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Treating blood pressure in two dimensions

When you get a blood pressure reading, the clinician always gives you two numbers – “120 over 70,” for example. These numbers correspond to your blood pressure when your heart is contracting (the first number, called systolic blood pressure) and when your heart is relaxing (the second number, diastolic blood pressure). Historically, cardiology guidelines have focused on lowering systolic blood pressure to lessen risk of stroke. But are we ignoring potential consequences of low diastolic blood pressure by focusing so much on systolic blood pressure?

A new look at SPRINT

new study from researchers at McGill University and the University of Toronto suggests that we need to be paying attention to both sides of the blood pressure equation. As part of an open data competition, internal medicine physicians and researchers Dr. Todd Lee (McGill University), Dr. Rodrigo Cavalcanti (Toronto’s University Health Network) and Dr. Emily McDonald (McGill University) organized a team of colleagues to do a secondary analysis of the SPRINT study, a randomized controlled trial that studied the effects of intensive and standard systolic blood pressure targets on cardiovascular outcomes. 

The team was interested in the effects of aggressive, or “tight” systolic blood pressure control, on patients’ diastolic blood pressure, and the effects on cardiovascular outcomes. The diastolic period is where the heart’s blood vessels receive blood, so there is concern that lowering the diastolic blood pressure too much may harm the patient, particularly if they have pre-existing heart disease. 

“When you lower one blood pressure number, you also lower the other,” said Dr. Lee. He cautioned, “maybe there’s a point at which the harm from lowering diastolic too much outweighs the benefit of lowering systolic blood pressure.”

How low can you go?

The researchers wanted to measure specifically the effect of treatment-associated low diastolic blood pressure, so they excluded patients from the sample who already had low diastolic blood pressure when they started the trial. They found that patients in the SPRINT study who developed low diastolic blood pressure as a result of treatment were at increased risk of having the combined outcome of dying or having a heart attack, stroke, heart failure, or other acute coronary syndrome. 

Measuring all-cause mortality rather than just cardiovascular mortality was a deliberate decision, to make sure to include potential deaths caused by low blood pressure. “In these big trials, it’s sometimes hard to classify death correctly,” said Lee, “Maybe people who died of low blood pressure didn’t get it measured because they fainted or fell at home, so the cause of death was not attributed to low blood pressure.” 

This finding doesn’t mean that we should leave high systolic blood pressure alone. Their findings showed that patients who had tighter systolic blood pressure and normal diastolic blood pressure had the best outcomes. However, it is possible that making the diastolic pressure too low probably took away some of the benefit of tight systolic control, said Lee. Maybe there could have been an even greater benefit to patients if they had stopped reducing systolic blood pressure before the diastolic dropped too low.

Implications for practice

What can clinicians do with this information? Dr. McDonald hopes this study will be an impetus for clinicians to talk with patients about finding a balance between the two numbers.

“If I knew it would take a patient several medications to get to 120 and their diastolic blood pressure is already low, I would talk to them about the risks and benefits of tight treatment before just prescribing the medications,” said McDonald. “We want doctors to think about the diastolic blood pressure, and realize that pushing it too low could take away some of the benefits of controlling systolic pressure.”

McDonald and Lee believe there needs to be more research that examines both aspects of blood pressure, so we can get a better idea of the costs and benefits of tight systolic control. “We think this study is an important contribution, but we still don’t know whether the relationship between low diastolic pressure and cardiovascular events is only an association or a cause,” says Lee, “We may need another trial!”

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