The effect of a change in ambient temperature on blood pressure in normotensives

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1 (2001) 15, Nature Publishing Group All rights reserved /01 $ ORIGINAL ARTICLE The effect of a change in ambient temperature on blood pressure in normotensives PM Jansen, MJ Leineweber and Th Thien Department of General Internal Medicine, University Hospital St. Radboud, Nijmegen, The Netherlands The aim of this study was to investigate the influence of ambient temperature on blood pressure (BP). BP measurements were taken in 20 normotensive volunteers who stayed in Greenland for a 6-week period. Measurements of systolic (SBP), diastolic (DBP) and heart rate (HR) were taken before (3 sessions), during (7 8 sessions) and after the journey (3 sessions). Each session consisted of five BP measurements in the supine position after at least 5 min rest. All five readings were averaged. Temperature data (mean s.d.), collected from meteorological services, before, during and after Greenland were , and C. SBP values were , and and DBP , and mm Hg, respectively. HR amounted to , and bpm. Significant differences existed between, before and during for SBP and DBP and between, during and after for SBP. Readings were grouped in four categories based on the temperature at the time of reading. For SBP as well as DBP a clear dose-response relationship was demonstrated between low temperature and high BP, although for DBP only a few correlations were statistically significant. Mean correlation coefficients for SBP and DBP against temperature were 0.44 (P 0.001) and 0.27 (P 0.005), respectively. Our results are in favour of a moderate, but both significant and relevant increase in SBP and DBP when moving from higher to lower ambient temperature. (2001) 15, Keywords: blood pressure; ambient temperature; normotensives; cold Introduction Several studies have shown seasonal variations in systolic blood pressure (SBP) and diastolic blood pressure (DBP), with the highest pressures recorded in winter and lowest pressures recorded in summer This seasonal variation is observed for clinic BP, 1,3,6,9,10 home BP 10 and ambulatory BP. 2,4,5,7,8,10 These seasonal differences suggest a correlation of BP with ambient temperature, although a direct relation is not certain. BP variations might in fact be also determined by any other variable showing seasonal fluctuation. Another problem is the population studied. In some studies BP measurements were recorded in a population of hypertensive subjects 1,4 whereas in other studies normotensive as well as hypertensive subjects were enrolled. 2,3,10 Argiles et al (1998) 9 recorded BP in patients with end-stage renal disease treated with hemodialysis and found a marked sea- Correspondence: Th. Thien, MD, Professor of Medicine, University Hospital St. Radboud, Department of General Internal Medicine, PO Box 9101, 6500 HB Nijmegen, The Netherlands T.Thien aig.azn.nl Received 6 March 2000; revised and accepted 31 July 2000 sonal BP variation. On the other hand, Brueren et al (1998) 11 found no relevant seasonal influences on BP in borderline hypertensive patients in a primary care setting. 11 Whether there also exists a seasonal effect on BP in normotensive subjects is still a matter of dispute, but has been reported by some studies. 5 8 The purpose of this study was to assess the effect of an acute change in ambient temperature on BP in normotensive subjects. BP measurements were recorded in a group of 20 normotensive volunteers who went to Greenland for a 6-week period, resulting in an almost instantaneous drop in ambient temperature. Design and methods Subjects Blood pressure (BP) measurements were taken in a study population of 20 adult normotensive volunteers who went to Greenland for a 6-week period. The characteristics of the study population are given in Table 1.

