High Blood Pressure: What's Considered Dangerous?

According to 2018 figures compiled by the American Heart Association (AHA), about 103 million Americans struggle with high blood pressure, a figure that accounts for almost half of the nation's adult population. Globally, high blood pressure is thought to affect nearly a third of all adults.

Given that high blood pressure significantly increases the risk for both heart attack and stroke, there's no question that it's a serious and often deadly concern. In fact, AHA researchers say that, in 2015, about 79,000 Americans died as a result, a number that represents a nearly 38 percent spike since 2005.

The AHA explains that blood pressure is essentially the amount of force exerted by blood as it pushes up against blood vessel walls. Blood pressure is deemed to be "high" when the force of that push is higher than it should safely be.

The problem is that high blood pressure — also known as hypertension — is often symptomless. Many people don't even know they have it until a cardiac emergency, such as a heart attack, strikes. Prevention by means of routine blood pressure screenings is key.

A blood pressure reading is delivered as a ratio of a top number (systolic blood pressure) and a bottom number (diastolic blood pressure). Both numbers are expressed as millimeters of mercury. The top number references blood pressure when the heart is beating, and the bottom number refers to the pressure when the heart is at rest between beats.

Read more: What Causes High Diastolic Blood Pressure?

According to the AHA, a normal blood pressure reading for adults is a top number below 120 combined with a bottom number under 80 — noted as 120/80 millimeters of mercury.

However, physicians start to get concerned when the top number heads north. "Blood pressure is considered 'mildly elevated' if it's between 120 and 129 over less than 80," says Willie E. Lawrence, Jr., MD, chief of cardiology with Midwest Heart & Vascular Specialists, in Kansas City, Missouri. "We define blood pressure greater than 130 over 80 or more as high blood pressure, or hypertension," he says. "Once it's above 130, that's certainly considered high."

Specifically, the AHA characterizes a blood pressure of 130 to 139 over 80 to 89 as "Stage 1" high blood pressure. Even more risky is "Stage 2," which is when a reading is between 140 and 180 over 90 to 120.

"Now, where we get particularly concerned is when the top number is found to be greater than 180," Dr. Lawrence says. "In truth, there are plenty of people who run around living their life with 180 and feel nothing. They may be asymptomatic. They may have no idea that anything is wrong. But unfortunately for them, in many cases, their first indication that something is very wrong ends up being a heart attack, a stroke or congestive heart failure."

Read more: Reasons for High Systolic Blood Pressure

"We view a top number of 180 or more over anything over 120 as a possible sign of a 'hypertensive crisis,' and at that point we certainly have to be very concerned," Dr. Lawrence says.

As Mayo Clinic researchers point out, blood pressure readings at that level threaten the integrity of blood vessels, which can become chronically inflamed. Once that happens, the vessels can start to leak fluid or blood, undermining the heart's ability to effectively pump blood throughout the body.

The result could very well be a stroke or heart attack. Short of that, in some instances, a person in a state of hypertensive crisis may experience symptoms, including severe chest pain or headache, nausea, shortness of breath or even a seizure, explains Mayo Clinic. However, there's no guarantee that any noticeable symptoms will arise.

The takeaway: Make blood pressure screenings part of your health care routine. The results can be literally life-saving.

Read more: How Does Blood Pressure Change During Exercise?