What’s the Difference between Systolic and Diastolic Blood Pressures?
Healthy blood pressure is a measure of overall good health in a person. For most adults, getting their blood pressure checked at the doctor is a regular occurrence, but many have no idea of what those two numbers mean. Here’s a quick explanation.
Blood Pressure Basics
Blood pressure (BP) readings are usually given as two numbers like 120/75 (pronounced one-twenty over seventy-five). More precisely, the reading is 120 mm Hg systolic blood pressure (BP) over 75 mm Hg diastolic BP, with the pressures measured in millimeters of mercury. The top number is systolic pressure, the pressure blood exerts on the arteries when the heart pumps. The bottom number is the diastolic pressure, the pressure blood exerts on the arteries between heartbeats (when the heart is at rest).
Healthy blood pressures range from 90/60 to 120/80. If a person has low diastolic BP (less than 60) and a normal systolic reading, they have an isolated diastolic hypotension. This can be caused by stiff arteries due to aging, diabetes, or fatty buildup in the arteries (atherosclerosis). Other symptoms include dizziness and loss of balance. Over time, it can lead to heart failure due to insufficient blood and oxygen reaching the coronary arteries.
Conversely, a high diastolic BP (over 80 mm Hg), hypertension, can cause symptoms including nose bleeds, heart palpitations, nausea, and headaches. It is found most often in young adults with no specific underlying cause found. But it can be caused by thyroid problems, kidney disease, and sleep apnea.
If the systolic pressure is low (below 90 mm Hg) the patient is considered to have low systolic BP. Causes include heart conditions, pregnancy, endocrine problems, dehydration, lack of nutrients in diet, medications, and diabetes.
Having above-average systolic BP (130 mm Hg or higher), hypertension, is probably the most dangerous condition . High pressure can cause tears in the arteries. A chemical, LDL cholesterol, can build up damaged blood vessels and form a layer called plaque. Plaque makes arteries narrower and raises blood pressure even higher. When that happens, the arteries carrying oxygen to the heart can get blocked and cause a heart attack. If oxygen-carrying blood is similarly blocked from reaching the brain, it can cause a stroke. Hypertension is caused by stiff arteries, diabetes or an overactive thyroid gland. This is the most common type blood pressure problem in people over 65.
Measuring Blood Pressure
In the early 1900s, Nicolai Korotkoff introduced a way of measuring systolic and diastolic pressure by using a sphygmomanometer (blood pressure gauge) and stethoscope.
Nurses usually take blood pressure readings on the left upper arm using a sphygmomanometer. This is because the brachial artery is relatively close to the surface of the skin and is closer to the heart than the right brachial artery. However, there are cases when that may not be possible, such as if the patient is a double amputee or has excessive spasticity in the arms. Then blood pressure readings are taken on the ankle or calf. These reading can be higher than those taken on the arm.
To take a manual blood pressure reading, the nurse wraps the gauge’s pressure cuff around the patient upper arm and fastens it with Velcro. The nurse uses a stethoscope to listen just above the crease of the elbow and locates the strongest pulse sounds. She then inflates the blood pressure cuff, which puts pressure on the brachial artery and increase that pressure until the pulse can no longer be heard—i.e. blood flow through the artery has been stopped.
Then, the nurse gradually deflates the cuff until she first hear a thumping or knocking sounds (called Korotkoff sounds, the sounds of blood flowing through the artery again) and notes the pressure. This is the systolic pressure—the pressure blood exerts on artery walls when the heart beats or contracts.
To get the diastolic pressure, the nurse continues to let air out of the cuff and the thumping noise will change to a “whooshing” sound, at which point she reads the pressure, the diastolic pressure. It is the pressure the blood exerts on the arteries between beats when the heart relaxes.
Blood pressure readings naturally fluctuate, so nurses commonly take readings several times before making a diagnosis of abnormality.
This method is reliable if done by a practiced person. To improve accuracy, the sphygmomanometer is often mounted to a wall so it is anchored and stable.
A healthcare practitioner can also take blood pressure readings without the stethoscope by feeling the presence or lack of it pulse in the patient’s wrist, a method called palpation.
Several factors affect blood pressure readings. In fact, a 2017 study found 27 factors cause false blood pressure readings for adults in clinical settings. For example, some patients experience the “white-coat effect,” where their blood pressures rises because they are nervous being in a doctor’s office. This can add 10 mm Hg to the systolic pressure.
Whether the patient is sitting or standing when the readings are taken also creates inaccurate readings, as could a lack of support for the patient’s back or arm while getting blood pressure taken or having the arm lower than the relative level of the heart.
Another major source of inaccuracies are sphygmomanometers that are not regularly calibrated. Researchers in Sweden studied how calibration affects accuracy. They concluded that the blood-pressure meters should have errors of no more than ±3 mm Hg. But they also found that most sphygmomanometers in the study had not been recalibrated within two years, and the accuracy of the readings went down significantly after going that long without being recalibrated.
The study also found that less than 40% of the physicians taking part could recall the last time their blood-pressure meters were recalibrated and that the average time between recalibrations was almost 5.6 years. That’s why best practice is to recalibrate them annually.
Another source of error applies to automated home versions of the sphygmomanometer. They use similar cuffs, but inflation and deflation are automated, microphones replace nurses and stethoscopes, and electronics use oscillometric algorithms to determine systolic and diastolic pressures
In these oscillometric-based devices, the machine detects the oscillations (Korotkoff sounds) due to blood flow when cuff pressure is the same as systolic pressure. As the machine slowly releases pressure from the cuff, the oscillations’ amplitude increases and then decreases as blood flow to the arm returns to normal. Although the oscillation with the greatest amplitude corresponds reliably with mean arterial pressure, the systolic and diastolic readers are often less accurate compared to the tractional method of having a nurse or doctor listen for the sounds.
In fact, oscillometric methods tend to overestimate systolic and underestimate diastolic pressures. A study of home blood pressure machines using the oscillometric technique found most of them were off by an average of 5 mm Hg. That’s not too bad, however, as one study found manual methods could be even less accurate—up to 15 mm Hg higher.
Mean Arterial Pressure
Health care staff use mean arterial pressures rather than systolic and diastole pressures. Mean arterial pressure (MAP) is the average pressure in the arteries from the beginning of one heartbeat to the beginning of the next. It’s considered a better indicator of flow to the organs than systolic blood pressure (SBP) and diastolic blood pressure (DBP). When treating patients with conditions such as strokes and intercranial bleeding, health care professionals often use MAP readings to determine the effectiveness of some treatments intended to reduce blood pressure.
In hospital settings, doctors use invasive methods that involve catheterization to put a sensor in the blood stream to measure MAP. There’s also a simple formula for calculating MAP based on conventional blood pressure readings:
MAP = 0.33(SBP) + 0.66(DBP)
The formula is based on the fact that the circulatory system is at DBP about two-thirds the time and SBP the other third. A MAP reading of 70 to 110 mm Hg is considered normal.
Automated blood pressure machines are now common sights in pharmacies, and they let people get their own blood pressure reading in minutes. But, health providers warn that those machines may not be accurate. That’s because there are no requirements for those machines to be tested for accuracy. To make matters worse, they might not be checked or regularly serviced. Even worse, they may not get maintained often or at all.
