Systolic vs. Diastolic Blood Pressure: What's The Difference? (2024)

The top number of a blood pressure reading is your systolic blood pressure. It measures the pressure blood applied against the artery walls when the heart squeezes (“systole”).

“Having a normal systolic blood pressure is important for maintaining good blood flow to the body’s organs,” explains Jim Liu, M.D., a cardiologist at The Ohio State University Wexner Medical Center.

When systolic blood pressure is too high or too low, there’s either too much or too little pressure against the artery walls, and blood isn’t flowing through the body properly. This imbalance can lead to a range of health complications.

While there is some disagreement among experts, according to the Centers for Disease Control and Prevention (CDC), a systolic blood pressure reading under 120 mmHg is considered normal. However, Dr. Liu adds that readings under 90 mmHg are an indicator of low blood pressure and can be a cause for concern, too.

High Systolic Blood Pressure

High systolic blood pressure occurs when blood exerts too much pressure against artery walls while the heart beats. There are three stages of this condition:

• If systolic blood pressure falls between 121 and 129 mmHg, blood pressure is considered elevated.
• When systolic blood pressure rises above 130 mmHg, it’s diagnosed as hypertension.
  • Stage one hypertension is marked by systolic blood pressure readings between 130 and 139 mmHg.
  • Stage two hypertension is marked by systolic blood pressure readings that reach 140 mmHg and above.

Research suggests risk of cardiovascular mortality doubles with every 20mmHg increase in systolic blood pressure over a baseline systolic blood pressure of 115 mmHg^[1]Tackling G, Borhade MB. Hypertensive Heart Disease. National Library of Medicine 2022. . Despite this health risk, hypertension exhibits few or no symptoms at most stages, giving it a reputation as a “silent killer.”

“Most of the time, high systolic or diastolic blood pressure doesn’t cause symptoms,” says Dr. Liu. “However, when hypertension goes unnoticed and uncontrolled due to a lack of symptoms, it can lead to long-term health consequences.”

Checking your blood pressure at home or having it checked by a health care provider regularly is often the best way to determine whether you have high systolic blood pressure. A diagnosis of hypertension is never made on a single office blood pressure measurement, and confirming values at home is often necessary.

Low Systolic Blood Pressure

A systolic blood pressure reading below 90 mmHg is considered low, though this threshold may vary depending on the individual. For instance, people who typically have high blood pressure can experience low blood pressure symptoms at seemingly normal values.

Low systolic blood pressure is less common than high systolic blood pressure, but it can still cause health repercussions. Low systolic blood pressure is often associated with over-medication, bleeding, infection and advanced heart failure. Certain medication side effects or dehydration can also cause low systolic blood pressure.

No matter the cause, low systolic blood pressure (hypotension) often manifests with the following symptoms:

• Blurred vision
• Feeling faint
• Confusion
• Sleepiness
• Physical weakness

Diastolic blood pressure is the bottom number of a blood pressure reading, and it measures the blood pressure applied against the artery walls when the heart is filling between beats (“diastole”). Healthy diastolic blood pressure falls in the range between 60 and 79 mmHg, says John Higgins, M.D., a cardiologist with McGovern Medical School at UTHealth Houston.

“Up to about 10% of patients may have isolated elevation in their diastolic blood pressure,” says Dr. Higgins. However, he adds that people are more likely to have an isolated systolic blood pressure issue or abnormal measurements for both blood pressure numbers.

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High Diastolic Blood Pressure

When diastolic blood pressure rises above 80 mmHg, a person is diagnosed with high blood pressure. Stage one hypertension is marked by diastolic blood pressure readings between 80 and 89 mmHg. Stage two hypertension is marked by diastolic blood pressure readings that reach 90 mmHg and above.

Like high systolic blood pressure, high diastolic blood pressure often goes undetected. People with high diastolic blood pressure usually don’t experience symptoms, though elevated diastolic blood pressure can increase a person’s risk for heart disease, heart attack and stroke.

Low Diastolic Blood Pressure

Any diastolic blood pressure reading below 60 mmHg is considered low. Low diastolic blood pressure indicates that the blood vessels aren’t holding on to the pressure needed to keep the blood flowing. This condition can lead to feelings of dizziness and more frequent falls. A low diastolic blood pressure reading could be an indicator of infection, a leaky aortic valve, abnormal blood vessel connections or a thyroid disorder.

While both blood pressure numbers are important, many studies suggest systolic blood pressure has a more significant effect on overall cardiovascular health—which is especially true for people over the age of 50.

“High systolic blood pressure is more common,” adds Dr. Higgins, “But high diastolic blood pressure is also unhealthy and can increase one’s risk of diastolic heart failure, heart attack, atrial fibrillation, peripheral arterial disease and abdominal aortic aneurysm.”

For these reasons, health care providers often consider blood pressure numbers to determine a patient’s overall blood pressure health. Both numbers are important and can be used to diagnose a patient with high or low blood pressure, either independently or together.