ≥ 90 mm Hg, or self report of physician diagnosis and currently receiving antihypertensive medication [23, 24]. Diabetes was defined as fasting glucose ≥ 126 mg/dL (≥ 7 mmol/L) or self-report of physician diagnosis and currently receiving anti-diabetic medication [25]. Smoking status (never, former and current) was recorded using an adapted version of the WHO STEPS questionnaires [26]. A current smoker was defined as having smoked within the last six months and a lifetime total of more than 100 cigarettes.

Mental health was measured using the General Health Questionnaire (GHQ-12) [20]. The GHQ-12, though previously used in Latin American settings, has not been validated in Peru. However after consultation, a group of local experts concluded it was appropriate for this setting. The GHQ-12 score ranges from zero to twelve points, and was dichotomised; a score of five or more was considered a positive case for a mood related disorder (depression and anxiety), based on a previous study conducted in Santiago de Chile [21].

Social capital, defined as the social relationships, bonds and perceptions within societies or groups of people, [27] may also change after migration. Social capital was measured using the Short Social Capital Assessment Tool (SASCAT). The SASCAT questionnaire has been previously validated in Peru [28, 29] and includes both a cognitive and a structural social capital component. In the cognitive component (trust, social harmony, sense of fairness) of the SASCAT, a score of three or more out of four points was considered as 'high' cognitive social capital [28, 29]. The score in the structural component (group membership, involvement in citizenship activities and support from individuals in the community) was initially categorised in quarters, and subsequently made into a dichotomous variable with the lowest quarter as the reference.

Exposures

The primary exposure was migration from a rural to an urban environment, defined by study group, i.e. rural, rural-to-urban migrant and urban groups.

Outcomes

The Spanish-version Rose angina questionnaire was used to record chest pain in the standard way, and from this we classified chest pain into three mutually exclusive groups:

Statistical methods

Prevalence estimates of non-exertional chest pain or possible/definite angina were calculated for each study group (rural, migrant, urban) and adjusted (age and sex, then with total cholesterol/HDL ratio, overweight or obese, smoker, diabetes, hypertensive and mental health). Mantel-Haenszel odds ratios were used to assess the risk of having pain in the migrant and urban groups compared to the rural group.

For general description of data, frequency analyses were calculated as number (percentages). Mantel-Haenszel odds ratios (OR) and 95% confidence intervals (CI) were used to assess the relation between risk factors and non-exertional chest pain or possible/definite angina after adjusting for age and sex.

Ethics

Ethical approval for this protocol was obtained from ethics committees at Universidad Peruana Cayetano Heredia in Peru and London School of Hygiene and Tropical Medicine in the UK. The purpose of the study was explained to each of the study participants and informed consent was obtained, following international standards for ethical research in developing countries.

Strengths and limitations

Few other studies exist that seek to assess the prevalence of and aetiological drivers of angina, an important early stage of the natural history of coronary disease, in the developing world. The analyses presented in this study further challenge researchers on the appropriate measurement of clinical phenotypes in low- and middle-income country settings. This study thus adds to this growing field by highlighting the complexities and challenges for the assessment of angina in the developing world. We appreciate that a greater sample size would have made our findings more precise.

Implications

Appropriate identification at incident symptomatic presentations of coronary disease such as angina may prevent or at least delay progression to more serious later manifestations of coronary disease such as myocardial infarction and coronary death if treatment is instigated at time of diagnosis. Whereas the measurement of a myocardial infarction is standardised through assays for biomarkers, angina is a clinical constellation of symptom descriptors, rendering quantitative measurement difficult. More discriminating measurement of angina symptoms, focusing more on the description of the chest pain, may complement existing angina assessment. Using a multi-centre chest pain clinic cohort, descriptors of chest pain using a simple symptom score were predictive of coronary events across sex and ethnicity [35]. A measure of the consistency of relation to exercise of chest pain may be a further discriminator in distinguishing those with and without coronary disease [30].

Our results also show the dangers of applying research results from the developed world to the health issues of low- and middle-income countries [36]. When facing the challenge of acting upon the epidemic of non-communicable diseases in developing countries, a measure which is of widespread use in one setting may have a different utility when applied to an alternative setting for which it was not designed for.