2 114 Table 1 Composition of the study population Effect of change in ambient temperature on BP Total Male Female participants participants n Age (years) 28 ± ± ± 2.0 (21 54) (21 54) (22 27) BMI (kg/m 2 ) 22.2 ± ± ± 1.3 ( ) ( ) ( ) BMI, body mass index; mean ± s.d. (range) are given. Protocol The expedition to northern Greenland took place in the period from 10 August to 25 September After some days of acclimatisation, a hiking tour was made across the icecap with all subjects. Thereafter, the study group was divided into two subgroups. One subgroup of 10 subjects stayed in the village of Qaanaaq (Thule) and a subgroup of 10 subjects made a second hiking tour across the icecap for 15 days. When the latter subgroup returned to Qaanaaq, the whole study group stayed in Qaanaaq for one further week till the expedition returned to The Netherlands. Measurements of SBP, DBP in mm Hg and heart rate (HR) in beats per minute (bpm) were taken before (3 sessions), during (7 or 8 sessions) and after the journey (3 sessions). All sessions took place within the period of 3 August and 16 October Measurements before and after the journey were taken in Nijmegen, The Netherlands. All measurements of SBP, DBP and HR were taken with a Takeda UA-751 BP measurement device. Each session consisted of five measurements taken shortly after each other. All five readings were averaged. The measurements were taken in the supine position after at least 5 min rest. All measurements were taken in the morning and by the same observers for the whole study period. Meteorological data from Qaanaaq/Thule were kindly provided by the Danish Meteorological Institute. Meteorological data for Nijmegen, The Netherlands, were purchased from the Royal Dutch Meteorological Institute. Temperature data during the hiking tour were collected from the Argos navigation system which was used. Sodium excretion Since during the whole expedition ready-made food was used we compared in four male subjects (age years), BMI kg/m 2 a 24-h urine collection at home using their normal diet and using the diet consumed during the expedition. Excretion of sodium (sodium intake), potassium (potassium intake), urea (protein intake) and creatinine (greatly a measure for quality of collection) was assessed for the normal diet and the Greenland diet (standard methods). Statistics Readings before, during and after Greenland were averaged for each individual subject before calculating the group average. To study the dose-response relationship between temperature and BP, readings were grouped based on the temperature at the time of reading. Four temperature categories were used. Category I consisted of readings with T 10 C, category 2 of readings with 5 C T 10 C, category 3 of readings with 0 C, T 5 C and readings at a T equal to or below 0 C were grouped in category 4. Readings for every temperature group were averaged for every individual subject before calculating the group average. Data were analysed using simple factorial ANOVA, paired t-test and Pearson linear correlation. P-values 0.05 (two-sided) were considered statistically significant. All statistical analyses were performed using SPSS for MS WIN- DOWS Release 6.1 (SPSS Inc., Chicago, IL, USA). All results are given as mean s.d. unless indicated otherwise. Results Mean ambient temperatures in The Netherlands were C before departure to Greenland, C in Greenland and C in The Netherlands after returning from Greenland. Figure 1 shows SBP, DBP and HR before, during and after the Greenland expedition. All readings before, during and after Greenland were first averaged for every subject. Values in Figure 1 represent the mean values for all subjects. Mean differences for SBP, DBP and HR during vs SBP and HR before Greenland were mm Hg (P 0.001) mm Hg (P = 0.002) and bpm (NS), respectively. Mean differences for SBP, DBP and HR during, vs SBP, DBP and HR after Greenland were mm Hg (P = 0.001), mm Hg (NS) and bpm (NS), respectively bpm (NS), respectively. at the time of reading. Table 2 shows mean temperature, SBP, DBP and HR for every temperature category. Means (s.e.) and levels of significance are depicted in Figure 2, except for HR. The HR as given in Table 2 was significant different between category 1 and 4 and between two and four differences were not clinically relevant. For every subject the Pearson correlation coefficient r was calculated between SBP and ambient temperature and DBP and ambient temperature. Mean r-value was 0.44 (range , P 0.001) for SBP and 0.27 (range 0.83; 0.40, P = 0.001) for SBP and 0.27 (range 0.83; 0.40, P = Table 3 gives the results of the urinary excretion of diet-related variables during the control normal diet and after having used the planned Greenland diet for a few days. There were no significant differences between the two diets, suggesting that both diets were roughly equivalent in electrolyte (eg sodium) and protein content.

