What happens to blood pressure during extreme heat? A 4.2 million-visit study offers a clue

A US study spanning more than 4.2 million summer clinic visits examined how blood pressure responds to extreme temperatures, including whether age and hypertension alter that response.

Study: Extreme Heat Exposure and Blood Pressure in Community Health Center Patients. Image Credit: M-Production  / Shutterstock

In a recent study published in the journal JAMA Network Open, a group of researchers assessed the association between short-term exposure to extreme heat and systolic and diastolic blood pressure among adults receiving ambulatory care at community health centers in the United States.

Background

What happens to blood pressure when extreme heat arrives? Heat exposure is recognized as a concern, while blood pressure responses outside laboratory settings remain uncertain. Short-term heat can trigger competing responses: vasodilation and redistribution of blood flow toward the skin may lower blood pressure, whereas dehydration, increased cardiovascular demand, and sympathetic activation may raise it. Research has reported varied blood pressure responses and uncertainty about vulnerability among people with hypertension. Community health centers serve low-income populations and routinely record blood pressure, offering an opportunity to examine responses in outpatient care. 

About the study

Researchers conducted a cross-sectional study using electronic health record data from OCHIN, a national network of 2,554 community health centers across 40 states in the United States. The study included adults aged 18 years or older who had a visit with a recorded pressure measurement in June, July, or August from 2019 through 2023. Encounters were the unit of analysis, and patients with multiple visits contributed observations. When multiple readings were available, their mean was used after restricting values to 70-269 mm Hg for systolic pressure and 50-149 mm Hg for diastolic pressure.

Heat exposure was derived from GridEX, a 500 × 500-meter gridded dataset that provides apparent-temperature estimates incorporating air temperature, humidity, and wind speed. Extreme heat was defined using the Excess Heat Factor, and intensity was quantified using the Extreme Heat Magnitude Indicator (EHMI). Clinic locations were geocoded and linked to grid-cell values, with exposure defined as an EHMI greater than 0 on the visit day. Models assessed exposure on the day of the visit and over an 8-day window. Mixed-effects models accounted for repeated patient visits and differences between facilities and adjusted for demographic characteristics, hypertension and medication status, comorbidity burden, year, region, and time of day. Analyses were limited to encounters with complete data.

Study results

The analytic dataset contained 4,221,866 ambulatory encounters during the summer months. The cohort had a mean age of 47.6 years (standard deviation, 17.0), and 63% of encounters involved females. Most encounters (81%) involved patients without a hypertension diagnosis; 18% involved patients receiving antihypertensive medication; and 1% involved patients with hypertension without medication. Extreme heat occurred during 106,428 encounters (2%), while 4,115,438 (98%) occurred without extreme heat. Mean systolic blood pressure was 124.8 mm Hg in unexposed encounters and 124.0 mm Hg in exposed encounters. Mean diastolic blood pressure was 76.5 and 76.2 mm Hg, respectively.

After adjustment, same-day extreme heat was associated with a 0.7-mm Hg reduction in systolic blood pressure (95% confidence interval, −0.8 to −0.6) and a 0.3-mm Hg reduction in diastolic blood pressure (95% confidence interval, −0.3 to −0.2). Patient-level intraclass correlation coefficients were 0.40 and 0.41 for systolic and diastolic blood pressure, respectively, while facility-level intraclass correlation coefficients were 0.05 and 0.08.

Systolic blood pressure differences were consistent across percentiles, with values approximately 1 mm Hg lower at the 10th and 50th percentiles and 2 mm Hg lower at the 90th percentile during extreme heat. Diastolic blood pressure showed no meaningful differences across percentiles. Cumulative distribution curves indicated a population-wide systolic shift rather than a large change concentrated in a subgroup.

Among adults aged 65 years or older, extreme heat was associated with a 1.1-mm Hg lower systolic blood pressure and a 0.8-mm Hg lower diastolic blood pressure. Among adults younger than 65 years, corresponding differences were 0.6 and 0.3 mm Hg. Interaction tests for both measures had P < .001. Hypertension status modified the systolic association (omnibus interaction P = .003). The adjusted systolic difference was 0.6 mm Hg lower among patients without hypertension, 1.0 mm Hg lower among those with hypertension receiving medication, and 0.6 mm Hg lower among those with hypertension without medication; the confidence interval for the latter included no difference. Hypertension status did not modify the diastolic association (omnibus interaction P = .84).

Distributed lag analyses showed the largest reductions on the encounter day and for extreme heat exposure one day earlier, with associations weakening by three days before the visit. Across lags 0 through 7, the cumulative difference was 0.8 mm Hg lower for systolic blood pressure and 0.5 mm Hg lower for diastolic blood pressure. Among older adults, encounters involving dizziness, dehydration, or heat-related illness were associated with lower blood pressure, and this population-level association persisted after exclusion.

The study had several limitations. Exposure was assigned based on clinic rather than residential locations; routine blood pressure measurement procedures were not standardized, and heat-related changes in healthcare use could have influenced which visits were captured. The study could not assess individual antihypertensive drug classes, and not every patient contributed visits under both exposed and unexposed conditions.

Conclusions

The study found that acute extreme heat exposure was associated with modest reductions in systolic and diastolic blood pressure among adults receiving outpatient care at community health centers. Reductions were slightly larger among adults aged 65 years or older. Hypertension status statistically modified the systolic association, though differences between groups were small, and did not modify the diastolic association. The observed differences were unlikely to be clinically meaningful individually, but the authors cautioned that these outpatient estimates may not capture patients with larger heat-related blood pressure reductions who seek emergency or urgent care.

The findings indicate that blood pressure responses to environmental heat in everyday community settings during extreme heat may differ from those observed under controlled laboratory conditions. They also suggest that acute blood pressure changes are unlikely to be the primary pathway connecting extreme heat with cardiovascular risk. Other physiological pathways may play a larger role in heat-related cardiovascular harm. More research is needed to clarify heat-related cardiovascular responses.