The guidelines also redefined the various categories of hypertension. It eliminated the category of prehypertension, which had been defined as systolic blood pressure of 120 to 139 mm Hg or diastolic pressure (the lower number in a reading) of 80 to 89 mm Hg. Instead, people with those readings are now categorized as having either elevated pressure (120 to 129 systolic and less than 80 diastolic) or Stage 1 hypertension (130 to 139 systolic or 80 to 89 diastolic).
The new guidelines stem from the 2017 results of the Systolic Blood Pressure Intervention Trial (SPRINT), which studied more than 9,000 adults ages 50 and older who had systolic blood pressure (the top number in a reading) of 130 mm Hg or higher and at least one risk factor for cardiovascular disease. The study's aim was to find out whether treating blood pressure to lower the systolic number to 120 mm Hg or less was superior to the standard target of 140 mm Hg or less. The results found that targeting a systolic pressure of no more than 120 mm Hg reduced the chance of heart attacks, heart failure, or stroke over a three-year period.
A nurse takes your blood pressure at your annual physical. The numbers are recorded and the checkup continues. But do you know where on the blood pressure chart your levels are? Are they healthy? Too low? Too high, meaning you have hypertension? If you have high blood pressure or are heading in that direction, you should know that hypertension is among the primary enlarged heart causes, and a major risk factor for heart failure.
The brain requires unobstructed blood flow to nourish its many functions. Very high, sustained blood pressure will eventually cause blood vessels to weaken. Over time these weaken vessels can break, and blood will leak into the brain. The area of the brain that is being fed by these broken vessels start to die, and this will cause a stroke. Additionally, if a blot clot blocks a narrowed artery, blood ceases to flow and a stroke will occur.
"Blood pressure guidelines are not updated at regular intervals. Instead, they are changed when sufficient new evidence suggests the old ones weren't accurate or relevant anymore," says Dr. Paul Conlin, an endocrinologist with Harvard-affiliated VA Boston Healthcare System and Brigham and Women's Hospital. "The goal now with the new guidelines is to help people address high blood pressure — and the problems that may accompany it like heart attack and stroke — much earlier."
Healthcare professionals use a stethoscope and a manual sphygmomanometer to measure your blood pressure. Typically they take the reading above your elbow. The sphygmomanometer has a bladder, cuff, bulb, and a gauge. When the bulb is pumped it inflates the bladder inside the cuff, which is wrapped around your arm. This inflation will stop the blood flow in your arteries. The stethoscope is used to listen for sound of the heartbeat, and no sound indicates that there is no flow. As the pressure is released from the bladder, you will hear the sound of the blood flowing again. That point becomes systolic reading. The diastolic reading is when you hear no sound again, which means that the blood flow is back to normal.
Peter, I’ve been using a blood pressure meter for nearly 30 years, so my response is based on my personal experience and information I’ve acquired over the years. First, I suggest that you take your meter to the doctor and have them check several readings of your meter against theirs. For example, If your meter consistently shows it’s 10 points lower than the doctor’s, just delete the 10 points from your meter reading (have them check both numbers so you can adjust both as necessary). Also, it’s common that many doctor’s offices take your blood pressure incorrectly (you should actually sit still for 5 minutes, with your feet on the floor and the cuff at the same level as your heart) . Some of us have”white coat” hypertension, so you may always be elevated at the doctor’s office. If you are a large man, you (and your doctor’s office) may need to use a larger cuff as the wrong size of cuff can affect your reading. Also, I spoke with customer service at one of the companies that makes many of the home & professional meters, and she told me that the automated machines are not very accurate if you have kidney disease or heart failure (I have both). Ask the doctor’s staff to always use the manual system and it will be more accurate than those noisy automatic ones.
The guidelines also redefined the various categories of hypertension. It eliminated the category of prehypertension, which had been defined as systolic blood pressure of 120 to 139 mm Hg or diastolic pressure (the lower number in a reading) of 80 to 89 mm Hg. Instead, people with those readings are now categorized as having either elevated pressure (120 to 129 systolic and less than 80 diastolic) or Stage 1 hypertension (130 to 139 systolic or 80 to 89 diastolic). 

As for when to check your blood pressure, the most important thing is to do it consistently the same time of the day (ask the doctor which time he prefers and also what time in relation to taking your medication). The following article has a lot of good information for someone just starting to monitor their blood pressure: https://www.drugs.com/cg/how-to-take-a-blood-pressure.html
If your blood pressure readings are consistently high, you and your doctor will probably discuss treatment strategies. Treatment for high blood pressure often begins with lifestyle changes such as a weight loss and exercise program as well as a low sodium diet. In fact, the AHA recommends adopting these strategies as a means of preventing the development of high blood pressure and heart disease. If these strategies are not successful in lowering your blood pressure, medications may be recommended. 

Typically, more attention is given to systolic blood pressure (the first number) as a major risk factor for cardiovascular disease for people over 50. In most people, systolic blood pressure rises steadily with age due to the increasing stiffness of large arteries, long-term buildup of plaque and an increased incidence of cardiac and vascular disease.
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