Modelling time trends, seasonality & the contribution of indoor environmental factors on child asthma hospital readmissions

Submission note: A thesis submitted in total fulfilment of the requirements for the Degree of Doctor of Philosophy to the College of Science, Health and Engineering, La Trobe University, Bundoora.

Child asthma readmission is a useful indicator of asthma morbidity. Hospitalization time trends provide a longitudinal view of child asthma morbidity and can provide in depth understanding of changes in asthma aetiology and/or prognosis. Asthma has important environmental risk factors and therefore examining hospitalization seasonal trends presents an indirect but important marker for assessing the impact of environmental exposures on asthma morbidity. The indoor environment is a significant risk factor for child asthma development and exacerbations especially as Western societies may spend 90 percent of their time indoors. Very little is known in Australia about child asthma readmissions in regards to their time trends, seasonality and associations with the indoor environment. This thesis contributes knowledge on these aspects of asthma morbidity. For all child asthma hospital admissions between fiscal years1997 and 2009 in Victoria, Australia, time trends and seasonality, including pollen season, were examined and their associations with age and sex assessed. Daily readmissions were modelled as a count time series. Admissions had reduced over time, however readmissions within 28 days had initially decreased and then increased from about 2002 to 2009 to reach their highest rates ever reported. Seasonality had a strong impact on admissions and readmissions and was modulated by age and sex. The model was used to predict readmission frequencies one year into the future and showed increasing frequencies especially for young boys. This model’s predictive capacity is novel and may prove helpful for health service planning and clinical management. To examine the role of the indoor environment and lifestyle factors in readmissions, indoor factors were measured in the homes of 44 children previously hospitalized for asthma and a questionnaire administered. Increasing levels of airborne yeast in the children’s bedrooms and increased frequency of vacuuming were associated with increased risk of readmission within 28 days. Respective odds ratios and 95 percent confidence intervals were 1.82 (1.04, 3.18) and 22.2 (3.17, 156.10). Collectively these results underscore the importance of the environment both outdoor and indoor, particularly in children’s bedrooms, for asthma morbidity and provide possible avenues for improving management to reduce the likelihood of asthma exacerbations.

This thesis was a recipient of the Nancy Millis Award for theses of exceptional merit.