Clinicians are told that patients should have an office blood pressure (BP) less than 140/90 and, in the presence of diabetes, less than 130/80. But we often see huge fluctuation in blood pressure readings. It’s said that most office-based blood pressure readings are inaccurate. Not only is there the well-known spike in BP often brought on by “white coat syndrome,” but pressure can vary significantly throughout the day, and small differences in a patient’s posture during testing can also move the mercury.

Just 10 mm Hg more or less can make a large difference to risk, yet repeated measurements can easily vary by as much as 30 mm Hg. The problem is further complicated by the fact that some patients really do have blood pressure that changes greatly throughout the day, while others don’t. We expect to see a fall in BP by 10% to 20% during the sleeping hours. Failure to fall — or an exaggerated nighttime fall in BP — is an adverse prognostic indicator.

Good office measurements

Two common errors in office-based measurements are: not supporting the patient’s back, and letting their feet dangle above the floor. Legs should be uncrossed and the arm should rest flat on a table so that the cuff is at heart level.

I currently try to have an average of 5 office blood pressure measurements before I see my patients. But the Canadian Hypertension Society notes that even among patients with five office BPs above target values, still 20% will actually have so-called normal blood pressures.

In fact, for this reason the Society recommends that all hypertensive Canadians should have a BP machine at home. That’s an ambitious target given that 95% of adult Canadians will develop high BP over a lifetime. Meanwhile, ambulatory BP monitoring is playing an increasing role in clinical practice.

Interpreting ambulatory numbers

I currently consider ambulatory blood pressure in patients who have fluctuating BP, to rule out white-coat hypertension or masked hypertension.

In patients taking multiple medications, I often want to see how BP is controlled over 24 hours. Ambulatory blood pressure is just a day in a person’s life and should never be interpreted alone, but a 24-hour average blood pressure less than 130/80 is nowadays often considered normotensive.

For someone on typical work hours, a good 24-hour average can be calculated from daytime measurements every 20 minutes from 9am till 9pm, and nighttime measurements every 30 minutes from 1am to 6am.

I encourage people to keep a log during the BP monitoring to note any activity or symptoms. Providing them with a copy of their results encourages patients to be actively involved in their health.