Seasonal variation in hospital admissions for chronic obstructive pulmonary disease in Finland

Hospital admissions in general are characterised by a marked seasonal variation. We studied the periodicity in hospital treatment for chronic obstructive pulmonary disease (COPD) in Finland where the unfavourable climate with great climatic differences between summer and winter may play an important role in causing the seasonality in COPD hospitalisation.

Data by month were obtained for the years 1972-92 from the National Discharge Register, which contains information on patients treated in all hospitals in Finland. The search was concentrated on principal diagnoses conforming to International Classification of Diseases codes 491, 492 and 496. There were 182, 723 admissions of COPD patients aged 55 years or over during the period in question. Time series analysis was carried out on retrospective data over a 21 year period and analysed by two age groups (55-74 years or > 74 years) and gender. The autoregressive integrated moving average (ARIMA) model was used to analyse seasonality.

The seasonality pattern showed a peak in winter (13.4% excess mean monthly admissions in January) and a trough in summer (10.0% deficit in below mean monthly admissions in July). This pattern was more prominent in women and in those aged 75 years or over.

The cold winter together with an increased incidence of respiratory infections may be the most probable cause of the periodicity noted here. Due to the unfavourable northern climate even a greater seasonal variation was expected.