Seasonal variations in blood pressure: a complex phenomenon.

easonal variations in cardiovascular morbidity andmortality show a winter peak and summer nadir, asreported since decades across different latitudes,ethnic groups and age strata. Mortality excess in wintermonths is mostly related to cardiovascular events includingacute myocardial infarction, sudden death, stroke andpulmonary thromboembolism [1–3]. In a landmark studybasedon300000cardiovasculardeathsfromtheCanadianMortality Database, Sheth et al. [4] observed a 19 and 20%increase in mortality, respectively, from acute myocardialinfarctionandstroke,inJanuarycomparedwithSeptember.In a community-based study carried out in Minnesota, USAfrom 1979 to 2002, a 17% increase in sudden cardiac deathwas found in winter compared with summer [5]. Theassociation between cold weather and sudden death wassimilar over years, across age and sex groups and wasstronger for individuals without a previous history ofcoronary heart disease. Available data are mostly derivedfrom countries exposed to cold/temperate climates charac-terized by large variations in seasonal temperature. Similarfindings, however, have been also reported by studiesperformed in countries with smaller variations in environ-mental temperature. In a population of almost 1 millionpersons in Israel, mortality from ischemic heart disease andstroke in men was, respectively, 51 and 48% higher in mid-winter than in mid-summer; the corresponding figures inwomen were 48 and 40%, respectively [6]. Seasonal vari-ations have been also reported for nontraumatic rupture ofthoracic and abdominal aortic aneurysms [7]. Exposure to winter weather conditions has been hypo-thesized to induce physiological and clinical changesincluding sympathetic activation, hemoconcentration,hypercoagulability, increase in plasma lipids and ininfections rate that, overall, may increase the incidenceof cardiovascular diseases [8,9]. As a direct effect of coldweather on the cardiovascular system, reflex coronary andsystemic vasoconstriction may occur. Platelet activationmay take place during viral infections; increased plateletstickiness and thrombus formation may provide a rationalefor the association between ischemic heart disease, strokeand influenza. Short-term and long-term effects of coldexposure on blood pressure (BP) have also been consist-entlydemonstratedinbothnormotensiveandhypertensiveindividuals. Thus, weather-related increments in BP areseen as major determinants of cardiovascular morbidityexcess in the cold season.TheseseasonalBPfluctuationsrepresent oneofthelongterm components of the variations that characterize BPbehavior in daily life. BP variability is indeed a dynamicand complex phenomenon including short-term andlong-term fluctuations as a result of intricate interactionsbetween behavioral, humoral and neural central or reflexinfluences. Although often overlooked, also environmentalfactors may importantly contribute to BP variations, asthey may potentially influence physiological mechanismsinvolved in BP regulation. Particularly, seasonal climaticchanges have been reported to be associated with BPvariations in humans, SBP and DBP levels presentingsignificant increases during winter months [10,11]. Seasonal influences on BP were first described in theearly1960sbyRose[10],whoanalyzedBPmeasurementsin56 middle-aged men affected by ischemic heart diseaseduring a 1–3 years follow-up period and found a clearseasonal trend, with a peak in spring and a trough in latesummer. Additional evidence on this phenomenon hasbeen provided by large population studies including bothnormotensive and hypertensive individuals, showing thatseasonal BP changes are not only limited to conventionalBP measurements performed in the clinic but also affectout-of-office BP levels. Either when considering clinic BPvalues, the average of self-BP measurements performed byindividuals at home or the mean of the 24-h BP valuescollected by ambulatory BP monitoring (ABPM), BP levelshave been reported to be lower during summer and higherduring winter [12]. In the last decades, studies performed indifferent settings (i.e. clinical trials, general population,