Influence of climate on emergency department visits for syncope: role of air temperature variability

Abstract

Background: Syncope is a clinical event characterized by a transient loss of consciousness, estimated to affect 6.2/1000 person-years, resulting in remarkable health care and social costs. Human pathophysiology suggests that heat may promote syncope during standing. We tested the hypothesis that the increase of air temperatures from January to July would be accompanied by an increased rate of syncope resulting in a higher frequency of Emergency Department (ED) visits. We also evaluated the role of maximal temperature variability in affecting ED visits for syncope.

Methodology/principal findings: We included 770 of 2775 consecutive subjects who were seen for syncope at four EDs between January and July 2004. This period was subdivided into three epochs of similar length: 23 January-31 March, 1 April-31 May and 1 June-31 July. Spectral techniques were used to analyze oscillatory components of day by day maximal temperature and syncope variability and assess their linear relationship. There was no correlation between daily maximum temperatures and number of syncope. ED visits for syncope were lower in June and July when maximal temperature variability declined although the maximal temperatures themselves were higher. Frequency analysis of day by day maximal temperature variability showed a major non-random fluctuation characterized by a ∼23-day period and two minor oscillations with ∼3- and ∼7-day periods. This latter oscillation was correlated with a similar ∼7-day fluctuation in ED visits for syncope.

Conclusions/significance: We conclude that ED visits for syncope were not predicted by daily maximal temperature but were associated with increased temperature variability. A ∼7-day rhythm characterized both maximal temperatures and ED visits for syncope variability suggesting that climate changes may have a significant effect on the mode of syncope occurrence.

Figure 1. Day by day values of maximal and minimal air temperature, heat index and of syncope observed from January 23^rd, 2004 to July 31^st, 2004.

The expected progressive increase of air temperature from January diverged from Emergency Department (ED) visits for syncope which remained stable until May, before decreasing. Maximal and minimal air temperatures fluctuate (temperature variability) on a day by day basis. The temperature variability was lower in June and July compared to the cooler months. Heat index has been computed only for values of maximal temperature >20°C and its spontaneous variability mirrors maximal temperature fluctuations.

Figure 2. Frequency domain analysis of maximal air temperature variability (upper panel), of daily Emergency Department (ED) visits for syncope variability (middle panel) and of their relationship (coherence, lower panel).

Broken line is the result of surrogate analysis. A major oscillatory component at 0.04 cycles×day⁻¹ corresponding to a period of 23.2 days could be identified in the power spectrum of maximal air temperature variability. Two other minor oscillatory components were also present at 0.15 and 0.3 cycles×day⁻¹, i.e. characterized by periods of ∼7 and ∼3 days, respectively. A significant non-random fluctuation in the pattern of ED visits for syncope (middle panel) was found at a peak frequency of 0.15 cycles×day⁻¹ (period ∼7 days). As obtained from coherence and surrogate analyses, maximal temperature and syncope ED attendances variability were linearly coupled in a frequency range between 0.15 and 0.20 cycles×day⁻¹ (between 7 and 5 days, respectively). This suggests a potential influence of maximal air temperature oscillations on the pattern of ED visits for syncope.