Hour of day and time of year has significant impact on blood pressure levels

The odds of measuring an elevated blood pressure can vary by as much as 40 percent depending on the time of day and time of year, according to researchers from the Center for Health Research and Rural Advocacy at Geisinger Health System in Pennsylvania. They recommend that clinicians anticipate changes in blood pressure due to these temporal factors when diagnosing and considering treatment options for their patients.

Cynthia Thomas, DO, MPH, presented the research, "Circadian and Season Changes Have a Substantial Impact On Measurement of Elevated Blood Pressure in Practice," at the American Society of Hypertension 20th Annual Meeting in San Francisco this past weekend.

"Our research shows that seasonal and hourly variations in blood pressure can be quite substantial, and should be taken seriously by healthcare professionals treating patients for hypertension," said Thomas, who conducted the research with Walter "Buzz" Stewart, PhD, MPH, and C. Craig Wood, MS, both of Geisinger’s Center for Health Research and Rural Advocacy, and James Buehler, MD, Rollins School of Public Health, Department of Epidemiology, Emory University.

Although variability in blood pressure is well known, the magnitude of daily and seasonal variations is uncertain and the impact on clinical value has been disputed.

"To evaluate these issues, we analyzed records from nearly 25,000 patients to see if temporal factors like time of day and month of year could indeed be sufficient enough to warrant consideration during the diagnosis and management of hypertension. And the answer is, yes," Dr. Thomas said.

She and her colleagues analyzed 203,927 blood pressure measurements from 24,904 patients that had been compiled at 57 Geisinger Health System community practice clinics over a 3-7 year period. Statistical analysis was used to determine the probability of identifying blood pressure values at or above four different cutoff points (140 mm Hg and 120 mm Hg systolic, 90 mm Hg and 80 mm Hg diastolic) at different times of day and year.

"We found that depending on the time of year and day, the chances that an individual will have an elevated blood pressure measurement vary significantly," said Dr. Thomas. "Even at the hourly level, the probability of having a high blood pressure measurement during the afternoon versus the evening varied by as much as 26 percent."

Dr. Thomas and colleagues found that the lowest BP levels occurred in the morning and at midday with the highest seen at the end of the day. When analyzed monthly, the lowest levels occurred during the summer months and the highest during the winter months. Using a diagnostic cutoff point of 140 mm HG for systolic blood pressure, the difference in the odds ratio for the evening blood pressure (7:00 p.m.) compared to midday blood pressure (1:00 p.m.) was 1.26 (p<0.001). With the same diagnostic cutoff point (systolic blood pressure of 140 mm Hg), the comparison between summer and winter months was 1.24 (p<0.001).

"Our findings suggest that blood pressure can vary by as much as 40 percent as a result of the combined influence of the time of day and year. As these variances are clearly clinically significant, awareness of the effects of these temporal factors should be weighed in when diagnosing hypertension and making therapeutic decisions," she said.

Dr. Thomas does warn caution in interpreting these results due to certain limitations.

"Our results were not confounded by age, gender, race, history of hypertension or treatment status," said Dr. Thomas. "However, because our study is an observational analysis of real-world patient records, our conclusions do have some limitations, and additional studies are needed to reinforce and reconfirm our findings."

Geisinger Health System is a physician-led, integrated healthcare system serving approximately two million people in 31 rural counties in Pennsylvania. The system includes a tertiary/quaternary medical center with level one adult and pediatric trauma centers, the nation’s first rural children’s hospital, a women’s pavilion, a heart institute, a cancer institute, a second hospital, 42 community practice sites, two research centers, a fleet of helicopters and a health plan.

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