24 January 2017
Norman swan
Apart from smoking, the biggest risk factor for heart attacks and strokes is high blood pressure. It batters and weakens arteries and is also linked to heart and kidney failure. So it makes sense for doctors to prescribe treatments that lower blood pressure. But how low should doctors go?
Last year, a US study suggested doctors should aim to lower blood pressure further than traditional guidelines recommend. Now an August study warns that might not be a good idea for everyone.
Blood pressure is measured in millimetres of mercury (mmHg), a reference to the old-fashioned sphygmomanometers that doctors still sometimes use. They wrap a cuff around your arm, which is attached to a glass column filled with mercury, and pump up the cuff while feeling and listening to the pulse in the brachial artery just below it. The cuff pressure is increased enough to stop the pulse in the artery.
Then with a whooshing sound, the air pressure is reduced until the doctor hears the thump thump of the pulse coming back. A reading of the mercury level indicates the systolic pressure, from the Greek word sustolē, meaning “to contract”. It’s a measure of the heart’s power stroke.
A normal reading is considered to be below 120 mmHg. More air is released, and the doctor listens for a switch to a softer thump or the sound disappears altogether. That’s the diastolic pressure, deriving from the Greek word for expansion, diastole.
As the heart expands to refill, the elasticity of the blood vessels maintains a back pressure, which is crucial to keep blood coursing through the vessels and tissues in between heartbeats. Normal is considered below 80 mmHg.
Patients are considered candidates for treatment if their systolic blood pressure is above 140 mmHg. That’s based on studies that have correlated systolic blood pressure with risks of heart attacks and stroke. We used to use diastolic pressure, but it’s no longer considered as good a predictor of risk.
The question, then, is what should be the target? Until recently doctors thought it was good enough to get below 130mmHg. But in 2015, the American SPRINT (Systolic Blood Pressure Intervention Trial) study published in The New England Journal of Medicine found that in people with multiple risk factors such as heart disease and raised cholesterol, aiming below 120mmHg significantly reduced the chances of stroke, heart attack, heart failure and dying prematurely. It came at the price of having to take more medications plus some side effects, including dizziness.
So should 120mmHg be the new target? It depends who you are. Last August, researchers at John Hopkins University in Baltimore published a paper in the Journal of the American College of Cardiology that suggested this low target could cause hidden damage to the heart in people with low diastolic pressure by lowering it further.
In some people with arterial disease, their stiffened arteries have lost the elasticity to provide good diastolic pressure. The result can be that their systolic pressure is a high 140mmHg but their diastolic is a low 65mmHg.
The researchers suspected that if the diastolic pressure was lowered even further by the aggressive use of drugs, their coronary arteries, which supply oxygenated blood to heart muscle when the heart expands, might not do their job properly. A heart that is missing out on its oxygen starts suffering muscle damage – a first step on the way to heart failure.
To test this idea, the researchers studied 11,500 people with an average age of 57 and compared their diastolic pressure with circulating levels of troponin – a protein that is released by a damaged heart. They found that people with a diastolic pressure less than 70 mmHg were more likely to have high troponin levels.
The take-away message for people being treated for high blood pressure may be that if they already have low diastolic blood pressure, then perhaps lowering blood pressure too aggressively could hurt their heart. In consultation with their doctor, they should take extra care until more studies are released that shed light on this perplexing and relatively ignored issue.