Understanding Systolic and Diastolic Blood Pressure

Every blood pressure reading consists of two numbers, written one over the other. Most people know that lower is generally better, but fewer know what each number represents. Understanding systolic and diastolic pressure makes your readings far more meaningful.

How the heart creates pressure

With each beat, the heart contracts and pushes blood into the arteries. Pressure in the arteries peaks at this moment. The heart then relaxes and refills, and the pressure falls to its lowest point before the next beat. A blood pressure reading captures both the peak and the trough.

Systolic pressure

The first, or top, number. It measures the pressure your blood exerts against artery walls when the heart beats. It responds quickly to activity, stress and emotion.

Diastolic pressure

The second, or bottom, number. It measures the pressure against artery walls while the heart muscle rests between beats.

Units

Blood pressure is measured in millimeters of mercury, written mm Hg, a unit dating from the first accurate pressure gauges, which used a column of mercury. A reading of 120/80 mm Hg is said as “120 over 80”.

Blood pressure categories

The American Heart Association uses the following categories for adults:

• Normal: systolic below 120 and diastolic below 80
• Elevated: systolic 120 to 129 and diastolic below 80
• High blood pressure (hypertension) stage 1: systolic 130 to 139 or diastolic 80 to 89
• High blood pressure (hypertension) stage 2: systolic 140 or higher or diastolic 90 or higher

Note the words “and” and “or”. If your two numbers fall into different categories, the higher category applies. A reading higher than 180 systolic or higher than 120 diastolic requires prompt medical attention.

Other countries use somewhat different thresholds. A diagnosis is made by a healthcare professional, based on an average of readings taken on more than one occasion.

Which number matters more?

Both. The American Heart Association notes that systolic pressure typically receives more attention as a major risk factor for cardiovascular disease in people over 50. In most people, systolic pressure rises steadily with age as large arteries stiffen and plaque builds up over time. However, an elevated systolic or an elevated diastolic reading alone may be used to diagnose high blood pressure.

Pulse pressure

The difference between the two numbers is called pulse pressure. For 120/80, it is 40. Doctors sometimes consider it alongside the main readings, particularly in older adults.

Why high blood pressure matters

Sustained high pressure damages arteries and forces the heart to work harder. Over years, this raises the risk of:

• heart attack and heart failure;
• stroke;
• kidney disease;
• vision loss;
• vascular dementia.

The World Health Organization describes hypertension as a major cause of premature death worldwide. Because it usually causes no symptoms, measurement is the only way to detect it.

Low blood pressure

Blood pressure that is lower than usual is not a problem for many people. It becomes a concern if it causes dizziness, light-headedness, fainting, blurred vision or fatigue. Report these symptoms to your doctor.

Why readings change

Blood pressure is not fixed. It is typically lowest during sleep, rises on waking, and varies with:

• physical activity;
• stress, pain and anxiety;
• caffeine, alcohol and nicotine;
• a full bladder;
• body position;
• medicines.

This is why readings are taken at rest, and why an average of several readings is more informative than any single one.

Seeing both numbers at home

An automatic monitor shows systolic, diastolic and pulse together. Our Automatic Upper Arm Blood Pressure Monitor displays all three and stores previous readings.

The bottom line

The top number is the pressure of each heartbeat; the bottom is the pressure between beats. Both count, and both are best judged as averages over time with your doctor. Find home monitors in our heart and blood pressure department.

Sources

1. American Heart Association. Understanding Blood Pressure Readings.
2. National Heart, Lung, and Blood Institute. High Blood Pressure.
3. MedlinePlus. High Blood Pressure.
4. World Health Organization. Hypertension.