Seasonality, ambient temperatures and hospitalizations for acute exacerbation of COPD: a population-based study in a metropolitan area

Almagro, Pere; Hernandez, Carme; Martinez-Cambor, Pable; Tresserras, Ricard; Escarrabill, Joan

Temperature and hospitalizations for COPD in outpatient visits for COPD for each Fahrenheit degree decrease. 23 These results are similar to those of two other studies done in Taiwan, which found an increase of 0.8% in outpatient consultations for COPD exacerbations for each 1°C decrease in temperature and an increment of emergency room consultations in the colder months. 24, 25 Our data yield similar conclusions in a different climate and with more severe exacerbations, since it is centered on hospital admissions. Finally, a population-based study in Scotland confirmed an increase in hospital admissions for COPD with a decrease in monthly temperatures, especially in groups of lower socioeconomic status. 26 Other studies link diurnal temperature range with emergency room visits for cardiovascular disease or mortality from COPD. [27] [28] [29] [30] The reasons for the increase in exacerbations with cold temperatures are complex and not entirely elucidated. The most plausible explanation is that the increase in viral infections acts as a trigger for exacerbation. It is well known that virus alone or associated with bacterial infection is present in around two thirds of COPD exacerbations. The most common virus is the human rhinovirus causing the common cold, which can be found in more than 50% of exacerbations, followed by respiratory syncytial virus, coronavirus, adenovirus, and others. 30, 31 Viral infections lead to increased basal inflammation being present in the airways of patients with COPD. 34 Most respiratory viral infections in the northern and southern regions of the world have a seasonal pattern not observed in the tropics, and they have been associated with low temperatures, humidity, variations in patient physiology, and changes in social behavior. 30 These respiratory infections are more common in winter, both in COPD patients and in the general population, and occur in a wide range of climates, being more closely related to temperature variations than to absolute temperatures. Recent research suggests that human rhinovirus infections, at least in children, are more severe in winter. 34 Another possible explanation for the increased number of hospitalizations for COPD in winter is the decline in lung function with the cold, and the increased airway inflammation and longer duration of exacerbation experienced by COPD patients with decreasing temperatures. [35] [36] [37] [38] Our study confirms the low impact of the influenza A (H1N1) pandemic in 2009 on the number of hospitalizations for COPD. The pandemic in 2009 affected predominantly the young population, probably because of residual immunity in the elderly from previous antigenically similar virus exposure. 39 Only 10% of patients hospitalized for influenza A in Catalonia in 2009 had a diagnosis of COPD. 15 Other factors associated with an increase in COPD exacerbations in some studies are low humidity and higher air pollution levels, although their role appears less relevant than that of falling temperatures, and in our study their significance disappeared after adjustment for temperature. 23, 30, 31, [40] [41] [42] Our study has some limitations. First, the collected data about the number of COPD admissions were based on hospital discharge codes, so we cannot rule out errors inherent in coding or data not collected with the etiology of the exacerbation. The definition of discharge codes based on ICD-9 is used in all previous studies using this methodology and their acc