3 Effect of change in ambient temperature on BP 115 Figure 1 Mean (± s.e.) systolic (SBP) and diastolic (DBP) blood pressure and heart rate (HR) before, during and after the Greenland expedition. (*P 0.05; **P 0.001). Table 2 Mean (± s.d.) of temperature, systolic (SBP), diastolic (DBP) blood pressure and heart rate (HR) for every temperature category (categories are based on ambient temperature; further definitions are given in the text) Figure 2 Mean (± s.e.) ambient temperature, systolic (SBP) and diastolic (DBP) blood pressure for four temperature categories based on ambient temperature at the time of reading (further definitions are given in the text). (*P 0.05; **P 0.01) Only significant differences are indicated. Table 3 Indication of mean ± s.d. (range) of the urine concentrations of sodium, potassium, urea and creatinine (all in mmol/24 h) using the usual diet and the Greenland diet Category Normal diet Greenland diet No. of subjects Sessions per subject 3.4 ± ± ± ± 0.7 Temperature ( C) 14.8 ± ± ± ± 3.0 SBP (mm Hg) 116 ± ± ± ± 18.0 DBP (mm Hg) 64 ± ± ± ± 5.8 Heart rate (bpm) 59 ± ± ± ± 9.4 Discussion The results presented in this study show a moderate, yet relevant influence of ambient temperature on BP. Low ambient temperature is associated with a rise Sodium 136 ± ± 19 (64 212) (87 132) Potassium 74 ± ± 13 (50 101) (73 98) Urea 332 ± ± 24 ( ) ( ) Creatinine 13.0 ± ± 0.8 ( ) ( ) in both SBP and DBP. Especially for SBP an inverse dose-response relationship could be demonstrated. Also for DBP, an inverse dose-response relationship was present, but not all correlations were statistically significant. These results reflect temperature changes within one season in contrast to other studies where temperature changes were always associated with seasonal changes.

4 Effect of change in ambient temperature on BP 116 Table 4 Studies of seasonal differences in systolic (SBP) and diastolic (DBP) blood pressure in mm Hg Ref. No. Subjects BP method Winter Summer BP Ambient T analysis c ( C) SBP DBP 1 a Mild hypertensives Cl 4 2 C High normotensive to mild hypertensive A M Normotensives and hypertensives Cl 12 6 C Hypertensives, indoor lifestyle A 5 2 M Normotensives A 3 3 M Normotensive females H/Cl 5/9 4/4 C Normotensives, smokers A 7 4 M n.d Normotensives, non-smokers A 3 3 M n.d End-stage renal disease patients treated with C 12 7 M 21 haemodialysis Normotensives and hypertensives A/Cl/H 5/10/9 2/4/3 C Borderline hypertensives A 3 1 M 15 This 20 Normotensives H 6 3 M 15 study b Ref, reference; n.d., no data about temperature are given; BP method, method of blood pressure measurement (A, ambulatory BP; Cl, clinic BP; H, home BP). a Brennan et al (1982) studied subjects with mild hypertension receiving treatment with bendrofluazide, propranolol or placebo. The results given here are derived from the placebo group. Number of patients in this subgroup are not presented. b In our study temperature changes were independent of seasonal changes. c BP analysis (C, BP differences calculated, eg, from regression; M, measured BP differences). For a correct interpretation of the results some other factors affecting BP should be considered. To assess whether a difference in sodium intake could contribute to the observed BP changes, the sodium excretion of four individuals was assessed while using their normal diet and using the Greenland diet. No significant differences were found. Possibly, a quantitative difference in the food could have contributed to a difference in sodium intake. An overview of population-based studies of 24-h sodium excretion and BP showed pooled regression slopes of 3.7 mm Hg/100 mmol sodium for SBP and 2.0 mm Hg/100 mmol sodium for DBP. 12 A doubling in dietary intake, which is highly unlikely to have occurred, would have explained a difference of 4 mm Hg in SBP and 2 mm Hg in DBP. Thus, increased sodium intake could have contributed to BP changes only for a part. Furthermore, the observed inverse dose-response relationship for temperature and BP suggests that temperature is the main factor affecting BP. A difference in alcohol intake between the periods is also a possible confounder. 13 Although the alcohol intake was not assessed, it can reasonably be assumed that the intake was low or similar during the expedition, considering the poor availability, in particular during the hiking tour. Whether a difference in levels of stress is important, is hard to assess. Certainly the expedition appealed to the physical performance of the participants. However, it should be mentioned that the participants were used to a physical and sportrelated life at home and were well-prepared for the expedition. Certainly, the increased levels of exercise during the expedition could have influenced BP. It is well-known that regular exercise has a BP lowering effect This is especially true for moderate exercise levels like as in long-distance walking, which the participants were subjected to in Greenland. To minimise the effect of exercise, BP measurements were always taken in the morning before walking. But in case of an influence of exercise on our results, this should in fact have led to an underestimation of the BP difference between high and low ambient temperature. Several groups have studied the seasonal variation in SBP and DBP using ambulatory, clinic or home BP measurements (Table 4). Most studies found some seasonal effect. Other studies confirm this. 7,17 Highest pressures were reported in winter and lowest pressures in summer. Interestingly, the data reported by Näyhä (1995) showed BP peaks in spring and late autumn and troughs in mid-winter and summer. 17 The explanation for this finding remains to be elucidated, but probably reflects the complexity of factors influencing BP. Large differences in BP changes dependent of the climate have been reported. This may in part be associated with the heterogeneity between the populations studied. In particular the temperatureassociated BP variation in normotensives is still a matter of debate. Some studies reported seasonal variations in normotensives 3,5,6,8 whereas another study found no relevant seasonal influences in borderline hypertensive primary care patients, suggesting that seasonal BP variations only occur in hypertensives. 11 Furthermore, from the above-mentioned studies it remains difficult to assess the contribution of ambient temperature changes per se on BP variations. In fact, any other seasonal factor might be associated with BP variation.

