Is There a Safe Blood Pressure Reading?

photo by marianoheluani / iStock

BLOOD PRESSURE, while clearly a concern for some, still confounds medical experts.  And everyone should pay attention when the thermometer starts rising in the summer heat.

My blood pressure, which hovers around 90/60, has always been considered a sign of good health in the United States, but if I were in Germany, for example, it would be deemed too low and worthy of treatment.  When I was in the middle of minor surgery in Brussels, where I lived for several years, suddenly medical personnel began racing around and pulling equipment off the walls as if my heart had just stopped.  When my husband mentioned that my blood pressure is usually low, the activity died down and the room became silent except for one quiet “Oh.”  The procedure proceeded as planned.

Early this year, U.S. physicians once again changed the blood pressure levels considered healthy. Despite several such changes and an enormous amount of research, however, no one is entirely sure what levels are “safe,” whether “unsafe” levels really cause the problems they are accused of causing, and whether the drugs that purport to lower the levels do a good job.

First, the acceptable levels.  Blood pressure is the force of blood against the blood vessel walls. High blood pressure, or hypertension, makes the heart work harder to pump the blood, and too much stress on the heart and arteries can damage them over time.  Hypertension contributes to hardening of the arteries and the development of heart failure, and creates a higher risk for heart attack, stroke and kidney disease.

In January, 2014, the highest level deemed safe for the systolic reading, the upper number in the equation, which is the pressure measured when the heart contracts to pump blood, was increased from 140 to 150mmHg (millimeters of mercury).   What has been considered the “healthy” goal of less than 120/80 was also questioned, because there is little evidence of any health risks below 140. (The lower number is the diastolic pressure, measured when the heart is at rest.)

Because blood pressure usually rises with age, doctors used to add 100 to the patient’s age, making 160 an acceptable upper systolic limit for a 60 year-old.  (In the U.S., those 60 years old and over take the majority of anti-hypertensive drugs.)  By the late 1960s, the system was deemed too casual, and the number 160/100 was applied to every age group.  The next change occurred in 1997 when the safe level was lowered to 140/90.  The Joint National Committee (JNC), which makes these decisions, is composed of leading cardiologists and experts on hypertension.

There is also the issue of timing: Blood-pressure readings are highest in the morning.  They can also vary from one arm to the other.  For some people, “white-coat hypertension” occurs when simply being at the doctor’s office, before the pressure is measured, causes the systolic level to rise by as much as 30 mmHg.  People who have this experience can do their own monitoring at home, which works best using spygmometers (blood pressure monitors) that fit around the upper arm, not the finger or wrist.

Hypertension, considered a silent killer because there are no symptoms, affects one in every three American adults, according to the American Heart Association.  About 95 percent of those with high blood pressure have “essential hypertension,” meaning there is no known cause.  Most of them are “salt sensitive,” so that any added salt increases their blood pressure.  People living in the north of Japan eat more salt and have the highest incidence of hypertension of anyone elsewhere in the world.

For high readings, doctors often prescribe medications, although evidence for their efficacy is “only moderate,” says the JNC.  And one of these, beta-blockers, has recently been shown to cause stroke and hypotension (low blood pressure) in surgical patients who are hospitalized.  Research on African-Americans has shown that antihypertensive medications should be started right after getting a high reading.  But for everyone else, the preferable recommendations are the usual: more exercise, less processed foods, more exercise, less salt, etc.

In the absence of a good spygmometer, one thing that can separate the low from the high in terms of blood pressure is the hot tub.   Those who feel lightheaded, dizzy or exhausted after getting out may have low or high pressure.  But those who feel this way while still in the tub definitely have low pressure caused by a myriad of factors: less gravity means the heart has less difficulty pumping blood; heat makes your blood vessels dilate, creating  less resistance to the heart’s pumping; and lower fluid volume is caused by dehydration as a result of sweating.  Mental and physical relaxation also play a role.

When outdoor temperatures rise, blood pressure can decrease during the day and increase at night.  In very hot weather, the heart has to work harder to pump blood to your skin’s surface to assist with sweating to cool the body.  (Blood pressure can also rise in cold weather when blood vessels constrict and the heart has to work harder to push blood through the veins and arteries.)  Tips to avoid blood-pressure changes in the heat include wearing lightweight and light-color clothing, staying hydrated, and, surprisingly, paying attention to your feet: most people sweat the most inside shoes, so well-ventilated footwear and wicking socks can be helpful.

People with low blood pressure can also nod off more easily, for example, during long drives or at evening cultural events – or at least that was my father’s excuse when we both fell asleep at the opera.

— Mary Carpenter