No Relevant Seasonal Influences on Office and Ambulatory Blood Pressure: Data from a Study in Borderline Hypertensive Primary Care Patients

Abstract

Our objective was to study seasonal influences on office and ambulatory blood pressure. We therefore designed a prospective 7-month study of 47 borderline hypertensive patients in a primary care setting. We used no interventions. Our main outcome measures were the differences between summer and winter office and ambulatory blood pressures and 95% confidence intervals. Results showed that winter minus summer differences ranged from 0 to 3 mm Hg. Only one significant difference was found: ambulatory systolic daytime pressure was significantly higher (3 mm Hg) in winter than in summer. Our results do not confirm the data of earlier studies in hypertensives. In view of the small and clinically irrelevant winter-summer differences, it seems unnecessary to modify antihypertensive treatment of borderline hypertensives according to the season. Am J Hypertens 1998;11:602–605 © 1998 American Journal of Hypertension, Ltd.

Mild hypertension, ambulatory blood pressure monitoring, office blood pressure, seasonal effects

Several studies suggest that there are seasonal influences on blood pressure in hypertensive patients, blood pressures being higher in winter than in summer.^1–5 The seasonal variation in blood pressure appeared to be larger in older than in younger subjects and was related to outdoor temperature.² No seasonal differences were found in normotensive subjects.³

We wanted to find out whether these differences also exist in borderline hypertensive primary care patients, as it is especially in this group of patients that there is a considerable risk of misclassification and unjustified drug treatment.^6,7

The data presented here were derived from a 7-month comparative, prospective study in borderline hypertensive primary care patients. Participating patients did not receive antihypertensive treatment, either at the start or during the course of the study.

Materials and Methods

Seventeen general practitioners participated in the study. All were given instructions on adequate techniques of blood pressure (BP) measurement.⁸ Phase V of the Korotkoff sounds was recorded as the level of diastolic pressure.

Patients

Patients were selected on the basis of an initially elevated office BP measured by the general practitioner during routine clinical care (case-finding). Inclusion criteria were mean of two systolic values (measured in one visit) between 160 and 200 mm Hg (160 ≤ SBP ≤ 200 mm Hg) or mean of two diastolic measurements between 95 and 115 mm Hg (95 ≤ DBP ≤ 115 mm Hg), and age between 20 and 75 years. Exclusion criteria were known hypertension or antihypertensive treatment in the year preceding the study, suspicion of secondary hypertension, congestive heart failure or unstable angina, or pregnancy.

Measurement Procedures

After inclusion by the general practitioner, all patients underwent four procedures of BP measurement. Because the aim of the study was to detect seasonal influences on office and ambulatory blood pressure, only two of these were relevant for the present analysis. One procedure consisted of duplicate measurements in one visit by the general practitioner in his office in weeks 2, 3, and 4 and months 2, 3, 4, 5, 6, and 7. The other procedure consisted of ambulatory blood pressure measurements during one working day in week 4 and month 7.

Measuring Devices

The general practitioner measured BP with a calibrated mercury sphygmomanometer, provided with a standard-sized cuff (12 × 35 cm). Ambulatory blood pressure was measured using a SpaceLabs 90207 (SpaceLabs Inc., Hillsboro, OR) automatic cuff-oscillometric device. This instrument received a B rating for both systolic and diastolic BP according to the criteria of the British Hypertension Society protocol, and satisfies the criteria of the Association for the Advancement of Medical Instrumentation.^9,10 Blood pressures were recorded automatically every 15 min from 6 AM till 10 PM and every 30 min during the night.

Data Analysis

The months of December, January, and February were defined as winter; the months of June, July, and August were considered summer. Only data from patients who had their first ambulatory measurement during the winter period and their second ambulatory measurement during the summer period, or vice versa, were analyzed. Office and ambulatory blood pressures of all included patients were analyzed in a way analogous to a two-period crossover design¹¹; 95% confidence intervals were computed for the differences between summer and winter blood pressures. P < .05 was regarded as statistically significant.