5 The present effects found in normotensives could be stronger in (borderline) hypertensives. Therefore these data warrant intensive BP monitoring in (borderline) hypertensives in the winter and, if necessary, a season-based treatment strategy. Furthermore, doctors should be aware of the spontaneous variation during seasons and not always suspect poor on non-compliance when BP is not adequately controlled during the winter. In conclusion: the present study shows that BP clearly rises when normotensives are abruptly exposed to low temperature, independently of the season. Ambient temperature appears to be the main factor causing seasonal variations in BP. Acknowledgements We would like to thank all members of the University of Nijmegen Student Expedition to Greenland 1998 for their willingness to participate in this study. References 1 Brennan PJ, Greenberg G, Miall WE, Thompson SG. Seasonal variation in arterial blood pressure. Br Med J 1982; 285: Giaconi S et al. Seasonal influences on blood pressure in high normal to mild hypertensive range. Hypertension 1989; 14: Woodhouse PR, Khaw KT, Plummer M. Seasonal variation of blood pressure and its relationship to ambient temperature in an elderly population. J Hypertens 1993; 11: Fujiwara T et al. Seasonal differences in diurnal blood pressure of hypertensive patients living in a stable environmental temperature. J Hypertens 1995; 13: Effect of change in ambient temperature on BP 5 Kristal-Boneh E, Harari G, Green MS, Ribak J. Seasonal changes in ambulatory blood pressure in employees under different indoor temperatures. Occup Environ Med 1995; 52: Imai Y et al. Seasonal variation in blood pressure in normotensive women studied by home measurements. Clin Sci 1996; 90: Kristal-Boneh E, Harari G, Green MS, Ribak J. Body mass index is associated with differential seasonal change in ambulatory blood pressure levels. J Hypertens 1996; 9: Kristal-Boneh E, Harari G, Green MS. Seasonal change in 24-hour blood pressure and heart rate is greater among smokers than nonsmokers. Hypertension 1997; 30: Argiles A, Mourad G, Mion C. Seasonal changes in blood pressure in patients with endstage renal disease treated with hemodialysis. N Engl J Med 1998; 339: Sega R et al. Seasonal variations in home and ambulatory blood pressure in the PAMELA population. J Hypertens 1998; 16: Brueren MM et al. No relevant seasonal influences on office and ambulatory blood pressure. Am J Hypertens 1998; 11: Elliot P. Observational studies of salt and blood pressure. Hypertension 1991; 17 (Suppl I): I3 I8. 13 Ascherio A et al. Prospective study of nutritional factors, blood pressure, and hypertension among US women. Hypertension 1996; 27: Kingwell BA, Jennings GL. Effects of walking and other exercise programs upon blood pressure in normal subjects. Med J Aust 1993; 158: Fagard RH. The role of exercise in blood pressure control: supportive evidence. J Hypertens 1995; 13: Puddey IB, Cox K. Exercise lowers blood pressure sometimes? Or did Pheidippides have hypertension? J Hypertens 1995; 13: Näyhä S. Adjustment of blood pressure data by season. Scand J Prim Health Care 1985; 3: