Multimorbidity and cardiovascular disease in the United Kingdom
Keywords: multimorbidity
electronic health records
cardiovascular disease

Background: Multimorbidity, the presence of two or more conditions in the same individual, is common, particularly in populations with cardiovascular disease. Addressing the evidence gaps in temporal trends, effects on treatment, and associations with relative mortality risk in the field of multimorbidity will inform health service planning, resourcing and delivery of quality patient care for an increasingly multimorbid population.
Aims: The aim of this thesis was to examine multimorbidity and cardiovascular disease using the United Kingdom’s Clinical Practice Research Datalink. To achieve this aim, the specific objectives of this thesis were to:
1. Select an appropriate framework for measuring multimorbidity, including selection of conditions, in the Clinical Practice Research Datalink
2. Prepare data in the Clinical Practice Research Datalink for analysis, including phenotyping of the selected chronic conditions
3. Investigate patterns and temporal trends of comorbidities in incident cardiovascular disease
4. Examine the association between multimorbidity and blood pressure & antihypertensive prescription
5. Investigate the association between multimorbidity and mortality in patients with cardiometabolic comorbidities
Methods: First, a literature review was conducted to select an appropriate method of measuring multimorbidity applicable to the Clinical Practice Research Datalink. Second, raw data in the Clinical Practice Research Datalink was extracted and converted to data for analysis, including phenotyping of chronic conditions. Third, age/sex-standardised prevalences of comorbidities were calculated for patients with incident cardiovascular disease, with stratification by age, sex and socioeconomic status. Fourth, a landmark cohort analysis using linear and Poisson regression investigated the association between number & type of comorbidity, and blood pressure & prescription of antihypertensives. Finally, a cohort study using Cox proportional hazards regression examined the association between cardiometabolic comorbidities and mortality, with adjustment for a range of additional comorbidities.
Results: The method of measuring multimorbidity was highly heterogeneous, and a pragmatic approach of examining multimorbidity by number of conditions, specific comorbid conditions, and condition groups, was chosen for subsequent analyses. Data in the Clinical Practice Research Datalink was extracted and processed for analytical use, including phenotyping of 60 chronic conditions. The prevalence of patients with multimorbidity in incident cardiovascular disease increased between 2000 to 2014. Furthermore, 4 out of the top 10 comorbidities were non-cardiovascular conditions, and patterns of comorbidities differed by age group and sex. The presence of comorbidity was associated with lower blood pressure in hypertensive patients, largely driven by a lower blood pressure at diagnosis, even when differences in health service use were accounted for. The mortality risk of patients with multiple cardiometabolic comorbidity was partly explained by additional comorbidities, and the combined risk of concomitant cardiometabolic conditions was less than the additive risk of the individual conditions.