Results

Data collection started in the autumn of 1991 and ended in the autumn of 1994. Outdoor temperatures in the months of data collection, defined as the mean temperatures over 1 month, ranged from 1.5° to 5.2°C in the winter months, and from 15.0° to 21.4°C in the summer months.¹²

Forty-seven patients were selected for analysis of seasonal differences: 25 of them had their first BP measurements during summer, whereas 22 had their first measurements in winter. Eight patients suffered too much discomfort from the ambulatory BP measurement during sleep and turned off the monitor during the night. They were excluded from the analysis of 24-h ambulatory blood pressures.

Characteristics of the summer-winter subgroup were as follows: 7 men and 18 women, mean SBP at entrance into the study 159 mm Hg (SD = 17), mean DBP at entrance into the study 99 mm Hg (SD = 5), mean age 54 years (range 33–69 years), and mean body mass index (BMI) 26 kg/m² (range 19–37 kg/m²). Characteristics of the winter-summer subgroup were: 15 men and 7 women, mean SBP at entrance into the study 166 mm Hg (SD = 17), mean DBP at entrance 103 mm Hg (SD = 6), mean age 49 years (range 28–71 years), and mean BMI 27 kg/m² (range 22–36 kg/m²). Table 1 presents means, winter-summer differences, and 95% confidence intervals of daytime ambulatory blood pressures, 24-h ambulatory blood pressures (N = 39), and office blood pressures. Only one significant difference was found: systolic ambulatory daytime pressure was significantly higher in winter than in summer.

Table 1.

Means, Number of Patients (N), Mean Differences, and 95% Confidence Intervals (CI) of Mean Differences

	Mean	N	Winter − Summer Difference	95% CI
Daytime ABPM
SBP	142	47	3	0 to 6*
DBP	90	47	1	0 to 3
24-h ABPM
SBP	138	39	2	−1 to 4
DBP	87	39	0	−1 to 2
OBP
SBP	153	47	2	−2 to 6
DBP	95	47	1	−1 to 3

Mean numbers of office measurements = 6 (summer) and 5 (winter).

ABPM, ambulatory blood pressure measurement; OBP, office blood pressure, measured by the general practitioner; SBP, systolic blood pressure; DBP, diastolic blood pressure.

*

Statistically significant (P < .05).

Discussion

Several studies have described seasonal influences on blood pressure, reporting higher levels in winter than in summer (Table 2). The aim of the present study was to investigate whether these differences are also apparent in borderline hypertensive primary care patients. The data presented here were derived from a prospective study in primary care patients.

Table 2.

Studies of Seasonal Differences in Systolic Blood Pressure (SBP), Diastolic Blood Pressure (DBP), and Ambulatory Blood Pressure (ABPM)

N	Summer SBP	Winter SBP	Summer DBP	Winter DBP	Reference	T*
56	144–145	145–146	86–88	87–88	1
†	143–145	146–148	90–91	92–93	2
9	147	154	96	103	3‡	24
22	No change	No change	84	89	4	13
	121	125	81	86	4 (ABPM)
50	144	148	85	89	5	16
	133	138	83	86	5 (ABPM)

*

T, difference between summer and winter temperature (°C).

†

Placebo group/routine follow-up examinations. Number of patients in this subgroup not presented.

‡

Borderline hypertensives, measurements in upright position.

Analyzing the data in a way analogous to a two-period crossover design, the only statistically significant seasonal influence found was in systolic daytime ambulatory blood pressure. The difference we found (3 mm Hg) is probably not clinically relevant. Our results do not agree with those of earlier studies in hypertensives. With reference to seasonal influences, borderline hypertensives seem to take an intermediate position between normotensives (no seasonal influences) and hypertensives (significant seasonal influences). Winter-summer differences may be related to the extent of BP variability, which is known to be larger in hypertensives than in borderline hypertensives and normotensives.^13,14 The larger variability of BP in hypertensives may be related to a lower resistance to external stimuli, such as outdoor temperature. However, further research is needed to test this hypothesis.

Our results do not confirm the data of Minami et al.⁵ However, their study population was not homogeneous, consisting of treated as well as nontreated hypertensive patients. As far as we know, our study is the only one in this field using a two-period crossover analysis. Moreover, the study comprised borderline hypertensives, a group particularly prone to misclassification, overdiagnosis, and overtreatment.

In view of the small and clinically irrelevant winter-summer differences, it seems unnecessary to modify antihypertensive treatment of borderline hypertensive patients according to the season.